Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 140857

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
357 AM EST Wed Feb 14 2018

High pressure will move away from the region today. A warm
front will cross the area tonight into Thursday, then a cold
front will slowly move through late Thursday night into Friday.
An area of low pressure will likely affect the region Saturday.
High pressure will return briefly Sunday into Monday before
another frontal system approaches for the middle part of next


Milder temperatures are expected today as low level SW flow
advects a warmer airmass into the region. Most of the day should
feature scattered to broken clouds with moisture advecting into
the area as well. Despite the cloud cover, the warmer airmass
should allow temperatures to reach well into the 50s and 60s
(coolest N MD, warmest central Shenandoah Valley). With lack of
appreciable large scale forcing, any precipitation today should
be confined to upslope areas along and west of the Allegheny

Isentropic lift ahead of a warm front coupled with a subtle
upper level jet max should result in rain breaking out this
evening into the overnight. The best forcing will be just north
of the CWA into PA, but still expect an intermittent rain to
develop, mainly north of US 50 tonight. Amounts should be light
(0.10-0.25"). The NAM is the most aggressive keeping rain mainly
north of the MD/PA border, and it`s possible there is little if
any rain for southern parts of our area tonight. There`s also
decent mid-level lapse rates (above a stable boundary layer)
with elevated CAPE and a decent low level jet late this
afternoon and evening, so a few rumbles of thunder can`t be
ruled out. Lows will be mild for mid-February (40s and 50s).


Another push of low level warm/moist advection moves up out of
the Ohio Valley into PA on Thursday, with the region again on
the southern reaches of the best forcing. Scattered showers are
expected during this time, most likely for the northern third of
the CWA. Temperatures will be very mild, possibly near record
highs. 850 mb temperatures as high as 14 C in persistent SW flow
should allow temperatures to approach or even exceed 70 F
Thursday afternoon, despite a good amount of cloud cover. It
will be a bit humid for this time of year as well, with dew
points approaching 60 F.

A wave of low pressure will develop near Chicago Thursday
afternoon and move eastward into New England by Friday. This
will slowly drag a cold front through the region, accompanied by
a period of showers. Deep layer westerly flow usually results
in a sub-par moisture fetch, so current thinking is intermittent
showers will result during the day Friday as the front crosses
(as opposed to a widespread and persistent soaking rain, despite
the anomalously warm and moist atmosphere). An isolated
downpour or two is plausible though given a little instability
and strong upper jet.

Much colder air will pour over the area Friday night in gusty
NW flow behind the front. Temperatures will fall quickly, but so
will the dew points. There should be enough of a drying wind to
preclude re-freeze, especially since rainfall isn`t expected to
be all that heavy, but temperatures will fall below freezing
for much of the area after midnight Friday night.


Main focus of the long term is Saturday and Saturday night. The
latest guidance has trended towards better odds of wintry

Guidance is in good agreement regarding a dry start to Saturday,
with high pressure centered to our north allowing chilly
Canadian air to dominate. A wave of low pressure will be
developing along the front which moves through Friday in the
southern Plains, however, and it will rapidly spread clouds and
precipitation east-northeast. The GFS, GGEM and ECMWF all
overspread the region with precipitation during the afternoon
Saturday, with precip ending from west to east as the low heads
out to sea Saturday night. There remains some uncertainty about
low and mid level temperatures, with potential for a mid-level
warm layer resulting in some sleet and freezing rain, as well as
potential for warm surface temps to limit accumulations or even
cause snow and ice to mix with rain. Greatest potential for this
is naturally across southern portions of the CWA, from
Charlottesville eastward towards Patuxent River, but mixing
could easily occur further north. In addition, not all models
show a significant snowfall just yet, with potential for the
wave to take a course a bit further north or south than the
current consensus. That all having been said, potential is
definitely there for a significant snow storm. This potential
will be closely monitored over coming days.

After this brief shot of wintry weather, we return to our
overall pattern of warmer than normal weather quite quickly.
Pacific high pressure builds in on Sunday, with temps already
rising back to near normal... this means widespread above
freezing temperatures, so whatever snow does fall won`t be
sitting on the ground for too long. Guidance brings another warm
front northward across the region on Monday, with an increased
risk of rain along with continued warming temperatures. That
front lifts north of the region Tuesday, with temps looking
likely to rise back into the 60s. However, low pressure will be
passing northeastward just to our northwest, which means there
will still be a chance of rain.


A brief period of MVFR or even IFR is being hinted at in the
LAMP/HRRR along the I-95 corridor first thing this morning.
Other guidance isn`t really catching on, but there may be brief
ceiling restrictions for the morning push. Lower ceilings would
develop over east-central VA and advect north, if it does

Otherwise mainly VFR is expected today with light south flow
around 10 knots. A period of light rain is likely this evening
mainly north of a line from IAD/DCA northward. Some CIG/VSBY
restrictions to MVFR likely for a time overnight. LAMP more
aggressive with widespread IFR, but NAM/GFS BUFKIT soundings
don`t seem to support this on a widespread scale. Patchy fog
possible near cool water as warm airmass moves overhead Thursday
into Friday.

Additional restrictions (mainly MVFR) are possible Thursday
into Friday as showers develop ahead of a cold front. SW flow
around 10 kts flips quickly to NW and becomes gusty Fri PM
behind cold FROPA.

Good odds of MVFR if not IFR conditions on Saturday and Saturday
night as a wave of low pressure brings the potential for
signficant wintry weather. Returning to VFR Sunday under high


Winds will be relatively light beneath a developing inversion
with warm air advection aloft. Mild air over cool water will
also result in poor mixing, so gusts today should be minimal.
Southerly flow increases just off the deck tonight, and some
gusts into SCA range are possible after midnight, though did not
have the confidence to go with a headline just yet given the
marginal setup.

Thursday will be very mild in the low levels so mixing should
again be poor over relatively cooler water. Patchy fog may
develop Thursday into Friday with warm air moving over cool

SW flow increases making SCA level gusts more likely Thursday
night into Friday. Gusty NW winds behind cold FROPA Friday
afternoon and evening could come close to gale. Forecast
soundings suggest a little low-level instability along the cold
front, which suggests gale force gusts could mix down in any
heavier shower elements.

Winds may remain light on Saturday as low pressure approaches
from the southwest. As the low strengthens and pushes northeast
off the coast Saturday night and Sunday, winds will likely shift
northwest and increase at least to Small Craft Advisory
strength. Gales aren`t impossible either, but still a lot of
uncertainty on this.


Unseasonably warm weather is expected on Thursday. While temps
will be over 20 degrees above normal, it will be difficult to
top the record highs in Baltimore and Washington. However, the
record highs at Dulles could be smashed. Below are the current
record highs for Thursday February 15:

DCA   77 in 1949  52 in 1909
BWI   77 in 1949  51 in 1949
IAD   68 in 1982  44 in 1984 and 1967




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