Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
637 am EDT Thursday Aug 21 2014
a frontal boundary remains nearly stationary across the area into
the upcoming weekend. High pressure from New England is slow to
build over the region from Sunday into early next week.
Near term /until 6 PM this evening/...
warm front has stalled over the forecast area...and will remain nearly stationary
today. Limited low ceilings and fog to start...then vrb clouds to pcldy again
today. Most of the day expected to be precipitation free. Heating...modest dewpoints
(for middle aug) and convergence in vicinity of stalled frontal boundary (ovr
interior portions of fa) expected to result in isolated to possibly scattered
convection by lt this aftn/eve. Weakening short wave from the west-northwest may bring
convection into far west-northwest areas lt in the day as well. Hi temperatures from
the m80s at the CST...to the u80s to l90s inland.
Short term /6 PM this evening through Saturday/...
northwest flow aloft remains from tonight into the weekend...W/ series of weak impulses
passing through the region. Initial impulse (wkng S/w) may keep at
least isolated convection over the forecast area tonight. A 2nd weak short wave pushes
through the forecast area on Friday will ptntl for additional isolated/scattered
convection by afternoon/evening hours.
A more vigorous wave drops across the region Friday night into Sat as
surface boundary remains over the forecast area. This should provide best chance of and
greatest areal coverage of showers/thunderstorms across the local area
during the short-term period due to more favorable/stronger upper
jet support. High pressure build over New England Saturday begins
to push S toward mdatlc region by lt in the day. This will push the
remnant front back S of the area.
Hi temperatures Friday mainly in the M/u80s Friday...then from the l/m80s on Sat
(psbly u70s along the Atlantic beaches on the eastern shore). Low temperatures mainly
be in the u60s to l70s Friday and Saturday mornings.
Long term /Saturday night through Wednesday/...
decreasing chances for precipitation and near seasonable...late Summer
temperatures expected through the extended period forecast.
Upper flow continues to be dominated by building high pressure over
the central Continental U.S. And low pressure/troughing off the NE coast. Flow
remains blocked over eastern Canada and the northestern US...resulting in
little progression of the central Continental U.S. Ridge through the period. The
result will be ongoing northwesterly flow into the early period. Backdoor
front will push well south of the local area sun as the trough
amplifies along the eastern Seaboard. Latest model guidance has trended
slightly wetter sun after previously bringing in drier air behind
the front. Surface high pressure over the NE states will ridge southward over
the local area in wake of the front...with subsidence indicated in
model soundings. Moisture also looks to be on the downtrend as model
derived precipitable waters drop below 1.5 inches. With that said...sun may end up
being dry...but will retain slight chance probability of precipitation over the Piedmont and
coastal areas as disturbances in northwesterly flow and model uncertainty
decrease confidence. Better confidence in dry conditions heading
into early next week as the central Continental U.S. High begins to nudge in from
the west and subsidence aloft wins out. Best moisture also gets
shunted to the south and west. Trend continues into Tuesday and Wednesday as
high pressure builds overhead and dry air and subsidence win out.
Onshore flow and low-level thicknesses yield daytime temperatures generally
in the upper 70s to lower 80s through Tuesday (-1 Standard dev for this time of
year). Return flow on Wednesday and warming thicknesses push temperatures back
into the middle 80s. Lows generally in the low-middle 60s through the period
as dewpoints remain in the low-middle 60s.
Aviation /12z Thursday through Monday/...
as of 12z...a frntal boundary is stalled over the middle Atlantic with hi
pressure located over the NE states. The combo of calm/light winds along
the front and decent low-level moisture has led to low ceilings and visibilities
Erly this morng. Any MVFR/IFR should end by later this
morng...with light winds and some low/mid-level clouds sticking
around through the day. Isolated/scattered rain showers/thunderstorms can be expected mainly
late in the day or during the eveng hours...but chance is not hi
enough to warrant a mention in the tafs.
Outlook...scattered mainly afternoon and evening thunderstorms can be expected Friday
and Saturday with dry weather indicated Sunday and Monday. A few
hours of MVFR/IFR can be expected around sunrise through Sunday
no headlines necessary with this forecast package. A stationary front will
remain just inland today over the middle Atlantic region. This will lead
to light winds over all waters (aob 10 kt) out of the S/se. Expect mainly
2 feet seas (3 feet this morng out 20nm). The front stalls over the waters
tonight as a wave of low pressure develops along the front...veering winds
behind the low to average out of the north on Friday. Hi pressure over the NE states
ridges southward over the waters Friday night into Sat as low pressure pushes off the
NC coast. A strengthening pressure gradient will result in increasing
northeasterly winds over the cstal waters late Friday night through Sat. Hi pressure
remains anchored off the NE CST through Erly next week resulting in
persistent onshore flow. At this time...speeds appear to remain
sub-sca...but seas are forecast to build to 4-5 feet late Sat through
Maryland...moderate rip current risk through this evening for mdz025.
NC...moderate rip current risk through this evening for ncz102.
Virginia...moderate rip current risk through this evening for vaz098>100.