Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 170053
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
740 PM EST Tue Jan 16 2018

.SYNOPSIS...
A strong Arctic cold front will approach from the west, crossing our
region very late tonight through Wednesday morning. High pressure
will build into the Carolinas Thursday, then remain anchored over
the Deep South through the weekend, leading to a warming trend.

&&

.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
As of 740 PM Tuesday...

Upper trof edging east is right on track, with mid cloudiness
reaching the Triad during the past hour. Timing of the precip is on
track as well, with latest HRRR soundings in the Triad indicative of
saturation in the -12 to -18 growth zone a little before midnight,
with the Triangle area following suit a couple of hours later.
Initial light precip will begin to blossom behind the front as it
moves into the western Piedmont after midnight, but may not reach
the ground for a couple of hours afterwards, with coverage
continuing to spread east through morning. Any changes prior to
arrival of 00Z guidance will be to update hourly grid trends.

Previous Discussion:

Summary and Main Points:

NWP models have trended wetter over the past couple of runs, which
has boosted forecast confidence that the strong dynamic support
aloft associated with the vigorous shortwave trough will support a
quick burst of significant snowfall across central NC between 12 to
18z Wednesday, with an increasing likelihood of seeing a broad snow
swath of 2 to 4 inches, with locally higher amounts of 5 to 6
possible across the interior portions of the RAH CWA. As such, have
upgraded all but the 2 most SE counties(Sampson, Wayne)to a Winter
Storm Warning. Due to the west-east difference in onset and ending
timing of precip, will segment the warning/advisories maps, with an
earlier start in the west and later ending in the east.

Details:

The well-advertised blast of Arctic air, that`s currently supporting
daytime highs in the teens and 20s across central and eastern TN, is
well on it`s way. There has been little change in the model timing
of the frontal passage through the area; between 06z-09 through the
NW Piedmont, to 12 to 15z across the southern Coastal Plain. On the
heels of this front, the high-amplitude positively-tilted trough
that currently extends from SE Canada back into the Southern Plains,
will assume a neutral tilt as it traverses the region on Wednesday,
a signal of the strengthening deep layer lift(coupled ascent from
strong s/w dynamics, vigorous jet divergence and deep layer f-gen)as
depicted by bufkit soundings that shows a classic cross-hair
signature(moderate to strong lift intersecting with the dendritic
growth zone(-10C to -18C))across central NC between 12 to 18z. As
such, expect the ana-frontal precip band to fill in and blossom
across the area during that time frame. So, it`s not a surprise to
see that models have trended wetter over the last 24 hours, with
average liquid equivalents now in the 0.20 to 0.40"range.

While we could see a brief period of rain or rain-snow mix at onset,
especially east of the US 1 where precip will move in after
daybreak, deep layer cooling will result in a quick change-over to
snow, with high snow:liquid ratios of 12:18 expected in the 3 to 6
hour window of strongest forcing, similar to the upstream ratios we
saw across Middle Tn and Central-Eastern Tn earlier this morning.
This would support a broad swath of 2 to 4 inches(0.5-1.0"/hour
rate)across interior sections of the state, with localized higher
amounts of 5 to 6 inches certainly possible given the favorable set
up for banding precip.  Elsewhere, along the western and SE fringes,
1 to 3 inches are possible. Aggressive drying aloft will overwhelm
the area as early as late morning in the west to mid afternoon in
the east. This would support a brief transition of freezing drizzle
before ending, resulting in only trace ice amounts. The drying and
subsidence aloft is so strong and abrupt, that western portions of
the forecast could see sunshine by the afternoon, indeed a quick
hitting system.

Additionally, the Arctic high building in the Southern US in the
wake of this system will support very cold temperatures and wind
chills Wednesday and Wednesday night. Daytime highs will likely
occur during the early morning at most locations, with wet-bulb
cooling and CAA leading to a quick drop into the 20s by midday. NWLY
wind gusts of 15 to 20 kts will make it feel even colder, producing
wind chills in the teens and 20s.

Lows Wednesday night are expected to be in the teens, with wind
chill values approaching the single digits across much of central
NC. Given these frigid temperatures, any snow or slush on roads and
bridges/overpasses will freeze, making for treacherous road
conditions Thursday morning.


&&

.SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
As of 200 PM Tuesday...

In the wake of the exiting system Thu, expect blustery and cold
conditions with afternoon temperatures 10-15 degrees below normal.
Dependent upon the extent of the snowpack, min temps Thu night may
end up a few degrees colder, possibly ranging from the mid teens to
around 20.


&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 200 PM Tuesday...

The extended forecast will feature a warming trend across the
Carolinas as the upper level pattern transitions from a highly
amplitude pattern with a west coast ridge and east coast trough, to
one of feature a broad, low amplitude trough across the entire U.S.

Moderation in the atmosphere begins in earnest Friday and continue
through the weekend. The dry air mass coupled with a wly flow should
result in a wide diurnal temperature swing of 25-30 degrees. Highs
Friday will be close to normal in the upper 40s and lower 50s, then
warm well into the 50s Saturday, and near 60-lower 60s Sunday.

The mild conditions will persist into early next week as the flow in
the mid-upper levels remain zonal. An upper level disturbance will
be lifting well to our nw Monday. This system`s attendant cold front
will approach and cross our region Monday and Monday night,
increasing the threat for scattered showers.

&&

.AVIATION /00z WEDNESDAY THROUGH MONDAY/...
As of 640 PM Tuesday...

24-Hour TAF Period: No major changes aviation-wise for the next 24
hours. Still expect snow to develop across Central NC tonight,
reducing ceilings and visibilities into the MVFR/IFR range along
with it. Light and variable winds early tonight will become north-
northwesterly and gusty Wednesday morning into the afternoon. -KC

Previous discussion: Precipitation falling through the dry sub cloud
layer will initially evaporate before reaching the surface but will
aid to lower ceilings into the MVFR category after 06Z Wed in the
Triad, and between 09Z-13Z elsewhere across central NC. The
precipitation will initially start out as rain but will quickly
change over to snow. the snow will pick-up in intensity across the
west after 09Z, and across the east after 12Z, with visibilities 1-
2SM common. The snow will taper off over the western Piedmont after
15Z, and by early-mid afternoon across the eastern counties. Sfc
winds will be light and variable through 10Z, then increase out of
the north with gusts 18-23kts probable. -WSS

Looking ahead: Aviation conditions will improve west-to-east late by
Wednesday night as a drier colder air mass overspreads the region.
VFR parameters expected Wednesday night through Sunday. -WSS

&&


.RAH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 AM to 9 PM EST Wednesday for NCZ078-
089.
Winter Storm Warning from 11 PM this evening to 4 PM EST Wednesday
for NCZ007-021>025-038>040-073>076-083-084.
Winter Storm Warning from 4 AM to 9 PM EST Wednesday for NCZ008>011-
026>028-041>043-077-085-086-088.

&&

$$
SYNOPSIS...WSS
NEAR TERM...CBL/mlm
SHORT TERM...WSS
LONG TERM...WSS
AVIATION...KC/WSS



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