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FXUS61 KLWX 280045

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
845 PM EDT Mon Mar 27 2017

A cold front will cross the area late Tuesday. High pressure 
builds over the region Wednesday into Thursday. Another
disturbance will approach the area late Thursday into early Saturday.


Although instability meager (around 500 J/Kg on 00z LWX RAOB),
it's been sufficient for a line of scattered showers to develop
in the Shenandoah Valley. This cluster is now (as of 8:30 pm)
emerging off the Blue Ridge headed toward DC. Mesoscale guidance
has been suggesting such a scenario since late this afternoon.
Behind that, the nearest precipitation is in the Ohio Valley.
However, dewpoints are on the higher side (in the upper 50s) and
positive vorticity advection will be increasing overnight.
Have targeted PoPs in the database for the evening hours, per
CAMs, but will be transitioning to a more widespread but low-end
probability for the pre-dawn hours as forcing grows. 

The other consideration will be the fog potential overnight, as
there will be plenty of low-level moisture and little flow (but
with an easterly component). Am still going with patchy wording
but for a larger portion of the domain. (Visibility values are 
lower too, although that's not reflected in public forecasts.) 
Will need to see how things transpire due to residual cloudcover,
which may be an impediment.


Another warm day tomorrow as moisture/warmth pools ahead of 
cold front. SPC continues to delineate the southern half (i.e.,
from DC south) and east of the Blue Ridge in a marginal risk 
for severe weather. While CAPE is higher tomorrow (e.g., 
~500-800 J/kg MLCAPE with the higher amounts to the south) shear
is also considerably less (i.e., ~20-25 knots). Thus, expect 
the possibility of a few poorly-organized marginally-severe 
thunderstorms across the risk area. Any threats expected to be 
isolated and confined to damaging wind gusts and small hail.

Chances for precipitation should be ending late Tuesday evening
as the shortwave moves off the coast and cold front pulls away 
from the area. High pressure returns Tuesday night and Wednesday.
Temperatures Wednesday cool nearly ten degrees from Tuesday (though
still remain above normal).


Thursday starts out dry as a high pressure builds into the NE 
CONUS. Meanwhile, a strong closed low will be moving northeast 
from the southern plains and into the Mid-Atlantic. This system
will bring decent amounts of moisture into the region, with 
rain starting Thursday night and remain into sometime Saturday. 
Many GEFS members bring the highest QPF amounts on Friday night.
Conditions become dry behind this system Saturday and into 
Monday. High temperatures will be in the 50s and 60s, and lows 
in the 40s.


A couple of showers are meandering near IAD this evening, but 
are not causing any flight restrictions. These showers heading 
toward DCA. Expect the same result. 

Flow will be light tonight. In the DC/Baltimore corridor, there
will be an onshore component, with relatively high dewpoints and
potentially patchy wet ground. That means that fog development
is a real possibility. Guidance has been emphatic that it once
again will be dense. Am uncertain if that is an artifact of 
model bias. It still looks like the best chances for fog or low
clouds will be at IAD/MTN/BWI/DCA. Went lower than previous TAFs,
but still kept forecast at IFR due to reduced confidence. 

A chance for thunderstorms returns to the terminals Tuesday
afternoon...with the possibility of gusty winds and small hail
in the strongest storms. Conditions should improve late Tuesday
evening through Wednesday with the return of VFR as high 
pressure builds into the area.

Dry/VFR conditions expected for Thursday before rain approaches
over the area Thursday night, possibly bringing sub-VFR 
conditions over the terminals Friday into early Saturday.


LIght winds on the waters tonight. Thunderstorms will again be
possible Tuesday afternoon, some of which could produce SMW-
level winds. Wind gusts will then increase Tuesday night into 
Wednesday behind frontal passage...with SCA possible as early as
early Wednesday morning...but likely during the day Wednesday. 
Winds expected to diminish later Wednesday.

Dry conditions expected for Thursday before rain approaches over
the area Thursday night bringing showers over the waters into part
of Saturday. Wind gusts will be near the small craft advisory 
threshold mainly Friday night into Saturday... therefore SCA possible
at times.





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