Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
411 am EDT Tuesday Sep 23 2014
high pressure builds across the northern middle Atlantic today as low
pressure develops off the southeast coast. This area of low pressure
will slowly move up the coast Wednesday and Thursday.
Meanwhile...high pressure will remain anchored across New England.
Near term /until 6 am this morning/...
current GOES WV imagery shows a highly meridional middle/upper pattern
across eastern North America...with a vigorous trough digging across the
Gulf Coast...and deep south-southwest flow along the East Coast. At the
surface...1026mb high pressure is centered over the Ohio
Valley...and the cold front that crossed the region yesterday
morning has stalled in vicinity of the Gulf Stream. This general
pattern will persist through the day. A considerable amount of
cirrus will continue to stream north-northeast within the south-southwest flow aloft...and
this will produce partly to mostly cloudy conditions. Low-level NE
flow will result in a cooler day...with highs ranging from the upper
60s along the coast...to the low 70s inland.
Short term /6 am this morning through Thursday/...
surface high pressure shifts to northern New England tonight and builds
to ~1035mb. Meanwhile...an inverted trough will begin to sharpen off
the southeast coast in response to the slow NE advancement of the
Gulf Coast trough. Forecast soundings indicate a significant layer
of dry air centered around 800mb...and given this most of the region
should remain dry overnight...with the exception of NE NC where
20-30% probability of precipitation will be forecast after 06z.
The pattern becomes highly blocked Wednesday into Wednesday night as
a closed 700mb high builds north of a weak low over the southeast
states. At the surface...high pressure remains anchored over New
England as a weak coastal low drifts up the coast. A substantial
plume of moisture will advect northward from the Gulf Stream
Wednesday into Wednesday night and hit a +2 St dev middle and surface
high north of the region. This will result in a strong easterly low level jet and
bring the potential for heavy rainfall. Model derived quantitative precipitation forecast on average
supports 1.5-2.0in storm total east of I-95...although there is the
potential for locally higher amounts. Spatial and timing differences
still exist amongst numerical model guidance. This forecast
generally follows that of the 23/00z NAM/ECMWF...which focuses rain
over southeast portions Wednesday afternoon/evening...and then up the coast
overnight Wednesday night...but confidence does remain rather low.
The pattern begins to relax Thursday...although lingering cloud
cover is expected along with a chance of rain continuing mainly over NE
A NE wind will become rather breezy Wednesday afternoon and evening
along the coast...with speeds averaging ~20mph...with gusts of
25-30mph. Highs Wednesday and Thursday should range from the upper
60s/around 70 over the Piedmont...to the low/middle 70s along the
coast...but will ultimately be dependent on cloud cover and rain.
Lows should generally range through the 60s...with upper 50s
possible over the northwest Piedmont counties.
Long term /Thursday night through Monday/...
model differences continue to make for a rather low confidence
extended period. Not quite buying the extended wet period (cut-off low)
European model (ecmwf) has painted across the area (given time of year and previous
performances) so have gone with the dryer GFS / Canadian solution. Gist
is that oceanic moisture will be battling the dry air from the high
prs to the north. Last time this happened...the high won out except
along coastal sections. Have trended this forecast in the same
manner meaning the best chances for precipitation will be along coastal sctns
with mainly dry conditions west of the ches Bay through the weekend.
Coolest day Friday. Highs in the l-m70s. 75-80 both Sat & sun. Lows
Next trough of low pressure moves east across the mts Monday. Low chance
probability of precipitation with this feature for now. Highs 75-80.
Aviation /08z Tuesday through Saturday/...
VFR conditions prevail early this morning over the taf sites as high
pressure builds in from the west. Pressure gradient over the region
has resulted in good mixing tonight and winds generally 8-12 knots. Northerly
surge over the Bay is producing locally stronger winds at
korf...with a few gusts up to 25 knots. Otherwise...bkn-ovc high clouds
at or above 20k feet above ground level spreading over the region from the SW.
For today...high pressure builds just north of the local area with a
surface ridge extending into the southeast coastal plains. Pressure gradient
between high pressure and a frontal boundary off the southeast coast will
result in NE winds 10-15 knots with a few gusts up to 20-25 knots during
peak heating. Otherwise...VFR ceilings/visibilities with a few middle level clouds
4-6k feet above ground level and high clouds at or above 20k feet. Measurable precipitation remains
south of the region.
Low pressure forecast to develop along the coastal front Wednesday and
lift toward the region into Wednesday night. This will bring periods of
rain...especially near the coast...along with reduced visibility and low
ceilings. Breezy NE winds also expected along the coast.
ongoing secondary cold air advection surge has produced northerly winds of 15-20 knots with
gusts to 25 knots over the waters. As a result...Small Craft Advisory headlines were
raised late last evening for the Bay/lower James/Currituck Sound.
Waves average 2-3 feet and up to 4 feet in the mouth of the Bay. Expect this
trend to continue through middle morning as high pressure builds over the
waters...relaxing the gradient winds and veering the flow to the NE.
Speeds average 10 knots over the Bay/rivers/sound and 10-15 knots over the
coastal waters today as high pressure builds into New England. Low
pressure forecast to develop along a stalled frontal boundary off
the southeast coast late tonight-weds...lifting northward over the waters by Wednesday
night. Pressure gradient strengthens as NE winds increase to Small Craft Advisory
conditions. Persistent NE winds will push seas to 4-5 feet in the southern
water (south of Cape Charles light) beginning late tonight.
Speeds/seas/waves increase across the rest of the waters through the
day Wednesday. Combination of strong low level gradient winds...NE flow
and warm waters will result in solid Small Craft Advisory conditions Wednesday into Wednesday
night. 20-25 knots...gusts to 30 knots for the coastal waters and Lower
Bay...15-20 knots with gusts to 25 knots elsewhere. Waves expected to
build to 4-5 feet with 3-4 feet in the lower James and 2 feet in the
sound. While gales appear possible...spatial differences in the
models lend to a lower confidence for gales...so will leave at Small Craft Advisory
with this forecast package. Seas will also respond...quickly
building upwards of 8-10 feet (possible higher 20 nm out). Flow and
wave direction will result in nearshore waves of 5-7 feet. High surf
advisories may be needed Wednesday.
Models bring the surface low over the Bay Wednesday night...but differ
thereafter. Some models keep the surface low over the region while others
push it out to sea Thursday. Have opted to push Small Craft Advisory headlines in the
coastal waters and Bay through 4th period (weds night). Waters remain
between high pressure to the northwest and low pressure over the western Atlantic
through the week.
persist NE flow is expected to develop Tuesday night and strengthen
Wednesday into Thursday as high pressure remains anchored over New
England...and low pressure develops off the Carolina coast. Current
guidance suggests tidal departures rise to between 1-1.5ft above normal
Wednesday night into Thursday. This would put several locations right
around the minor flooding thresholds.
Maryland...moderate rip current risk through this evening for mdz025.
NC...moderate rip current risk through this evening for ncz102.
Virginia...moderate rip current risk through this evening for vaz098>100.
Marine...Small Craft Advisory from 7 am Wednesday to midnight EDT
Wednesday night for anz635>638.
Small Craft Advisory until 10 am EDT this morning for
Small Craft Advisory from 7 am Wednesday to 6 am EDT Thursday
Small Craft Advisory from 10 PM this evening to midnight EDT
Wednesday night for anz633.
Small Craft Advisory from 10 PM this evening to 6 am EDT
Thursday for anz656-658.