Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Wakefield Virginia
439 am EST Friday Jan 30 2015

Synopsis...
a cold front will move off the coast this morning...followed by
cold high pressure late tonight through Saturday night. A strong
low pressure system will affect the area Sunday into Monday. High
pressure returns to area through middle week.

&&

Near term /until 6 PM this evening/...
shortwave energy dropping out of the Great Lakes begins to
overtake and merge with shortwave energy moving across the
Carolinas early this morning...thus becoming a stronger and more
organized upper level trough. This occurs in vicinity of the frontal
boundary...which is causing a deformation zone to develop in the
Carolinas. The best lift with this feature is somewhat elevated
and given the large dewpoint depressions across the area...can
justify no higher than 15% pop in NE NC right along the Albemarle
Sound. Farther north across the Northern Neck...middle peninsula...
and Maryland/Virginia Eastern Shore...weak radar echoes in clear air Mode are
beginning to Blossom as a jet streak and weak shortwave move
across this area. Once again...can justify no higher than 15-20%
pop especially since surface dewpoint depressions are too large. Given
the better jet support within this area...there may be just enough
weak lift through the dendritic growth zone (dgz) to carry a
slight chance of sleet mixed with rain across the Maryland Eastern Shore
through this morning.

Otherwise...cold and dry modified Arctic air moves into the region
behind the departing clipper front thus causing the surface pressure
gradient to tighten significantly. Throw in strong cold air
advection and a favorable mixing environment...and you get a
recipe for gusty northwest winds resulting in a brisk day. Winds will
become gusty by middle morning and maximum out around middle-late afternoon/early
evening. The strongest wind gusts are expected to occur across the
Maryland/Virginia Eastern Shore with gusts up to 40-50mph...while interior
Virginia/NE NC should experience gusts up to 30-35mph. A Wind Advisory
has been issued for the Maryland/Virginia Eastern Shore from early this
evening through midnight tonight. Temperatures in the 40s today
will yield wind chill factors mainly in the 30s.

&&

Short term /6 PM this evening through Sunday night/...
Canadian high pressure slowly builds into the middle Atlantic region
from the west tonight and then settles over the region Sat. Northwest
winds remain breezy along the coast on Sat with gusts of 25-30 miles per hour.
Wind chills Sat morning start out from 0-15 degrees above zero and
warm through the day. Mostly clear skies and dry/stable conditions
will keep temperatures on the cold side Friday night/Sat. Lows Friday
night in the upper teens most areas and in the low-middle 20s far southeast.
Highs Sat in the lower 40s most areas...upper 30s along the coast
from the Northern Neck to Virginia Beach...and in the low-middle 30s Maryland/Virginia
Eastern Shore. High pressure slides off the southeast coast late Sat
night into sun as a cloud shield from the next approaching system
starts to overspread the area. The clouds will prevent lows from
reach their full potential...therefore expect warmer lows Sat
night in the 20s.

A southern stream low pressure is anticipated to track into the
area from the southwest late Sun afternoon/early evening into Monday.
While isentropic sfcs show moisture moving into the region through
Sun morning...time heights/cross sections reveal dry low levels
and it is assumed that the moisture present is from
increasing/thickening clouds. Have therefore removed precipitation
chances from the forecast for Sun morning. The best lift/Omega
moves into the area from the west centered around 21z...so do not
anticipate precipitation to begin until late afternoon/early evening in
northwest areas of the forecast area (from roughly South Hill to Richmond to
salisbury). The incoming low crosses the region Sun night and
widespread precipitation is anticipated. The track of the low has
trended northward which is keeping temperatures warmer during Sun
night. Therefore...best chance for snow is across far northwestern
counties from Louisa to Salisbury. Light snow accumulations are
possible (2 inches or less with this forecast package). A
rain/snow mix will be possible from roughly Farmville to the
Richmond metropolitan area to the Virginia Eastern Shore...otherwise the rest
of the area should experience rain. Overall quantitative precipitation forecast amounts are less
than 0.50 inches Sun night. Temperatures may be a bit tricky sun
as warming is expected under drier conditions Sun morning. Highs
around 50 south to upper 50s central Virginia to lower 40s Northern Neck and
Maryland/Virginia Eastern Shore. Temperatures Sun night should remain fairly
steady overnight (with slight variations of a degree or two) due
to the steady precipitation present. Low Sun night low-middle 30s northwest
to upper 30s to lower 40s southeast.

&&

Long term /Monday through Thursday/...
the precipitation then ends from west to east on Monday with hi temperatures only in the low
30s north to low 40s S under strong cold air advection. The general idea with this
storm is that the current track favors mainly a rain event for
much of the forecast area with some wintry precipitation mixed in at the beginning and
end of the event especially over north and west areas. This is still
several days out and slight changes in the track will make a
difference over where the rain/snow line develops. Any precipitation assctd
with Monday storm is offshore by 00z Tuesday. Clearing skies and cold Monday
night as 1030 mb high apprchs forecast area. Lows in the teens to l20s southern
most counties.

Ridge axis moves ovrhd Erly Tuesday then shifts offshore. Mostly sunny
and cold. Highs l-m30s. Clouds increases Tuesday night ahead of next systm.
Kept it dry for now as any precipitation should stay s&w of forecast area through 12z Wednesday.
Lows may occur in the evening then steady or rise aftr midnight as a S
wind develops. Lows in the 20s to l30s southeastern areas.

Models solutions differ a bit with the middle week systm. GFS takes surface low
NE into the Great Lakes region with the trailing cdfrnt prognosticated across the
forecast area Wednesday night. It also develops a low on southern end of boundary. This allows
said boundary to stall just offshore allowing the low to track NE along
the southeastern US coast but stay just south of akq forecast area. This scenario
brings milder air in ahead of the boundary for mainly rain...but a mix
with then change to snow Wednesday night as cold air advection develops behind it. European model (ecmwf) has
the low going into the Great Lakes but shows a strngr and wetter southern
stream low moving NE along the middle atlntc coast before lifting NE away
from region Thursday. Went with chance (liquid) probability of precipitation Wednesday. May start off as a
brief rain/sleet mix across Piedmont Erly depending on how fast the
precipitation moves in. Highs in the 40s. Went with chance probability of precipitation for a rain to
snow scenario Wednesday night for now. This a rather low confidence call
and will likely all depend on how much moisture is available when
the cold air ovrsprds the region. Lows m20s-l30s.

Cold with some lingering rain/snow showers along the coast Thursday morning.
Otw...clrd and cold as strong high prs builds in from the west. Highs
m30s-l40s.

&&

Aviation /09z Friday through Tuesday/...
southerly winds averaging 10-15 knots will shift to the west-southwest between
09-12z...and then sharply to the northwest after 12z. There are
indications that a few showers may affect mainly eastern portions of
the area with the front...so have added -shra to korf/kphf/ksby
from 09-13z. Still do not expect ceilings to drop to more than about
4000 feet and visibilities will only drop slightly so retained p6sm
wording.

Gusty northwest wind will develop after 14z...with gusts increasing to
around 30 knots from 18z-22z. Winds slowly diminish later this
evening but should still be gusting to 20-25 knots (highest at ksby).

Dry/less windy Sat as the area of high pressure slides across the
region. A more moist frontal system impacts the region Sunday afternoon into
Monday. This looks to be mainly rain although there is a chance
for precipitation to mix with or change to snow at kric/ksby on the back
side of the system before the precipitation ends on Monday.

&&

Marine...
decided to cancel all ramp-up Small Craft Advisory headlines now that gales are
within 12 hours of occurring...otherwise only minimal changes to
overall thinking with the upcoming event. Winds will shift to the northwest
later this morning and increase...but widespread gale force
winds may hold off until late afternoon as cold air surge will lag by
several hours. Think the strongest winds...with gusts to 45 knots over
northern coastal waters...35-40 knots in the Bay and southern coastal
waters...and ~35 knots in most of the rivers will occur this evening.
This is when the pressure gradient will be the tightest and when
much colder air (850 mb temperatures dropping below -12c) really surge over
the relative warmth of the waters (+5 to +8 c). Seas & waves respond
to these gusty winds. Seas average 7-11 feet with waves in ches Bay 4-6
feet.

Given the temperatures/wind gusts and current water temperatures...have a freezing
spray advisory for the coastal waters north of Parramore Island.
However...have delayed the headline until after midnight as surface
temperatures over the waters will generally be above freezing until later in
the evening. This is where some moderate icing is
likely. Kept light icing mentioned farther south & in the ches Bay
at this time.

Winds diminish on Sat...though gales will persist in the morning
over northern coastal waters...with sca's over the remainder of the
marine area. Winds quickly diminish below Small Craft Advisory levels by Sat evening
as high pressure builds into and over the area Sat night.

Next low crosses the area Monday with another cold air advection surge seen behind
this system Monday afternoon and night. Generally have winds in Small Craft Advisory range
for now...though as more details in timing and location with this
system arise will fine tune the forecast and could see a period of
gales.

&&

Akq watches/warnings/advisories...
Maryland...Wind Advisory from 4 PM this afternoon to midnight EST tonight
for mdz021>025.
NC...none.
Virginia...Wind Advisory from 4 PM this afternoon to midnight EST tonight
for vaz099-100.
Marine...Gale Warning until midnight EST tonight for anz635-636-638-656-
658.
Small Craft Advisory from 10 am this morning to noon EST
Saturday for anz633-637.
Gale Warning until 4 am EST Saturday for anz630>632-634.
Gale Warning until 7 am EST Saturday for anz652-654.
Freezing spray advisory from midnight tonight to noon EST
Saturday for anz650-652.
Gale Warning until noon EST Saturday for anz650.

&&

$$
Synopsis...bmd
near term...bmd
short term...bmd
long term...mpr
aviation...lkb/dap
marine...lkb

National Weather Service Glossary of Abbreviations