Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
137 am EDT Tuesday Apr 21 2015
rain showers will taper off overnight as a cold front crosses
the area and offshore by Tuesday morning. High pressure builds
back in from the southwest on Tuesday. Another cold front crosses
the area late Wednesday into Wednesday night. High pressure
returns Thursday and Friday.
Near term /until 6 am this morning/...
a line segment of thunderstorms has developed from north-central NC into far
southern Virginia overnight in vicinity of a pre-frontal surface trough. The
environment remains strongly sheared with effective shear values
around 40-45kt across the coastal plain. Additionally...MUCAPE
values are ~1000 j/kg. An isolated severe threat will continue
overnight across the watch area...before convection weakens later
tonight. Meanwhile...the surface cold front currently sits from north central
Virginia down into SW/scntrl Virginia and will move off the coast by sunrise
Tuesday morning. Temperatures are mild ranging from the upper 60s
to low 70s. Eventually...lows should be in the middle/upper 50s west
of I-95 and in the upper 50s/low 60s eastward to the coast.
Short term /6 am this morning through Thursday/...
stacked upper low anchors over the Great Lakes region and southern
Canada through the short term period thanks to blocky flow over the
high latitudes. In wake of tonights (mon nights) cold front...high
pressure forecast to build across the southeast states Tuesday...extending
northeastward over the local area. The result will be dry conditions and a
westerly wind. Highs forecast in the upper 60s east to low 70s west under a
mostly sunny sky. Series of upper level disturbances will round
the base of the upper low...with the first rounding the base Tuesday
night- Wednesday. Associated fast moving cold front drops into the Ohio
Valley Wednesday. High pressure pushes off the southeast coast Wednesday...with
increasing southwesterly low level flow over the local area. Weak front
reaches the central Appalachians Wednesday afternoon. Will maintain
slight chance- chance probability of precipitation across the northern local area Wednesday
afternoon...closest to best moisture/dynamics. Highs Wednesday
generally in the low-middle 70s. Front drops across the local area
Wednesday night. Will maintain slight chance-chance probability of precipitation along the
coast Wednesday night. Frontal boundary stalls just south of the local
area early Thursday morning. Wave of low pressure prognosticated to develop
along the stalled boundary Thursday morning as southern stream energy
traverses the Gulf states. GFS/European model (ecmwf) solutions keep measurable
precipitation south of the local area Thursday...but NAM/sref solutions
spread precipitation into S and southeast Virginia/NE NC. Will carry slight chance
probability of precipitation Thursday across those areas based on uncertainty. Cooler Thursday
with west-northwest flow behind the front. Highs generally in the low-middle
Long term /Thursday night through Monday/...
cold front moves south of the area Thursday night followed by cool high
pressure building in from the northwest. Dry with lows both Thursday and Friday
nights 40-50 xcpt a few u30s northwestern most counties. Highs Friday M-u60s.
Next systm prognosticated to track across the Carolinas Sat. European model (ecmwf) somewhat
further south with its moisture fields than GFS. Best sprt for precipitation
will be across southern half of forecast area where chance probability of precipitation will be maintained.
Slght chance across the nrth. Cool Sat with highs 60-65. Precipitation ends Sat
evening as systm exits off the coast. Lows in the 40s.
Canadian high pressure dominates Erly next week with any precipitation
staying south of forecast area. Dry with a slow moderating trend. Highs sun
in the l-m60s. Lows Sun night in the 40s. Highs Monday M-u60s.
Aviation /06z Tuesday through Saturday/...
as of 06z...a cold front was situated over central Virginia and
is expected to cross the taf sites between 10 and 14z. Convection
has been rather isolated so far and with daytime heating cut
off... widespread thunderstorms are unlikely. Cannot rule out a thunderstorm
overnight at orf/phf/ecg but not included in the taf due to low
S winds gradually veer becoming west/northwest following the frontal
passage. Winds are forecast to gust to around 20 knots and back to
the west this afternoon.
Outlook...the surface high moves offshore Wednesday. A cold front
crosses the area Wednesday night with a chance of showers mainly at ric
and sby. Dry weather returns for the rest of the week.
10 PM update...Small Craft Advisory hoisted for the Bay and lower James for S
winds 15 to 20 knots and 4 to 5 feet waves at the mouth of the Bay.
Conditions are expected to subside during the early morning hours
with the Small Craft Advisory currently scheduled to expire at 4 am.
Seas remain at or above 5 feet...so will continue the Small Craft Advisory headlines with
an emphasis on subsiding conditions through Tuesday. A cold front
crosses the area late tonight. SW winds ahead of this boundary will
shift to the west but remain below 20 kts...so no headlines (wind
wise) expeceted behind the front.
Next trough / front prognosticated across the waters Wednesday night / Erly
Thursday switching the winds to a northwest then north direction. Once
again...surge behind this boundary not strong enough to generate
headlines. Northwest flow sets up behind this boundary into the weekend
with conditions remaining below Small Craft Advisory levels.
overnight rainfall of one to one half inch resulted in rises over
area rivers during the day. Minor flooding on the Rivanna river at
Palmyra should end before midnight. Rises are expected overnight
tonight for the James River...with the Richmond Westham gauge
approaching minor flooding Tuesday night and Wednesday.
southerly flow into this evening will help maintain ~1.0 feet
anomalies in the Middle Bay through tonight. Bishops Head will
approach the minor threshold late tonight. A coastal flood
statement has been issued to account for this. Other locations in
the Middle Bay should stay below minor thresholds. Tidal
anomalies will begin to subside Tuesday as the flow becomes
Marine...Small Craft Advisory until 4 am EDT early this morning for
Small Craft Advisory until 7 am EDT this morning for anz654-
Small Craft Advisory until 1 PM EDT this afternoon for anz650-