Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 201142

Area Forecast Discussion
National Weather Service Raleigh NC
642 AM EST Mon Nov 20 2017

High pressure over the area today will shift offshore
late tonight and into the day on Tuesday. A weak low pressure will
develop off the Florida coast on Tuesday, and then track
quickly northeastward along the Southeast coast Tuesday night. A
cold front will move through the area on Wednesday.


As of 640 AM Wednesday...

High pressure and a deep dry air column will dominate today with
nary a cloud in the sky until the arrival of some thin cirrus clouds
late this afternoon/early evening. It will be a chilly start this
morning, with 12z low-level thicknesses averaging around 20 to 23
meters below normal, more typical of late December as oppose to
mid/late November. Highs today a good 5 to 7 degrees below normal,
ranging from mid 50s north to upper 50s/near 60 south.

Aforementioned thin cirrus Monday evening will spread north and
possibly thicken tonight within the increasing SWLY flow aloft, in
response to a southern stream shortwave trough advancing east
across the western and central GOM. Meanwhile, high pressure over
the area, will quickly scoot east and offshore. The NAM is quicker
with the onset of southerly low-level moisture return late
tonight, advecting a stratus cloud deck into southern/southeastern
portions of the forecast area during predawn hours. Lows in the
lower to mid 30s.


As of 340 AM Wednesday...

The aforementioned southern stream shortwave trough over the GOM
will progress eastward across the Florida peninsula during the day
on Tuesday, and then lift northeast along the SE Coast Tuesday
evening/night. The associated weak spin-up of surface low pressure
area over the eastern GOM will quickly track NE up the SE coast,
remaining offshore. This wave will support yet another quick hitting-
light precip event across the area late Tuesday afternoon and
Tuesday night, with bulk of measurable precip expected to remain
along and east of US 1, where low-level moisture transport will be
strongest. Expect QPF amounts to range from a trace across the
western Piedmont a tenth of an inch across the Coastal Plain

Under a thickening cloud cover, temperatures will range
from lower to mid 50s across the NW Piedmont to lower/mid 60s SE.
Much warmer overnight lows in the lower 40s NW
to near 50 SE.


As of 400 AM Monday...

Medium range guidance is in relatively good agreement with the
planetary scale pattern, which is forecast to be a stable one
consisting of a persistent mean ridge centered near Baja CA and
another strong ridge extending from the w-cntl N. Atlantic nwwd
through the Davis Strait and to the North Pole. Between these to
ridges, a high-amplitude, longwave trough will become established
from near Hudson Bay swwd into the e-cntl GOM mid-late week, and
then reload this upcoming wknd. Numerous shortwave perturbations are
forecast to round the wrn NOAM ridge and amplify into the base of
the ern NOAm trough; and details regarding the timing and amplitude
of these shortwave troughs --and their interaction with a persistent
frontal zone draped from the ern GOM newd along and offshore the
sern US coast-- arise in the model guidance with respect to the
timing and wwd influence of at least a couple of episodes of
cyclogenesis from the FL peninsula newd along the offshore frontal

Otherwise at the surface, a lead moisture-starved nrn stream cold
front will collapse sewd, likely in backdoor fashion through the srn
middle Atlantic states, late Wed-Wed night, with following cP high
pressure poised to expand across most of the ern US through the end
of the week. A reinforcing and similarly moisture-starved, nrn
stream cold front is then forecast to sweep across NC Sat night-Sun.

Sensible weather conditions are consequently forecast to be mainly
dry, and mild in the upr 50s to lwr 60s Wed and Sat, ahead of the
respective fronts, with cooler 50s, and some upr 40s on Thu,
otherwise. At this time, any precipitation chances on the far nwrn
edge of the aforementioned coastal cyclones are expected to remain
confined to the far sern RAH CWFA, most likely on Thanksgiving Day,
and perhaps again with the second low early this wknd. However, the
wavelength of the larger scale pattern and associated breadth of the
ern US trough would certainly support more amplification than
currently indicated over the sern US, so there is room for a wwd jog
in the low level frontal zone and associated surface waves, and wrn
extent of precipitation into the Carolinas.


As of 640 AM Monday...

VFR parameters across central NC will persist through Tuesday as an
area of high pressure will be in control of our weather. Sfc winds
tonight through Tuesday will be variable at less than 10 kts.

MVFR ceilings are possible Tuesday afternoon through early Wednesday
as an area of low pressure skirts northeastward up the Southeast
U.S. coast. VFR parameters will return on Wednesday and should
remain VFR through the majority of the upcoming week.




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