FXUS61 KLWX 250153

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

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


As low pressure lifts northeast toward the forecast area, the
moist conveyor will be lifting north into Pennsylvania and New
Jersey. East/onshore flow remains, with overrunning/isentropic
lift present. Thus, the rain has transitioned into a light
rain/drizzle scenario, with low clouds and areas of fog. As the
center of the low approaches, believe that the upglide will
decrease somewhat-- enough to gradually decrease PoPs from cat
to likely...and for central Virginia/Potomac Highlands, to a
high end chance by dawn. With such a setup, temperatures won't
be falling far tonight...with upper 40s west and lower 50s east
by dawn.


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.


The synoptically forced precipitation has lifted north of the
terminals. With residual moisture, lift, and onshore flow...
low clouds and fog has developed in the light rain and drizzle.
Terminals are bouncing between MVFR and IFR, but believe that
IFR and even LIFR will prevail overnight. Aside from CHO, am
forecasting no lower than IFR at this time. If it were to go
lower, believe that cigs would be the culprit.

Gradual improvement expected Wed as the low moves northeast of
the terminals, but it will be a slow process. If we're lucky,
then VFR will return mid-late afternoon. Otherwise it will wait
until early evening.

No flight restrictions anticipated Thu. The first part of the
day may be breezy (from the northwest) in the pressure gradient
in the wake of the departing low, but gusts should not
significantly exceed 20 kt.

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


East flow across the waters this evening in advance of low
pressure. With low clouds and fog, mixing not ideal. Have
cancelled the Small Craft Advisory for the Potomac above Indian
Head. Believe an impulse will be supporting a burst of better
wind travelling up the Bay tonight, so will be keeping the rest
for now.

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.


Water levels running about a foot above astronomical normals
this evening, with most of that increase coming during the
daytime/evening hours Tuesday. The departures are a pinch higher
than that for the tidal Potomac below the 301 Bridge. Based on
these trends and current projections, Piney Point will brush the
minor threshold tonight. With continued south/east flow into
tomorrow, departures will be steady or continue to rise
slightly. Therefore, anticipate a repeat tomorrow morning.
Therefore, have issued a Coastal Flood Advisory for St Marys Co
for the next two tide cycles.

Although water will be running high, it is unclear whether any
other sites will be affected. If there isnt enough northwest
wind to push water out on Wed (which is a real possibility),
then Annapolis may briefly touch minor flood too. Also, SW DC
forecast to come just inches away for the next few cycles. Will
be monitoring those sites, but not enough confidence to issue
anything at this time.


MD...Coastal Flood Advisory until 2 PM EDT Wednesday for MDZ017.
MARINE...Small Craft Advisory until 4 AM EDT Wednesday for ANZ530>534-
Small Craft Advisory until midnight EDT tonight for ANZ536.


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