Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 130051

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
851 PM EDT Mon Mar 12 2018

Low pressure off the Carolina coast will rapidly intensify as
it moves northeastward toward the Canadian Maritimes through
Tuesday night. Canadian high pressure will build southward into
the central U.S. and eventually the southeastern U.S. by
Thursday. A weak cold front will push through the region
Thursday with Canadian high pressure building overhead for the
first half of the weekend.


Precipitation consolidation continues this evening as low
pressure strengthens off the Delmarva coast. A deformation band
crossed the I-81 corridor toward sunset leaving behind light
(half-inch or less) accumulations. That area becoming more
intertwined with an arc of more intense radar echoes along the
Atlantic coast. The forecast challenge tonight will be how far
west this area expands as the low tracks up the coast.

Presently light precipitation, mostly snow, can be found along
the Baltimore-Washington corridor. Based on visibilities, would
suspect that if there are any accumulations they would be
confined to northeast Maryland (metro Baltimore). Meanwhile,
NHK/NUI still reporting rain. While the general consensus of
Hi- Res guidance keeps more substantive precip on the eastern
shore, the verdict isn`t quite as clear for the Baltimore area.
Am keeping a chance (30-40%) of snow through the late night in
light of these more robust members.

Temperatures will be at or above freezing during this period of
precip. More vigorous cold advection will arrive in the wake of
the precip, driven by rapid coastal cyclogenesis. Little change
required in the low temperature forecast; lows subfreezing
areawide, in the teens in the mountains.

Then we turn to a rather prolonged upslope snow event, given
persistent W/NW winds starting tonight. A few inches of snow may
occur at the highest elevations through Tuesday, with models
showing a ramp-up in the precip intensity by Tuesday afternoon
as additional upper energy pivots into the region. I did not get
a chance to give this the look it deserves due to ongoing
weather, so this will be further assessed on upcoming shifts.
Elsewhere, Tuesday will be breezy/blustery. Even though air
temps may reach the mid/upper 40s east of the Blue Ridge, wind
chills hold in the 30s all day.


The duration of the short term period, Tuesday night through
Wednesday night, will feature continued upslope snow. I can`t
rule out a shower escaping further east on Wednesday, but kept
chances in the 10-20 percent range east of the Highlands. It`s
possible that enough snow could fall Tuesday through Wednesday
to warrant advisories, but the most likely scenario right snow
seems prolonged light snow with moderate periods that could
tally several inches.

For most of us though, this period will simply be cooler than
normal for mid-March, and still a bit breezy.


On Thursday an upper level shortwave will allow for some snow
showers mainly west of the Allegheny Front, but cannot rule out
having them elsewhere in our CWA. Thursday into Friday a high
pressure system centered over the Gulf Coast states will be
shifting east allowing for return flow to settle over our region
with dry conditions. On Friday a boundary will be sliding south
of our area as a high pressure then builds from the north. The
high will be over us on Saturday and then offshore on Sunday
allowing for continued dry weather. A boundary may affect us on
Monday increasing the PoPs over our area.

High temperatures will be in the 40s and 50s Thursday into Saturday,
then in the 50s and 60s Sunday and Monday. Low temperatures will be
in the 20s and 30s Thursday and Friday night, increasing to the 30s
and 40s Saturday and Sunday night.


There was a brief trip into IFR earlier this evening as a band
of snow crossed MRB. That should be the exception tonight.
However, as this axis crosses the metros, MVFR ceilings will be
possible. It has already happened at MTN. Confidence further
southwest (ie: IAD) lower, but there`s enough of a chance to
include in TAFs.

Otherwise, VFR is expected at the TAF sites through Wednesday,
aside from perhaps a passing shower mainly at MRB, but with
periods of gusty winds especially during the daytime hours.

Expecting mainly VFR conditions Thursday but some passing snow
showers cannot be ruled out. These may bring periods of sub-VFR
conditions. Then, VFR conditions expected Friday and Saturday.


Low pressure will strengthen off the Delmarva coast tonight. A
strong pressure gradient will reside over the waters through
Wednesday as low pressure churns near New England and Canadian
high pressure slides southward in to the central and
southeastern U.S. The gradient will support a strong Small Craft
Advisory with periods close to gales.

In light of the rapid deepening of the low and its proximity,
have opted to upgrade the Chesapeake Bay from Drum Point-Smith
Point including Tangier Sound to Gale Warning for tonight. The
rest of the waters will have a Small Craft Advisory in effect. A
Small Craft Advisory remains in effect for all waters through
the day on Tuesday, and continues for all but the upper and
middle tidal Potomac on Tuesday night. Will allow later
forecasters to decide if Gales need to extend longer or wider

Wind gusts stay above the Small Craft Advisory criteria
Wednesday into Thursday, and perhaps even Friday.


MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ530>533-
     Small Craft Advisory from 6 AM Tuesday to 6 AM EDT Wednesday
     for ANZ534-543.
     Gale Warning until 6 AM EDT Tuesday for ANZ534-543.
     Small Craft Advisory until 8 PM EDT Tuesday for ANZ535-536.


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