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

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.

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!

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).

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.

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.

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: