Wednesday, April 24, 2013

April 24, 2013: Showers, Tstorms Coming This Evening

An approaching cold front will sweep through the cities this evening.  A narrow band of showers and tstorms has developed ahead of the front, and is presently moving across the Appalachians.  The line will move through the cities b/t 8-10 PM tonight.   Expect showers, gusty winds and lightning. 

High-resolution forecast models and radar trends suggest that this activity may be more of a threat north of the Mason Dixon Line.   The strongest storms are located over central-southern PA, with isolated severe thunderstorm warnings there.    The Storm Prediction Center expressed a 40% probability that a Severe Thunderstorm Watch would be needed this evening, encompassing PA and north-central MD, as far south as Baltimore, Howard and Carroll Counties.  However, as of 5 pm, no watch has been issued.  Conditions (instability, wind shear) are marginal for sustained or widespread severe tstorms.

Friday, April 19, 2013

April 19, 2013: Severe Tstorms Possible This Evening

The NWS has issued a Tornado Watch - which also includes the probability of severe tstorms (i.e. damaging wind gusts, heavy rain, small hail) - for the entire D.C.-Baltimore region until 10 pm tonight.

What we are tracking:  A broad band of showers and thunderstorms, forming ahead of a cold front approaching from the west.  The front is presently crossing the Appalachians.   Individual storm cells will move in from the southwest at about 40 mph.  The timing is most likely between 6-10 pm.

What are the odds?   Severe tstorms require strong wind shear and an unstable atmosphere.   This afternoon, because of the overcast, the sun has not been able to heat the surface, and this has diminished the instability.   However, uplift along the front is quite vigorous, and the increase in wind speed and wind direction changes with altitude (wind shear) are very strong.   When severe thunderstorms form in this type of setting (weak instability and strong shear) they tend to be low-topped supercells or short, bowing line segments.   The "mini-supercells" can produce a weak or moderate intensity tornado.  Bowing lines (called bow echos) can create strong straight-line winds called downbursts, and isolated small tornadoes.

Heavy rain is likely because the moisture content of this air mass is very high, and storms have the potential to "train" or repeatedly hit the same location over and over again.   One thing that may catch folks by surprise - there may be little or no lightning associated with these "thunderstorms" because of the limited instability.

Here are the 4 pm radar and lightning maps, showing intense storm cells crossing the central Appalachians (top panel).   At the same time, the nearest cluster of lightning is along the NC-VA border (bottom panel).  There are active severe thunderstorm and tornado warnings several hundred miles to our southwest.

The NWS Storm Prediction Center feels that the greatest likelihood of severe weather is over central NC and southern VA.  This is where the air mass is most unstable, and overlaps with the region of strong wind shear.  They state the probability of a tornado for any one location in our region is 5% and damaging winds is 15%.

The most likely weather impact at any one location:  2-3 hours of heavy showers and strong wind gusts, with some limbs down, and a few rumbles of thunder.  Isolated pockets of power outages.  I would not be surprised to see the Tornado Watch canceled earlier than 10 pm.

Thursday, March 28, 2013

March 28, 2013: One More Shot of Cold Next Week

In my earlier post, the models were suggesting that a new coastal storm would develop around the middle of next week, on the heels of an arctic air mass dropping south out of Canada.

Over the past couple of days, the models have trended away from a storm...but maintain a strong late-march surge of arctic air.  The arctic front will sweep south across the Mid Atlantic early next week, pushing all the way offshore into the Gulf of Mexico by Thursday morning:

With strong high pressure in control of the Mid Atlantic's weather next week, it does not appear that any storminess will develop.  After highs near 65 F on Monday, temps will drop into the mid-upper 40s on Wednesday-Thursday, but weather will remain fair.

Tuesday, March 26, 2013

March 26, 2013: Spring Ain't Here...Not By A Long Shot

Enjoy the slow ramp up to temps in the low 60's this weekend - because another blast of arctic air will likely arrive around Monday of next week.  And...two of the major medium-range prediction models are creating yet ANOTHER coastal low scenario around Wednesday of next week (April 3)...with the suggestion of more wintry weather/snow for our region.  Nothing like receiving the dead of winter three months late!

Here is the latest ECMWF model forecast, next Thursday morning:

Another snowstorm?  Courtesy Weatherbell Analytics.
Don't let the mild weather this weekend fool you!  The NAO (North Atlantic Oscillation) is still negative, the Greenland Block is still in place, and Mother Nature is reloading the Siberian Express for our region. 

Here is the latest GFS model forecast for snow accumulation, valid next Tuesday:

Not a lot of snow for D.C. - Baltimore, but the presence of an Arctic Front in the Mid Atlantic makes me nervous.  Courtesy of Weatherbell Analytics.

As they always's gonna get worse around here, before it gets any better.   Look for persistent, mild, springlike weather around the 3rd week of April. 

Saturday, March 23, 2013

March 23, 2013: Slushy Mess Still On Tap Monday

***10 PM UPDATE***
See the latest NWS snow accumulation map for our region, below, through 8 pm Monday night.

My forecast is still on track, based on all the reasons I gave in my blog yesterday (March 22).

Here are some accumulation numbers.

First - the probability, according to the NOAA Weather Prediction Center, that D.C.-Baltimore will see > 4" of snow is about 30%.  There is zero probability that the Metro region would see at least 8".  The probability of at least 4" is much higher in the elevated terrain to our west and north:

Probability of at least 4" of snow.  Blue = 10%, green = 40%, red = 70%.
Second - the NWS forecast office in Sterling, VA is beginning to issue snow accumulation maps.  Their latest (10 pm) is shown below. 

Friday, March 22, 2013

22 March, 2013: A Slushy Monday AM Commute

We are on track for an East Coast storm Sunday afternoon through Monday morning.   This is not expected to be a high impact storm for several reasons:

1)  The storm is split into two parts - a vortex over the Ohio Valley and a coastal low.   As the Ohio Valley storm weakens, the coastal takes over.  Anytime this transfer process occurs i.e. one storm robs energy and moisture from the other, the result is often a disorganized, weak field of precipitation left in between the two systems - conincident with the D.C.-Baltimore corridor;

2)  The coastal low, while intensifying, will be pulling its precipitation and energy away from the coast fairly rapidly;

3)  There is no deep, subfreezing air mass to our north, and thus no feed of intensely cold air into either storm system.   This has been a major reason why coastal storms have failed to produce measurable snow in our region all winter;

4)  Enough mild air may be pulled in off the warm Atlantic to keep the lowest few 1000 feet of atmosphere above freezing for some of the time - thus facilitating more of a wintery mix;

5)  The coldest air will be over the higher elevations to our west and northwest...and as usual this winter, these regions will likely experience the most prolonged period of all snow;

6)  There is limited moisture inland in between this complex of systems, generally 0.5" of total precipitation...this does not translate into large snow accumulation particularly with rain and sleet mixing in.

Many government and private forecast agencies are starting to issue snow accumulation maps.  The one I feel most comfortable with thus far, in terms of painting the general snow accumulation across the Mid Atlantic, comes from Accuweather:


Thursday, March 21, 2013

March 21, 2013: Wintry Mix Likely Sunday-Sunday Night-Monday

We are now 3 days out from this next winter weather event - even though it is now Spring :)

The various models (ECMWF, GFS, Canadian) continue to portray different evolutions of a complex, double-barrel storm system forming over the East.   All models develop heavy snow within the Mid Atlantic as the primary low over the Tennessee Valley transfers its energy to a coastal low Sunday night.  A shallow layer of cold air will remain wedged in between the systems, dammed up against the Appalachians.

However, the trends and consensus among the models support the notion that the D.C.-Baltimore region will NOT be in the heavy snow bulls eye;  that will unfold over the higher elevations either to our west, or to our north and west.   Warm enough air will invade on east winds off the Atlantic to prevent an all-out heavy snow storm.  What appears to be on tap for here is a 12-18 hour period of "wintry mix" meaning rain, sleet, and wet snow.

Here are two of today's model runs, and their snow accumulation predictions:

WeatherBell Analytics
This one is from the GFS model.   The snow bulls eye, up to 1 foot, is over Garrett County and he WV Panhandle.   D.C. and Baltimore pick up about 3".

Here is the Canadian model's forecast snow accumulation:

Weatherbell Analytics
Note the heavy snow bulls eye (again, 1 foot) over the PA Poconos and Delaware Water Gap region.  Zero frozen precipitation occurs over our Metro region.

The NOAA Weather Prediction Center states that the chance of a major D.C. - Baltimore - Philadelphia snow storm is 10%.

Again, there is a complex storm evolution involved here, so future refinements to this scenario are probable.

Wednesday, March 20, 2013

March 20, 2013: A Storm Looking More Certain Sunday Night

A consensus is emerging that a complex coastal storm will develop off the Eastern Seaboard this Sunday into Monday.

However, most computer models suggest that there will be two pieces of energy - a primary system over the Tennessee Valley, and a secondary or coastal piece of energy off Cape Hatteras. 

There are significant differences in the models on which storm will predominate, and in particular their tracks and intensities. 

For now, a wintry mix is on the table for the D.C. - Baltimore metro region, with elevation-dependent snow (possibly heavy) over the mountains to our north and west. 

Once the storm departs Monday morning, a reinforcing blast of chilly arctic air returns to start the first week of April.

Here is the best guess concerning positions and intensities of the two storms, from the NOAA Weather Prediction Center:

Forecast Map, Sunday Morning
In this map, note the two areas of low pressure (Tennessee Valley and Georgia).   A weak wedge of cold air damming remains in place between the two systems, over the Mid Atlantic (blue "H" over NY-PA).

Forecast Map, Monday Morning
Note how the two systems remain separate. Eventually the primary storm over the Tennessee Valley weakens, drifting northeast, while the secondary of coastal system deepens as it moves northeastward away from Cape Hatteras.  Neither storm is very intense.  This is a bit of an "outside track" for a coastal low to clobber us with heavy snow.  However, with the western piece of energy and enough cold air in place, we could still pick up accumulating snow in some parts of the region.

Bottom line:  I expect a rain-sleet-snow mix here overnight Sunday, with a low chance of a major snowstorm. 

Stay tuned as the details of this complex forecast continue to get better resolved.

Tuesday, March 19, 2013

March 19: Next Potential Storm Still Highly Uncertain

In year’s past, I have blogged about the lessons learned from the 1942 Palm Sunday (March 29) snowstorm, which buried Baltimore in 22 inches of heavy, wet snow.    It was outright crippling in terms of downed trees, broken utilities and clogged transportation arteries.    There was a real “shock and awe” factor with this one – as the following pictures attest:
Library of Congress

Library of Congress
So, even though this is an extreme “outlier” event, there is precedent;  it could happen again.    Today, the computer models continue to advertise a significant coastal low or Nor’easter in the March 25-26 timeframe.    This, thanks to an unusually cold late-March air mass coming down from Canada.    When air this cold moves along the warm Atlantic’s Gulf Stream the Polar Front becomes prime breeding ground for a coastal storm.
With the possible storm still about a week out, there continues to be a lot of uncertainty in the model projections…in terms of storm location, intensity, track and manner in which it taps the cold air.    Odds are against a major snow storm during early spring, due to high Sun angle and warmer air masses to our south and east.  Also, the western Atlantic water is very warm, and if this air invades the D.C. – Baltimore region (in the lowest few thousand feet), it means a rainy day, not snow. 
At this point, I give equal probability to all of the following options:  All rain, mix of rain and snow, a big snow, nothing at all – which means I have about zero confidence in what may happen seven days from now.
Stay tuned, and as usual, this next “event” will probably have everyone on edge as the weekend draws near.

Sunday, March 17, 2013

17 March, 2013: Slippery Monday AM Commute Likely

Winter has returned.  First, let's detail the moderate impact, wintry event on tap for tomorrow morning.  I label this storm "moderate" because even though total frozen precipitation is expected to remain light (1" or so across the Metro region), the timing is rotten, with various forms of frozen precipitation (snow, sleet) falling through the Monday AM rush.

The NWS has issued a Winter Weather Advisory for the western D.C. and Baltimore suburbs, effective tonight through tomorrow late morning, for slippery travel:

The Metro regions are barely on the edge of this advisory.   In Central Maryland, the region of more significant impact is actually upper Montgomery and Howard Counties, where elevations are higher.

Here is the NWS accumulation map:

Like the last winter storm, this one is expected to be strongly elevation-dependent, with heaviest snow tied to colder air at the mountain tops.   The Metro region is far from the expected snow accumulation bulls eye.

What's causing this wintery scenario over the Metro region?  A weak area of low pressure approaching from the Tennessee Valley will pull in Atlantic moisture and create uplift of air.  A classic pattern of Appalachian Cold Air Damming will supply marginally cold air over the low elevations.  The damming is a subfreezing layer of air just a few thousand feet thick, wedged up against the eastern slopes of the Appalachians.   The cold air is being pumped southward by high pressure over eastern Canada.  The forecast map below shows the interaction of these elements:

The blue H = high pressure over Canada, while the red L = low pressure over Kentucky.  Shaded colors indicate intensity of precipitation.

We will have an air layer cold enough for snow and/or sleet over the Metro region, with Atlantic moisture overrunning the cold dome.   As the high pressure moves eastward through the day on Monday, the wind direction will shift from a northerly to an easterly flow, bringing in warmer air off the mild Atlantic.  This changes frozen precipitation to rain sometime Monday morning.  Plain rain will persist through Monday night.

What could go wrong with this forecast?   (1)  The warmup will be delayed, leading to a longer period of frozen precipitation through the day Monday.   Whenever we deal with cold air damming scenarios, this is always a possibility.  The cold, dense air is hard to "scour out" and the prediction models often make the transition to warmer air too quickly.  (2) The primary low to our west will transfer its energy to a coastal low, which then tracks up the New England coastline (dumping heavy snow on interior New England).  The details of how the transfer will play out can impact available moisture, timing and intensity of uplift, and the wind field - all of which confound the forecast of multiple wintry precipitation types across our region.

How about the potential for a major snow storm late next weekend?   Several cards are still on the table, including (1) a source of deep, subfreezing air moving southeast out of Canada;  and (2) formation of a coastal storm, somewhere along or off the Atlantic coast.   The details of the storm's formation 7+ days out remain problematic.  Last night's model runs (both the GFS and ECMWF) developed a major, slow-moving cyclone off Cape Hatteras, with a frigid air mass in place over Washington-Baltimore - meaning big time snow.   However, the afternoon runs of these same models have shifted the region of cyclone formation further south and east, over open ocean, and create the storm later, starting Tuesday, March 26.  If these latest model runs are true, the impact here is minimal, or none at all.

The two sets of graphics below illustrate the large difference between the earlier and later model runs (the ECMWF is shown, but the GFS had very similar trends):

Earlier ECMWF run:  Major Baltimore Snow Storm (Weatherbell Analytics)
Later ECMWF run:  A Miss for Baltimore (Weatherbell Analytics)
The bottom line:  The models want to create a powerful Nor'easter off the southeast U.S. coast early the week of March 25, feeding off an unseasonably cold air mass spreading east out of Canada.   A lot also depends on what is happening at jet stream level, which I am not showing here.  I expect that the models will waffle back and forth on various scenarios as we head into the coming week.  Our confidence will grow once a stable trend emerges.  This is one to keep a close eye on. 

March 17, 2013: Crushing Blizzard Possible Next Weekend

Folks, winter is a long way from ending around here.   Absolutely bone-chilling air is moving down into the central and eastern U.S. this coming week.   Now, several of the forecast models (GFS, European) are predicting a major east coast Nor'easter with heavy snow and high wind over the Carolinas and Mid Atlantic late next weekend.   The development of this type of intense, late March system is not unprecedented and is wholly consistent with an arctic air mass moving toward the East Coast.   This could be a crippling, 1+ foot snowstorm for D.C.-Baltimore. 

I just want to toss the idea out there now, and I will have much more to say about this storm, as we follow the forecast every day this coming week. 

In the meantime, we are still on track for a 1" or so slushy snow-sleet-rain mix tonight and tomorrow morning.  I'll have more to say on this storm around 4 pm stay tuned.

Friday, March 15, 2013

March 15, 2013: Spring Ain't Here - More Snow Coming?

Well, it sure feels mild out there in the strong mid-March Sun, but the large-scale weather patterns have another idea in mind, to close out this month.

One of the big indicators of short-term (7-10 day) climate variation, called the North Atlantic Oscillation (NAO) Index, has swung highly negative - in fact the most negative it's been all winter long.  A negative phase of the NAO means a jet stream pattern that favors persistent, abnormally cold air over the Eastern U.S. 

You can see it here:

Courtesy NOAA
The red curves at the very end of the top panel indicate that the NAO index is forecast to remain very negative through the rest of March. 

Unseasonably cold air masses will visit our region over the next week.  Tomorrow through Monday, two weak waves will move through, each producing light precipitation - mainly rain.  However, the disturbance Sunday night has the potential to work with some really chilly air, stuck against the east slopes of the Appalachians.   This is called Cold Air Damming.  The sub-freezing air layer may spell a period of light, wintry mix Sunday night into Monday morning:  Cold rain, sleet, snow.  The NWS thinks that any accumulation (again, light) will occur along the Mason Dixon Line and in the higher western elevations.   But we're in for a 2-3 day ride of chilly, gray, damp, miserable weather - much more of a mid-January type of weather pattern.

One of the medium-range forecast models, the GFS, is hinting at a more significant winter storm for our region next Saturday-Sunday, as we get into the backside of a coastal low with an air mass cold enough for all snow.  Well, we'll see.  Such a storm is consistent with the strongly negative NAO.   But climatologically, a big snow event is not very likely after the vernal equinox (which occurs late this coming week).  And I am very reticent to make any type of forecast call based on one run of a single model 9 days in the future.   It's worth noting, for the record, but so very much can change in the models over the next 8-9 days.  I would not bet money on this scenario. 

Wednesday, March 6, 2013

March 6, 2013 3 PM Snowstorm Winding Down

This is the storm that The Weather Channel (TWC) has named Winter Storm Saturn.   Otherwise known around here as the Sequesterstorm, or The Big Slop.

The coastal low, while intensifying this afternoon, is out to sea and is exiting our region.  It was a very compact system with a heavy but relatively small precipitation shield - with the bulls eye of liquid water content centered right over us.  Truly a Mid Atlantic-Only storm.

As of 3 pm, the precipitation shield is finally on the move, ejecting toward the ocean.   The mesoscale prediction models predict that the back edge of solid precip will clear D.C.-Baltimore by 6 pm this evening.

Here are some images of the storm, from different vantage points, at 3 PM.  The first is the weather radar, which indicates that most of the precipitation has turned to light rain across the region:

The bright yellow-red region over the Eastern Shore is an artifact of the radar at Wilmington, DE called a "bright band" and it does NOT indicate a cluster of thunderstorms.

Next up is the false-color infrared image, which gives information about cloud top temperature and height.   The storm does not look nearly as well-organized in this presentation, but the swirl of low-level cloud about the storm's center of circulation is evident:

Here's the view of the storm from the water vapor channel.  The circulation is pulling in extremely dry air on its south side (orange shades), from as far away as Florida!

Here is the final snow total accumulation prediction from NWS Sterling.  Verification of these values is going to be difficult. The wet nature of the snow makes it very hard to measure, because of compacting, partial melting, flowing and ponding on surfaces.  

The general gradient in snow from east to west was well-predicted for this storm.  The nature of the snow - heavy and wet - was also a good prediction.  Here are some surprises or "curve balls" this storm threw into what proved to be a very challenging forecast:

1)  Timing.  The storm got ahead of the forecast by about 6 hours.   Pure snow started falling area-wide by midnight, whereas most forecasts held off the arrival of all snow until daylight or mid- morning.  Likewise, as the bulk of the precip will depart the region by 6 PM, this is sooner than called for yesterday;

2)  Type of Precipitation.  In many locations, light snow and rain were mixed together through much of the day.   This cut down the snow accumulation predictions compared to last night, especially for the D.C. - Baltimore corridor;

3)   Melting Through The Day.   Temperatures remained several degrees above freezing, and even rose slightly during the day at some locations.   Early, wet snow that started to stick on trees melted through the day.  That, and the gusty winds, allowed the trees to shed their burden, sparing us from widespread power outages.   Anticipation of power outages was perhaps the most dreaded aspect of this storm;

4)   Stronger Wind Gusts Than Forecast:   Last night, peak gusts were predicted to approach 35 mph, when in fact several locations were reporting gusts to nearly 50 mph early this afternoon - the province of Wind Advisory Criteria.

5)  The Enormity of the Snow Accumulation Gradient:   This is about as extreme as I've seen in the Mid Atlantic.  2" along I-95, vs. 20" along I-81 - a factor of ten difference.  By all aspects, what defined the heavy snow region was altitude dependance.    Here is a preliminary set of observed totals:

This will be the final post on this storm event.

March 6, 2013 Heart Of Storm Has Arrived

The snow storm is in full swing across the region, and will continue through the day.

The coastal low has formed over night and is off Norfolk, VA this morning.  A compact band of precipitation is wrapping around the north and west sides of the storm, setting up shop from southern PA to WV to northern NC.

Bands of moderately intense snow are moving from east to west across our region.  Warm air is invading from the ocean, pushing the rain-snow line very close to the DC-Baltimore region:

Heavy, wet snow is falling over the cities, in proximity to this melting line.

Winds have also picked up, presently from the NE, gusting to near 30 mph, as air streams into the low pressure to our south.

There is concern that a heavy snow band will set up, somewhere W and N of the storm, as the storm continues to intensify.  This would lead to 1"-2"+/hr snow accumulation over a localized area.

Timeframe:  The storm will slowly move toward the E-NE through the day.  By noon, we should see the wind back around to the N, bringing in colder air - ensuring continued production of snow.   Snow
continues through the afternoon.  By late afternoon, the back edge of the storm and colder air may produce a drier, more granular type of snow.   Most of the snow exits the cities around 6-7 pm this evening.  Winds will continue gusting to 30-35 mph at times, particularly as bands of snow move through.

The latest accumulation totals from NWS Sterling:

Bottom Line:  Baltimore straddles the 6" accumulation line, while 6"-8" will fall over the DC region.

Tuesday, March 5, 2013

March 5, 2013, 11:30 PM Storm Throws A Curve Ball

All storm total accumulations have been increased across the board.  Snow has started to fall around the region...many hours earlier than anticipated.  A solid band of moderate precipitation moving up from the southwest is the cause.  Initially, rain falling through a very dry air layer about 5,000 feet thick has evaporated vigorously, chilling the air to freezing...changing rain to snow.    Most meteorologists, myself included, were surprised by this early and rapid transformation. 

The snow band is shown below:

The new snow totals from NWS Sterling, showing a significant increase in snow totals to 8"-10", at 11 PM:

In the battle of temperature in this storm, it appears that more snow than rain will be the rule across much of the region.    The storm is proving to be very efficient at manufacturing its own cold air supply. 

March 5, 2013: Heavy Snow Accumulation In Metro Region Still Uncertain


My forecast for 4"-6" is still on track!   In fact, the NWS has lowered their accumulation totals across the Maryland Piedmont (compare this new afternoon forecast with the one issued this morning, below):

The Sweet Spot is still 100-150 miles to our southwest, over the high terrain.  There, the threat of power outages from heavy snow and high winds is very real.

Once again, arguments can be made that enough mild air will flow into the system, on E-NE windst after midnight, to keep the precip all rain over the metro area for several hours.   Once the cold air does arrive from the north, there will be sufficient moisture to generate several inches of snow, which will start off wet and slushy.   Locally higher amounts may fall if and where a snowband sets up.   The 4"-6" range is borderline Winter Storm Warning criteria.   Winds will gust up to 30 mph from the north tomorrow night, and then from the NW on Thursday, as the low pressure system exits (the storm is intensifying as it exits).

Timing on the snow is shifted a bit:  Rain should transition to all snow later tomorrow morning - by around 9 AM.  Moderate snow will fall in the afternoon.  Snow will start to taper around 6-7 pm, but may linger a few hours into the evening, adding perhaps another inch - and it may not be as wet as the earlier period of snow.


My forecast for 4"-6" heavy wet snow is still on track across the Metro region, for all the reasons I gave yesterday.  NWS is struggling to sort through competing processes, some of which would cool the atmosphere and favor more snow, some of which would warm things up and favor rain mixing in.

My own assessment:  There is no cold air mass in our region that can sustain heavy snow.  To get heavy snow, you either have to go up in altitude - where it's cold enough - or the storm has to manufacture its own cold air supply.  It can do this, under the right circumstances, through vigorous uplift of air - rising air cools.   Dry air beneath the cloud layer promotes evaporation, and this also chills the air.  But at the same time, the storm will pull in milder air off the Atlantic, and the condensation and freezing of oceanic moisture releases heat inside the clouds, warming them.
It boils down to a delicate balance of warming and cooling processes, which some of the forecast models do not have the best handle on.  We may not know until the event is underway, which process will win out.

There is a range of professional opinion on how much snow our region will pick up. Other weathercasters and meteorologists this morning are calling for 3"-7" across the Metro region.  Some are going for higher amounts.  The divergence of opinion reflects our lack of understanding of the temperature balancing act that will play out tomorrow.

Here is the latest snow accumulation map put out by NWS Sterling.  They have upped the totals from yesterday...mainly because there is one prediction model, the NAM, which is hanging on to a cold and snowy solution.   One of the other main models, the GFS, has warmed up, decreasing snow totals across the metro area.  The NWS may have raised their totals, to accommodate the cold bias in the NAM, as a worse-case scenario:

As new model runs come in, totals will get re-adjusted.  This process will continue throughout today and tonight and even tomorrow as the event gets under way.   This is not a high confidence forecast for our region, so stay tuned.  Next update today around 4 PM.

Monday, March 4, 2013

March 4, 2013 3:30 PM Major Snowstorm Remains Possible, Devil In Details


Still on track with forecast as laid out below;  no change in reasoning;  still thinking 4"-6" heavy wet snow across the metro belt.   Evening runs of the forecast models keep solid accumulation bulls eye over mountains of NOVA.   One of the models (GFS) has tapered off the snow totals across central MD.   NWS forecasters have tweaked back their numbers a bit, thinking that lack of cold air will prolong changeover of rain to all snow. 


Here are the NWS forecast snow totals for Wednesday's storm.  The Jackpot is in the mountains, well to our west and south.   NWS foresees about 4"-6" for DC and Baltimore.   This is entirely consistent with my initial call for a 4"-6" swath encompassing Baltimore, made at 3:30 pm today (read below).


The overall thinking outlined in my 9:30 AM blog from today remains intact.

The core of the event is still 48 hours away, so we are just beginning to codify likely zones of accumulation...subject to significant adjustments in the next 24-36 hours.

The reason this one is difficult, for the metro region:  Somewhat mild air is expected to filter in from the east (off the Atlantic) in low levels, setting up a rain-snow line somewhere along the I-95 corridor, that may very well shift around from hour to hour. But the warming influence may be countered aloft by strong "dynamical cooling" of the atmosphere, caused by vigorous uplift of air.   Over the mountains to our west and southwest, the elevation effect means cooler prevailing surface temperatures from the start...combined with dynamical cooling...producing all snow from start to finish - hence the highest snow accumulation potential there.

Today's models are showing the intrusion of low-level air with temps 37-39 F, on strong easterly winds off the Atlantic, as the storm intensifies over Norfolk:

The left panel shows the NAM model, the right the GFS model, at 7 AM Wed.  The westward push of milder air off the Atlantic gets over the I-95 corridor, and remains through early afternoon.  The dark red solid line is the 32 F isotherm;  near-surface temperatures east of this line are warmer than freezing.  If there is enough low-level warmth to melt snow, this will significantly cut down the snow accumulation.

Here is the NWS Hydrometeorological Prediction Center's probability of snowfall > 8" for the Mid Atlantic region, issued 3 pm this afternoon:

The metro area's probability hovers at 10%...and as discussed, the bulls eye for accumulation is over the mountains of NOVA, WV and western MD.

Here is what seems most likely at this time:

1)  About 12-15 hours of precipitation, with snow starting 7 AM Wed, tapering 7 PM Wed -- but the changeover could be delayed along I-95 if the warm air intrudes (the scenario discussed above);
2)  Heaviest precipitation falling from late morning until mid-late afternoon;
3)  Snow starting heavy and wet, remaining wet for several hours...ending up a bit drier and fluffier toward the end, as colder and drier air filters in at all levels;
4)  Likely total precipitation 0.5" to 1.0" across the metro region and a 10:1 average snow:liquid ratio would give about 5"-10" BUT this assumes early transition from rain to snow, and does not consider likely changes in the snow:liquid ratio as the storm evolves;
5)  Highest snow totals in mountains of NOVA and western MD, possibly > 12";
6)  Winds gusting to 25-30 mph during Wed afternoon and evening (higher in mountains);
7)  Likelihood of a heavy snow band setting up north and west of storm center Wednesday afternoon -
where this happens, expect heavy snows (2"+/hr) and higher local accumulations

At this time a SW to NE oriented swath of 4"-6" seems reasonable cutting through the Baltimore metro region.  Of course, this figure is subject to change!!!!

Next update:  11 PM tonight

March 4, 2013 9 AM Major Winter Storm Coming

Quick Morning Update:

1.   Winter Storm Watch:  Posted for the entire Metro region;
2.   Timing:  Rain/Snow early Wed AM, becoming snow, moderate to heavy at times during day,
      tapering Wed evening
3.   Accumulation:  Several inches, perhaps 6"+ but with a large gradient from NE to SW across
      central Maryland and NOVA;  bulls eye appears to be in the Appalachians to our SW (12"+);
      DC may pick up quite a bit more than Baltimore
4.   Type of snow:   This will follow the same gradient as the accumulation:  Heavy, wet snow
      (7:1) to the SW, fluffier farther NE (10:1)
5.   Other impacts:  Gusty winds, to 30-35 mph, with highest gusts in mountains
6.   Likelihood of power outages:  Greatest to the SW, where snow is wettest and gusts are strongest;
      however, folks in the metro region should be prepared

Tricky Aspects To This Forecast:

1.   Storm track:   Slightly further south reduces amounts in the metro;  slightly further north
      increases the amounts;
2.   Storm intensification:  This draws in colder air on the west side, increasing snow rates;  this
      also draws in more warm ocean air on the north side, increasing likelihood of sleet/rain mixing
      in with snow - there will be issues related to exact location of rain/snow line;
3.   Heavy snow band:  Where this sets up determines location of heavy snow i.e. 2"+/hr lasting
      several hours, leading to very localized heavy accumulation;  thundersnow possible in this band

Next Blog Update:  3:30 PM Today

Sunday, March 3, 2013

March 3, 2013: Significant Snowstorm Possible This Wednesday

It may be the start of Meteorological Spring, but it appears that there is still a lot of winter to get through, as the next 7-10 days offer up the possibility of significant, late-season snow.

The next threat is a strong coastal storm - which has not yet formed - but all computer models take dead aim at the Mid Atlantic, this Wednesday. 

Let me start by saying that the next few days are going to be very frustrating, because of the great uncertainty in storm track, intensity, speed, moisture content and availability of air cold enough for snow.

Statistically, major March snowstorms are not as likely as in February and January.  However, there have been several powerful, even crippling March Nor'easters and heavy snowfalls, including the 1993 Storm of the Century and the 1962 Ash Wednesday Storm.

But there is consensus among the forecast models slowly emerging, one that tracks a developing coastal low to our south, emerging off the coast of North Carolina, rapidly intensifying along the way.
The two forecast charts below illustrate the evolution of this system:

The track and intensification put us in a space-time "sweet spot" for significant snow accumulation - namely, on the northwest side of the storm, during the timeframe of its rapid deepening.

Here are factors in favor of heavy snow in the metro region:

1)  Storm track and intensity - not just at the surface, but at the jet stream level, which involves the optimal phasing of a northern and southern stream piece of energy;
2)  Ample moisture, pulled in off the Atlantic;  some models are generating upwards of 2" of liquid precipitation equivalent - making this potentially a very wet storm;
3)  Strong dynamics in the middle and upper atmosphere, which favor a process called "dynamic cooling" - that is, air forced to ascend vigorously cools strongly, enough to couteract surface temperatures that area a few degrees above freezing.

Now, there are plenty of uncertainties involved in this forecast;  here are some of the factors that would argue against a heavy snow in the metro region:

1)  Time of day and time of year - with some of the heaviest precipitation falling during the day on Wednesday, and the ever-increasing solar angle, it may be tough to get surface temps below freezing;
2)  Ground temperatures that remain too warm - especially at the onset - ameliorating heavy snow accumulation on roadways;
3)  No major source of sub-freezing air at the surface:   Lacking a strong, cold anticyclone over New England and Eastern Canada, surface temps in the metro region will be marginally cold enough for snow (this is why the heavy snow bulls eye may set up in the higher, colder elevations to our west and north);
4)  As the storm intensifies, enough warm air may get pulled in at low levels to change snow to sleet, cutting down on accumulations.  The strong, warm air inflow argues for mainly a rain storm along the Eastern Shore.

Here is the 2 PM Sunday guidance on heavy snow likelihood, from the NWS Hydrometeorological Prediction Center - note the bulls eye in the Central Appalachians:

So at this juncture, in the metro region, there are many issues revolving around (1) timing involving type of precipitation;  and (2) gradients in precipitation type i.e. the exact setup of a rain-snow line.   But there are enough "positives" for a significant snow event, here in the metro region, such that this storm should be taken seriously, and preparations started for possible big impacts.   The likelihood of seeing some accumulating snow is increasing.  The likelihood of a moderate- impact event i.e. 4"-6" is significant.  There is even a possibility that this storm will deliver a crippling blow to our region - a heavy, wet snow lasting for hours, combined with wind gusts of up to 35-40 mph, which could create widespread, multi-day power outages.

Here's a scary forecast scenario, consistent with a crippling snow - which is based on the GFS forecast model from 2 pm this afternoon.  Yes...Washington and Baltimore get the bulls eye of 12"-18" of snow!!!!

This is only one of several models, and one of many more forecast cycles to come, as the predictions become better refined.   This clearly is the most extreme of the spectrum of forecasts being generated.

In terms of impacts, this one is going to have us all on the edge, right up to 6-12 hours before the event.

Sunday, February 17, 2013

Feb 17, 2013: Rare Lake Effect Snow Showers Impacting Region This Afternoon

After an arctic front passage, the Great Lakes region often "clears up stormy", which refers to the development of Lake Effect snow showers.   Very cold air and high winds moving over much warmer lake water (as long as the lake surface remains unfrozen) causes warming and moistening of the air layer just above the lakes.   This destabilizes the air mass and causes it to convect (overturn), leading to showers of snow along and downwind of the lee shores. 

Typically, these lake effect bands - which are quite narrow but can extend over 100 miles downwind - fizzle by the time they approach the Mason Dixon Line.  But this afternoon, the arctic air mass is unstable enough...the wind trajectory is just right...and an upper air disturbance is helping to sustain uplift of air...allowing lake effect snow squalls to extend all the way into central MD and the Baltimore metro region.   The skies are very scenic, with whitish streaks of snow falling from beneath deep cumulus clouds.  Although the air beneath the cloud bases is very dry, some of these flakes are surviving to the surface.   No one should receive any more than a dusting from these snow showers.  Here is how the situation looks on weather radar - presenting as a very unique signature:

So enjoy a little taste of Lake Effect snow this afternoon!

Tuesday, February 12, 2013

Feb 12, 2013: Nuisance Snow Event Wednesday Night

An area of low pressure beginning to form along the Gulf Coast, today, will intensity and track northeast tomorrow (Wednesday), passing south of Baltimore Wednesday night.   This afternoon's weather map shows the low organizing a sizeable shield of precipitation across the southern Plains and southeast U.S.:

The forecast centers on two main questions:  (1) How far to our south will the heaviest precipitation track; and (2) what form will the precipitation take?  The latest model runs suggest we will pick up between 1/4"-1/2" precipitation:

Forecast map, 1 AM Thursday, Feb 14 (Unisys Corp)
The form of the precipitation - liquid vs. solid - depends on the thermal profile in the atmosphere's lowest 5,000 feet.  Most scenarios suggest enough warm air initially present to start as rain, but with sub-freezing air pulled in on the backside of the system, the rain changes to snow.  Assuming an 8:1 to 10:1 snow:liquid equivalent, we're talking 1"-2" in general.  The rain-soaked ground will probably melt much of this on contact.  If roads are adequately pre-treated, and treated during the event, they may remain wet or lightly slushed.   So this event should mainly be a light, grass-surface snowfall.   The system is not very intense, and it moves through our region briskly - both factors that will also limit accumulation.

Here is the official NWS snow accumulation forecast:

The snow amounts are higher to the north and west of the metro, for two reasons:  (1)  these regions will remain in the subfreezing air for a longer time;  and (2) the air will be sub-freezing at higher elevations in the mountains.   A shift in the forecast track can change this pattern, and if a small-scale snowband sets up - as some models are suggesting - snow amounts may be locally higher by several inches (often these bands are not well forecast).

Thursday, February 7, 2013

Feb 7, 2013: Light Wintry Mix Early Friday AM; Monster Snowstorm To Our North

First, the local weather picture.  For all the reasons given in my blog yesterday, a minor precipitation event is on tap for our region, Friday morning.   A light wintry mix of sleet and freezing rain is possible very early Friday AM, starting before the morning rush.  Temps warm through mid-morning (after 7 AM), changing precipitation to light rain.   Here is the latest NWS forecast which reflects trace amounts of ice...bearing in mind, of course, that just a mere trace of black ice can cause a very significant rush hour impact:

Again, like last Saturday evening's dusting of snow, when traffic was heavy, just trace amounts of wintery precip can create all manor of havoc to traffic and pedestrian traffic in this region.  Hopefully road crews will pre-treat before the morning rush.  It's all timing, timing, timing...

The second major story is the development of a powerful Nor'easter off the Carolina Coast tomorrow night.   This is the same storm that will bring us light precipitation tomorrow.   As it moves offshore, then northeast off the coast (passing by the Delmarva shore) it will literally "bomb out" off the New England coast...bringing a bona fide blizzard to coastal New England... with up to 2 feet of snow and winds gusting in the 50-60 mph range.  Not good for travel, nor for keeping the lights and heat on.  It will be a close scrape on the very backside of the storm for us.   Winds from the west-southwest will increase here Friday night into Saturday morning - possibly reaching Wind Advisory criteria at higher elevations to our west and northwest. 

Here are the forecast maps showing this enormously powerful storm, which The Weather Channel is calling Winter Storm Nemo:

Forecast map, Friday 7 AM, showing coastal low developing off Hatteras.  We pick up light precipitation on the northwest side of the storm, as it slides out to sea.

Forecast map, 7 PM Friday.  Nor'easter Nemo is rapidly deepening off the New England coast - pummeling the region with heavy snow and high, sustained winds.   Conditions become breezy but clearing over Baltimore.

Wednesday, February 6, 2013

Feb 6, 2013: Light Icing, Then Rain, For Thursday Morning Rush

Winter continues with a succession of mainly weak areas of low pressure, containing limited moisture, in frozen form.

This has been our pattern for a couple weeks, because the jet stream at 30,000 feet is racing across the U.S. in a largely west-to-east configuration.   When the jet stream flow in straight configuration, without significant meanders or bends, storm systems at the surface remain weak and fast-moving.

There are a few elements expected to come together Thursday morning, bringing the threat of a light wintry mix.  These are shown in the forecast map below:

Surface forecast chart, Wednesday evening.
First is a blast of chilly, sub-freezing air from the northeast, as strong high pressure (the 1038 mb anticyclone sitting over Ontario, above) circulates cold air southward on clockwise winds.   The cold, dense air hangs near the surface and piles up against the Appalachians.  As the dome of chilly air builds over the Piedmont and metro areas, precipitation (forming as rain 5,000 feet above the surface) falling through the sub-freezing air layer will freeze into sleet (ice grains) or freezing rain (glaze ice - freezing on contact with the surface).  The dome of cold, terrain-trapped air is called the cold wedge or cold air damming.

The source of moisture aloft is the second element shown above - a developing coastal low pressure system over the southeast U.S.   The system will slide toward the northeast on Thursday morning, to a location off Cape Hatteras.

Now, let's advance the forecast map by 12 hours:

Surface forecast chart, Thursday morning.
The coastal low has drawn closer to us, overspreading limited moisture.  Light precipitation falls as a wintry mix.  However, as the day progresses, two things happen:  (1) the strong anticyclone supplying cold air retreats to the northeast;  and (2) the coastal low pulls in milder air off the Atlantic, warming surface temperatures above freezing.  These two changes erode the cold wedge, and precipitation changes to rain.  Precipitation amounts are expected to be light, since the coastal low stays largely to our south and east of our region.

Monday, February 4, 2013

Feb 4, 2013: Quick Dusting To Light Accumulation Tonight

Once again, another Arctic Clipper - a weak, fast-moving, moisture-starved area of low pressure - will approach from the Ohio Valley, crossing our region over night.

You can see the approaching low pressure system below, over Illinois-Indiana, spreading a region of light snow (blue colors) our way:

The regional-scale (high resolution) forecast models suggest that more of the snow will fall on the western slopes of the Appalachians (this type of event is called Upslope Snow).   Air forced to ascend the 3,000-4,000 ft slopes, on a westerly wind, cools, and water vapor freezes out as snow.   When the air comes back down, crossing the eastern slopes, the air warms and dries.  Thus in the urban corridor (Baltimore-Washington) we often see greatly reduced precipitation (and sometimes, no precipitation at all) - in these situations, we experience a "snow shadow" effect.

You can see the upwind, upslope region of enhanced snow (green) and the downwind snow shadow in the tonight's regional forecast model graphic:

The NWS is forecasting 1"-2" across Baltimore.  The event will be over by the morning rush, but side streets will likely be untreated.  Note the high amounts being forecasted just west of the Appalachian Divide: