Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 211201

Area Forecast Discussion
National Weather Service Raleigh NC
700 AM EST Tue Nov 21 2017

High pressure will drift offshore today. Meanwhile a weak area of
low pressure will develop off the Florida coast, and then track
quickly northeastward along the Southeast coast tonight. A cold
front will move through the area on Wednesday.


As of 340 AM Tuesday...

Surface high over the area this morning will shift offshore, in
advance of a northern stream trough and attendant cold front
progressing SE into the Ohio and Tn Valley. Meanwhile, the southern
stream shortwave trough over the central GOM this morning will lift
northeast across the Florida peninsula today and then up along the
SE Coast late this evening and tonight. Associated weak spin-up of a
low pressure area will move NE up the SE coast overnight, remaining
well offshore. Brief surge of low-level moisture transport in
advance of this southern stream wave, coupled with strengthening
ascent in the jet entrance region, will yield yet another quick
hitting and light precip event across central NC this evening and
tonight. Bulk of measurable precip is expected to remain along and
east of US 1, where low-level moisture flux will be strongest, with
QPF amounts ranging from a trace across the western Piedmont to a
tenth of an inch across the Coastal Plain counties.

Although modulated by a thickening cloud cover, the southerly return
flow will support warmer temperatures than yesterday with afternoon
highs ranging from mid/upper 50s across the NW Piedmont to lower/mid
60s SE. Much warmer overnight lows in the lower 40s NW to near 50


As of 350 AM Tuesday...

Aloft, a lead shortwave trough now over the cntl GOM will be lifting
newd through ern NC and VA Wed morning, while another pair of
shortwave troughs now amplifying sewd across the nrn Plains and
Intermountain West, respectively, form and cause a positively-tilted
trough to edge ewd from the middle Atlantic states swwd to the cntl

At the surface, a cold front now stretching from s-cntl Ontario swwd
through the upr Great Lakes, then wwd into across the cntl Plains,
will progress sewd and lie invof the cntl Appalachians by 12Z Wed,
where it will be slowed and subsequently arc swd in backdoor fashion
through the srn middle Atlantic states late Wed and Wed night.

After a lingering (slight) chance of rain Wed morning over the nrn
Coastal Plain, in association with the lead shortwave trough
forecast to lift newd from the GOM, wnwly deep layer flow will favor
drying/clearing Wed. It will otherwise remain mild ahead of the
aforementioned cold front, with high temperatures exactly where they
should be relative to the 30-year avg - upr 50s to around 60 degrees
over the nrn Piedmont, and 60s elsewhere. Lows should be in the 30s,
coldest north, where relative calm and clear will support good
radiational cooling conditions. Increasing mid-high level moisture,
downstream of the next shortwave trough lifting newd from the GOM,
will meanwhile stream into srn and sern NC and favor lows in the
upper 30s to near 40 degrees there.


As of 400 AM Tuesday...

Little change needed to the long term forecast. Medium range
guidance remains 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, the latter of which will cause a relatively
strong negative NAO pattern to develop. Between these two ridges,
and locked in place by the negative NAO, 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 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-- remain 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 zone.

Otherwise at the surface, cP high pressure will expand across the
middle Atlantic states and Carolinas through the end of the week. A
reinforcing and moisture-starved, nrn stream cold front is then
forecast to sweep across NC Sat night-Sun, with another cP high to
follow for Sun-Mon.

Sensible weather conditions are consequently forecast to be mainly
dry, with generally below avg temperatures except for in a briefly
mild, pre-frontal airmass on Sat. As indicated last night, the
wavelength of the larger scale pattern and associated breadth of the
ern US trough would allow for one or more of the aforementioned
shortwave troughs to amplify strongly and perhaps briefly close off
from the sern US to the GOM, which would in turn favor a wwd jog in
the low level frontal zone and associated surface waves, and wrn
extent of precipitation into the Carolinas, most likely on Thu and
Sat. Indeed, the 00Z/21st guidance has trended in that direction for
Thanksgiving Day; and if these trends continue, current slight
chances of precipitation will need to be increased for the srn and
cntl Coastal Plain on Thu, with continued dry conditions elsewhere.


As of 700 AM Tuesday...

Sfc high pressure over the area this morning will drift eastward and
offshore throughout the day. Resultant SELY return flow will lead to
gradual moistening and broken/overcast ceilings between 3 to 5kt
during the mid to late afternoon. Low-level moisture advection in
advance of a sfc low that will move northeast off the SE Coast
overnight, will support the potential for MVFR ceilings in light
rain late this evening and overnight, especially at the eastern
terminals(KRWI and KFAY).

A cold front will cross central NC Wednesday. Cold dry air advection
will quickly scour out any sub-VFR ceilings Wednesday morning. VFR
parameters should prevail across central NC Wednesday afternoon
through Saturday.





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