Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 241853

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
253 PM EDT Tue Apr 24 2018

.SYNOPSIS...Low pressure over South Carolina will slowly move
up the mid-Atlantic coast tonight before passing by to our east
Wednesday. A cold front will pass through Wednesday evening and
high pressure will return for Thursday. Low pressure may impact
the area overnight Thursday into Friday before a cold front
passes through Saturday. High pressure will return by the end of
the weekend.



Rain has been very slow to move into northern VA today, but it`s
finally moving in. Strengthening onshore flow rest of today
into this evening will lead to a period of rain tonight ending
by midnight in the south but lingering into the early morning
hours in northeast MD. Model QPF has trended lower this cycle
with amounts likely to remain below an inch in most places
except in the south and on the eastern slopes of the Blue Ridge
mountains where amounts greater than an inch have been reported
in Highland county and approaching an inch in Augusta county.
Mid-level dry slot will start working into southern areas by
09Z as low pressure moves over southern MD.



Steady rain will end by 12Z everywhere as onshore flow
weakens and low pressure passes just to the east. Not much in
the way of forcing available to generate precip, but can`t
completely ruled out a few showers as weak instability develops
with passage of upper level trof axis. Otherwise, most cloudy
skies with some breaks in the clouds possible over the VA
piedmont in the afternoon.

Very weak high pressure builds Wed night and Thu. A second area
of low pressure will move across the area Thu night with
increasing chances of showers.



On Friday, a upper level 500mb trough will be situated over the
Midwestern US. A 500mb shortwave will move through the system and be
over our region on Friday. The associated sfc low pressure
system will shift into the region from the southwest. The exact
location of this sfc low is still uncertain as the 12Z GFS keeps
the low further north and west in northern Maryland and PA. The
00z Euro has the low approaching and moving east north east
just to our south. The difference between solutions is that the
GFS tries to keep the majority of the precipitation to the north
and west of our forecast area while the Euro tries to take the
low further southward. The 12Z Euro run may bring the low
further north like the GFS increasing the the chances for higher
precip amounts. As of now, this seems like a fairly weak
frontal passage that will bring light precipitation. The front
associated with this sfc low will likely move out of our
forecast area by early Saturday morning. Temperatures will rise
Friday up into 60s with some areas reaching the 70s due to warm
air advection from the south to southwest. A isolated
thunderstorm wouldn`t be out of the question on Friday.

Behind the weak frontal passage, a strong high pressure will build
into the area from the Upper Midwest and the Great Lakes
region. The high will remain over the region through early next
week leading to clear skies and temperatures close to average
for this time of the year with temperatures in the upper 60s to
low 70s.



Developing MVFR/IFR conditions tonight in rain if they haven`t
already as low pres over SC lifts ne and onshore continues to
strengthen. Winds will remain gusty through 00Z-03Z, then start
to diminish after 06Z. Gradual improvement in cigs expected Wed,
but still plenty of clouds.

Clouds will increase on Friday as a low pressures system moves in
from our south. Brief lowering of cloud bases and visibility will be
possible if showers and isolated thunderstorms move through the
region. Sub-VFR conditions will be possible on Friday. Clearing skies
along with light winds are expected over the weekend leading to VFR



SCA conditions expected through 03Z-06Z tonight, then winds
diminishing rapidly as sfc low moves overhead. Light winds
expected on Wed, then strengthening somewhat Wed night behind
departing low. Marginal SCA conditions possible, but confidence
low attm for any SCA.

A low pressure approaching from out south on Friday will lead to a
southerly flow and increasing clouds.  The southerly flow will
likely warrant the need for small craft advisory on Friday as winds
flow up the chesapeake bay. Over the weekend, a strong high pressure
will be present leading to clear skies and lighter winds. SCA
unlikely Saturday and Sunday as of now.


MARINE...Small Craft Advisory until 4 AM EDT Wednesday for ANZ530>534-
     Small Craft Advisory until 10 PM EDT this evening for ANZ535-


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