Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Wakefield Virginia
521 am EDT Thursday Apr 24 2014

Synopsis...
cool Canadian high pressure slides over the region today and
becomes located off the southeast coast by this evening. The next
cold front crosses the middle Atlantic region on Friday...followed
by a series of weak and generally dry fronts through the weekend.
A period of unsettled weather is expected early next week.

&&

Near term /through tonight/...
a cool Canadian surface high pressure is centered over the Great Lakes
region and extends across the Ohio Valley early this morning. The
surface high will slide southeast over the middle Atlantic region today and
become located off the southeast U.S. Coast by this evening. The
pressure gradient will remain somewhat tightened along the Atlantic
coast today as the high moves in and a strong surface low well east of
the northern New England coast slowly pushes eastward. With high pressure
overhead...sunshine will be plentiful and temperatures will warm nicely
today. Cirrus clouds will into the region from the northwest as the afternoon
progresses but this should have a minimal impact on high
temperatures today. Expect highs in the upper 60s inland Virginia/NE NC
and in the low-middle 60s closer to the immediate ches Bay/Atlantic
coasts. In addition...dry weather and fine fuels will prevail
today. The dry conditions coupled with breezy northwest winds will allow
min relative humidity values to fall into the upper teens-low 20s most areas by
this afternoon. Increased fire potential is a concern once again for
today...but will be limited to the Maryland/Virginia Eastern Shore. Please refer
to the fire weather section below for more information.

Winds will turn southeast-S and be light as high pressure continues to
move southeastward overnight. Meanwhile...a cold front and associated upper
level shortwave will push across the middle-Mississippi Valley and
into the Ohio Valley overnight. High clouds will increase and
thicken overnight as the surface front approaches the middle Atlantic
region. The combination of cloud cover and increasing
moisture/dewpoints will prevent temperatures from dropping quickly
overnight. Lows should be near normal with readings in the
middle- upper 40s areawide (except low 40s possible for interior
Maryland eastern shore).

&&

Short term /Friday through Saturday night/...
timing of the frontal passage has slowed and the upper level
shortwave strength is waning as both features cross the middle
Atlantic region Friday into Friday evening. Short range models are
in fair agreement with slowing the onset of precipitation and limiting it
to areas west of Interstate 95 from middle-morning to early afternoon. The
presence of a Theta-E gradient (ahead of the Theta-E ridge axis)
means that a slight chance of thunder is possible during this
timeframe...despite an initial lack of preferred/favorable surface
instability in the morning. Convective potential improves during
the afternoon/early evening as daytime heating and incoming shortwave
energy destabilizes the atmosphere and deep lift/Omega
accompanies the frontal passage. Model soundings show a primarily
unidirectional wind profile with slight veering with height in the
lowest levels (roughly below 850-900 mb) which implies that
thunderstorm line segments may be possible. However...wind shear
from 0-6 km is averaging about 30 knots which suggests that the
potential for organized convection is low. In addition ...
short-lived low level SW flow will advect modest precipitable waters into the
region (0.75 to 1.40 inches). Therefore if any thunderstorms
develop...they should primarily be unorganized with periods of
moderate to heavy rainfall with sub-severe wind gusts of 30-40 miles per hour
under the strongest storm cores. Keeping one caveat in mind...if
the upper level shortwave digs farther south over the next 24
hours (as of Thursday morning)...then the amplification of upstream
ridging will likely stall the frontal passage until Friday evening
thus causing an additional delay in the onset of precipitation to the area
and decreasing thunderstorm potential due to unfavorable timing.
Will need to monitor 12z model trends this morning as well as
shortwave placement in water vapor imagery to determine best
timing/impacts. High temperatures in the middle-upper 70s are anticipated
most areas while the Maryland/Virginia Eastern Shore should be in the low-middle
70s.

The cold front is expected to move offshore late Friday night as
a secondary dry cold front (associated with an upper low over the
great lakes) approaches from the northwest. Gradually clearing skies from
west to east Friday night should result in lows in the upper 40s to
lower 50s west of Interstate 95 and middle-upper 50s east of
Interstate 95 (lower 50s Maryland/Virginia eastern shore). The dry front will
sag down across the area on Sat which will result in breezy
westerly/downslope flow and increasing low level thicknesses. This
should allow daytime temperatures to warm into the upper 70s (possibly
80 degrees) for NE NC and interior Virginia west of ches Bay and into
the middle 60s to lower 70s for the Maryland/Virginia Eastern Shore. A narrow
band of moisture and measurable precipitation is expected to stay north
of the local forecast area Sat/Sat night. Lows Sat night will be
at to slightly above normal with temperatures of 45-50 degrees northwest of
Richmond and the Maryland/Virginia Eastern Shore...and in the lower 50s
elsewhere.

&&

Long term /Sunday through Wednesday/...
models continue to point to a highly amplified/blocky pattern over
the Continental U.S. Next week...resulting in an increasingly wet period.

Models continue to struggle with this blocky flow...mainly in
location of frontal boundary prognosticated to be draped across the region
along with timing of several batches of moisture prognosticated to rotate NE
from the deep south. What todays models show different from
ystrdys is a slower start to the precipitation Monday...somewhat higher
confidence for an extended prd of wetness from Monday night through Wednesday
along with a frontal boundary that will likely play havoc with
temperatures.

Kept sun dry/seasonable as 500 mb ridge builds across the area. Warm
front apprchs the region from the SW Sun night...but guidance suggests
any moisture stays west of forecast area through 12z Monday. Lows Sat/sun nights in
the m40s-l50s. Highs sun 70-75 west of the Bay...M-u60s along the
coast.

Forecast rather problematic next week as the models differ on
where this frontal boundary sets up. This will likely determine temperatures
and precipitation types (convective vs stratiform rain). Upper level low
prognosticated to slowly eject east from the nations middle section Monday
night only to the Ohio vlly by Wednesday. A moist east-southeast flow will likely
add to the moisture content resulting in periodic chances for precipitation
through the prd. Frontal boundary may also try to make a run farther north
by Wednesday as surface low begins to cross the mts. Trailing cold front
will eventually push all this out to sea...but that might not be
until Thursday. Upshot will carry high chance probability of precipitation (shwrs) Monday
through Wednesday. Temperatures coolest (l-m60s) along the coast due to an
onshore flow. Highs u60s-l70s west of the Bay. Lows in the 50s.

&&

Aviation /09z Thursday through Monday/...
hi pressure continuing to build over the region through today...resulting in
dcrsg north-northwest winds. A little bit gusty to between 15 to 20 knots into this
morning...especially near the CST. Winds diminish over inland locations to
around 5 knots by lt today. VFR today through tonight.

An area of low pressure and an associated cold front will bring
scattered showers and a few thunderstorms Friday. Brief periods of
IFR will be possible at the taf sites. High pressure builds into the
region during the weekend with VFR and dry weather taking over
through Monday. The next weather system brings a good chance for
precipitation by Tuesday.

&&

Marine...
lingering scas on the Bay into middle/lt morning hours...on the ocn until
middle/lt afternoon. Surface hi pressure is slo to build over the waters this
aftn/eve...resulting in gusty north-northwest winds this morning waning this afternoon.
Vrb winds tonight less than 10 knots...become south-southeast on Friday ahead of
approaching low pressure from the west. Speeds to increases Friday into Friday night...
though will keep below Small Craft Advisory...then become west-northwest Friday night and continue
through Sat.

&&

Fire weather...
dry conditions (min relative humidity values 15-25%) will occur again today.
However...winds will be lighter than Wednesday. This will preclude
the need for any red flag warnings over the local area. Northwest winds
do increase to around 15 miles per hour with gusts of 20-25 miles per hour during the
late morning/early afternoon hours on the lower Maryland/Virginia Eastern Shore.
Given how low relative humidity values will be Thursday afternoon coupled with low
fuel moisture conditions...have continued the increased Fire
Danger Statement for our Maryland counties (after coordination with Maryland
fire officials) and added Virginia Eastern Shore counties to the
increased Fire Danger Statement.

&&

Akq watches/warnings/advisories...
Maryland...increased fire danger from 11 am EDT this morning through this
afternoon for mdz021>025.
NC...none.
Virginia...increased fire danger from 11 am EDT this morning through this
afternoon for vaz099-100.
Marine...Small Craft Advisory until 11 am EDT this morning for
anz630>632-634.
Small Craft Advisory until 4 PM EDT this afternoon for anz650-
652-654-656-658.

&&

$$
Synopsis...bmd
near term...bmd
short term...bmd
long term...mpr
aviation...alb/lsa
marine...alb
fire weather...bmd