Scientific Forecaster Discussion
fxus61 kakq 210827
Area forecast discussion
National Weather Service Wakefield Virginia
427 am EDT Fri Sep 21 2018
high pressure will remain over the area through today. A
weakening cold front will pass through the middle Atlantic on
Saturday, then stall out just south of the area by Sunday.
Near term /until 6 PM this evening/...
as of 240 am EDT Friday...
* National Weather Service surveys have now determined that there have been 9 confirmed
tornadoes from 6 different storm cells on 9/17/2018. See
https:/weather.Gov/akq/sep172018_tornadoes for details.
Generally quiet weather expected to continue today as sfc hi
pres exits east of New England and strong ridge aloft slides by S
of the local area (over the carolinas). Starting out W/ broken-overcast
St spreading over (far) inland areas in Virginia (where there will be
isolated shras) and from central/interior ern NC. Will likely
start out partly-mostly cloudy over the local area...then
transition to partly- mostly sunny mid-late morning through the
afternoon. Cannot rule out an isolated tstm over parts of the
Piedmont late today. Highs in the l80s near the coast to the
Short term /6 PM this evening through Sunday/...
as of 240 am EDT Friday...
Center of the ridge aloft shifts S and east tonight resulting in
height falls while a weakening cold front approaches from the
north. The front slowly settles S across the area Sat. This front
combined with the increasing instability just S of the boundary
will yield increasing chance for tstms...esp S of I-64 by later
Sat afternoon. Do not expect severe wx W/ very weak wind flow
aloft of less than 30kt. The front stalls just south of the
area Sat night W/ wedge building in across the nrn and wrn areas
in the forecast area as a strong 1030mb high builds across New England.
Meanwhile, a very weak disturbance aloft moves through the forecast area
sun in SW flow aloft. This will allow for additional shras
through the day sun. Trending cloudier for at least nrn/central
areas sun...vrb clouds-partly cloudy southeast.
Highs Sat from around 80f north to the u80s over far srn/southeast Virginia and
NE NC. Lows Sat night in the l60s north and west to around 70f at the
coast in southeast Virginia-NE NC. Highs sun from the 60s across central Virginia
to the l80s in NE NC.
Long term /Sunday night through Thursday/...
as of 400 PM EDT Thursday...
Looking like yet another unsettled pattern for the medium range
period, at least from Sun night through Tue as strong sfc high
pressure (to ~1040 mb) builds from Ontario Canada to northern New
England and then off the Canadian Maritimes. Meanwhile, a frontal
boundary looks to stall in the vicinity of southern Virginia or NC, with a
trough of low pressure lingering along/off the southeast US coast. Rather
strong onshore east to NE flow will prevail Sun night/Mon/Mon night
along with high chance to likely pops (40-60%). Highs will remain on
the cool side over northern and western sections Mon with cad/in-
situ wedge pattern (upper 60s/low 70s), with highs in the upper
70s/lower 80s southeast. Lows in the 60s most areas to around 70 f (se) Mon
night. The front gradually lifts north Tue but may be slow to clear the
County Warning Area (especially north/northwest zones), so highs Tue in the lower 70s north to the
lower 80s southeast. Pops will be lower Tue night/Wed in the warm sector
ahead of an approaching cold front from the NW, though models do
linger some moisture not too far off the coast so will still
maintain 20-30% pops W/ partly-mostly cloudy skies. Highs Wed in the
80s. Models differ on timing of the front Thu, GFS being faster and
ecwmf slower. Splitting the difference/siding close to wpc frontal
positions yields lowering pops from northwest to southeast on Thu, with highs
mainly in the upper 70s to lower 80s.
Aviation /09z Friday through Tuesday/...
as of 230 am EDT Friday...
VFR conditions at all taf sites to start the 06z taf forecast
period with light S to southeast winds. MVFR ceilings will likely
develop after 08-10z across the region with the highest
potential coverage across the west and close to the immediate
coast. Any MVFR ceilings are expected to dissipate after 14z.
Patchy fog may also try to develop early this morning...but it
is not expected to be as widespread as Thu morning. VFR
conditions expected for all taf sites by mid-late this morning
and will through the remainder of the forecast period. Winds
turn to the S and eventually the SW across the region...generally
aob 10 kt. Shras and tstms return Sat as a cold front settles
over the local area. Localized MVFR conditions possible. MVFR
or even IFR conditions possible sun into Mon as the front
stalls/lingers just S of the forecast area and NE winds bring low level
moisture into the area along with scattered shras. Highest probs would
be at ric and sby. The front returns back north early next week.
as of 330 am EDT Friday...
Latest analysis indicates ~1026mb high pressure off the New
England coast, with sfc ridging extending south-southwest into
southern/southeastern Virginia. With the sfc ridge axis just inland, winds
have diminished to 5-10kt and have slowly turned to the southeast.
With the persistent onshore flow during the past day or so, seas
are around 4-4.5 feet over the northern coastal waters/4.5-5
feet south of Cape Charles. Will allow the Small Craft Advisory for the southern
coastal waters to continue until 7 am. Seas decrease to 3-4 ft
during the day today as the wind becomes S/southeast at 10-15 kt over
the waters. Expect a slight increase in the wind as it veers to
the SW from this evening through early Saturday am as the sfc
ridge moves southeast of the coastal waters. This (coupled with an
approaching cold front from the nw) will lead to a tightening
pressure gradient. Could see a few gusts to 20-25 kt over the
nrn coastal waters, with gusts to 15-20kt on the Bay. Opted to
not issue an Small Craft Advisory given the marginal (and short-lived nature) of
the peak wind gusts this evening-Sat early am.
A pre-frontal wind shift to north occurs over the nrn half of the area
Saturday, but the latest 21/00z guidance is still in agreement that
the cold front and stronger NE winds do not arrive until Sunday
morning (and likely last through at least monday). Strong high
pressure builds over New England Sunday into Monday, before sliding
offshore Tuesday. Meanwhile, weak low pressure develops in vicinity
of the Gulf Stream. The pressure gradient tightens resulting in an
east-northeast wind of 15-20kt and potentially 20-25kt over the ocean and Lower
Bay where seas/waves build to at least 5-7ft/4-5ft. Small Craft Advisory conditions
are likely late Sunday into early next week.
as of 930 PM EDT Thursday...
Cancelled the river Flood Warning at Matoaca so now the only
remaining gauge in flood is the Appomattox river at Mattoax.
See flsakq for more site-specific details.
as of 430 am EDT Friday...
While tidal departures are still 1-1.5 feet above normal,
am expecting to see departures fall during the day today as the flow
shifts from southeast to south-southwest. However, water levels may approach low-end
minor flooding in parts of the Upper Bay during high tide late this
morning through the afternoon. Issued a coastal flood statement for
the Northern Neck and lower Maryland Eastern Shore to account for this.
Marine...Small Craft Advisory until 7 am EDT this morning for anz656-