Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 160101

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
901 PM EDT Thu Mar 15 2018

Low pressure will linger over the Canadian Maritimes through
Saturday. A weak cold front will continue southeast of the area
tonight and surface high pressure will build toward the region
Friday. Another weak system will pass through the area Saturday
before high pressure returns for Sunday. A stronger storm system
will likely impact the area during the middle portion of next


Deep surface low remains over Atlantic Canada. Winds have
shifted to the northwest after a weak cold front pushed through
the area. Those winds quickly subsided with sunset and skies
cleared, even along the western slopes. Expect clouds to
increase (more so on the western slopes) and winds to tick up a
bit later tonight as the next upper level disturbance crosses
the area. This disturbance will combine with upslope flow to
produce some snow showers along the Allegheny Front, but at this
point think it`s no headlines for snowfall.
The vort max is fairly strong, so it could be enough forcing to
bring a stray snow shower or flurry east of the mountains along
the Mason-Dixon line, but odds are low. Lows tonight will be
below freezing once again.

Friday is more of the same, though with weakening upslope flow
as we head through the day, the snow showers along the Allegheny
Front should diminish. It will stay breezy and cool, however,
with highs pushed back down into the 40s across most of the
region behind today`s cold front.


High pressure will build back in briefly on Friday night,
allowing winds to drop off overnight. Clear skies should result
in another chilly night with most places dropping into the 20s

A weak system passing through the region on Saturday is of some
concern. With temps likely below freezing region-wide at dawn
Saturday, the speed of precip`s progression into the area will
likely be the key to whether we have any icing issues. In
addition, soundings at the leading edge of precip, as well as
towards the northern side of the system near PA, could result in
a bit more snowfall. For now, given significant disagreement
among various models regarding timing and placement of precip,
not to mention some differences in temp profiles, have gone
fairly general, with chance of mixing at the start and likely
rain later Saturday. Best chance of ice and snow is west of the
Blue Ridge, and this potential remains in the HWO. Temps are
also uncertain Saturday, with potential for precip and wedging
holding temps low, but right now most guidance brings temps up
towards the 40s. Will need to watch this closely, however.

Guidance generally clears all precip from the region early
Saturday evening, which will turn out partly cloudy and cold
again. Some may linger in the far south (central VA) close to
the front this system is traveling along, so have pops linger


The large scale pattern will continue to be dictated by two
stalled upper level lows: one over the Canadian Maritimes and
one over the northeastern Pacific/western North America. The
latter of the two upper lows will eject energy eastward across
the United States, while the Canadian Maritimes low blocks
energy from ejecting harmlessly into the Atlantic. The resultant
troughing over the eastern U.S. will lead to a continuation of
below normal temperatures over the Mid-Atlantic through much of
next week.

High pressure will build from the Ohio Valley eastward across
the Mid-Atlantic Sunday into Monday. Current guidance suggests
the next piece of energy/upper shortwave will cross the central
Plains and mid-Mississippi Valley Sunday night into Monday,
moving eastward through the Tennessee/Ohio Valley by Monday
night, then exit the east coast Tuesday. As is to be expected at
this time range, uncertainties exist in overall finer scale
system evolution, but there is a high likelihood of the
development of another coastal low pressure system along the
eastern seaboard during this time. At least some wintry
precipitation is possible, but by mid-March things would have to
come together just right to result in a more significant winter
storm east of the mountains. Have added this threat to the HWO
based on 12Z probabilistic guidance, though exact details are
subject to change.


VFR through Friday night. Winds have relaxed with sunset, but
may increase later tonight as an upper disturbance passes. The
more notable increase will occur after sunrise on Friday when
gusts of 20-30 kt will become possible. They should finally drop
off for a while later Friday night as high pressure briefly
builds in.

Saturday could bring issues as a weak wave of low pressure
brings precip across the region. Most likely, this will just be
rain at the terminals, but some wintry precip is also possible,
especially at the onset. Cigs and vis could be reduced below
VFR as well, especially if there is some front-end snow (most
likely at MRB) or if rain persists for a while (most likely at
CHO). Improvement back to VFR looks likely Saturday night.

Mainly VFR Sun-Mon as high pressure moves from Ohio to over the
Mid- Atlantic then eastward into the Atlantic Ocean. Winds Sun
will be northerly around 10 kts, then go light and variable Sun
night as high pressure moves overhead. Light easterly flow is
anticipated Monday as the high moves offshore, but depending on
finer scale details of the evolution of the pattern, winds could
be more northeasterly.


Continued northwest flow around the low pressure lingering to
the northeast will keep Small Craft Advisory conditions going on
the waters on and off through Friday evening. Winds actually
subsided below 10 kt in many areas with sunset, but will limit
changes to the headlines since an upper level disturbance will
likely increase mixing and thus wind gusts later tonight.
Widespread gusts of 20-30 kt will occur after sunrise Friday.
The probability of any gale force gusts appears to be low.

High pressure then builds briefly overhead late Friday night,
allowing the gradient to weaken and in turn, the winds, so it
should drop below SCA late Friday night. A weak wave of low
pressure then crosses the region later Saturday, but winds
should remain below SCA. There will likely be some rain with
this wave, however.

Generally light winds (N Sun, E Mon) are expected as high
pressure moves from the Ohio Valley eastward over the waters and
then out to sea.


Low pressure will remain over the Canadian Maritimes through
Friday. High pressure will approach from the north and west
during this time. A tight gradient between these systems will
bring gusty winds to the area, especially from the late morning
through the early evening hours Friday. A downsloping west to
northwest flow will cause low relative humidity.

For Friday...more gusty winds are expected. Somewhat chillier
conditions are expected as well with high temps in the 40s but
fuels should be noticeably drier. The relative humidity will
drop to between 15 and 25 percent across central Virginia and 25
to 35 percent across northern Virginia, eastern West Virginia
and Maryland. The strongest winds will be displaced from the
lowest RH values (strongest winds over Maryland, northern
Virginia and eastern West Virginia). An enhanced threat for the
spread of wildfires is possible Friday given the fact that fuels
will be dry and RH values will be low. After review, believe
that the Red Flag conditions may occur locally but with fuel
moistures still marginally above criteria and the wind/RH
collocation missing, have opted to not issue any headlines for
fire weather yet. An SPS may be again needed for Friday, but
will let mid shift make final determination on its placement.


MARINE...Small Craft Advisory until midnight EDT Friday night for
     Small Craft Advisory until midnight EDT tonight for ANZ535-536-
     Small Craft Advisory from 9 AM Friday to midnight EDT Friday
     night for ANZ535-536-542.


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