Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
620 am EST Friday Nov 27 2015
Synopsis...high pressure will extend over the region through
Saturday... then weaken Saturday night into Sunday. A backdoor cold
front will drop southward through North Carolina Sunday night into
Near term /today through tonight/...
as of 305 am Friday...
Another dry day and night... albeit with a period of clouds and fog
in the northern and NE County Warning Area this morning. A portion of the surface
ridge that has extended across the region for a few days will break
off into a smaller surface high that will stay centered over the southern
Appalachians today through tonight. Narrow ridging holds over the
forecast area in the middle levels... sandwiched between middle level lows
NE of the Bahamas and a larger/deeper low over NE/UT. Once any
shallow low clouds and fog mix out this morning with heating...
expect generally mostly sunny skies. High clouds should be on the
increase tonight... as a strong polar stream shortwave trough
crossing southeast Canada nudges the westerlies southward slightly into the
midatlantic region. Low level thicknesses support highs in the middle
60s to lower 70s today... on the upper range of statistical
guidance. Lows tonight in the middle 40s with fair skies. -Gih
Short term /Saturday and Saturday night/...
as of 320 am Friday...
Precipitable water values will start out below normal... but signs of a change in
the quiet pattern are starting to emerge. The surface high centered
over the far southern Appalachians early Sat morning will drift slowly SW
and weaken through Sat night... as strong frontal zone pushes from
the Ohio Valley slowly southeast then southward toward NC. The southern nudge
of the westerlies continues... leading to a steady thickening of
clouds from the northwest and lowering of cloud bases along with increasing
precipitable water to over 1 inch. By late Sat and especially into Sat night... a
solid deck of middle clouds will be overspreading northern and central
portions of the forecast area. Despite a modest increase in upper
divergence and middle level DPVA particularly across the north to
contribute to weak forcing for ascent... there should be enough dry
air in the lowest 10 kft to keep conditions dry over central NC
through Sat night. Expect highs Sat in the upper 60s to lower
70s... as the uptick in thicknesses balances out the increasing
cloud cover. Lows Sat night in the upper 40s to around 50... around
10-12 degrees f above normal. -Gih
Long term /Sunday through Thursday/...
as of 322 am Friday...
Weak high pressure over the area on Sunday will keep conditions dry
but cloudy as above normal temperatures prevail with highs in the
low to upper 60s northwest to southeast. Combined with the surface high...zonal
flow aloft will keep a cold front north of the area for much of the
day before progressing through the area Sunday evening. Rain chances
with the front are uncertain but better across the northern counties
Sunday night and into Monday when high pressure over New England
slides into a favorable position for cold air damming and the
development of a wedge front across the central NC.
Therefore...expect temperatures on Monday to be on the order of ten
degrees lower...generally in the 50s.
As the surface high to the north moves offshore on Monday
night...attention turns to a stronger low pressure system over the
upper Midwest that will take over control of the stalled out front
over the Carolinas and push it back northward as a warm front on
Tuesday that will warm temperatures back into the 60s for the
eastern half of the area with some lingering cad wedge holding in
the northwest Piedmont...keeping temperatures there in the middle 50s. As the
low progresses over the Great Lakes on Tuesday night into Wednesday
morning...timing differences in the model data arise with the GFS
much more progressive with the cold front...moving it completely
through the area by Wednesday afternoon. The European model (ecmwf) is slower with
the system and keeps some precipitation chances in behind the front through
early Thursday morning. A blend of these solutions is favored and
thus carry chance probability of precipitation through much of this period. High
temperatures will drop back into the 50s for Thursday as a colder
airmass moves in.
Aviation /12z Friday through Tuesday/...
as of 620 am Friday...
VFR conditions dominate most central NC terminals currently...
however rwi will see a period of IFR/LIFR conditions with fog and
scattered low clouds this morning lasting until 14z-15z. A ridge of
high pressure both at the surface and aloft remains extended across
NC. Low level moisture has been steadily increasing the last 24 hours
due to a low level flow off the Atlantic... and this in combination
with nearly calm surface air has facilitated development of areas of
IFR/LIFR fog/clouds. These sub-VFR conditions may expand briefly to
the other taf sites (int/gso/rdu/fay) prior to 15z... however
confidence in this is not high. Aviators should still be alert for
potential adverse aviation conditions near the taf airfields. Any
sub-VFR conditions will mix out by middle to late morning... yielding a
trend to partly cloudy then mostly sunny skies by midday. VFR
conditions will hold through the end of the taf valid period.
Looking beyond 12z Sat: another period of sub-VFR shallow fog is
possible early Sat morning. VFR conditions will dominate Sat into
sun. Clouds will increase and thicken with lowering bases late sun
into Sun night as cold front moves in from the north. Ceilings are
expected to drop to MVFR then IFR/LIFR Sun night... lasting through
Monday and perhaps into Tuesday as the frontal zone stalls over the region.