Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 010809

National Weather Service Raleigh NC
409 AM EDT Thu Oct 1 2020

Pacific high pressure over the Southeast will yield to a cold front
that will cross VA and NC this afternoon and early tonight. Canadian
high pressure will follow and extend across the middle Atlantic
through the weekend.


As of 350 AM Thursday...

Within highly amplified flow aloft, an observed 150kt jet at 250-300
mb over the Dakotas last evening will dig a shortwave trough from
the Great Lakes/upr MS Valley this morning to the OH Valley by 00Z
Fri, before then pivoting newd along the cntl Appalachians through
12Z Fri. Preceding 500 mb height falls and large scale forcing for
ascent will maximize between 40-50 meters over cntl NC tonight.

At the surface and in the lower levels, a cold front stretching at
08Z from w-cntl PA wswwd across the mid-South and srn Plains will
merge with an Appalachian-lee trough over wrn NC and VA this
afternoon, then strengthen while moving sewd across cntl and ern NC

It will be warm and sunny for most of the day ahead of the front,
with high temperatures between 75 and 80F. However, the passage of
front and accompanying 4-8 thousand ft cloud band will result in the
development of  cloudy or mostly so conditions in the Triad as early
as 22-00Z, then ewd across the remainder of cntl NC tonight, with
the relative greatest coverage across the nrn Piedmont, where
glancing influence of the shortwave trough will be maximized.
Additionally, given the strength and depth of frontogenesis and
associated saturation centered between 925-700 mb, and
juxtapositioning of that mesoscale forcing beneath the larger scale
ascent from the shortwave trough, that cloud band will also likely
be accompanied by a band of light rain showers. Weak MUCAPE of
around 100 J/kg based around 850 mb, combined with a shallow layer
of CSI in the developing frontal inversion and stability below, will
result in only spotty, light precipitation amounts of up to a few
hundredths across the nrn Piedmont, with non-measurable sprinkles
elsewhere. CAA-driven lows tonight are expected to be comparable to
those of this morning, mostly in the low-mid 50s.


As of 400 AM Thursday...

A highly amplified longwave trough will persist from QC sswwd into
the n-cntl Gulf of Mexico, downstream of an equally-amplified ridge
over wrn NOAM. Within that flow, a modest shortwave trough will
amplify across the mid MS and TN Valleys on Fri, then pivot newd to
the middle Atlantic coast through 12Z Sat. Associated weak forcing
for ascent will be acting upon a dry and deeply stable environment
behind the stronger shortwave trough that will be lifting away from
cntl NC early in the day, so the only notable impact in cntl NC
will be some passing, thin cirrus.

At the surface, 1024-1025 mb high pressure will build east of the
Appalachians on Fri and across the middle Atlantic Fri night. The
incoming high and associated nnwly flow will result in cooler
conditions than previous days, with highs in the upr 60s to lwr 70s.
And while light nnwly stirring will continue for much of the night,
outlying areas over the wrn Piedmont may experience calm and
relatively strong radiational cooling to near 40F, ranging through
the 40s elsewhere.


As of 310 AM Thursday...

The weekend will begin with high pressure sitting to the north of
North Carolina, bringing cool northeasterly flow to the region. The
high will take its time moving to the east, reaching the NYC metro
area by Sunday morning and continuing to shift offshore. Meanwhile,
the GFS and ECMWF both show a wave of low pressure developing along
an offshore front that should bring a chance of precipitation to
eastern locations. The 00Z models have trended showers a little bit
further to the east, so also pulled precip chances slightly farther
to the east as well. In addition, both the GFS and ECMWF show a low
near Chicago Sunday morning that will shift east. The ECMWF has the
track of the low slightly farther south than the GFS, which is the
primary reason for all locations having a chance of rain Sunday
night. As both lows move northeast, have kept some slight chance
pops Monday morning due to the ECMWF`s more southerly solution, but
any showers should depart by Monday afternoon. High pressure moves
across the Southeast for Tuesday and Wednesday. Temperatures will be
below normal throughout the entire extended forecast, beginning
about 10 degrees below normal and warming to 5 degrees below normal
by Wednesday.


As of 150 AM Thursday...

There will be a chance of shallow radiation fog this morning at RWI.
Otherwise, confidence is high that VFR conditions and clear skies
will exist until a cold front and following/post-frontal 4-8
thousand ft ceilings move southeast across cntl NC between around
00Z/2nd at INT/GSO, 03Z at RDU, and 06Z at RWI/FAY. Light sswly
surface winds will become nwly and increase into the 7-12 kt range,
with occasional gustiness into the teens kts, behind that front.

Outlook: The aforementioned front is forecast to strengthen as it
crosses cntl NC tonight; and this will result in lowering ceilings
with time and ewd extent, such that MVFR ceilings will be possible
after 06Z at RDU; FAY; and especially RWI. A coastal low will spread
light rain inland, perhaps as far west as RWI, Sun-Sun night.





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