Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 090132

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
832 PM EST Sun Dec 8 2019

Low pressure will develop over the central United States and lift
into the Great Lakes through Monday. A warm front developing east of
this area of low pressure over the Carolinas is expected to lift
northward across the Mid-Atlantic during this time. The
aforementioned low will drag its trailing cold front through the
region Tuesday into Wednesday. Strong Arctic high pressure will
build from the Great Lakes toward the Mid-Atlantic Wednesday
night through Thursday night, then shift offshore of New
England as low pressure develops over and approaches from the
southeastern United States.


Low deck clouds developing east of the Blue Ridge this evening
and are expected to overspread the area overnight. Isentropic
lift will increase tonight as well, with clouds lowering and
the chance for rain or drizzle just about everywhere by

The main focus for rain will likely be in two areas:
1) southeast of I-95 associated with enhanced low-level upglide
as a weak SLP min/low develops along a northward advancing warm
front, aided by proximity to the Atlantic, and 2) over eastern
West Virginia closer to more appreciable height falls ahead of
the approaching upper trough. Latest guidance, however, has
trended drier between these two features. Have backed on
precipitation slightly to the west of the theta-e ridge. Given
magnitude of warm advection though, did not want to pull back
too far toward daybreak as there is still a chance at measurable

Temperatures tonight will be 10 to 15 degrees warmer than
Saturday night due to the warmer air moving in on southerly flow
and increased cloud cover. Lows are likely to be achieved this
evening, with temperatures expected to hold steady or slowly
rise after midnight.

Some earlier runs, led by the NAM, suggested patchy freezing
rain possible in the mountains, but have pulled back on that
outcome given drier precipitation trends...making a rather low
probability event even more unlikely. By the time rain does
arrive, believe there should be just enough warm advection to
have temperatures above freezing.

Elsewhere, the period of steadiest rain is expected Monday
morning through about mid afternoon, focused mainly along and
northwest of the US-29 corridor closest to the parent trough
over the Midwest. A relative min in QPF is likely over portions
of the Shenandoah Valley due to moist SW flow in the low levels
being intersected by upstream terrain.


A lull in the steadiest rain is expected for much of the Mid-
Atlantic east of the Appalachians Monday night into first thing
Tuesday morning. This is due to strongest low level forcing from
the warm front lifting northward out of the area. But as the
upper trough and surface cold front trailing low pressure moving
across the Great Lakes approaches, showers should re-develop
Tuesday into Tuesday night.

The surface front is expected to move east of the area Tuesday
evening, but the RRQ of a strong (170+ kt) 250 hPa jet and PVA
ahead of the trough lags behind the front, resulting in an
anafrontal precipitation event. Precipitation will be fighting
some dry advection in NW low level flow, but given the strength
of forcing aloft and residual moisture in the column, there
should still be enough ingredients to result in rain changing to
snow before it ends late Tuesday night into Wednesday morning.

At the present time, the most likely area for accumulations
appears to be in a band from east-central West Virginia
northeastward through northwestern Virginia, and northern
Maryland, with the highest accums most likely over the higher
terrain, including the Blue Ridge mountains. The latest model
guidance is a bit aggressive in terms of both anafrontal QPF
amounts, and subsequent low level cold air/snowfall. Nudged
previous forecast upward but not nearly as aggressive as the 12Z
NAM, or even quite as high as the latest ECMWF, given lower
certainty in this setup climatologically speaking, and model
variability several days out. Of note, 9Z SREF show 4-6 C temp
spread at 850 hPa at 12Z Wednesday over the I-95 corridor,
which shows there`s still a fair amount of uncertainty in just
exactly when cold air gets here.

Precipitation should wind down as cold/dry air continues to
rush into the region ahead of an Arctic high building in by
Wednesday afternoon.


Canadian high pressure will build to the north Wednesday night into
Thursday morning, resulting in much colder temperatures.
Thursday will be much colder than normal as well, with highs
only in the 30s. Dry conditions are expected throughout this
period, as the aforementioned cold front should get far enough
south and east to be of no concern.

By Friday, though, things get more active. A potent mid-upper
cutoff low in the southern stream will swing across the
southern US, phasing with northern stream shortwave diving down
across the Plains. Attendant surface cyclogenesis will likely
occur along the Gulf Coast and moving northward into the Ohio
Valley Friday into Saturday.

Timing differences between the GFS and ECMWF for precip arrival
exhibits where the forecast becomes more uncertain. The GFS
brings precip into our region by midday Friday, while the ECMWF
delays precip arrival by roughly 12 hours. The exact timing of
precip arrival, as well as the magnitude of residual low level
cold/exact low track will determine p-types at onset. What
models do agree on is a surface ridge and a strong in-situ cold
air damming signature Friday into Saturday. While temperatures
gradually begin to warm aloft, temperatures at the surface still
remain in the low to mid 20s. As high pressure continues to
slowly move off the East Coast, it is unclear if southerly flow
will be strong enough to scour out the cold air at the surface
prior to precip arrival. While confidence is low in any outcome,
the 12Z guidance does show the potential for a wintry mix at
onset, (especially for areas west of the I-95 corridor) before
temperatures warm throughout the entire column enough to
continue the p-type as plain rain.

Rain should taper off by Sunday morning as low pressure moves
away from the area. However, areas along/west of the Allegheny
Front could see some lingering northwest flow rain/snow showers.
Several ensemble members also try to develop a second area of
low pressure, which could renew precip chances.


VFR flight conditions continue this evening, but low deck clouds
(CIGs 040-050) have already overspread the terminals east of
the Blue Ridge. CIGs should continue to drop Monday morning,
likely falling to IFR. Rain will increase during this time as

LLWS is highly likely given 40-50+ kt LLJ down as low as 2 kft
AGL. SW Surface gusts to near 20 kts are possible at times. The
forecast mention of this will be in the afternoon, especially
the second half. Model soundings/time-heights suggest the LLJ
will continue until about 02-03 UTC.

A relative lull in precip is expected Monday night, before
showers increase again Tuesday, then changeover to snow before
ending Wednesday morning. Most likely terminals to experience
accumulation appear to be near MRB, with lower probabilities
especially near/east of I-95.

VFR conditions expected on Thursday, with high pressure overhead.
Sub-VFR conditions likely Friday into Saturday as low pressure
approaches from the south, possibly resulting in mixed pcpn.


South winds should stay below SCA criteria through tonight,
though the gradient will be increasing. Strong winds just above
the surface are expected Monday, but there will also be a steep
low level inversion present on the north side of an approaching
warm front. Even so, given the strength/height of the
inversion, there should be enough to mix down gusts to around 20
kts or so, with highest confidence over the more open waters
where SW to NE fetch is longest. Small Craft Advisory in effect
Monday afternoon for the Mid Chesapeake Bay and lower Tidal

The warm front will lift north, permitting some gusty winds to
mix to the surface in spite of less than ideal mixing. Have
extended/expanded the Small Craft Advisory to all Chesapeake Bay
waters (and the lower tidal Potomac/tribs) Monday night. The
SCA will likely need to be extended through Tuesday as the
inversion weakens in SW flow. Gusty NW winds are expected behind
a cold front approaching from the west Wednesday, with SCA
likely during this time as well.

Winds should become light Thursday into Friday as high pressure
build over the waters.


MARINE...Small Craft Advisory from 6 PM Monday to 6 AM EST Tuesday for
     Small Craft Advisory from 1 PM Monday to 6 AM EST Tuesday for


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