Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Wakefield Virginia
401 PM EDT Thursday Apr 24 2014

Synopsis...
high pressure slides offshore tonight. Low pressure tracks from the
Ohio Valley across the northern middle Atlantic Friday...and off the
coast Friday night. A cold front drops across the region Saturday
night...with high pressure building in from the Great Lakes Sunday.

&&

Near term /until 6 am Friday morning/...
current WV imagery indicates an upper trough steadily tracking west-east
across the Central Plains...with another trough downstream of the
local area over Atlantic Canada. A narrow ridge axis exists over the
southeast and Ohio Valley...with surface high pressure underneath
this feature extending from the eastern Great Lakes through the middle
Atlantic.

The Central Plains trough will lift NE through the middle Mississippi
Valley toward the Great Lakes overnight. Meanwhile...surface high
pressure will slide offshore. The sky should be mainly clear through
the evening and early overnight hours...before clouds begin to
increase over western portions late. The temperature should drop
quickly through the evening hours...with cool dry high pressure over
the area...and become steady after midnight. Lows should range from
the upper 30s to low 40s over interior portions east of I-95...with
low/middle 40s west due to increased mixing (from light southeast flow) as the
high shifts offshore and the approach of low pressure from the west.

&&

Short term /6 am Friday morning through Sunday/...
the parent trough lifts through the Great Lakes Friday...with
surface low pressure tracking across the northern mid-Atlantic. A
prominent (surface and middle-level) Theta-E ridge precedes the trough
axis. This should produce an increase in boundary layer moisture
through the morning. Forecast soundings show a period of rather deep
moisture along with a period of modest lift passing across the local
area largely in the 18-00z timeframe. Given this there is high
confidence of rain showers and embedded thunderstorms and rain passing across the area. The
best destabilization occurs over southeast portions. This will be in the
presence of modest deep layer shear (~35kt 0-6km)...and ~30kt of
0-1km bulk shear. At this time instability appears too marginal to
support organized severe weather...but there is enough shear for a
minimal threat mainly across far southeast Virginia/NE NE. Highs should generally
be in the low/middle 70s (locally cooler along the coasts).

The middle-level cold front (cold Theta-E boundary) moves off the coast
late Friday night...with drier air arriving from the west. The surface
boundary is mainly a wind shift...so temperatures Saturday should
generally be a few degrees warmer than Friday. Highs are expected to
range from the middle 70s over eastern portions (locally cooler along
the immediate coast)...to around 80 along a west of I-95. This will be
after morning lows ranging from the upper 40s northwest...to upper 50s southeast.

Saturday should begin mostly sunny...before clouds increase
(especially n) ahead of a cold front approaching from the northwest. At
this time probability of precipitation will be held below 15%...but a few sprinkles are
possible across the far northern portions. This follows more of a
12z GFS/European model (ecmwf) solution rather than the more moist 12z NAM due to the
expectation of limited boundary layer moisture.

The cold front drops through the area Saturday night...with high
pressure building in from the Great Lakes Sunday. Lows should range
from the middle 40s to low 50s Sunday morning...followed by highs
Sunday afternoon in the middle 60s NE to low/middle 70s SW under a mostly
sunny/clear sky.

&&

Long term /Sunday night through Thursday/...
a highly amplified/blocky pattern will result in an unsettled prd
next week. Todays models continue to slow the arrival of precipitation until
Monday afternoon when the stalled frontal boundary across the Carolinas begins
to move back north as a warm front. Kept Sunday night and Monday morning
dry as high prs to the north is slow to retreat. Lows m40s-l50s.

Main upper level low prognosticated to slowly eject east from nations middle
section Monday to the Ohio/tnn vlly Wednesday. Models continue to show a
scndary low dvlpng over the middle atlntc region Thursday with a trailing
cold front pushing deepest moisture offshore late Thursday followed
by lagging upper level moisture across the area.

Numerous forecast challenges during this prd. Prime of which will be
the location of frontal boundary now prognosticated to hover somewhere around
the NC/Virginia border Tuesday through Thursday. The uncertainty here will make
a problematic forecast for temperatures...wind direction and precipitation type
(i.E. Convective vs stratiformed). Better confidence today that
the Tuesday through Wednesday time frame will be the wettest...so have
increased to likely probability of precipitation then. Otw...kept precipitation showers County Warning Area wide
for now given the low confidence this far out. Could very well end
up being more of a stratiformed rainfall across the Piedmont if a
cool east-NE wedge sets up as well as having to add thunder to the
southern zones given a frontal boundary location farther north.
All-in-all...expect a cool and wet prd. Highs gnrlly from the
l-m60s northern half of forecast area to u60s-l70s southern half. Lows u40s-
m50s.

&&

Aviation /20z Thursday through Tuesday/...
VFR conditions expected through the 18z taf period for all akq
sites. Surface high pressure building overhead along with upper
level ridge has created dry and mostly clear conditions across the
mid-Atlantic. Winds gusting to near 20 knots at ksby will subside
around 22/23z as the pressure gradient weakens to the north.

An area of low pressure and associated cold front will bring
scattered showers and a few thunderstorms Friday afternoon. Brief
periods of IFR will be possible at the taf sites. High pressure builds
into the region during the weekend with a quick moving cold front
passing through the area Sat night.

&&

Marine...
no headlines tonight and Friday morning as high prs builds over the
waters. Expect variable winds to become S-southeast by morning. Prs gradient
increases ahead of apprchg cold front Friday. Data showing higher
confidence for at least minimal sca's across the ches Bay Friday afternoon.
Frontal passage Friday evening with a wind shift to west-northwest aftr midnight. Although
data shows little in the way of cold air advection...expect a few hour prd of minimal
Small Craft Advisory right behind frontal passage so will keep sca's through the night across
the ches Bay. Otw...expect winds & seas to stay below Small Craft Advisory levels across
coastal waters at this time.

West winds Erly Sat ahead of a weak trough prognosticated to cross the waters
Sat afternoon. Wind shift to the north behind this feature...but kept winds
below Small Craft Advisory levels as most models progg weak advection behind this ftr.
GFS somewhat stronger with its surge and has winds into minimal Small Craft Advisory
levels Sat night. Will take a closer look at this tomorrow.

Large upper level systm expeceted to apprch the area next week. Although
winds not that strong...wna showing increasing swell with seas 5-7 feet by
the middle of next week. Have split the difference but incrsd seas
to 4-6 feet for now.

&&

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 until 6 PM EDT this evening for
mdz021>025.
NC...none.
Virginia...increased fire danger until 6 PM EDT this evening for vaz099-
100.
Marine...Small Craft Advisory from 1 PM Friday to 4 am EDT Saturday for
anz630>632-634.

&&

$$
Synopsis...ajz
near term...ajz
short term...ajz
long term...mpr
aviation...dap
marine...mpr
fire weather...bmd