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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
246 PM EDT Monday Apr 27 2015

an upper level low pushes east across the region through Tuesday
as high pressure briefly builds in. Low pressure will impact the
region midweek with high pressure returning for the weekend.


Near term /through tonight/...
upper level low remains positioned over the Canadian Maritimes
through tonight...extending back to the Great Lakes and south into
the mid-Atlantic. A passing shortwave this morning brought broken-overcast
sky conditions to the region. Another shortwave/vorticity maximum currently
noted in the water vapor imagery over Ohio/WV/PA will dig south
through the remainder of this aftn/evening. This energy combining
with minimal instability could result in isolated-scattered rain
showers...with light snow mixing in at the higher elevations of
the Potomac Highlands.

Any precipitation that does occur will taper off this evening...with the
entire area becoming dry for the second half of the night. Only other
concern for tonight was the frost potential...mainly over the
southern Shenandoah Valley areas. Current forecast drops overnight
lows into the middle to upper 30s mainly...with isolated spots nearing
freezing. Thinking the winds should stay elevated enough and the
clouds clear out late enough to keep temperatures as forecasted.
However...if the trend this evening looks to have clearing quicker
and/or winds dropping more...could see the need for a frost advisory
in Augusta/Rockingham/Nelson. Counties to the west are yet to enter
the growing season.


Short term /Tuesday through Wednesday night/...
upper level low pivots off to the east Tuesday. High pressure builds
in at the surface...keeping the area dry. Subsidence building in
behind the departing low will help to clear skies Tuesday
morning...but could see scattered cumulus by Tuesday afternoon. Downsloping winds
under the scattered skies will push temperatures into the middle to upper 60s.
Lows Tuesday night stay in the 40s for most of the area...limiting
the frost concern.

Weak high pressure will be over the area early Wednesday. The high
will move offshore by early afternoon...and be replaced by a wave of
low pressure developing over the Appalachians. A second area of low
pressure will become more organized as it passes through northern Florida
and move offshore. Clouds will overspread the forecast area during
the day...with potential for light rain developing over portions of
The Highlands...the central Shenandoah Valley and the central
foothills. Otherwise dry weather expected. Return flow is expected to
develop for the afternoon with the high offshore. Temperatures should be a
few degrees warmer than Tuesday...with highs mainly in the upper 60s
and low 70s.

Weak low pressure persists over the mountains Wednesday night. The low that was
over Florida during the day will move offshore and strengthen. There
remains some spread among models about the strength and trajectory
of this system...and therefore uncertainty about timing and location
of rainfall onset overnight. Will continue with chance probability of precipitation across
the forecast area for the time being. Lows for most areas will be in
the middle to upper 40s...with low to middle 50s likely along the I-95


Long term /Thursday through Monday/...
model uncertainty still remains rather large with track of low
pressure moving out of the southeastern United States during the
middle of the week. Depending on which model or even model cycle
you choose...our area could see anywhere from a couple of inches
to less than a half an inch of rainfall. 00z GFS most aggressive
with track...with the low hugging the coastline as it moves to our
east/northeast. However...12z GFS now in better agreement with 00z
European model (ecmwf)...and keeps low far enough offshore that overall impacts to
our area are less. For now...leaned more toward the 00z European model (ecmwf)/12z
GFS solution due better initializations. Will need to monitor this
closely as a small change in the track could significantly change
impacts for our area.

Regardless of what happens with the late week system...high pressure
returns this weekend. Temperatures will be above climatology and precipitation chances
will be low.


Aviation /19z Monday through Saturday/...
VFR conditions through the taf period. Brief rain shower possible this
afternoon...mainly 19-22z...with minimal categorical impacts. Occasional
gusts possible late this afternoon...with the gusts ending 23-02z this
evening. Occasional gusts will be possible again at kbwi/kmtn on Tuesday.

Primarily VFR Wednesday. Light northwest winds in the morning
become southerly in the afternoon. Possible sub-VFR with potential rain developing
Wednesday night. Winds S-SW less than 10kt.

Uncertainty remains large with track and impact of low pressure
system tracking along coast on Thursday. The more the low hugs the
coast...the greater the aviation concerns will be.
Regardless...would expect sub-VFR conditions in any showers that
occur Thursday into Friday.


Small Craft Advisory remains in effect through this evening for all waters...then
continues on the Chesapeake Bay and southern tidal Potomac through
Tuesday afternoon. Sub-Small Craft Advisory conditions on all waters by Tuesday evening.

Light northwest winds on the waters Wednesday morning...becoming S-southeast 5-10kt by
the afternoon. No advisories expeceted. Low pressure will emerge into the
Atlantic off the Florida coast Wednesday night and strengthen as it begins to
move to the north. Winds become SW around 10kt...with potential for
some gusts up to 15kt. Light rain possible.

The low pressure system tracking along the coast Thursday into Friday has
the potential to cause periods of Small Craft Advisory or gale conditions.


Lwx watches/warnings/advisories...
District of Columbia...none.
Marine...Small Craft Advisory until 4 PM EDT Tuesday for anz530>534-
Small Craft Advisory until 8 PM EDT this evening for anz535-


near term...Sears
short term...Sears/KS
long term...mse

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