469
FXUS62 KRAH 050245
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
945 PM EST Wed Dec 4 2024

.SYNOPSIS...
Southwesterly flow will result ahead of an Arctic cold front that
will move across NC on Thursday. Following Arctic high pressure will
modify while building from the northern Plains to the Southeast
through Saturday.

&&

.NEAR TERM /TONIGHT/...
As of 920 PM Wednesday...

The only notable forecast change was to further increase low
temperatures, to mid 30s to around 40, given the previously noted
southwesterly surface wind that will remain steady and continue to
gust at times overnight.


.PREV DISCUSSION... /ISSUED 330 PM EST Wed Dec 04 2024/

South-southwest winds have increased in the last hour or so, with
winds in the 10-15 kt range and gusts up to 20 kt. Temperatures were
in the mid to upper 40s in most locales. Surface analysis shows a
992 mb surface low over the upper Great Lakes and a 1027 mb high
just off the SE coast. An increasing pressure gradient will continue
to build over the region tonight ahead of a strong Arctic cold front
slated to be on our doorstep by early Thu morning. Model soundings
show little in the way of low-level moisture ahead of the front, so
any clouds will be largely high-base with no threat of rainfall amid
developing downslope flow. Forecast soundings also show winds
remaining elevated tonight, between about 12 and 15 kt from the SSW.
Some gusts up to 20 kt are possible overnight. As a result of this
and increasing WAA, overnight lows were forecasted a touch higher
than MOS guidance in the mid to upper 30s, except some low 30s in
outlying areas.

&&

.SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
As of 945 PM Wednesday...

Point forecast soundings suggest there will be a short period of
surface wind gusts of 30-35 kts, near 40 kts on a more-isolated
basis, between 14-17Z, as diurnal heating and stronger/deeper mixing
taps into lingering low-level jet momentum. The strongest of these
wind gusts will likely be in the form of pre-frontal swly ones in
the ern Sandhills and Coastal Plain and post-frontal wly ones over
the nw Piedmont.

Additionally, a sprinkle may accompany the Arctic front across the
nrn Piedmont and nrn Coastal Plain between 12-16Z, where point
soundings depict a fleeting, several thousand ft saturated layer and
non-zero MUCAPE.

Lastly, a blend of latest model guidance has increased the gradient
of surface dewpoints along the passing cold front and yielded
widespread single digits behind it over the nrn Piedmont during the
afternoon and area-wide Thu evening. Some minimum RH values between
20-25 percent may consequently result over the nw Piedmont Thu
afternoon, when Red Flag Warning conditions may be met.


.PREV DISCUSSION... /ISSUED 330 PM EST Wed Dec 04 2024/

* Increased Fire Danger for the Triad Thursday

The surface cold front is forecast to through the region in the
morning hours over the Triad and Piedmont sections of central NC,
then over the Coastal Plain closer to midday. Immediately ahead of
the front, temperatures will reach the mid 40s in the NW to the
upper 50s in the SE. Along and behind the front, winds will be gusty
in the 15-20 mph range, gusting between 25 and 30 mph, switching
from SW to WNW. Dewpoints behind the front are forecast to lower
into the teens across the Triad, resulting in RH levels dipping into
the upper 20s. This combined with the wind gusts will increase the
threat of fire spread. After coordination with the NCFS, an
Increased Fire Danger was issued for the Triad counties from 11am to
5pm Thu.

The pressure gradient is expected to relax somewhat Thu night as
high pressure builds into the TN valley, though forecast soundings
still show some mixing to prevent optimal radiational cooling.
Nevertheless, the Arctic airmass will result in lows some 15+
degrees below normal in the upper teens to lower 20s.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
As of 400 PM WEDNESDAY...

Arctic high pressure will dominate the first part of the extended
period, bringing sunny skies and dry weather from Friday through
Sunday. CAA from NW flow will bring high temperatures around 15
degrees below normal on Friday, only in the upper-30s to mid-40s.
For reference, while records for low maximum temperatures are not
expected to be broken, the current forecast isn`t too far off from
the records of 38F at RDU and 39F at FAY for 12/6. The surface high
will slowly move east from the mid-MS Valley to the southern
Appalachians by Friday night, allowing for near ideal radiational
cooling conditions. Lows on Saturday morning are expected to be in
the mid-to-upper-teens in many outlying areas, and close to 20F in
the urban centers. The record lows at GSO, RDU, and FAY for 12/7 are
13F, 15F and 17F respectively, so again the forecast isn`t far off
even though the records should be safe. As the surface high moves
offshore on Saturday and the low-level flow turns southwesterly,
temperatures will begin to modify. Temperatures are expected to
reach the mid-to-upper-40s on Saturday with lows in the mid-to-upper-
20s on Saturday night, which is still below normal. Conditions will
turn slightly warmer than normal by Sunday and Sunday night, with
highs in the mid-to-upper-50s and lows in the mid-30s to lower-40s.

As the surface high moves farther east to become centered near
Bermuda on Monday, a shortwave trough aloft will eject NE from the
Southern Plains to the TN Valley and northern Mid-Atlantic. It will
shear/flatten out as it does so, but it should still bring some
light precipitation to central NC from early Monday morning into
Monday night. Rainfall amounts on the latest guidance are only in
the tenth to quarter inch range at most, and POPs are mainly in the
high chance category. Forecast highs on Monday are in the mid-50s to
lower-60s, with lows Monday night in the upper-40s to lower-50s.

Another potentially more potent southern stream upper disturbance
and associated surface low may bring more significant rain to the
region on Tuesday and Wednesday, as moist SW flow aloft persists.
However, there are timing differences between the models with the
deterministic GFS and ECMWF having later timing for the heaviest
precipitation compared to a lot of ensemble guidance. So for now
just keep high chance POPs through the period. Forecast highs are
mid-60s to 70 on Tuesday, but the temperature forecast after that is
also very uncertain as it will depend on the timing/position of the
frontal system, with the more suppressed ECMWF bringing through the
front faster compared to the inland low track on the GFS.

&&

.AVIATION /23Z WEDNESDAY THROUGH MONDAY/...
As of 615 PM Wednesday...

TAF period: High confidence in VFR conditions through the TAF
period. There is a very small chance of an MVFR ceiling Thursday
morning at INT/GSO, but there is not enough confidence to include it
in the forecast. The wind will remain somewhat strong overnight,
around 10-12 kt, but it appears that there will be enough of an
inversion to prevent gusts. However, a strong low-level jet will
result in LLWS, primarily speed shear, although there will be some
directional shear as well. 50-55 kt could possibly be a slight
underestimate of what is observed. The LLWS will come to an end as a
strong cold front moves through in the morning and mixing resumes,
somewhere between 14Z in the west to 16Z in the east. It appears
that initial gusts could be as high as 30 kt, although values should
come down slightly through the rest of the day. Gusts will likely
end before the end of the 00Z TAF period, but have not included that
in this forecast for simplicity.

Outlook: Gusty NW winds Thursday will weaken Thu night and Fri. VFR
should prevail through Mon but unsettled weather could return Tue.

&&

.FIRE WEATHER...

Following the passage of a dry cold front on Thursday morning,
relative humidities will fall into the upper 20s to around 30 in
some locations across the Northwest Piedmont. Winds will be gusty
along and behind the front between 25 and 30 mph, which will be
conducive for the quick formation and spread of fires. After
coordination with local land managers and forestry officials, a Fire
Danger Statement has been issued. Consult your local burn-permitting
authority to determine whether you may burn outdoors. If you do
burn, exercise extreme caution.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS/Kren
SHORT TERM...MWS/Kren
LONG TERM...Danco
AVIATION...Green/Kren
FIRE WEATHER...Kren