Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Wakefield Virginia
350 am EDT Friday Sep 4 2015

a backdoor cold front will move south across the area later today
and tonight. Strong high pressure across the northern states will
slowly build south across the middle Atlantic region Saturday and
Sunday. Low pressure develops off the southeastern coast late in
the weekend.


Near term /today/...
kept patchy fog in grids through 12z per current surface observation. Models agree
in building an upper level ridge over the northestern Continental U.S. Through the
Holiday weekend. Large scale confluence over eastern Canada allows
strong surface high pressure to build into New England...pushing a
backdoor front across the forecast area later today. Before this occurs...
anthr Summer-like day with temperatures about +1 Standard dev above normal.
Expect partly to mostly sunny skies with highs ranging from the
m80s along the coast (where winds will gradually turn ne) to the
u80s-l90s west of ches Bay.

A warm/moist and moderately unstable air mass (mlcape values 1000-
1500 j/kg) will remain over the forecast area in advance of the bkdr cold front.
A more stable airmass prognosticated along the coast due to the onshore
flow. High res data suggests a dry morning then a rthr quick buildup
in cumulus/tcu/sct cb's as the convective temperatures are reached around noon.
Models show best sprt for convection will be along and west of the
i95 corridor (along a weak wind convergence bndry) with little if
any sprt east of the ches Bay. Thus...kept probability of precipitation below 14% along the
coast...20-30 pop from the ches Bay West to the i95 corridor with
40- 50 pop across the Piedmont. No severe weather expect...but could see gusty
winds and lclly heavy downpours given precipitable waters at or above 1.5 inches.


Short term /tonight through Sunday/...
front pushes south across the region this evening before weakening
into a elongated trough off the Carolina coast ivof the Gulf Stream
wall. Thus...kept thunder chances across the Piedmont through 00z. Airmass
becomes more stable by late evening and overnight...however the combo of
lingering moisture at different levels and the increasing NE wind flow leads
to the need to keep at least slght chance probability of precipitation going through the night
under pt to mostly cloudy skies. Becoming breezy at the coast as well with
gusts up to 25 miles per hour. Highest probability of precipitation late tonight shifts toward the
coast due to the moist flow off the ocean. Lows u60s-l70s.

What once looked like a dry and cooler weekend now appears to be
rthr unsettled due to little movement in synoptic features. Add to that
a dvlpng area of low prs off the southeastern coast results in periods of
low level cloudiness and a rthr gusty NE wind along the coast (25-30
mph). Latest data also suggests weak middle level disturbances tracking
across the region resulting in the need to keep 20-40 pop across the
southern half of the forecast area Sat. Driest across the north closer to the center
of the high. Cooler given the above scenario with highs u70s-l80s.

Precipitation chances (20-40%) continue to shift towards the coast Sat night as the
high prs extends SW across the mts keeping thw western half of the forecast area
dry. Lows Sat night m60s-l70s.

Moisture from the coastal trough and the persistent onshore flow will likely
lead to addntl showers (20-40 pop) moving onshore and making as far west
as the i95 corridor Sunday before shifting towards the coast once
again Sunday night. Highs Sunday u70s-l80s.


Long term /Sunday night through Thursday/...
long term period starts off Sun night and Monday with surface low pressure
spinning off the Carolina CST with hi pressure north and west of the forecast area.
Resultant onshore flow will lead to temperatures only making it into the
middle 80s Monday under a prtly-mostly sky. With the moist onshore flow
did hold onto some slight chance-chance probability of precipitation over much of the forecast area
sun ngt/Mon. Kept thunder out due to a lack of instability. The
surface low weakens and starts sliding east Tuesday allowing for drier conds
and temperatures rebounding into the middle/upper 80s under a prtly cloudy
sky. Dry weather continues midweek with temperatures further increasing as southerly
flow develops ahead of the next cold front...with hi temperatures in the
upper 80s to near 90 Wednesday and Thursday. The front will cross the area
sometime late Thursday or Friday.


Aviation /08z Friday through Tuesday/...
areas of MVFR fog/haze expected early this morning...with a chance
for brief IFR visibilities at kphf and kecg between 09z-12z. Otw...VFR
conditions to continue today outside of isolated to widely scattered afternoon
convection...mainly along and west of Interstate 95.

Outlook...periods of sub VFR conditions will occur in areas of
showers and in early morning fog/stratus Saturday through Monday
with onshore flow. In addition...gusty winds are expected along
the coast on Saturday.


a weak trough of low pressure over the waters this morning is
producing light north-northwest winds. NE flow develops all waters by late
morning/afternoon as a backdoor cold front drops south towards the
region and high pressure overspreads the NE states. Winds will
slowly increase through the day over all waters...reaching 10-15 knots
everywhere by early evening and 15-20 knots over the northern ocean
zones. Winds increase a bit more Friday night so that sca's will
be necessary for the Ocean/Bay/lower James River...with waves on
the Bay building to 3-4 feet and seas reaching 4-6 feet (highest
north). The start time of the Small Craft Advisory for the southern coastal waters
(south of cape charles) was pushed back about 12 hours (til 7am sat)
as it will take the longest for Small Craft Advisory conditions to be realized
there. All sca's will then run through the day Sat. May also need
to include the Currituck Sound during the day Sat but not
confident enough at this time due to short duration of highest winds.
Conditions improve Sat night for most of the Bay and lower James
so that the sca's can end. Still expecting waves to remain around
4ft at the mouth of the Bay into Sunday morning...with seas
lingering around 5 feet. Sub-Small Craft Advisory conditions appear likely to start
next week.

In addition...the onshore flow and high nearshore waves will enhance
the rip current threat for the Holiday weekend.


Akq watches/warnings/advisories...
Marine...Small Craft Advisory from 10 PM this evening to 7 am EDT
Sunday for anz634.
Small Craft Advisory from 10 PM this evening to 1 am EDT
Sunday for anz632.
Small Craft Advisory from 10 PM this evening to 10 PM EDT
Saturday for anz630-631.
Small Craft Advisory from 1 am to 10 PM EDT Saturday for
Small Craft Advisory from 1 am Saturday to 7 am EDT Sunday for
Small Craft Advisory from 7 am Saturday to 7 am EDT Sunday for
Small Craft Advisory from 7 PM this evening to 7 am EDT Sunday
for anz650-652.


near term...mpr
short term...mpr
long term...mas

National Weather Service Glossary of Abbreviations