Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1019 PM EST Wednesday Feb 10 2016
Synopsis...an upper level disturbance will cross the southeast
states late Thursday through Friday...while a surface frontal zone
retreats north and supports coastal cyclogenesis off the southeast
United States coast. Arctic high pressure will build in from the
northwest late Friday through the weekend. A complex storm system
will affect the region early next week.
Near term /through Thursday night/...
as of 1010 PM Wednesday...
Tonight: 00z radiosonde observation data indicate heights aloft continue to
rise throughout the southeast and Middle Atlantic States...as an
eastern Continental U.S. Trough briefly retreats/lifts up the northeast coast. A
strong low-middle level warm air advection regime and axis of low-middle level fgen on the
west side of the departing trough --beneath the exit region of a
nearly 150 knots northwesterly upper jet streak from the upper Midwest
to the Tennessee Valley-- has supported a narrow stripe of light snow from
the northern plains to mostly recently...the southern Appalachians.
The nwp models insist that the upper jet and associated forcing below
will weaken as it migrates across the Carolinas...around the base of
the departing trough axis...overnight. This weakening forcing and
dispersing of moisture/clouds...in conjunction with the northwesterly
flow rain-shadow effect in the Lee of the Appalachians...supports
this weakening trend already noted in regional radar imagery during
the past couple of hours. Dry conditions are consequently anticipated
throughout central NC overnight.
At the surface...a weak/secondary cold front now crossing central
NC will result in a secondary surge of cold air advection and bl mixing tonight...
but with periods of calm and maximized radiational cooling over
the Piedmont especially late tonight...as a weak (1024 mb) surface
high across the Tennessee Valley builds east into the central-southern
Appalachians. Associated forecast low temperatures are close to the
previous forecasts...in the middle teens to lower 20s...with minimum
wind chill values ranging from the high single digits to middle
Thursday and Thursday night: the upper trough will begin to relax as
the surface low pressure system off of New England continues out to
sea. Thicknesses will take a while to recover however and the warmer
thicknesses will remain to our south. Therefore not much change in
high temperature for Thursday with upper 30s to lower 40s across the
area. Skies will be mostly clear with plenty of sun however with but
still a little breezy with gusts up to 15 kts or so. Thursday night
will feature a gradient in the low temperature from north to south
with upper teens north to middle 20s south as a low pressure system
begins to develop south of the area late Thursday night into Friday
Short term /Friday and Friday night/...
as of 230 PM Wednesday...
With the longwave trough holding over eastern noam... a perturbation now
coming into the Pacific northwest will ride atop the western ridge
and shear out as it dives into the trough base and eventually
crosses the Carolinas Friday generating weak DPVA. The forcing for
ascent with this feature will be augmented by upper divergence in
the left exit region of the jet diving into the central Continental U.S. Toward
the southeast states... and this will support surface low pressure
development... tracking across Arkansas and the Gulf states/southern Georgia/Southern SC
Thursday/Thursday night before shifting off the SC coast Friday morning. The
overall lift is not terribly impressive however... and the latest
model runs do not substantially deepen the surface low until it is
already well off the coast and moving east-northeast away from NC... keeping it
rather weak and flat without a strong thermal advection pattern.
Moisture is also unimpressive... with forecast soundings indicating
multiple unsaturated layers and inconsistent moistening above -10c
that brings doubt as to the presence of ice aloft. With models
continuing to depict very light to no quantitative precipitation forecast except as one gets closer
to the central coastal area... will take probability of precipitation and amounts downward
over much of the County Warning Area. Thicknesses and the vertical thermal
structures noted on forecast soundings support a mostly rain/snow
mix over the far southeast County Warning Area... and while this is the area that is most
likely to see some slightly heavier banded precipitation on the northwest side of
the departing low... the high probability of a mixture will keep any
accumulations very low... with only a slushy coating at most.
Further inland... within the colder thicknesses... any precipitation should
be light flurries... yielding little more than a light dusting with
no travel impacts. Expect considerable cloud cover Friday... with
clearing Friday night as the polar front drops southeast through the
area... most likely occurring after midnight. Highs 36-42 Friday and
lows 21-28. -Gih
Long term /Saturday through Wednesday/...
as of 330 PM Wednesday...
..bitterly cold air this weekend with a wintry weather threat early
Sat/Sun: the aforementioned Arctic front dropping southeast through the County Warning Area
during the predawn hours Sat will introduce strong cold air
advection and a plunge in thicknesses (to around 80 M below normal)
as a dense polar surface high starts to build in from the northwest. The
core of the high will move from the Midwest Friday night across the southern
Great Lakes and Ohio Valley to the midatlantic by sun evening while
extending down through NC. Expect decent sunshine with little more
than flat cumulus Sat afternoon with mixing and residual moisture beneath
the subsidence inversion aloft... and increasing high clouds Sunday
ahead of the next storm system. The thicknesses projected by the
models suggest highs below freezing Sat along/north of Highway 64...
and below freezing areawide on Sunday... with lows around 8 to 15
Sat night. The thickness climatology indicates that single digit
lows are possible Sat night... but with expectations of some mixing
persisting overnight... have opted on the "warm" side of this
climatology. See climate section below. Lows Sun night in the upper
teens to lower 20s as the surface high drifts NE toward New England.
Monday through Wed: model differences Widen heading into Monday as another
round of energy shifts into the plains before amplifying over the
eastern states by middle week. In short... the European model (ecmwf) solution (and the
Canadian is similar) was preferred over the GFS... with the former
appearing more reasonable and fitting better with climatology.
According to this solution... a potent perturbation will swing
through OK Monday then deepen over the lower miss valley Monday night
before crossing the Tennessee Valley/se/Carolinas/midatlantic Tuesday into Wednesday.
The European model (ecmwf) brings strong moisture advection and deep lift starting
Monday and continuing through Tuesday night... finally shifting to our NE
Wednesday... all the while taking a Miller-b type cyclogenesis pattern
through the region. There remains a great deal of uncertainty
regarding the vertical thermal structure as well as quantitative precipitation forecast... but
confidence in at least some precipitation Monday through Tuesday night is high
enough to bump up probability of precipitation into the likely category. And if the European model (ecmwf) is
correct... we'd be looking at a very wet event... with a chance of
mostly snow Monday trending up to mostly rain Monday night as probability of precipitation
increase... indicating a lower overall impact from wintry weather.
But a lot can change over the next several model runs... so everyone
should keep a close eye on the latest forecasts. Temperatures should start
to warm up into the 30s Monday... into the 40s Tuesday as we get into the
warm sector... and perhaps low 50s (close to normal) Wednesday as the middle
level low moves NE away from our area. -Gih
Aviation /00z through Monday/...
as of 635 PM Wednesday...
24 hour taf period: high confidence of VFR conditions through the
taf period. Wind gusts will be the issue that lingers this evening.
West northwesterly winds at 10 kts gusting 15-20 kts will be
possible at all taf sites but especially at kgso/kint through the
VFR conditions are expected through Thursday.
Long term: a coastal system on Friday with a shortwave moving across
the north will bring a chance for sub-VFR conditions particularly at
eastern taf sites. The next more significant system is prognosticated to
enter the region Monday night into Tuesday.
Here are the record lows and record low maximum temperatures for Sat
Feb 13th and sun Feb 14th at gso... rdu... and fay:
Rec yr rec yr
low low maximum
02/13 11 1955 31 1986
02/14 6 1905 22 1914
02/13 4 1899 10 1899
02/14 -2 1899 27 1916
02/13 14 1973 32 1955
02/14 12 1968 33 1916