Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 240130
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
930 PM EDT Sun Apr 23 2017

.SYNOPSIS...
High pressure near Long Island will slide out to sea tonight.
Low pressure in Georgia will very slowly move eastward, reaching
the South Carolina coast on Monday, the North Carolina coast
Tuesday, and moving east of Mid-Atlantic on Wednesday. A cold
front will reach the region late Thursday or early Friday.

&&

.NEAR TERM /THROUGH MONDAY/...
Latest surface analysis depicts low pressure over Georgia, with
a stationary front extending eastward towards Wilmington, NC and
a cold front extending south into the Gulf of Mexico. To the
north, high pressure is centered near the Jersey Shore and over
the southern Great Lakes, with another center in the central
Plains. Aloft, we have a closed low over western Tennessee, with
a shearing shortwave over southern New England and another ridge
over the southern Plains.

This closed upper low will be moving at a snails pace over the
next few days, and its associated surface features will do
likewise. Overall, this is acting like a mid-winter storm,
except that its too warm for any snow. Other than the lack of
snow, the cool northeast wind, minor coastal flooding and long
period of precipitation also reflect its rather winter-like
nature.

Another element of similarity will be the excruciatingly slow
progress of the precipitation into our area. As the low pushes
east tonight, the confluence over our region will only very
slowly give way tonight and Monday, and the drier air in place,
fed into the region by the elongated high pressure ridge
(classic banana ridge often present on the northwest side of
winter storms) will continue to feed dry air into it. This trend
has continued into the evening, with the very low clouds still
confined to far southern Virginia. Light rain has made it up to
Staunton and Charlottesville, but it`s northward progress is
somewhat uncertain overnight. Even though there will be better
chances for a steady rain as Monday progresses, model QPF
suggests it may be of little substance north/east of the Potomac
River.

As the best forcing stays south, significant heavy rain is
generally not expected across the region, so no flood watches
have been issued, though if any were needed, it would be in
central VA near the Blue Ridge where the upslope flow and upper
level forcing is maximized. Lows tonight will be in the 40s with
highs generally in the 50s on Monday. Have nudged highs down in
the southwest portion of the CWA where steadier rains are
expected. As a proxy, daytime temperatures in southwest VA
today were in the mid 40s to around 50.

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
The slow moving low will only reach Cape Hatteras on Tuesday and
won`t even make its closest approach to the region until Tuesday
night, though at that point it should be starting to weaken with
diminishing precip across our region. Bottom line is that we`ll
continue to see periods of rain through Tuesday evening, which
should then diminish later Tuesday night. Northeast winds will
continue and temperatures will stay in the upper 40s to mid
50s. A period of heavier rain is possible late Monday night into
Tuesday as the low lifts northward and forcing increases, by
which time the dry air feeding into the system from the north
should be mostly eliminated, but again, no flood watches, as
uncertainty about placement and timing remains rather
significant. From Monday night onward, the setup also becomes
more favorable for low clouds and drizzle when it`s not raining.
Have also nudged down high temperatures Tuesday in
central/northern portions of the area where models are likely
too fast in dislodging the rain-cooled CAD wedge.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
A cut off low moves from the Mid-Atlantic coast into the NE,
and away from the coast on Wednesday. Some isolated to scattered
showers possible, mainly to the eastern half of our CWA on
Wednesday. High pressure builds Wednesday night into Thursday
before a cold front approaches from the west. This front is
expected to stall over or near our CWA Friday and into Sunday
keeping warm and humid conditions over our region, allowing for
showers and maybe thunderstorms at times.

Temperatures will be above normal during this period, with highs
reaching the 70s and 80s, lows in the 50s and 60s.

&&

.AVIATION /01Z MONDAY THROUGH FRIDAY/...
VFR now, but deteriorating slowly tonight and Monday as low
pressure moves slowly toward the region with rain, a northeast
wind, and lowering CIGs and VIS. For 00Z TAFS, made some
significant adjustments to slow down timing of MVFR/IFR cigs.
NAM is the fastest model to deteriorate, so have incorporated
its solution as "SCT" groups, but with dry air feed from the NE,
think it will take well into Monday for lower cigs to reach the
metros/MRB. While rain chances increase, it will likely be light
through much of Monday and unlikely to cause vsby issues (except
at CHO).

IFR does become likely by Monday night and Tuesday as the low
continues slowly moving northeast up the coast. Period of
heavier rain also appears likely late Monday night into Tuesday
morning. LLWS could also be an issue during this time. Some
improvement likely to start Tuesday night as the low starts
heading into the Atlantic.

Sub-VFR possible for the terminals early on Wednesday before
VFR conditions return on Wednesday night as low pressure moves
away. VFR conditions continue into Thursday, and sub- VFR
conditions possible Friday with a lingering front nearby.

&&

.MARINE...
Most of the waters are still well below SCA conditions this
evening, although there is an uptick in winds near the mouth of
the Potomac. Any stronger winds will likely remained confined to
this region overnight. Expect widespread SCA by Monday
afternoon as a slow moving low moves its way up the eastern
seaboard. Right now not expecting gales, but could be close in
the middle bay. Lowest visibilities with low clouds/fog/moderate
rain will be Monday night into early Tuesday. Winds may start
diminishing by Tuesday night.

Isolated to scattered showers possible early on Wednesday
before dry conditions return Wednesday night as low pressure
moves away. Dry conditions continue into Thursday, and showers
and maybe thunderstorms possible on Friday with a lingering
front nearby. Wind gusts should remain below the small craft
threshold.

&&

.TIDES/COASTAL FLOODING...
Have issued a Coastal Flood Advisory for St. Marys again for the
overnight high tide, as even a slightly lower trend would still
produce minor flooding. Guidance suggests Annapolis will also
need to be watched overnight, although anomaly trends this
evening currently suggest it would fall short.

A persistent onshore flow will continue over the waters through
Tuesday. Elevated water levels are expected to continue. The
onshore flow will strengthen a bit for Monday through Tuesday.
Minor flooding is possible near times of high tide. The flow
should gradually turn north Tuesday night into Wednesday as the
low moves away from the area. Water levels should decrease
during this time.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Coastal Flood Advisory until 3 AM EDT Monday for MDZ017.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 AM Monday to 6 AM EDT Tuesday for
     ANZ530>532-535-536-538>542.
     Small Craft Advisory until 6 AM EDT Tuesday for ANZ533-534-537-
     543.

&&

$$
SYNOPSIS...RCM
NEAR TERM...ADS/RCM
SHORT TERM...ADS/RCM
LONG TERM...IMR
AVIATION...ADS/IMR/RCM
MARINE...ADS/IMR/RCM
TIDES/COASTAL FLOODING...ADS/RCM



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