Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 201401

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
901 AM EST Sat Jan 20 2018

High pressure to the south will dominate the weather through
the weekend. A cold front will approach the region Monday before
passing through Monday night into Tuesday. High pressure will
return for Wednesday through Thursday of next week before moving
offshore Friday.


High pressure will remain over the southeastern United States
through the day today leading to dry conditions and light
southwest surface flow. Aloft west/northwest winds will lead to
downsloping flow which will provide for warming conditions and
mostly sunny skies through the day, aside from some high cirrus
passing overhead and some cumulus along the westerly facing
slopes. Highs should reach the 50s area-wide to potentially
60F in some locations.

Tonight, with the high sliding a bit further east, a bit more
low level warm/moist advection looks likely to take place, which
appears to result in weak frontogenesis over the region. This
could result in a bit of a wedge forming over the region late
tonight and Sunday, which would cause a milder night but a
cooler, cloudier Sunday. However, the NAM is one of the models
showing this, and it has goofed these scenarios before by
moistening it too fast. This is problematic in particular given
how dry the air mass remains at present. Thus, have half stepped
between the NAM and GFS for cloud cover and highs on Sunday,
bringing them down from today`s forecast but not as cool as the
NAM would suggest. If the NAM is right, it could be stuck in the
40s most of the day Sunday, while the GFS would suggest another
very mild day like we are expecting today.

Further west, along the Allegheny Front, the warm/moist
advection looks more certain to significantly moisten the
atmosphere, so have some chance pops along the ridgeline late
tonight and Sunday, but no further east. With warmer temps,
expect precip to be in the form of rain, but it might be close
to freezing late tonight.


Whatever wedge manages to form on Sunday will likely linger
Sunday night before trying to scour out on Monday as southerly
winds increase ahead of the approaching cold front (currently
located over the Rockies). However, once again, the NAM tries
to hold it in place, suggesting a much cooler day. Again have
half-stepped this, given we are expecting little if any precip
to reinforce the cold wedge, though a little light rain and
drizzle will still be possible along the Allegheny Front and
Mason-Dixon line. The uncertainty with highs on Monday is the
largest of the forecast period, with the NAM suggesting high
40s in much of the area while the GFS brings temps into the
60s. Did go warmer than Sunday given the increasing southerly
flow expected. Monday night the cold front moves in with rain
showers. Forcing is decent and there could be some elevated
instability, but surface inversion may preclude thunder if the
NAM is right. For now have not included any thunder. Temps will
likely only drop a little from Monday`s highs, whatever they may
be, and if the cool NAM is ultimately right, temps may in fact
rise Monday night. Regardless, southerly winds should increase,
and the jet just above the surface looks quite strong, so if
there is any convection late, some strong gusts should try to
mix down if there is little to no inversion.


50+ knot low level jet and several hundred joules of CAPE ahead
of a cold front/negatively tilted upper trough yield some
concern of gusty surface winds Tuesday morning. Question will be
if surface/boundary-layer air is still relatively cool as
surface high pressure wedge slowly retreats to the north and
east. Regardless, anomalously high moisture combined with
strong frontogenetic/jet forcing will result in a soaking
period of rain showers during the morning, quickly coming to an
end from west to east as drier air filters in.

High pressure building in behind the front will bring a return
to more seasonable temperatures (highs in the 40s, lows in the
20s) for the second half of next week. Otherwise, mainly dry
conditions are expected. One exception is a secondary upper
trough that pivots through early Thursday which could increase
clouds and result in a few upslope snow showers.


VFR should prevail through today at all terminals. A backdoor
cold front may then drop south across the region tonight, with
an increasing risk of MVFR or IFR cigs later tonight into
Sunday. Not certain on this, however, as the atmosphere is
currently quite dry, so a lot of moistening has to take place
for these low clouds to form. If they do form, they may linger
through Monday, before an approaching cold front also causes
reduced visibility via rain showers Monday night.

IFR seems likely Tuesday morning with showers ahead of a front.
Low-level wind shear and perhaps gusty surface winds would be a
concern well given 50+ kt low-level jet forecasted by most

VFR should return for the second half of Tuesday. Winds will
shift from S to NW after frontal passage. VFR Wednesday.


High pressure will provide light winds through Monday.
Increasing southerly winds ahead of an approaching cold front
will likely lead to SCA gusts Monday night, especially after
midnight and especially over the middle portion of the bay.

Strong gusty winds out of the south area possible especially in
heavier showers Tuesday morning. Winds will become NW and
remain gusty later Tuesday through Wednesday behind the front w/
SCA likely.




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