Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
355 am EST Monday Dec 22 2014

Synopsis...
a weak area of low pressure will track up the middle-Atlantic coast
today into Tuesday. Stronger low pressure will push north along
the Appalachians on Wednesday...sending a cold front eastward
Wednesday night. Breezy conditions will follow the front on
Thursday...then high pressure will build back into the region to
close out the week.

&&

Near term /through tonight/...
challenging forecast early this morning. County Warning Area is dry at this time...but rain
has already pushed northward into southern Virginia at 08z and will
continue to spread northward through the early morning hours as low
pressure develops off the southeast coast. Temperatures across much
of the County Warning Area were below freezing...and as precipitation moves in from
the south it will encounter this sub-freezing air.

A period of sleet is possible initially as precipitation begins over
the County Warning Area...but primary p-types to contend with over the next 12-24
hours will be freezing rain or rain depending on surface
temperatures.

Challenges revolve around the timing and north/northwest extent of
precipitation today and also how quickly temperatures will be able
to climb above freezing.

Trend of the models has been to slow down the beginning of precipitation by
a little bit. Have trended this way...but precipitation should still make
it into the Virginia foothills/Piedmont by sunrise. Will be
expanding the Freezing Rain Advisory a tier east to the I-95
corridor as light quantitative precipitation forecast is expected to fall while temperatures are
near freezing from Spotsylvania up to Prince William County.
Temperatures in these areas should rise above freezing by noon.

Precipitation is also likely to make it further north into northern
Virginia during the late morning/midday hours. Several suites of
guidance suggest temperatures sitting around the freezing mark.
Therefore have expanded advisory a tier north as well for areas of
freezing rain which may continue into afternoon. Have opted not to
expand advisory all the way to the Mason Dixon line however as
confidence is quite low in precipitation being able to make it that far
north through middle-afternoon even if temperatures are cold enough to
support freezing rain.

Finally...expiration time of existing advisory has been extended
from noon to 6 PM. Again...several suites of guidance suggest
temperatures continuing to be at or just below freezing for areas
near/along and west of the Blue Ridge into the afternoon.

Weak low pressure moves up the middle-Atlantic coast tonight. Highest
probability of precipitation will be closest to the Bay...lower in the mountains. Much of
the County Warning Area looks warm enough for primarily rain tonight but there still
may be pockets of sub-freezing air west of the Blue Ridge and any
rain or drizzle may freeze and advisory may need to be extended into
tonight for those areas.

&&

Short term /Tuesday through Wednesday night/...
in the wake of the low...mid-Atlantic remains in a wedge/light flow
regime on Tuesday therefore cloudy skies and the chance for light
rain or drizzle will continue. Temperatures should be warm enough
for just liquid precipitation.

While there is general synoptic agreement in the computer models
through the middle of the week...there is still quite a bit of
spread in the smaller scale features. Amplified upper level pattern
through the middle portion of the week. Trough over the central
Continental U.S. Will lift east...inducing cyclogenesis over the western Gulf
of Mexico Tuesday night. This low will ride up the western side of
the Appalachians...reaching central Quebec by Thursday.

A strong low level jet ahead of the deepening low will transport
moisture up the eastern Seaboard Tuesday night...bringing increasing
chances of rain. Temperatures drop little from tuesdays highs...so
no frozen precipitation concerns. While probably not continuously rainy
through the period...90-100 probability of precipitation are warranted...with moderate rain
possible at times. The nature may become more showery on
Wednesday...but not a compelling reason to break that out at this
time. 00z NAM/met guidance much cooler for highs on
Wednesday...likely due to a much weaker surface low than the global
models...which allows the cold pool to linger east of the mountains.
However...the other guidance of 60s may be a bit overdone...except
perhaps east of I-95. If this occurs...will also have to monitor for
the potential of strong wind gusts if convective elements develop.

The cold front trailing the low will push through Thursday
night...bringing an end to the precipitation except some possible upslope.
However with the warm start...it will take until late in the night
for the temperatures to drop cold enough to support any snow. All
told...impressive precipitable water values and a dynamic system could lead to
storm total rainfall in excess of an inch...especially east of the
mountains.

&&

Long term /Thursday through Sunday/...
Thursday will be breezy behind the front. Any upslope snow showers
should gradually come to an end. There will only be a glancing blow
of cold air advection...as high pressure will be building in from the southwest.
This high will remain over the southeast states through Saturday and
temperatures are expected to remain above normal. The next chance of
precipitation will arrive as the next trough approaches Saturday into
Sunday. Model solutions still differ quite a bit...so probability of precipitation remain
low. There will be potential for temperatures to cool back down
behind this system for the second part of the weekend.

&&

Aviation /09z Monday through Friday/...
precipitation is expected to spread north today. Period of light freezing
rain/sleet is most likely at cho/iad. May be too warm at dca/BWI for
freezing rain...but initial period of sleet can/T be ruled out
before turning to rain. Models have trended a little slower with
onset of precipitation and have followed suit in tafs. Expect conditions to
drop into MVFR this afternoon...then likely IFR or LIFR tonight and
continuing into Tuesday.

Below-VFR ceilings will linger through Wednesday night. Visibility could drop
below VFR in heavier rain/showers at times from Tuesday night
through Wednesday night. Winds will shift from southerly to westerly
Wednesday night and could gust above 25 knots on Thursday. No
significant weather expected after Thursday evening.

&&

Marine...
weak low pressure moves off the middle-Atlantic coast tonight. Marginal
Small Craft Advisory expected on the lower portions of the waters this afternoon
otherwise winds below Small Craft Advisory through Tuesday.

A strong low pressure system will move through the region between
Tuesday and Wednesday night...bring moderate rain at times over the
waters. A Small Craft Advisory is likely Wednesday through Thursday...with the
strongest gusts expected on Thursday. Winds will diminish Thursday
night as high pressure builds in.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...Freezing Rain Advisory until 6 PM EST this evening for vaz025-
026-029-036>040-501-503-504-507-508.
Freezing Rain Advisory until noon EST today for vaz050>052-055-
056-502.
Freezing Rain Advisory from 10 am this morning to 6 PM EST
this evening for vaz027-028-030-031-505-506.
WV...Freezing Rain Advisory until 6 PM EST this evening for wvz505-
506.
Freezing Rain Advisory from 10 am this morning to 6 PM EST
this evening for wvz055.
Marine...Small Craft Advisory from 10 am this morning to 6 PM EST this
evening for anz534-537-543.

&&

$$
Synopsis...ads
near term...bpp
short term...bpp/ads
long term...ads
aviation...ads/bpp
marine...ads/bpp

National Weather Service Glossary of Abbreviations