Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
1001 am EDT Sat Jul 26 2014
high pressure slides off the middle Atlantic coast today as a warm
front lifts north across the area this afternoon through tonight.
A strong cold front approaches the region on Sunday and crosses
the area Monday into Monday night.
Near term /through tonight/...
weak middle level short wave moving east across northern Virginia is responsible for the
broken cloud deck across the forecast area this morning. Latest radar loop
showing scattered showers across northern Virginia and Maryland with even a few lightning
strikes across northern del. This feature will continue to move east
this afternoon. Latest short range models are a bit more bullish with
precipitation chances this afternoon...basically overcoming the cap that was
present on soundings past few days. Best sprt for isolated showers/tstrms
will be along & north of a occluded frontal passage-ixa-wal line. Middle level cap will be
harder to overcome across southern half of forecast area where it should remain dry.
Dewpoints alrdy in the M-u60s with only minor suppression during
afternoon mixing as winds become S-SW at or below 10 miles per hour. Look for highs around
90 inland...M-u80s along the coast.
tonight...the region becomes well-rooted in the warm sector this
evening and dewpoints around 70 degrees will be common areawide. A
thermal/Lee side trough develops in the wake of the warm frontal
passage overnight...thus causing the surface pressure gradient to
tighten ahead of an approaching cold front from the west (s winds
between 5-10 mph). A muggy night will be on tap with temperatures/dewpoints
both in the lower 70s. Despite conditions being near
saturation...mixing overnight should prevent fog from developing.
Short term /Sunday through Monday night/...
the thermal trough strengthens on Sunday and the surface pressure
gradient tightens up even more as a strong cold front approaches
the region from the west. Shortwave energy ejects from a parent
low/trough swinging through the western Great Lakes on Sunday is
prognosticated to cross northern Virginia sun aftn/evening. Confidence is increasing
regarding severe weather potential during this timeframe. Not only will
ample moisture be present across the region (pwats around 2.00
inches and dewpoints in the lower 70s) but 0-6km bulk shear
increases to 50-55 knots with increasing levels of 850-700mb MUCAPE
ranging from 500-2500 j/kg...depending on which model solution is
preferred. Surface lifted index values are also very unstable (-3 to
-7 during the aftn/evening). Model cross sections/time heights
reveal plenty of elevated lift (including through the hail growth
zone) and soundings confirm this. Soundings also show increasing
winds with height with dry air entrainment in the middle levels (sw
surface gusts to around 20 miles per hour in the aftn). NAM soundings show the wind
profile veering with height while the GFS is more unidirectional.
All of the above convective parameters suggest that organized (and
possibly linear) convection will be possible on Sunday. Some
storms may become severe with damaging winds...large hail...and
very heavy rainfall. Storm Prediction Center confirms this in their day 2 outlook
which has the entire Wakefield County Warning Area in a slight risk area for
damaging winds and large hail. Supercells may also be possible due
to favorable shear profiles...which means isolated tornadoes
cannot be ruled out. Will highlight severe potential in the severe weather potential statement.
High temperatures of 90-95 degrees are expected on Sunday. The
combination of heat and dewpoints in the lower 70s will bump heat
index values to 100-104 degrees (sub-advisory). If the area
experiences more sunshine early in the day...especially southeast Virginia/NE
NC...then heat indices could push into advisory levels (105-109
degrees). Will need to monitor closely.
A brief reprieve from adverse weather may be possible late Sunday night
into Monday morning pre-fropa. The cold front is expected to gain
momentum from a strong shortwave swinging across the Ohio Valley
into New England during the day. This should push the front
quickly through the area Monday aftn/evening. Thunderstorms are
expected to become focused across southeast Virginia/NE NC during the frontal
passage. The aforementioned convective parameters from Sunday will
also be in play on Monday. Although overall values are slightly
weaker...there is a continued threat for damaging winds...large
hail...and very heavy rainfall. High temperatures will be tricky due to
exact placement of thunderstorms...how much drying can occur Post-
frontal...and how afternoon mixing keeps temperatures elevated due to breezy
SW-west winds with gusts to around 25 miles per hour. Current forecast is for
highs in the lower 90s regionwide. A cooler and drier airmass
filters in from the north Monday night as precipitation comes to an end
across the southeast. Lows should range from the lower 60s northwest to lower
Long term /Tuesday through Friday/...
going with a combination of the 12z GFS and 12z European model (ecmwf) for the
extended period. Longwave trough digs into and over the middle Atlantic region
Monday night into early Thursday...before retrograding back into the Ohio and MS
valleys Thursday afternoon through Friday. Cold front will push off the CST Monday night
and Tuesday...lingers farther off the CST Tuesday night through Thursday...then
possibly drifts back toward the CST late Friday. Other than a slight or small
chance for showers over extreme southeast Virginia and NE NC Monday night into Tuesday
morng...will have a dry forecast for the remainder of the extended
period. A more comfortable airmass will dominate the area Tuesday into
Thursday...before a little more humidity returns on Friday. Maximum temperatures will
generally be in the lower to middle 80s through the period. Min temperatures will
range from the lower 60s to lower 70s Tuesday morng...range from the upper 50s to
middle 60s Wednesday and Thursday morngs...and range through the 60s Friday morng.
Aviation /14z Saturday through Wednesday/...
surface hi pressure off the CST...W/ light south-southeast winds at this time. Winds light
enough...and recent rainfall ptntlly leading to patchy fog (and MVFR
conds) next few hours (through at sunrise). Expecting scattered-broken cloudiness
today...W/ low probability for any convection.
S-SW winds gusting to 15 knots and scattered-broken cloudiness expected sun as a cold front
approaches the region from the west. Ptntl for MVFR conds in any scattered
convection lt morning through afternoon. The front is expected to cross
the region on Monday.
Note: observations from ksby are not readily available due to a
communications problem. Return to service is unknown at this time.
However...current information may be obtained by calling into the
ASOS site directly. Amend not schedule will be appended to the ksby taf
until further notice.
no headlines expected through Monday. Hi pressure slides east of the waters through
tonight. S to SW winds will at or below 15 knots today...increases a bit tonight...especially on
the ocn (to about 20 kt). In advance of approaching cold front...similar
conds (ssw winds avgg speeds at or below 15 kt) sun through Monday. That cold front
to cross the waters Monday...then off the CST Monday night. Winds will turn
to the northwest behind the fnt into Tuesday (w/ a possible surge in speeds).