Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
1011 PM EDT Friday Apr 17 2015
a weak trough of low pressure pushes across the Middle Atlantic
States overnight. High pressure builds over the area
Saturday...then shifts off the coast on Sunday. A warm front lifts
northeast Sunday night with a cold front crossing the region late
Near term /until 6 am Saturday morning/...
current analysis depicts weak surface trough lingering across the
region...with relatively light west-northwest flow aloft. Earlier convection
diminished fairly rapidly over the past hour...just some isolated
light showers over Southside Virginia remain. Some locations that
received rain earlier are already seeing patchy fog...mainly across
northwest sections of the County Warning Area. Otherwise...variably cloudy skies with
light flow and rather high amounts of low level moisture should
promote some expansion of the fog after midnight. Have added 20%
probability of precipitation overnight across extreme southern Virginia and NE NC as area of more
widespread precipitation over Georgia/SC tracks east-northeast and may brush these zones.
Elsewhere...continuing mention of low clouds/patchy fog overnight given
lingering low level moisture and light winds. Low temperatures in the 50s to
around 60 f.
Short term /6 am Saturday morning through Monday/...
becoming a nice day on Sat (aft any low clouds/fog dissipates).
West-northwest flow aloft results in mostly sunny conds north...partly sunny S.
Again...there may be scattered rain over the Carolinas...staying S of the
forecast area. Hi temperatures M-u70s along the coast (xcp near 70f right at the
beaches) to the u70s-l80s inland.
Surface hi pressure builds S across the region Sat night shifting the winds to a
north-NE drctn. Dry/cooler with low temperatures in the u40s north to the l-m50s
Hi pressure shifts off the CST Sun morning west/ low level flow becoming east-southeast.
Meanwhile...low pressure will be tracking toward the lower Great Lakes
states...pushing a warm front NE toward the mdatlc region. Starting out
sunny-partly cloudy...then incrsg cloudiness expected midday through the
afternoon (fm W-e). Latest models generally holding serve with respect to timing of
arrival of any rain. Rain may reach the Bay by evening...will have 30-60%
probability of precipitation east-west from there. Right at the CST...keeping probability of precipitation at or below 14%. Hi
temperatures from the u60s/around 70f west of i95 and right at the beaches...to
the l/m70s elsewhere.
Significant inflow of deep layered moisture into the region Sun night as
warm front makes progress NE through the forecast area. May be periods of MDT/heavy
rain...and would not rule out isolated (elevated) thunder. The area of
rain expected to shift off the CST Monday morning...W/ trough aloft
approaching from the west by lt Monday afternoon (ptntlly leading to scattered
shras/tstms). Otherwise...vrb clouds-partly sunny midday/afternoon on Monday.
Hi temperatures l/m70s east...u70s-nr 80f west.
Long term /Monday night through Friday/...
cold front exits the middle-Atlantic coast to begin the extended period
Monday night as the cut-off low lifts into the Great Lakes region.
Will maintain chance probability of precipitation (solid chance along the coast) Monday
night...but latest models have sped up frontal passage.
Thereafter...blocky flow prognosticated to prevail through the end of the
period as the upper low remains over the Great Lakes/New England.
Surface high pressure builds across the southeast states/southern middle-Atlantic
region Tuesday. Westerly/downslope flow and a mostly sunny-partly cloudy sky
will result in slight above normal temperatures...generally in the low 70s
inland and middle-upper 60s coastal areas. High pressure pushes off the
coast Wednesday as a shortwave dives down the backside of the upper low
over the Great Lakes region. Associated cold front forecast to drop
over the Ohio Valley Wednesday. Temperatures Wednesday near normal...generally in the
middle-upper 60s. Cold front crosses the local area late Wednesday night-
Thursday morning...but parallel flow aloft will stall the front over
the middle-Atlantic region. 17/12z GFS stalls the front over the local
area with more moisture than its 17/12z European model (ecmwf) counterpart...which
pushes the front through with little fanfare. Have opted for slight
chance probability of precipitation Thursday-Thursday night at this time due to uncertainty. Near normal
again Thursday...before cooling slightly into the low-middle 60s inland to
middle-upper 50s coastal areas.
Aviation /02z Saturday through Wednesday/...
02z update...showers have virtually dissipated and removed mention
out phf taf. Added 2sm fog at ric at 07z where wet ground from
rain this evening will promote fog despite broken to overcast cloud
Kept IFR fog at sby where both the GFS and NAM MOS have 2 miles
there by 06z. As clouds increase...mixing and cloud cover should
prevent conditions from getting lower than MVFR. VFR conditions
return midday Saturday if not earlier.
A weak surface trough northwest of the District of Columbia metropolitan area will move S/southeast and
pass through the taf sites overnight. Wind shift to north will
probably not be realized until after 10z. Winds eventually shift to
NE during the Saturday.
Outlook...dry weather prevails as weak high pressure builds over
the area with dry and VFR conditions Saturday night into Sunday. A
complex frontal system affects the Middle Atlantic States late Sunday
through Monday evening. Rain develops from the west late Sunday
and continues across the area Sunday night and Monday. Periods of
IFR can be expected. There will also be a chance for thunderstorms Monday.
Dry weather returns for Tuesday and Wednesday.
latest weather analysis depicts low pressure off the SC coast...high
pressure over the Ohio Valley and a surface trough over the local area.
Gradient between these features is weak...with southerly winds generally at or below
10 knots over the waters this afternoon. Waves average 1-2 feet and seas 3-4
feet. Flow backs to the north-northwest tonight as the surface trough crosses the
waters...remaining at or below 10 knots. North-northwest winds persist Sat as a weak
backdoor front drops southward over the northern middle-Atlantic region. NE flow
increases late Sat night-Sun morning to 10-20 knots as the front drops
over the waters and high pressure builds over the St Lawrence
Valley. Speeds appear to remain below Small Craft Advisory conditions...but seas
prognosticated to build to 4-5 feet late Sun morning-Sun afternoon. High
pressure slides off the NE coast late sun...with flow becoming Ely.
Low pressure lifts over the Great Lakes region late sun-Sun night.
This will result in an uptick in gradient winds Sun night. Small Craft Advisory
conditions are expected with gusts 20-25 knots in the Bay and 25-30
knots in the coastal waters. Seas increase to 5-7 feet. Flow becomes
southeasterly Monday as the backdoor front lifts back to the north as a warm
front. Winds diminish to 10-15 knots and seas subside to 3-5 feet. Cold
front approaches from the west late Monday...crossing the waters
late Monday night. At this time...sub-sca conditions are expected Post