Air Resources Laboratory banner image
Air Resources Laboratory web site National Oceanic and Atmospheric Administration (NOAA)

dc discuss

Office: LWX

FXUS61 KLWX 201911

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
311 PM EDT Tue Mar 20 2018

One coastal low will head off the coast this evening while
another low develops to the south and heads northeast along the
coast on Wednesday. This low will head east away from the region
Wednesday night with high pressure returning Thursday. Another
system may impact the region over the weekend.


Complex storm system impacting the region is now looking more
and more likely to the the biggest snowfall of the winter across
most of our region... except, its now SPRING! Latest analysis
depicts one low pressure east of Norfolk. This is the system
which has brought today's precipitation so far. It will head
east off the coast tonight, with decreasing impact on our
region. We also have high pressure sitting over southern Canada.
It is supplying the cold air supply which is going to allow
precip to change over to snow across the region by early
Wednesday as it continues trying to build southeast. Finally, we
have our second low pressure system. It is currently sitting
over South Carolina and will towards Cape Hatteras by late
tonight, then head north off the coast for a spell on Wednesday
before heading east off the coast Wednesday night. It is this
low, in combination with the upper low still sitting out over
western Kentucky, which will provide the region with significant
additional snow later tonight into Wednesday.

With the first low moving off the coast and forcing dwindling,
expect precip to continue diminishing through the rest of today,
though light rain/sleet/snow may linger across the region
through this evening. With accumulation rates expected to be
very low as we head into the evening, have not expanded advisory
any further south and east as impacts over the metro should be
minimal for the evening rush. Further west, where we had
existing advisories and warnings, decided to just extend/upgrade
the existing headlines, even though a relative lull is likely in
these areas as well. That all having been said, motorists should
be cautious this evening even in the metro, given light frozen
precip may be ongoing (though pavement temps remain elevated for
the moment).

Things change considerably late tonight. Both the surface and
upper lows will be strengthening as they head east and northeast
toward us, and as divergence and isentropic lift increase late
tonight, snow, heavy at times, will overspread the region later
tonight from west-southwest to east-northeast. Have started
warning/advisory in areas without current headlines just before
this heavier band of precip is expected to move in. One thing if
note is that a warm layer could remain aloft (as depicted by
various guidance such as the NAM) into the overnight, so the
steadier precip may start as a mix of snow and sleet before
transitioning to all snow. As the low continues to strenghten
during the day Wednesday and gradually gets captured by the
upper low, heavy snow could potentially continue much of the day
across the CWA, and our totals could very well be conservative.
Would not be surprised to see additional significant revisions
to the forecast even at this late hour. Winds will be northeast
to north gusting 20-25 mph, so not a blizzard, but perhaps a
factor for severity of conditions. Accumulations will be QPF and
ratio dependent, with our current graphic including the snow
which fell today. Thus, highest totals for the whole event are
still expected for northwestern areas, with totals locally over
a foot most likely in these areas. However, given the intensity
of the precip band expected to cross the region on Wednesday
with the second low, would not be shocked if some places that
haven't seen much so far could still manage to get close to a
foot. Temps through the next 24-30 hours will continue to be
near or a bit below freezing, with not much change overall.


system will be winding down as it heads out to sea Wednesday
night. Some snow may linger especially eastern areas early, but
by midnight or thereabouts it should be dry CWA wide. Gusty
north to northwest wind will continue so don't expect much
radiational cooling despite the snowcover. Lows mostly in the

Thursday will be warmer with the sun returning as high pressure
slowly but surely builds back in, though temps will still be
below normal. Highs mostly in the 40s. Winds will be lighter by
Thursday night with temps dropping back into the 20s.


.LONG TERM /FRIDAY THROUGH TUESDAY/... Surface high pressure
will be in control on Friday as an upper level low swings over
the northeast US later on Friday. Some precipitation is possible
in the form of rain or snow as suggested by the thermal
profiles. Dry conditions are expected then on Saturday as a
frontal boundary and a low pressure system approaches the area.
The actual track and timing of these systems is still uncertain,
moving closer to us Saturday night into Sunday. Guidance has
shifted to a more southern track, but still a few models are
suggesting a track closer to us. This will define the impacts we
might experience from it in the form of snow or rain or if
anything at all.

High pressure returns later on Sunday with dry condtions and
remains in control into Tuesday.


Poor conditions at present should relax over the next few hours
as we get into a lull between systems, so MVFR quite possible
for a while late afternoon/evening. However, widespred IFR to
LIFR expected late tonight and Wednesday as snow, heavy at times
overspreads the region. Significant accumulations will also be
a big impact on runway ops.

Conditions improve significantly Wednesday night and VFR should
return Thursday if not late Wednesday night. It should remain
VFR thru Thursday night.


Gale continues on the middle bay at this hour but will drop to
SCA tonight. SCA should continue all waters thru Wednesday as
coastal low takes shape off the coast, but winds may relax on
the Potomac Wednesday night as the low heads offshore. SCA may
then return on Thursday all waters.

Winds are expected to be near or above the SCA threshold on
Friday. Wind gusts will then decrease below criteria on Saturday
and remain below through Sunday.


Tide levels have surged significantly in the lower Maryland Bay
and lower Tidal Potomac this afternoon. Anomalies range from
zero in the upper Bay to around two feet near the mouth of the
Bay in Virginia. Have gone near or above all tidal guidance with
the afternoon forecast, keeping tidal anomalies of 1.5-2.0 feet
in the areas that are already near that level. This puts St.
Mary's County at the minor threshold this evening, and well
above it tonight. A Coastal Flood Advisory has been issued for
the evening and morning tide cycles, and may need to be extended
for a couple more cycles.

Calvert (Solomons) and King George/Charles (Dahlgren) get
close. How far up the Bay and Potomac the surge gets, given a
stiff north wind, remains to be seen, and will have to be
closely monitored.


DC...Winter Storm Warning from 2 AM to 8 PM EDT Wednesday for
MD...Winter Weather Advisory until 8 PM EDT this evening for MDZ006-
     Winter Storm Warning from 8 PM this evening to 8 PM EDT
     Wednesday for MDZ006-503-505-507-508.
     Winter Storm Warning until 8 PM EDT Wednesday for MDZ003>005-
     Winter Storm Warning from 2 AM to 8 PM EDT Wednesday for
     Winter Storm Warning from 6 AM to 8 PM EDT Wednesday for
     Coastal Flood Advisory until 9 AM EDT Wednesday for MDZ017.
VA...Winter Weather Advisory until 8 PM EDT this evening for
     Winter Storm Warning from 8 PM this evening to 8 PM EDT
     Wednesday for VAZ025>027-029>031-503>505-507-508.
     Winter Storm Warning until 8 PM EDT Wednesday for VAZ028.
     Winter Weather Advisory from 2 AM to 2 PM EDT Wednesday for
     Winter Storm Warning from 2 AM to 8 PM EDT Wednesday for
WV...Winter Weather Advisory until 8 PM EDT this evening for WVZ505-
     Winter Storm Warning from 8 PM this evening to 8 PM EDT
     Wednesday for WVZ505-506.
     Winter Storm Warning until 8 PM EDT Wednesday for WVZ050>053-
MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ530>533-
     Small Craft Advisory until 6 PM EDT Wednesday for ANZ535.
     Small Craft Advisory from 6 PM this evening to 6 AM EDT
     Thursday for ANZ534-543.
     Gale Warning until 6 PM EDT this evening for ANZ534-543.
     Small Craft Advisory until midnight EDT Wednesday night for