Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 130854

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
354 AM EST Mon Nov 13 2017

A weak low pressure system will move east into the Atlantic
Ocean today, followed by high pressure on Tuesday and
Wednesday. Another front will cross the area Wednesday night
into Thursday morning with high pressure moving back in for


Current analysis shows one surface low over northern West
Virginia and a second developing low near the VA/NC coast. The
low in WV is weakening as the one near the VA/NC coast takes
over, but they are moving pretty quickly eastward. It turned out
that the dry slot was further west than some guidance depicted,
and most of the CWA got rain overnight, but that rain is
starting to clear the region. With the low strengthening near
the coast, rain will likely linger/keep regenerating for several
more hours in southern MD, but areas further west will probably
be dry by dawn or shortly thereafter. Even in southern MD, the
rain should clear the area by noon, with a dry afternoon with
partial clearing expected nearly everywhere. The exception is
the upslope areas along the Allegany Front, where some showers
will likely continue much of the day. Assuing we do get some sun
east of the mountains, we should rebound close to 50 along I-95.

Tonight, high pressure will slowly build in from the west. We
will have some lingering clouds and low level moisture along
with a light northerly breeze. This may be enough to keep fog
from forming, as suggested by stat guidance, but model soundings
suggest fog is possible. For now have not put in forecast just
yet, but definitely a concern if we can clear enough and the
winds can drop off enough. Lows will be in the 30s.


High pressure will slowly cross the area Tuesday and Wednesday
before a front approaches Wednesday night. This high will be a
bit dirty, with a lot of lingering low level moisture, so the
potential for fog and low clouds is elevated - however, at this
point we have not explicitly included it in the forecast. It is
just something to watch for. Otherwise, temperatures should be
closer to normal, but not quite back to normal, with highs
reaching the low 50s in the warmer spots and higher 40s in the
cooler spots, with lows in the 30s overall.

Cold front moves into the region Wednesday night, with a few
showers. Timing for showers right now looks best west of I-95
before midnight, and then a bit later east of I-95. However,
forcing is weakening as it moves through, so most likely showers
will be weakening as well as they push across the CWA. Lows will
be a little milder, but still a lot of 30s and 40s.


A surface cold front will depart east of the area Thursday
morning. Clouds and a stray shower will be possible along the
Mason-Dixon line with upslope showers likely through Thursday
afternoon as the upper trough axis pivots across.

Ridging at the surface and aloft will quickly pivot across the
eastern U.S. on Friday. Meanwhile, a large/deep trough will dig
towards the Great Lakes. This trough will induce strong surface
cyclogenesis over the eastern Great Lakes by Saturday, dragging
a strong cold front across the Mid-Atlantic. Showers and breezy
conditions are likely during this time, but precipitation should
be all liquid as we will be solidly in the warm sector.

Surface low pressure occludes. This surface feature and the
parent upper low (which becomes cut off by this time) will stall
or move slowly across the Northeast Sunday into Monday. This
will leave the Mid-Atlantic in a cold and blustery northwest
flow. Below to well below normal temperatures are strongly
favored during this time. Upslope snow showers would seem likely
as well given the pattern, and a few flakes could even jump
east of the mountains given the strength/instability of the
upper trough Sunday into Monday.


MVFR cigs and vis are most common out there early this morning
with the rain pulling through. There remains an opportunity
towards dawn for some IFR cigs to envelop the area, perhaps even
some high IFR vis as well, but odds seem lower than they looked
earlier. As the westerly flow increases and insolation
increases as we head towards noon, clouds should lift and any
vis restrictions should clear, resulting in a VFR afternoon.
Tonight looks mainly VFR, though model soundings suggest
possible fog. Stat guidance is solidly against it at this
juncture, so have not included yet. Fog potential exists again
Wednesday morning, but otherwise Tuesday and Wednesday should be
VFR with light winds as high pressure passes on by. Cold front
on Wednesday night could bring sub-VFR cigs/vis in showers.

Mainly VFR Thu-Fri. N flow Thu 10-15 kts becoming light
variable Thu night-Fri as high pressure builds in. Good model
agreement leads to a high confidence forecast for this time


Low pressure pulling east away from the region today should
cause rain this morning to break for some sun later today. Winds
look mostly sub SCA, but if there is just enough mixing, we
could touch SCA criteria late today and tonight, or at times
during the day Tuesday. Probability of sustained SCA looks low
at this time so have not pulled trigger on one. Wednesday looks
solidly sub-SCA as high pressure pushes across the area before a
cold front arrives at night.

SCA gusts likely Thursday into Thursday night in northerly
channeling behind a cold front. Winds go light as high pressure
builds in late Thursday night through Friday.




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