Scientific Forecaster Discussion
fxus62 krah 251708
Area forecast discussion
National Weather Service Raleigh NC
110 PM EDT Thu may 25 2017
an upper level trough will cross central NC this afternoon. Weak
high pressure will follow and extend across the southeastern U.S.
Through Friday night. A warm front will develop east across the
Appalachians and become quasi-stationary across NC and Virginia through
Near term /through Friday/...
as of 110 PM Thursday...
A mid-upper low over western Appalachians is pivoting east southeast
this afternoon. Cold air aloft associated with the upper low
combined with solar heating is supporting convection in central
North Carolina with small hail. Isolated stronger cells have the
potential to produce up to 1 inch diamater hail.
There will be a related conditional threat of severe hail,
particularly east of U.S. Hwy 1, where both instability and sswly
mid level flow/bulk shear will be relatively maximized in a brief
window from 17z-21z. Strong to damaging straight line winds, driven
by both the background mid level flow and sub-cloud
evaporational/diabatic cooling, may also result.
After about 21z, the passage of the trough axis will cause the mid
level flow to weaken and veer and consequently mitigate an earlier
severe threat, while small hail will remain possible through early
Lastly, mixed character precipitation (ie. A stratiform shield of
light rain with some convective elements) will likely pivot across
the nrn Piedmont as the parent mid level moist axis swings around
the base and wrn side of the upper trough; and this may result in a
few sprinkles there this evening, until the moist axis lifts away
Lows tonight in the 50s, with a light but steady swly breeze in the
mean sea level pressure gradient between sub-1000 mb low pressure that will be
lifting away from the middle Atlantic coast, and incoming 1015 mb
high pressure from the nrn gom and Gulf Coast. Ridging and drier
airmass on Friday will support clear to scattered clouds with Max
temperatures 80 to 85.
Short term /Friday through Friday night/...
as of 115 am Thursday...
The mid level low over the northeast early Fri will continue moving
to the NE along the coast of the Canadian Maritimes through Fri
night, with its trailing trough axis lifting NE away from NC Friday
morning. Rising heights aloft and a drying/stabilizing air mass --
as agreed upon by the latest model runs -- supports plenty of
sunshine with no pops. Low level thicknesses start the day well
below normal but do start to recover in the afternoon, indicating
highs just a tad under normal, 79-84. Fast mid level winds from the
west-northwest Fri night will likely draw some high level moisture across the
region, and we could see a little orographic enhancement of these
high clouds, based on the forecast thermal and moisture profiles.
Expect lows of 60-64. -Gih
Long term /Saturday through Wednesday/...
as of 410 am Thursday...
Sat/Sat night: a very broad and flat anticyclonic flow will stretch
from northwest Mexico across the Southern Plains and mid miss valley to the
mid Atlantic region, north of a weak ridge centered over the northwest
Gulf, to begin the Holiday weekend. At the surface, a weak and
elongated Bermuda high will extend across the southeast states, with
a weak frontal zone to our north across VA/KY. Models show at least
one subtle wave (which may be an old mcv) tracking within this
fairly fast mid level flow across the mid Atlantic Sat
evening/night, prompting convection chances across northern and
eastern NC late Sat but particularly Sat evening and into the night,
accompanying a weak surface low tracking along the surface front
boundary. With good model agreement on this general picture, will
ramp up pops across the north late Sat afternoon, showing a Max in
the NE County Warning Area Sat evening before decreasing slowly Post-wave overnight.
Thicknesses will have rebounded back above normal, so expect highs
from the mid 80s far northwest and near the Virginia border to the lower 90s southeast.
Lows Sat night 65-70.
Sun-Mon: starting sun, a mid level low over west Ontario will deepen
and drop southeast across the north Great Lakes, as broad troughing begins to
dig over the central and eastern Continental U.S.. this trend from gently
anticyclonic or zonal to cyclonic flow with the trough axis to our
west indicates a moist return flow and increasing chances for mainly
afternoon/evening convection, triggered in part by weak
perturbations tracking from the west Gulf Coast northeastward across
the Carolinas. At the surface, we're likely to see a cold front
holding well to our NW, but with a weak prefrontal surface trough
extending through the Piedmont. This will likely serve as a focus
for the expected storms, with models suggesting the best coverage
during the afternoon and evening of Memorial Day (unfortunately for
many). Will go with a good chance of showers/storms each afternoon
and evening. The aforementioned cold front is expected to drop southeast
into NC by late Mon, but appears likely to wash out before having
any major impact on temps, as the mid level trough axis will still
be to our west. Highs both days in the 80s to around 90 (with sun
perhaps a degree or two hotter than mon) with muggy lows in the 60s
to near 70.
Tue-Wed: as the mid level trough over central and eastern noam
broadens and shifts eastward, we should see a reinforcing front
dropping through the area, helping to bring in drier and slightly
cooler temps by mid week. Expect low shower/storm chances to be
confined to southeast sections Tue, with dry weather everywhere Wed as weak
high pressure starts to build in from the west. Highs from around 80 to
the mid 80s, and lows around 60 to the mid 60s. -Gih
Aviation /18z Thursday through Tuesday/...
as of 110 PM Thursday...
Convection continues to intensify at 17z from Albemarle to Chapel
HIll to Roxboro. Isolated hail up to 1 inch diameter with stronger
cells. Convective cells moving east northeast at 25 knots and will
pass through rdu vicinity 1745-1830z. Afternoon solar heating will
support additional convection mainly northern half of area as an
upper level trough and associated very cold temperatures aloft pivot
northeastward across the forecast area.
Outlook: at least a couple of clusters of showers and storms, some
strong to severe, and with gusty outflow winds surging well away
from them, will return to portions of central and/or northern NC for
Sat afternoon through Sunday. The approach and stall of a couple of
cold fronts east of the Appalachians may result in a band of MVFR
ceilings over central NC Mon night, and also a small chance of
diurnal convection both Mon and Tue.