Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
112 am EDT Tuesday Apr 28 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 today/...
upper level low remains positioned over the Canadian Maritimes
through tonight...extending back to the Great Lakes and south into
the mid-Atlantic. Shortwave energy embedded within the northerly flow has
resulted in widely scattered rain showers. However...low level air mass dry...which has
precluded most of this moisture from reaching the ground.
Am noting a downward trend now that sunset has occurred. Believe
this will continue...and will run with a dry forecast overnight.
Clouds will be a bit more substantive though...then part in the wake
of the vorticity. In turn...the clouds will keep temperatures up...which is
reflected in lamp guidance. Going temperature forecast seems to be on track. Have
made no temperature changes...which means do not believe there will be much of
a frost concern in the central shen valley.
Short term /tonight through 6 PM Wednesday/...
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 /Wednesday night 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 /05z Tuesday through Saturday/...
VFR conditions through the taf period. Gusts have diminished somewhat.
Will have northwest flow through the night which will continue into tmrw. Occasional ceilings
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.
Due to gusts inxs of 20kt on the central ptmc have extend the Small Craft Advisory to
the upper 2/3 of the ptmc through 6 am. This may need to be extend into
the daytime hours.
Continue the Small Craft Advisory through the day Tuesday. Still believe that 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.
District of Columbia...none.
Marine...Small Craft Advisory until 4 PM EDT this afternoon for
Small Craft Advisory until 6 am EDT early this morning for