Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
743 am EDT Tuesday Aug 4 2015
a cold front crosses the area this afternoon and evening...then
stalls over the Carolinas Wednesday into Thursday. This boundary
moves back north as a warm front Thursday night.
Near term /through tonight/...
scattered-broken middle to high level clouds will cross the forecast area through middle
morning. For the rest of today and tonight...middle and upper level
troughing resides over the NE Continental U.S. From the Great Lakes to New
England... resulting in westerly flow aloft over the middle Atlantic
region. Meanwhile...a weak surface trough extends from the Delaware-Maryland-Virginia
into the NC coastal plain and is expected to shift toward the coast
throughout the day. Model cross sections and soundings indicate
that moisture is rather shallow/limited and overall atmospheric
lift is weak. However...showers and thunderstorms should develop
along and east of the trough axis as a surface cold front and its
associated shortwave energy cross this boundary. In addition...the
fringe of a jet streak is expected to cross the Delaware-Maryland-Virginia region
this afternoon/evening (60-80kt) thus supporting better chances for
thunderstorm development across northern counties (mainly north of a
Louisa-Richmond-Melfa line). With precipitable waters on the order of 1.00 to
1.50 inches...quantitative precipitation forecast amounts will be rather light with 0.25 inches or
less of rainfall anticipated. Temperatures will be slightly warmer
than Monday as the thermal trough axis nudges farther northward into
central Virginia today and breezy south-southwest winds promote afternoon mixing. Expect
highs in the low-middle 90s inland and upper 80s to low 90s at the
immediate coast. Shower activity should taper off late this
evening and come to and end along the coast around midnight. High
pressure starts to build in from the west-northwest late tonight and will
help to cool overnight temperatures into the upper 60s for far northwestern
counties. Low-middle 70s inland...mid-upper 70s closer to the coast.
Short term /Wednesday through Thursday night/...
high pressure continues to build into the region from the west-northwest as
the surface trough gets compressed and the surface cold front becomes more
east to west oriented across NC. There is even less moisture/lift
available to work with on Wednesday and anticipate an overall dry
forecast...with the exception of locations along and south of the
Virginia/NC where a slight chance of shower/storms will be possible late
in the afternoon through the evening. Slightly cooler and more
comfortable with highs in the lower 90s (mid-upper 80s at the
beaches) and dewpoints in the low-middle 60s inland and upper 60s to
lower 70s closer to the coast. Lows Wednesday night in the
middle-upper 60s far northern counties...lower 70s inland...middle 70s at the
The front remains stalled across the Carolinas and possibly far
southeast Virginia into Thursday. Weak low pressure is expected to develop
along this boundary and lift a warm front toward the forecast area on Thursday
night...bringing more widespread showers and thunderstorms to the
area from the afternoon through the overnight hours. This scenario is
supported by precipitable waters increasing toward 2.00 inches Thursday night as
well as cross sections showing ample (and deeper) atmospheric lift
as the Theta-E ridge axis builds along the warm frontal feature.
Quantitative precipitation forecast amounts could be pretty hefty...especially under stronger
storm cores... with rainfall amounts of 0.30 to 1.00 inches
possible during this 24 hour period. Cool high temperatures continue on
Thursday with slightly warmer dewpoints bringing an uptick in
humidity levels. Expect highs in the middle-upper 80s northwest of Richmond
to the lower 90s for metropolitan Richmond and locations to the southeast. Lows
Thursday night could be tricky due to rain-cooled areas. However...
temperatures are currently expected to range from the upper 60s northwest to
low-middle 70s southeast.
Long term /Friday through Monday/...
unsettled weather to start off the long term period with frequent
chances of rain due to an upper-level trough over the far eastern Continental U.S.
And transient shortwave troughs passing through the middle Atlantic region.
12z GFS/European model (ecmwf) continue to differ on exact timing of precipitation...but for
now the forecast calls for a 30-40% chance of rain Thursday night through
Friday night. Drying trend then commences into this weekend as the
upper-level trough axis pulls offshore and surface hi pressure builds in from the
west. As for temperatures...highs will range through the 80s every day with
lows avgg in the middle 60s to lower 70s.
Aviation /12z Tuesday through Saturday/...
VFR conds prevail aside form some spotty areas of MVFR ceilings across
far southeast Virginia/ NE NC. A weak cold front will push into the County Warning Area late
this afternoon/evening and will produce widely scattered showers and
thunderstorms (confidence and coverage far too limited to include in tafs
at this point but have included vcsh at ksby after 21z). The
front will push off the coast early Wednesday morning and usher in
slightly drier conditions with limited shower/thunderstorm chances. Weather
becomes more unsettled later Thursday into Friday as low pressure and its
associated cold front affect the area.
S/SW winds have diminished over the lower James so have allowed
Small Craft Advisory to end there as well as coastal waters S of Parramore Island
where seas are below 5 feet. Seas average 4-6 feet across northern coastal
waters and have extended the headlines there through 10 PM
tonight. Seas average 3-4 feet elsewhere. Waves in the Bay average 2-3 feet.
Surface pressure gradient and low level nocturnal jet influences look
to weaken by later this morning so should see winds trend down to
average 10-15 knots and will probably be able to end the Bay headlines.
However...expect winds to increase again later this afternoon and may
need another round of sca's for the Bay (or if winds do not drop
off later this morning dayshift may just extend sca's through the
aftn/evening). The front drops through the area late tonight...with
relatively light north/northwest flow Wednesday following the frontal passage. Sub-Small Craft Advisory conds
then expected into Thursday night...though pattern now suggests the
presence of a weak warm front lifting through the waters on Friday
and conditions may approach Small Craft Advisory thresholds for the Bay.
Marine...Small Craft Advisory until 10 am EDT this morning for
Small Craft Advisory until 10 PM EDT this evening for anz650-