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.