Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
401 am EDT Thursday Sep 3 2015
a weak trough of low pressure will persist over the middle Atlantic
today with weak high pressure centered over the Ohio Valley.
Stronger high pressure then slowly drops down from the northeast
states Friday through the weekend.
Near term /until 6 PM this evening/...
latest GOES water vapor imagery depicts a shortwave dropping over
the region early this morning...with the subtropical high suppressed
over the southeast states. Surface trough remains over the Piedmont into
northern Virginia with high pressure over the Ohio Valley. Earlier showers have
pushed south of the Albemarle Sound...resulting in dry conditions
over the local area. Stratus has developed over southern Virginia/NE NC...with
patchy fog elsewhere.
For today...fog and stratus will burn off/lift shortly after
daybreak. Shortwave drops over NE NC late this morning into the
early afternoon as the surface trough lingers over the eastern half of the
local area. Enough low level convergence is expected along the surface
trough and seabreeze boundaries to warrant isolated to widely
scattered showers and thunderstorms this afternoon in the southeast.
Another area of showers/thunderstorms expected over the central
Appalachians...but hi-res models keep convection west of the local
area. Weak flow/shear will limit overall organization.
However...locally heavy rainfall will be possible due to weak flow.
Similar 850 mb temperatures and low level thicknesses compared to 24 hours ago
will again result in highs in the low to middle 90s (upper 80s along
the coast). Heat index values will approach 100 degrees. Sky averages
Short term /6 PM this evening through Saturday/...
anomalous upper ridge builds over the eastern Continental U.S. During the short
term. A weakness under the ridge locates over the middle-Atlantic
region...resulting in weak flow aloft. Large scale confluence over
eastern Canada and New England will allow strong surface high pressure to
build southward into New England...pushing a backdoor front into the
region Friday morning-afternoon. Surface winds turn to the NE
Friday...resulting in a cool down. A warm/moist and moderately unstable
air mass (sref MLCAPE values around 1500 j/kg) will remain over the
local area in advance of the front. More stable along the coast due
to onshore flow. Have maintained slight chance to chance probability of precipitation east
and central...but increased to solid chance over the Piedmont as
weak vorticity maxima drop over the region. Weak flow and eff shear will
result in little organization...but also little storm motion. Precipitable waters
around 1.5 to 1.8 inches and corfidi/mbe vectors at or below 5 knots will favor
locally heavy rainfall. A partly cloudy to mostly cloudy sky is
expected Friday thanks to NE surface winds. Thickness also drop
off...resulting in highs in the middle 80s north to around 90 south.
Period of low clouds/stratus possible Friday night into Sat as winds
pick up from the NE. Gusts up to 20 miles per hour near the coast late Friday
night. Drier air mass Ushers into the region Sat...limiting highest
probability of precipitation to the south/southeast along remnant coastal boundary. Cool and
breezy start to the Holiday weekend Saturday...with highs in the low
to middle 80s. Gusts up to 25-30 miles per hour southeast and east portions.
Long term /Saturday night through Wednesday/...
long term period will feature mainly dry weather with surface hi pressure in
the vicinity. For Sat night/Erly sun...NE flow persists across the
forecast area as surface hi pressure centers over the northern middle Atlantic. With some moist
onshore flow did hold onto some slight chance-chance probability of precipitation over southeast
areas Sat night. Temperatures bottom out in the 60s Sat night and reach only
the low/middle 80s Sun afternoon. Flow becomes Ely Monday than southerly by Tuesday and
Wednesday as the hi shifts offshore. This will allow for slowly rising
temperatures each day with highs in the low/middle 80s Monday to middle/upper 80s Tuesday
and Wednesday. Bulk of precipitation is expected to stay S of the area through
this time period with any chance of rain (20%) confined to extreme
southeast Virginia/NE NC.
Aviation /08z Thursday through Monday/...
convection assctd with a southeastern moving trough has moved into NC Erly
this morning. Skies quickly clear behind the convection. Attention now
becomes one of determining how much fog develops before 12z due to
the wet grnd and nearly saturated airmass. Models offer up different
solutions on any fog development so indicated at least MVFR cndtns by 12
xcpt at orf. Guidance suggests local IFR visibilities in fog possible at both sby/ecg.
Expect any fog to burn off by or shortly aftr 12z with VFR cndtns expeceted
through the day (sct cu/ci). Winds west-northwest winds at or below 10 kts through middle morning
with coastal taf sites shifting to an onshore drctn due to a sea breeze.
Convection chances this afternoon/evening lower than ystrdy so just indctd scattered
cumulus for now.
Outlook...scattered diurnal convection possible again Friday. Dry weather
and VFR conditions expected through the weekend.
no headlines in short as winds remain below 15 kts through Friday morning.
Wind drctn rthr chaotic due to a lingering trough today. Expect the west-northwest
flow this morning to become onshore this afternoon due to local sea breezes
then back to a variable drctn tonight as high prs remains ovrhd.
Models forecast a 1025mb high across the Great Lakes and into New England
late Friday through Sat. This allows a bkdr cdfrnt to push S across the
waters Friday afternoon and night. Increasing prs gradient between the high to the nrth
and low prs dvlpng off the southeastern CST will result in an extended prd of
NE winds this weekend. Latest data continues to suggest solid Small Craft Advisory cndtns
across the coastal waters and southern ches Bay dvlpng north-S Friday night and continuing
Sat before dmnshg Sat night. Winds/waves/seas continue to decrease sun
as the surface hi weakens and flow becomes more out of the east than NE. Winds
then turn to the southeast then S Monday into Tuesday with sub-Small Craft Advisory conds expected
during this time. Held off on any headlines with this forecast package
since this is a 4th-5th prd event but do expect Small Craft Advisory headlines to be
issued with later forecasts.
In addition...the onshore flow will likely result in nearshore waves
of 3-4 feet which will enhance the rip current threat for the Holiday
weekend. Guidance continues to build tides to between 1/2 to 1 foot above normal by
Sat...but remain below minor flooding thresholds.