Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
410 PM EDT Monday may 4 2015
high pressure will slide farther out to sea tonight into
Wednesday. A backdoor cold front approaches from the north
Tuesday night...then drops into Virginia Wednesday into early
Thursday...before dissipating. Low pressure will spin off or near
the southeast coast Thursday into Friday.
Near term /until 6 am Tuesday morning/...
hi pressure centered off the middle Atlantic CST will provide the area with dry
and mild conditions tonight. Under a clear or mostly clear sky...min
temperatures will be in the upper 50s to lower 60s.
Short term /6 am Tuesday morning through Thursday/...
flattened high pressure remains across the Carolinas Tuesday as
the New England trough drags a relatively flat frontal boundary
along the Mason-Dixon line Tuesday into Tuesday night. Latest
00z model cross sections are now showing a slight increase in
available moisture with slightly stronger Omega fields during the
late afternoon/early evening timeframe (or during best daytime
heating/instability). However...2-d precipitation fields also
indicate that available moisture is getting sheared apart as it
approaches the middle Atlantic region. Will need to keep an eye on
water vapor trends as the current complex in the Midwest shifts
toward New England with the trough/surface frontal boundary. Have
therefore held onto slight chance probability of precipitation from Louisa County to the
Northern Neck to the Maryland lower Eastern Shore beginning late Tuesday afternoon...
increasing to chance probability of precipitation (no higher than 30%) Tuesday evening across
the Northern Neck and Maryland/Virginia Eastern Shore. SW winds and temperatures warming
into the middle 80s for highs will help to destabilize the atmosphere...
and thunderstorms may be possible in the afternoon/early evening. Model
cross sections indicate that moisture fields diminish after
midnight Tuesday night...however light showers may linger over the
Maryland lower Eastern Shore overnight Tuesday so kept slight chance
probability of precipitation here during this time. Another night with mild lows in the
upper 50s to lower 60s.
Forecast models then diverge in their solutions beginning
Wednesday. High pressure builds down from Canada and north of the
relatively flat surface front near the Mason-Dixon line. Meanwhile...
low pressure developing off the East Coast of Florida is expected
to lift toward the southeast coast into Wednesday night. There is
some agreement amongst models to keep precipitation over the area through
Wednesday as the surface boundary sags southward and a leeside trough starts
to develop. The exact location and timing of precipitation is what is in
question and have therefore tried to split the difference between the
more aggressive GFS...the drier NAM...and the middle of the Road
European model (ecmwf). This should keep light showers over the northern half of the forecast area
(north of richmond) Wednesday morning...which should then shift
westward during Wednesday afternoon/evening as additional instability
triggers showers near the Lee trough boundary. Thunderstorms are
possible once again Wednesday afternoon/early evening. Any precipitation
that occurs is expected to taper off through the evening with dry
conditions overnight. Southeast Virginia and NE NC will remain precipitation-free on
Wednesday/Wednesday night. Highs Wednesday 80-84 inland...
middle-upper 70s at the immediate coast and for the Maryland/Virginia Eastern
Shore. Lows Wednesday night remain mild with lows in the upper 50s
to lower 60s.
Long term /Thursday night through Monday/...
medium range period highly dependent persistent upper ridge over the
east, as well as the development and eventual progress of expected
development of an area of low pressure off the East Coast of
Florida. Models are in general agreement of the development and
initial slow drift northward on Wednesday. The hurricane center is
continuing to monitor this feature for potential subtropical
development. Surface ridging to the north and west-northwest flow aloft should
force the low to meander around a position along/just off the
southeast coast through the weekend. The gefs and Euro ensembles
both continue to take the low inland along the SC/Georgia coast, as does
the new 12z/04 GFS. The operational European, meanwhile continues to
keep the system offshore. A solution towards the GFS remains the HPC
preference, and have edged the forecast in this direction. This
scenario would yield slight chance to low chance pop for mainly diurnally
driven showers/T-storms at most across our area. Lowest chances will
be across the north with surface ridging in place north of the area.
Maintained low probability of precipitation (20-30%) with the (subtropical?) Low still
meandering to the E-se. Other headline for the period will be the
warmer temperatures. East-southeast flow will bring a modest increase in
humidity, and increasing low level thicknesses favor warm highs in
the 80s under a partly to mostly sunny sky. Early morning lows look
to remain well into the 50s to middle 60s late week into early next
Aviation /20z Monday through Saturday/...
high pressure is centered off the middle Atlantic coast. A cold front
over the Great Lakes slowly approaches the region from the north/northwest
As of 18z...scattered cumulus with bases around 7-9 k feet covered the County Warning Area.
Winds were from the S/SW at around 10 to 15 knots with a few higher
gusts. Winds diminish to around 5 to 10 knots by early evening and
remain at those speeds overnight and into Tuesday morning. The sky
will be mostly clear by sunset and remain that way through Tuesday.
Outlook: there is a chance for thunderstorms Tuesday evening at sby. There
will be a daily chance for mainly afternoon showers and thunderstorms
Wednesday through Saturday...especially southern portions. They are
not expected to be widespread. Otherwise mainly VFR conditions are
expected during the period. Late in the week...patchy fog will be
possible around sunrise.
latest observation reflect S-SW flow 10-15kt across the waters
this afternoon west/surface high pressure oriented along/off the NC coast. Winds
have increased into the 10-15 knots range as pressure gradient has
tightend slightly in response to low pressure moves through Quebec
as high pressure remains just offshore. Still appears as if there
will likely be some gusts to 15-20 knots by tonight...but should be
too infrequent/marginal to raise any headlines in the Bay as mixing
will not be optimal (very warm airmass over relatively cool waters).
Seas in northern coastal waters 3-4 feet...2-3 feet farther south. Gradient
slackens a bit on Tuesday...then increases Tuesday night/early Wednesday as a cold
front drops south across the waters in backdoor fashion. The front
looks to gradually push into the area on Wednesday and dissipate Wednesday night
as surface high pressure builds in from the north. Combined with previously
referenced surface low off the southeast could bring a modest increase
in wind speeds as directions shift to the east/NE Thursday with tightening
pressure gradient...and have continued with a GFS weighted GFS/CMC
solution for this period. While latest wavewatch guidance shows
increase in seas for later Thursday into Friday...current track of surface low
stays too far south to bring any significant increase in seas and
winds to the local area. Will continue to monitor these trends and
any lingering coastal flood threats over the next 24 to 36 hours.