Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 200954

Area Forecast Discussion
National Weather Service Raleigh NC
554 AM EDT Fri Oct 20 2017

A ridge of high pressure will extend across the eastern United
States through the weekend.


As of 340 AM Friday...

Mid-upper ridging now centered from the delta of the MS River to the
cntl Gulf of Mexico will amplify and build across the sern US
through tonight, downstream of significant trough amplification from
the Pac NW to the front range of the Rockies.

At the surface, 1026 mb high pressure analyzed over the Cumberland
Plateau will likewise build ewd, over or just east of the cntl
Appalachians by Sat morning.

With no change in, but another day of modification of, the air mass
associated with that ridge of high pressure, temperatures today will
be similar to, to perhaps a category milder than those of Thu.
Patchy, thin cirrus in nly upper level flow this morning will get
shunted offshore by this evening, as that flow backs to wly with the
approach of the aforementioned upper ridge. As such, skies will be
sunny/clear, except mostly so during the time of passing cirrus
early today. Strong radiational cooling will favor low temperatures
aob consensus statistical guidance tonight, mostly in the mid-upper


As of 350 AM Friday...

Mid-upper ridging will linger in the vicinity of the sern US coast,
while high pressure drifts off the middle Atlantic and Northeast
coast, while also strengthening. The result will be a generally
persistence forecast, though with an increase in high cloudiness
that may tip sky conditions into the partly cloudy range by Sat
night, with associated slightly milder low temperatures in the upper
40s to middle 50s. High temperatures again in the middle 70s to
around 80 degrees, or so.


As of 340 AM Friday...

The ridge over the southeast will move east Sunday as a trough over
the Midwest and Mississippi Valley deepens. We`ll have another nice
day Sunday to round out the weekend`s weather, featuring partly
cloudy skies (mainly high clouds) and return flow resulting in warm
temps with highs in the upper 70s to around 80.

Monday will feature increasing and lowering clouds as a closed
trough ahead of the upstream longwave trough moves across the TN
Valley and moist S/SW flow aloft increases over our area ahead of
this trough.

GFS and ECMWF solutions diverge Monday night and Tuesday. The ECMWF
shows the lead short wave getting absorbed into the larger longwave
trough and ejected to the northeast 12+ hours faster than the GFS.
The ECMWF solution would result in a band of showers quickly moving
across our CWA early Tuesday morning before exiting to our east by
noon Tuesday.  The slower GFS brings multiple bands of showers
across our area Monday night through about mid-day Tuesday, with
perhaps a brief opportunity for a few gusty tstms during the late
morning Tuesday before the showers exit to our east.  This thanks to
the proximity of the passing mid-level trough and subsequent
stronger wind fields and resultant shear; however GFS forecast
sounding show limited instability, thus perhaps a high-shear/low-
CAPE setup with isolated strong wind gusts the main concern if any
severe weather threat materializes. Finally, there will be another
brief opportunity for a few showers late-day Tuesday with the
passage of the sfc cold front.  Coverage and intensity should be
limited though, thanks to the earlier passage of the lead short wave
and subsidence and mid-level drying in its wake.

Tuesday night we`ll drying and strong CAA commence in the wake of
the cold front, with additional CAA behind the passage of the
longwave trough axis which is progged to move across our CWA late
Wednesday.  The main weather story for Wednesday and Thursday will
be the chilly temps, with highs only in the low 60s both days, and
potential for lows dipping down into the upper 30s in some spots
Thursday morning.


As of 550 AM Friday...

High pressure building east across the middle Atlantic states, and
associated strong radiational cooling and risk of "persistence"
(relative to similar conditions 24 hrs ago) radiation fog at RWI
this morning, will otherwise result in mainly clear, VFR conditions
with light and variable to light n/nely surface winds, through the
TAF period.

Outlook: Continued persistence radiation fog will be possible mainly
at RWI each morning through the weekend, followed by a higher
probability of more-widespread sub-VFR conditions in stratus and
fog, in increasingly moist east to southeasterly low level flow, Sun
night-Mon morning. An approaching frontal system will then result in
a good chance of sub-VFR conditions, showers and storms, and
strong/shifting winds with height late Mon-Tue.





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