Area forecast discussion
National Weather Service Wakefield Virginia
144 PM EDT sun may 19 2013
Synopsis...
a weak front over southern Virginia and lifting slowly north and will
dissipate across Maryland on Monday. A ridge of high pressure off
the southeast coast will dominate into the middle of the week. A
cold front will pass through the Middle Atlantic States around
Friday.
&&
Near term /until 6 PM this evening/...
1 PM update...increased probability of precipitation toward the coast over southeast Virginia and NE NC
where widespread showers are developing in areas that received
sunshine this morning.
10am update...made small changes to the forecast and bumped probability of precipitation
up slightly over southeast portions so that the entire County Warning Area has
likely showers. Most of the area will remain mostly cloudy to
cloudy but significant breaks should continue to appear over southeast Virginia
and NE NC which will promote shower development in the afternoon. Maximum
temperature forecast remains mostly in the middle to upper 70s with near
80 degrees expected over interior southeast Virginia. Lowest
readings will be in northern portions and along the coast. Continued
with isolated thunder except in portions of the Maryland Eastern Shore.
Went with isolated due to convergence and cape being weaker than they
were Saturday.
Previous discussion...
latest water vapor imagery continues to show stubborn
upper low over western Kentucky with a rather broad middle/upper level trough
situated from Kentucky down into the Carolinas. At the surface...stationary
front/boundary still located over northern NC. Area of showers and
embedded thunderstorms developed along the boundary in north central NC
earlier this morning...but have since weakened as they moved north
into the region. Cloud tops have warmed...but a few shallow
showers ongoing over the Eastern Shore...aided by middle level
energy/shortwave. RUC 700 mb Omega handles this area of showers
well...with the showers pushing offshore in the next few hours. In
addition...very moist boundary layer...warm air advection and calm winds have
resulted in areas of fog this morning...with some visibilities as
low as half a mile in the Richmond area.
Additional weak impulses will eject eastward from the dying upper low
into a warm/moist airmass...with the first arriving later this
morning. Cooling cloud tops already observed this morning over western
NC. Precipitation waters still around 1.5 inches (+1 Standard dev). Expect
showers to increase in coverage across the eastern Virginia Piedmont by middle
morning. For this afternoon...weakening upper low will remain in
the vicinity of western Kentucky as the boundary begins to lift northward as a
warm front. Shortwave will lift through Virginia...combining with weak
divergence aloft to provide forcing for ascent...albeit weak.
However...with abundant moisture in place...it will be enough to
trigger additional showers this afternoon...mainly across the
Piedmont into central Virginia. Instability will be limited...but good low
level lapse rates and Theta-E advection will be enough for
scattered afternoon thunderstorms....mainly across southern Virginia. S/southeast
flow will limit the amount of instability along the coast and over
the Eastern Shore. Middle level wind fields are rather weak...so the
main threat from thunderstorms will be heavy rain.
Another cloudy/wet day will prevent temperatures from reaching their full
potential...with highs generally in the middle/upper 70s. Colder
along the immediate coast and over the Eastern Shore.
&&
Short term /6 PM this evening through Tuesday/...
remnants of the upper low will eject over the Appalachians Sunday
night into Monday as a potent upper trough reaches the Midwest. At
the surface...warm front will locate north of the forecast area Monday afternoon
as high pressure located over the western Atlantic extends westward into
the Lower Middle Atlantic and southeast states. Warm/moist airmass will
build in as S/SW flow advects deep moist from the Atlantic...with
precipitation water values approaching +1.5 Standard dev. Weak shortwave and
divergence aloft will provide subtle forcing for ascent...but
expect most forcing and resultant convection to be diurnally
driven. While Theta-E advection and warm temperatures will provide
marginal instability (500-1000 j/kg cape)...a weakening middle level
wind field will provide limited shear. The result will be
scattered thunderstorms with very little organization. The
shortwave axis prognosticated to be around the eastern Virginia Piedmont...with the
best chances for showers/thunderstorms being to the east of the
axis over central and eastern Virginia.
Trough axis will locate along the coast Tuesday as the central Continental U.S.
Trough amplifies the upper ridge over the east. Residual moisture
and subtle uvm will be sufficient for showers along the coast
Tuesday afternoon. Dewpoints in the middle 60s and afternoon temperatures in
the low/middle 80s will result in greater instability (~1500 j/kg
cape and ~-6 lifted index) for convection/thunderstorms. Forcing
will be mostly diurnally driven. However...shear remains weak so
expect no organized thunderstorms.
Near seasonable normal temperatures Monday (upper 70s/low 80s) before a
warming trend Tuesday. Increasing heights/thicknesses...as well as
850 temperatures approaching +1 Standard dev...will result in temperatures generally
in the middle 80s.
&&
Long term /Tuesday night through Saturday/...
models similar over this time period with the timing of another trough
deeping at 500 mb toward Friday and Saturday. This will usher a
stronger cold front through the region Friday into Saturday. The
front will have some showers and thunderstorms with it as it moves
through. Before that a frontal boundary remains nearly stationary
across the Great Lakes and New England. South of this area will have
general south to southwest flow across the area. Could see an
isolated afternoon thunderstorm. Any thunderstorms that form north
along the boundary could push into the northern part of the area
during the evenings. Temperatures will be at or above normal until
the front passes. Then temperatures will drop back on Saturday to at
or below normal.
&&
Aviation /18z Sunday through Friday/...
a mix of VFR and MVFR conditions across the region this afternoon
with showers developing from ecg to orf and south of ric. Expect
mainly MVFR conditions at ric into this evening with a few showers
moving through after 19z. At orf conditions should remain VFR
until heavier showers reach the Airport around 19z reducing visibility
to MVFR and perhaps even temporarily IFR. Northeasterly flow at orf is due
to a weak Bay breeze and will become southerly this evening. Just to the
north phf has been slow to improve and will remain MVFR for the
afternoon and evening. Sby has continue to see IFR conditions but
this will temporarily improve to MVFR by this evening before
returning to IFR overnight.
Showers will dissipate with loss of heating overnight outside of a
few stray showers here and there. Despite most of the guidance
keeping conditions above IFR due to the low level moisture and
continues boundary across the region expect IFR ceilings and fog to
re-develop overnight at all taf sites. These IFR conditions will
linger into Monday morning improving slowly through middle morning.
More widespread showers are expected Monday afternoon and evening
with more coverage of heavy showers than compared to Sunday.
&&
Marine...
S/southeast winds prevail across the marine area this morning...with
speeds below Small Craft Advisory criteria everywhere even into the afternoon. Winds
generally 10 knots through late morning increasing to 10-15kt on the
Bay/rivers/sound...and 10-20 knots on the ocean /highest north of Cape
Charles Light/. These conditions expected to persist next 2-3
days...before winds become south-southwesterly/southwesterly in the Tuesday time frame.
Seas on the coastal waters continue to run around 4-5 feet and
have been slowly decreasing over the last few hours. Wind forecasts
do not appear to be sufficient to cause seas to increase to 5+
feet. However...wavewatch guidance persists in placing most of the
coastal waters in 5-6 foot seas today/tonight. Will maintain Small Craft Advisory
for the northern 2 coastal zones for seas through 00z Monday as winds may
temporarily increase this afternoon pushing seas in the north of
near 5 feet. Seas expected to subside below 5 feet after 00z Monday.
&&
Hydrology...
river level forecasts have been lowered substantially due to lack
of significant rainfall in the headwaters.
&&
Akq watches/warnings/advisories...
Maryland...none.
NC...none.
Virginia...none.
Marine...Small Craft Advisory until 11 PM EDT this evening for anz650.
Small Craft Advisory until 8 PM EDT this evening for anz652.
&&
$$
Synopsis...lkb/lsa
near term...Sam/lsa
short term...Sam
long term...jab
aviation...jao
marine...worse
hydrology...
