Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Wakefield Virginia
721 PM EDT Wednesday Apr 23 2014

Synopsis...
cool Canadian high pressure slowly builds in from the northwest
overnight. High pressure then slides over the region into Thursday
morning...locating off the southeast coast Thursday afternoon. A
series of weak fronts will cross the area Friday into the weekend.
A period of unsettled weather is expected over the area early next
week.

&&

Near term /until 6 am Thursday morning/...
upper trough axis has shifted offshore per latest water vapor
imagery...as surface high pressure builds southward from the Great Lakes
region. The result will be clear skies overnight and decreasing northwest
winds. With the high pressure to the west of the area...another
round of cold air advection overnight will push temperatures into the middle-upper 30s
inland and into the low 40s near the coast. Winds will not
completely Delaware-couple tonight...ranging from 5-10 miles per hour inland to 10
to 15 near the coast. Combine light mixing with rather dry ground
conditions and low dewpoints...do not anticipate widespread frost
and will leave out of grids. However...patchy frost is possible in
sheltered locales.

&&

Short term /6 am Thursday morning through Saturday/...
surface high pressure centers off the southeast coast Thursday as an upper
ridge builds over the eastern Continental U.S.. upper trough over the
intermountain west will Delaware-amplify and eject northeastward into the plains
Thursday...pushing an associated cold front into the MS valley. Flow
returns to the south late Thursday-Thursday night as a warm front lifts
over the region. Middle-level westerly flow over the Gulf states into the
middle-Atlantic will prevent much of a moisture return into the
region....so dry conditions will prevail. Northwesterly winds will not be
as strong Thursday afternoon...but min relative humidity values in the upper teens-
low 20s and dry fuels will again result in potential fire weather
concerns. Please refer to fire weather section below for more
information.

Ridge axis pushes offshore Friday morning as the upper wave reaches
the Ohio Valley. Questions still exist on strength of the upper wave
as it crosses the middle-Atlantic region Friday afternoon...with the
latest GFS trending sharper than its previous runs and coming
more in line with its NAM/European model (ecmwf) counterparts. Cold front will push
across the region Friday afternoon-evening. Short lived low-level SW
flow will be able to advect modest precipitation water values of 1-1.25
inches into the region...but dewpoints only prognosticated to warm into
the upper 40s to near 50 Friday. With the local area in the warm
sector Friday...temperatures will warm into the upper 70s to near 80
(depending on frontal timing). Combination of warm temperatures and
Theta-E advection will result in a marginally unstable airmass
(mlcape <500 j/kg)...mostly over eastern Virginia/northestern NC. Also...little
instability/lift prognosticated in the mixed phase layer so anticipate
only isolated/slight chance thunder in convection. Just like
yesterdays front...biggest limitations are west-southwest flow and low
dewpoints. Shear is also marginal (~30-35 kt) so do not anticipate
organized convection along the weakly forced front. With that
said...will maintain chance probability of precipitation over the region Friday afternoon
along/behind the front. Quantitative precipitation forecast amounts will generally be less than
one to two tenths.

Front pushes offshore Friday night as a secondary dry cold front
associated with an upper low over the Great Lakes approaches from
the northwest. Breezy westerly/downslope flow Sat and increasing low level
thickness values will result in daytime temperatures around 80 degrees most
locales...even behind fridays front. Narrow band of moisture and
measurable precipitation forecast to remain north of the local area late
Sat-Sat evening.

&&

Long term /Saturday night 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 drapped 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 stratiformed 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 /23z Wednesday through Monday/...
as the front continues to push farther off shore...the pressure
gradient will tighten up near the immediate coast where west-northwest-northwest
winds should average close to 20g30kt before tapering off during
the evening and overnight hours. Northwest wind speeds briefly increase
Thursday morning and then drop off through the afternoon as high pressure
slides across the area and becomes located off the southeast coast
by Thursday evening.

The next cold front approaches the region Thursday night...causing northwest
winds to back around to the S-se. The front is expected to cross
the middle Atlantic region Friday evening...bringing showers and a
slight chance for thunderstorms to area taf sites. Behind this
front...winds will generally be northerly around 15 knots or less through
the rest of the weekend.

&&

Marine...
mixing still producing gusty winds late this afternoon across the waters.
Expect winds over the rivers and Currituck Sound to slowly dmnsh next
few hours...so Small Craft Advisory will continue through 7 PM there....10 PM for lower James
ivof orf. Secondary surge noted in models keeps Small Craft Advisory headlines for
ches Bay / coastal waters into Wednesday morning. 25-30 knots gusts possible as well.

High prs builds into the area Wednesday afternoon then shifts offshore Wednesday
night and Thursday. This results in dmnsh winds and improving seas.
Thus...sca's will gradually end Wednesday...latest over coastal waters
where it may take seas a few hours longer to drop below 5 feet. Southeast flow
becomes S then SW as by Friday as return flow establishes itself on
back sode of retreating high.

Next front prognosticated across the region Friday night with a secondary trough
crossing the waters Sat. Expect conditions to remain below Small Craft Advisory levels
through Sat...although another weaker northwest surge is possible Sat afternoon.

&&

Fire weather...
dry conditions (min relative humidity values 15-25%) will occur again on
Thursday. However...winds will be lighter than today. 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 20-25 miles per hour during
the late morning/early afternoon hours on the lower Maryland Eastern Shore.
Given how low relative humidity values will be Thursday afternoon coupled with low fuel
moisture conditions...have opted for an increased Fire Danger
Statement for our Maryland counties...after coordination with Maryland fire
officials.

&&

Akq watches/warnings/advisories...
Maryland...increased fire danger from Thursday morning through Thursday
evening for mdz021>025.
NC...none.
Virginia...none.
Marine...Small Craft Advisory until 10 am EDT Thursday for anz630>632-
634.
Small Craft Advisory until midnight EDT tonight for anz638.
Small Craft Advisory until 4 PM EDT Thursday for anz650-652-
654-656-658.

&&

$$
Synopsis...Sam
near term...jdm/Sam
short term...Sam
long term...mpr
aviation...dap
marine...mpr
fire weather...jdm