Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 241045

National Weather Service Raleigh NC
645 AM EDT Sat Oct 24 2020

Weak high pressure will hold over the region this morning. A cold
front will approach from the northwest this afternoon, then push
slowly southeast and south into North Carolina tonight, ushering in
cooler air for Sunday as Canadian high pressure noses into the area
behind the front. This cool high pressure will weaken and dissipate
Monday, as warm air returns through mid to late week.


As of 405 AM Saturday...

Weak high pressure remains over central NC this morning according to
the latest surface analysis, with a strong cold front stretching
from Lake Ontario SSW through the central Appalachians and E TN to
LA. Aside from the patchy fog early this morning, we should see
plenty of sunshine through at least early afternoon NW and mid to
late afternoon SE, although afternoon scattered to broken cu is
expected areawide with heating and ascent of the moist near-surface
air. Mid and high clouds ahead of the front will also begin
streaming across our NW sections by afternoon, advecting out of the
potent mid level shortwave trough now over MS. While the precip
associated with the cold front is mostly anafrontal, we will see
modest low-mid level moisture advection ahead of the front, leading
to a band of PW near 1.5" over the NC mountains/foothills/N Piedmont
by late in the day. This, combined with surface CAPE reaching
several hundred J/kg during the afternoon, will support scattered
showers and an isolated storm or two across the Piedmont this
afternoon, mainly late. With continued WAA and a delay in frontal
cloudiness, highs should be near or a bit higher than yesterday, in
the upper 70s to mid 80s. The cold front should dip southward in
backdoor fashion into N NC this evening before progressing southward
through much of central NC overnight. Precip will transition to more
stratiform rain through the night as the aforementioned shortwave
trough tracks ENE with the corresponding surface low moving along
the frontal zone. Will continue good chance pops in the evening
across all but the far SE where the drier column holds on longer. As
moist isentropic upglide strengthens and deepens overnight with the
approaching shortwave trough, rain is expected to increase in
coverage, warranting a trend to likely pops (and this may need to be
upped to categorical if trends hold) across most of central NC
except good chance SE. Lows from the mid 50s N to mid 60s S as the
cool Canadian air mass starts nosing in from the N. -GIH


As of 420 AM Saturday...

Rain chances should hold in the good chance to likely range Sun
morning as the shortwave trough crosses the Carolinas, with the
surface front settling just to our S and short-lived cold air
damming developing. While the parent high depositing the cooler air
into central NC will be progressive, with its center moving from S
Quebec E to the Canadian Maritimes, the low level CAA will be
enhanced by flow around surface low pressure which forms off the GA
coast early Sun then deepens as it tracks NE just off the Carolina
coast. This should ensure considerable cloudiness on Sun, with
overcast skies over much of the area (may see some late-day holes in
the clouds in our SE) and a lingering chance of light rain or
nighttime drizzle as the surface-based layer remains moist and
stable through the lowest 1-1.5 km, especially over the Piedmont.
Will trend pops down to lower chance W to E Sun, then retain a
mention of drizzle overnight within the stable pool. Temps should
peak early Sun across the N, with readings steady or falling a bit
during the afternoon, while readings should rise just 7-10 degrees
in the S, yielding highs from the upper 50 N to lower 70s S. Expect
lows from around 50 N to 60 S. -GIH


As of 455 AM Saturday...

- Mostly cloudy skies over the forecast area as a cool wedge remains
in place across the NW Piedmont region.
- Precipitation is not expected to impact central NC.
- High temperatures from the upper 60s NW (due to CAD still in
place) to low/mid 70s in the SE. Lows Monday night will be well
above average and be in the upper 50s and along the coastal plain in
the lower 60s.
- Surface high lifts northeast and moves offshore.
- Model guidance from the ECMWF and GFS are not in agreement of a
coastal low moving up along the Mid Atlantic coast. GFS show no
coastal low and a drier trending forecast whereas the newest model
run of the ECMWF(24/00z) shows a low forming off the coast of
Florida and making its way up the Mid Atlantic coast. If that would
be the case this would bring higher precipitation chances to NC.
- Minimal PoPs were introduced to the forecast for Tuesday/ Tuesday
night with Slight PoP in the NW Piedmont.
-  Due to the warm air and moisture advection, highs will be well
above average in the mid/upper 70s to lowers 80s.
Tuesday night through Friday:
- A cut off low will move from the Four Corners region eastward and
across the Miss and TN Valleys by Thursday night.
-  Beginning Wednesday, PoPs increase as the low continues to move
eastward and isolated to scattered rain chances continue through
- Moisture from potential TC Zeta coming up from the Gulf will get
picked up by the cold front, and will tremendously increase
precipitation chances on Thursday and Friday.
- Temperatures remain above average in the mid to upper 70s before
the front moves through.
- After the cold front moves through, temperatures on Friday are
forecast to be near normal with highs in the mid to upper 60s.


As of 645 AM Saturday...

MVFR to IFR fog across central NC has remained quite patchy this
morning, despite the warm moist air and lack of clouds and wind.
This may be the result of dry air just off the surface. While fog
could still occur at the main TAF sites prior to 13z, the
probability appears low and will downplay this in the updated
terminal forecasts. Any fog is expected to burn off prior to 14z,
leaving a high probability of VFR conditions lasting through
nightfall. As a cold front approaches from the NW late this
afternoon, there is a risk of a few showers mainly after 21z at
INT/GSO and after 02z at RDU. This should be accompanied by a trend
toward MVFR to IFR cigs at INT/GSO starting mid evening (after 00z)
as the front settles into the N Piedmont. IFR conditions are
expected to continue at INT/GSO through the night with conditions
trending to IFR at RDU/RWI after 06z and at FAY after 09z. Areas of
light rain are expected to expand Sat night areawide across the W
and N (all but FAY) 06z-12z. Light surface winds will hold from the
SW and WSW today at all sites, but becoming northeasterly behind the
front after 00z at INT/GSO/RDU as the front passes through. A few
gusts are possible from the NE late tonight.

Looking beyond 12z Sun, sub-VFR conditions will persist Sun morning.
As the front continues settling southward, surface winds will be
from the NE areawide through Sun, with a good chance of rain and sub-
VFR conditions Sun, and patchy drizzle and poor conditions holding
through Sun night. Slow improvement to MVFR then VFR is expected Mon
with dry weather, but sub-VFR fog may again form Mon night/early Tue
morning. Rain chances and sub-VFR conditions are expected to return
Tue into Wed as another cold front moves into the area from the NW
and N and stalls out. -GIH




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