Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170850

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
350 AM EST Sun Feb 17 2019

Low pressure will pass through the area this afternoon and
evening. High pressure returns for Monday. Stronger low
pressure will likely impact the area during the middle portion
of the week. High pressure will briefly return late week.


A ridge of high pressure extends down across the northeastern United
States early this morning. Some high clouds noted on derived
satellite. However, temperatures have dropped below freezing across
all except the urban downtowns and along the shoreline of the
Chesapeake Bay.

Low pressure resides in the lower Mississippi Valley early this
morning. This low will be tracking toward the area this afternoon
and evening, with ample warm advection aloft (850 mb) ahead of this
system. There will also be some diurnal warming at the surface.
However, the layer around 925 mb will remain cold (subfreezing)
through the day. So, when precipitation arrives, we`ll be looking at
a mix of precipitation types.

Initially, marginally cold air will reside across the Potomac
Highlands/northern Maryland, so could see some wet snow there. For
just about everyone else within the forecast area, the initial
precip would be rain due to surface/boundary layer as well as 850 mb
temps, mixed at times with sleet due to low dewpoints. However,
freezing levels will be hovering at the 500-1000 ft level, and will
be changing little. That means that, like a few days ago, there will
be a layer (the hilly terrain) where temperatures will be below
freezing, and any rain that falls will be freezing rain. The highest
mountain tops in the Appalachians may be above this level. With warm
advection, positive vorticity advection, and the nose of the upper
jet all overhead, believe that there will be more than enough
forcing for precip to fall, with reasonable consensus at onset
during the afternoon. Therefore, likely PoPs during the afternoon
increasing to cat PoPs tonight. Northeast Maryland (Frederick to
Baltimore/Harford) may not see precip til dark, so have started out
those zones at chance today, but still rising to 90% tonight.

Given this logic, have expanded the Winter Weather Advisory for all
counties between the Blue Ridge and Appalachians, as well as the
northwestern suburbs of Baltimore and Washington, with staggered
start and end times. Accretion is hard to gauge; am pegging this at
tenth-inch or less for now. If rainfall rates remain low and winds
light, this may need to be reevaluated.


Low pressure will be transitioning offshore by early Monday and
pulling away during the day. Would not rule out a little lingering
precipitation in the morning, so have added slight chance/chance
PoPs to start the day. Am optimistically counting on enough warm
advection to erode the cold layer by then, as forecast highs will be
in the 40s to lower 50s. Otherwise, cold air sweeping in on
northwest flow would transition moisture back to snow showers...
primarily for the mountains. Would not be surprised to see
temperatures non-diurnal, with highs in the late morning/early
afternoon. Am also expecting a breezy day with good mixing. Mean
layer mixing would yield 25-30 kt gusts (30-35 kt resides at the top
of the mixed layer).

High pressure building Monday night and Tuesday will briefly provide
tranquil conditions. Temperatures near or slightly below climo.


The broad area of low pressure will continue to spread plentiful
moisture across the region on Wednesday, with a fair amount of low
level cold air in place. Aloft, warm air advection will be
increasing under strong southerly flow at 850mb, so a wintry mix is
expected for much of the area during the day before a transition
over to rain, at least across our southern zones. Further north,
mainly north and west of the metros, snow will hang on the longest
before switching over to a wintry mix Wednesday afternoon. Locations
along and west of the Blue Ridge will continue to see a threat for a
wintry mix into Wednesday night, transitioning to rain/freezing rain
as that low level cold air remains stubborn to erode. Further south
and east, a cold soaking rain is on tap Wednesday night. Ensemble
probabilities are quite high with regards to an inch of QPF being
achieved by Thursday morning for much of the CWA.

Warmer air will finally win out area wide by Thursday morning, with
rain expected as low pressure pulls away from the coast. High
pressure will build to our north the second half of Thursday, likely
bringing some reprieve from the wet weather. Another wave of low
pressure may impact the area once again Thursday night and into
Friday, but global guidance differs a bit on the strength and timing
of this feature. Daytime temperatures both Thursday and Friday would
support all rain, while a rain/snow mix may be possible Thursday
night and Friday night, mainly along our northern zones.

High pressure may provide another break from precipitation the first
half of Saturday, but yet another broad area of low pressure may
impact the area late Saturday and through the remainder of the
weekend. With the parent low passing well to our west, rain can be
expected with temperatures running above normal for late February.


VFR conditions for the morning hours, with deteriorating flight
conditions (MVFR to perhaps IFR) during the afternoon and evening as
precipitation overspreads the terminals. For most of the airfields,
this should be rain, perhaps mixed with sleet. However, there will
be a layer between approx 500-1000 ft up to near 3000 ft where
temperatures will be below freezing, and freezing rain/icing will be
a concern. That likely will affect MRB during the afternoon and
evening hours, and could linger overnight.

There may be a little leftover precipitation Monday morning. Due to
continued warming, this should be just rain. After that, strong
northwest winds will kick in (25-30 kt gusts), with flight
conditions returning to VFR.

High pressure builds across the terminals Monday night and Tuesday.
VFR should prevail.

IFR/LIFR conditions likely Wednesday into Thursday as a broad area
of low pressure crosses the region. A mix of wintry precipitation
Wednesday will transition from south to north to mainly a rain event
Wednesday night, with the potential of RA/FZRA lingering at MRB/IAD
into late Wednesday night. Improving conditions possible the latter
half of Thursday as high pressure builds to our north and
precipitation winds down. Another wave of low pressure may impact
the terminals Thursday night, with RA as the main ptype, and sub VFR
conditions returning.


Northeast winds near 10 kt will veer southeast today as low pressure
approaches. However, winds won`t increase substantially until the
low exits, which will be on Monday. Small Craft Advisories will be
likely, with gusts 25-30 kt. Gales aren`t completely out of the
question. This gradient will relax Monday night and Tuesday as high
pressure builds.

A broad area of low pressure will cross the waters Wednesday into
Thursday. A weak gradient will yield sub SCA conditions during this
time, with a wintry mix Wednesday transitioning to rain by Wednesday
evening, likely resulting in visibility reductions on the waters.
Another wave of low pressure possible Thursday night, with rain
causing additional visibility reductions, but winds look to remain
sub VFR.


MD...Winter Weather Advisory from 3 PM this afternoon to 3 AM EST
     Monday for MDZ003>006-503-505-507.
     Winter Weather Advisory from 1 PM this afternoon to 1 AM EST
     Monday for MDZ501-502.
VA...Winter Weather Advisory from 11 AM this morning to 9 PM EST
     this evening for VAZ025-026-036-503-504-507-508.
     Winter Weather Advisory from 3 PM this afternoon to 3 AM EST
     Monday for VAZ028-030-031-505.
     Winter Weather Advisory from 1 PM this afternoon to 1 AM EST
     Monday for VAZ027-029.
WV...Winter Weather Advisory from 11 AM this morning to 9 PM EST
     this evening for WVZ505-506.
     Winter Weather Advisory from 3 PM this afternoon to 3 AM EST
     Monday for WVZ051>053.
     Winter Weather Advisory from 1 PM this afternoon to 1 AM EST
     Monday for WVZ050-055-501>504.


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