Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
1005 PM EDT Tuesday Apr 21 2015
high pressure will slide across the Gulf Coast states and off the
southeast coast tonight into Wednesday morning. A cold front
approaches from the northwest Wednesday morning...and crosses the
middle Atlantic region Wednesday afternoon into Wednesday evening.
High pressure returns for Thursday and Friday.
Near term /until 6 am Wednesday morning/...
update...only minor tweaks made to ongoing forecast through
Previous forecast...dry and seasonably cool tonight...as hi pressure
slides across the Gulf CST states and off the southeast CST. In the
meantime...a cold front will push through the Ohio Valley to just west of
the mountains clear or mostly clear through early tonight...then increasing
hi clouds over northwest or west counties toward morng. Min temperatures will range from
the middle 40s to lower 50s.
Short term /6 am Wednesday morning through Friday/...
deep layer SW flow...deep mixing...and a dry low-level airmass
should limit coverage of showers/thunderstorms ahead of and right in
advance of the cold front Wednesday afternoon into early Wednesday eveng. The
system is rather dynamic however...so have gone with 30-50% probability of precipitation
over western and northern counties...and 20% probability of precipitation over southeast Virginia and NE NC. Steep
low and middle level lapse rates are expected Wednesday afternoon...but with weak
surface instability (li's 0 to -2)...thunderstorms are not expected to have
deep strong long-lived updrafts. However...the 700mb flow is
prognosticated to be ~60kt...and this will be above a deep well mixed
convective boundary layer. So...any thunderstorms have the potential of
producing strong wind gusts. Highs on Wednesday should average in
the lower to middle 70s.
Any lingering precipitation will end Wednesday eveng...as that front will push
off the CST and south-southeast of the area Wednesday eveng/night into Thursday morng.
Cooler and even drier air will then filter into the region for Thursday
through Friday...as hi pressure builds into and over the area from the northwest. Lows
Wednesday night ranging through the 40s to around 50. Clear or mostly clear Thursday
through Friday. Highs on Thursday ranging through the 60s. Very chilly Thursday night
with lows ranging from the middle 30s to middle 40s. Highs on Friday in the
lower to middle 60s.
Long term /Friday night through Tuesday/...
weak high pressure over the area Friday night keeps the forecast area dry. Data
supports clouds increasing after midnight ahead on next systm apprchg
from the SW. Cool with lows from the u30s northwestern most counties to the
u40s southeastern coastal areas.
Low pressure prognosticated to move across the southeastern US this weekend. Both
GFS/European model (ecmwf) hold off on precipitation overspreading the forecast area until late morning
and afternoon. A cool start to the weekend as the rain results in an
insitu-damming scenario. Highest probability of precipitation remain across the southern counties
where best sprt for precipitation will be. Lght NE flow holds temperatures in the
u50s across the Piedmont areas to l60s at the coast. Chance probability of precipitation continue
Sat night before ending west-east aftr midnight as the low moves
offshore. Lows in the 40s.
Cool northwest flow sets up across the region Erly next week. European model (ecmwf) wants
to keep a weak trough lingering across the region before moving southeast on
the forecast area Sunday night. Kept slght chance probability of precipitation Sunday across NC counties to
account for this. Highs in the m60s. Lows in the 40s.
Dry and a bit warmer Monday as high pressure moves north of the
region. Highs 65-70. Lows m40s-l50s.
Anthr low pressure systm tracks NE across the Ohio vlly Tuesday. An
assctd warm front lifts north across the area Tuesday with trailing cloud
front Tuesday night. Chance probability of precipitation this far out. Highs Tuesday 65-70.
Aviation /02z Wednesday through Sunday/...
VFR conds tonight with mostly sky clear and VFR conds expected through the
taf period. SW/west winds tonight and generally 5-10 knots. Middle to high
clouds will increase tomorrow with a chance of some widely scattered
showers or maybe an isolated thunderstorm Wednesday afternoon. Pressure gradient
tightens tomorrow morning ahead of deepening low pressure. Gusts
from the SW at 25-30kt will be possible Wednesday afternoon.
A cold front crosses the area Wednesday night with a chance of
showers mainly at ric and sby. High pressure and dry weather
return through Friday.
seas across Maryland coastal waters are quickly subsiding so will let current
headline expire with coastal waters forecast issuance. No headlines expeceted tonight as SW
winds average at or below 10 kts. Next in a series of fast moving cold frnts apprch
the area Wednesday night. Tightening prs gradient ahead of it will result in
SW winds increasing into Small Craft Advisory range (15-25 kts) across the entire marine
area. Thus...started Small Craft Advisory headline by late Wednesday morning and continued
it through 23z...although confidence rather marginal over the coastal waters.
Forecast confidence for Small Craft Advisory headlines becomes a bit lower behind the
cold frontal passage Wednesday night. The surge behind this mornings boundary
was strngr than any model indicated and am concerned the same trend
continues with respect to the next surge given the strength of the cold front.
Thus...have continue Small Craft Advisory headlines for the ches Bay through 10z Thursday for
this secondary surge after midnight despite a several hour period of sub
Small Craft Advisory level winds after sunset Wednesday evening.
Anthr cold frontal passage late Friday will lead to yet anthr surge
across the ches Bay Friday night. North-northwest flow sets up Saturday through
Monday with below Small Craft Advisory conditions xptd.
a river Flood Warning continues for the James River at Richmond
Westham. The river has leveled off at 12.0 feet...which is also
minor flood stage. Water levels will hover around 12.0 feet
through Wednesday afternoon before falling well below 12.0 feet.
Marine...Small Craft Advisory from 10 am to 7 PM EDT Wednesday for
Small Craft Advisory from 10 am Wednesday to 6 am EDT Thursday
Small Craft Advisory until 7 PM EDT Wednesday for anz650-652.