FXUS62 KRAH 220006

Area Forecast Discussion
National Weather Service Raleigh NC
807 PM EDT Sat Oct 21 2017

High pressure will extend across the Carolinas through Sunday. A
strong cold front will approach from the west Monday, and cross
central NC on Tuesday.


As of 807 PM Saturday...

High pressure over the area will drift slowly eastward, eventually
shifting offshore on Sunday. Similar to last night, higher BL
moisture/dewpoints across eastern NC will support the development of
fog, especially if the veil of broken cirrus currently over the
region thins out/wanes with time. Lows overnight will continue to
moderate under the increasingly low-level easterly component. Lows
ranging from upper 40s/near 50 in the typically cooler outlying
areas to lower-mid 50s elsewhere.


As of 305 PM Saturday...

High pressure at the surface and aloft will drift offshore Sunday as
a s/w digs into the lower MS Valley. The backing mid-upper level
flow will usher moisture into our region, leading to an increase in
mid-upper level cloudiness, primarily in the late afternoon through
Sunday night. Still expecting enough sunshine to boost temperatures
well into the 70s with locations across the south half hitting 80

Clouds will continue to increase and thicken Sunday night as the s/w
deepens and becomes a closed low over the western TN Valley by early
Monday. Strengthening isentropic upglide ahead of this system may
generate a shower or two late Sunday night, primarily across the sw
third of the forecast area. Southerly flow and thickening cloud
cover will yield overnight temperatures mainly in the 55-60 degree


As of 155 PM Saturday...

Model consensus has improved greatly concerning the cutting off of a
vigorous short wave moving into the Tennessee Valley Sunday night.
This short wave subsequently lifts northeast into the Ohio Valley as
it is absorbed into a stronger long wave trof which will be digging
deeply into the eastern CONUS through midweek. A strong cold front
associated with the lead short wave will move into the mountains
early Monday, and race east across central NC during the afternoon
and evening.

Low level flow increases dramatically and very quickly ahead of this
stacked system, with southerly low level jetting exceeding 35Kt by
midday providing a deep surge of moisture (PW`s rapidly approaching
2.0 inches) ahead of the surface front. This initial surge of low
level moisture and warm air advection will likely produce a pre-
frontal QLCS, which would develop and move into the southwest
Piedmont by early afternoon. The low level jetting continues to
strengthen, with H85 winds exceeding 50 knots as the initial line of
convection moves across central NC, and will likely be followed by
another line of convection accompanying the surface front a couple
of hours later. Instability will be unimpressive and somewhat of
limiting factor, but the low level forcing/convergence and upper
diffluence associated with the short wave lifting north of the area
will be very strong. Damaging wind gusts could accompany stronger
storms, and the strongly sheared low level profile would support
rotating storms, so a tornado or two look to be possible. Will
transition categorical PoPs across the area from 18-06Z, with a 6-
hour window of 90% PoPs area-wide centered around 00Z tomorrow
evening. Warm air advection will partially offset increasing
cloudiness and highs will reach the low 70s west to near 80 in the
southeast. The convective zone will move rapidly east of the area
after midnight, with clearing and weak initial cool air advection
over at least the western half of the area by sunrise. Mins will be
mostly in the lower 60s, with some upper 50s in the west.

Drier and cooler air will be filtering into the area on Tuesday,
with potentially some instability showers across the northern tier
as the main upper trof rotates across the Ohio Valley. Highs Tuesday
will be mostly in the upper 60s to lower 70s, with further cooling
into the low and mid 60s for Wednesday through Thursday. Mins will
bottom out Wednesday night as the upper trof axis shifts east of the
area and the drier airmass cools under better radiational conditions.
Some climatologically cooler locations will fall to the upper 30s,
with widespread lower 40s elsewhere. Shortwave ridging and return
flow as surface high pressure moves offshore will give us a warmup
late in the week, with highs Friday and Saturday mostly in the mid
60s to lower 70s.


As of 800 PM Saturday...

High pressure will remain in control through Sunday. Much like last
night, higher BL moisture/dewpoints across eastern NC will support
the development of MVFR visibility restrictions in fog, particularly
at KRWI and KFAY, and possibly at KRDU. Pockets of IFR/LIFR
visibility may occur close to daybreak in proximity of KRWI.
Any fog should lift and disperse shortly after daybreak. Light
and variable winds will become SELY Sunday afternoon.

Strengthening SELY moist advection in advance of an approaching cold
front will support an increasing threat of threat for sub VFR
ceilings late Sunday night/Monday morning, followed by breezy
conditions Monday with a high likelihood of showers and a few
thunderstorms late Monday afternoon/evening through early Tuesday.
Strong wind gusts of 40 to 50 mph will be possible with any storm.
VFR conditions will return Tuesday afternoon.






NWS RAH Office Area Forecast Discussion