Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 192324

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
724 PM EDT Thu Oct 19 2017

High pressure will hold sway across our region into the start of the
upcoming weekend. Temperatures will gradually edge up a few degrees
during that time. Meanwhile, a low pressure system, and frontal
pattern, will begin to impact parts of our region late Sunday and
into early next week.


As of 730 PM EDT Thursday: Hourly temperatures continue to run
warmer than forecast, but with sunset and the dry airmass in place
temperatures should start dropping off a little more quickly.
Hesitation is that right now dewpoints across the Upstate and NE GA
are a little higher than forecast overnight lows, so will monitor
dewpoint trends over the next few hours in case adjustments need to
be made for the late evening update.

Otherwise, an upper ridge will amplify over the Southeast and Ohio
Valley tonight, with the axis shifting east to the Carolinas by 00z
Saturday. At the sfc, high pressure will move little tonight thru
Friday, with the center drifting north from eastern TN to
southwestern PA. This high pressure system should keep the area
clear with light winds. Temps will continue a warming trend with
lows tonight and highs tomorrow expected to be a category or two
warmer than today. Fog is expected to develop again tonight in the
mountain valleys and around area lakes, but with slightly higher
dewpoint depressions and warmer min temps, coverage may not be as
much as last couple nights.


As 2:30 PM EDT Thursday: A large upper level high pressure system
will be anchored across our forecast through Saturday. Meanwhile
surface high pressure will extend from parts of the New England
states down through the Carolina`s and northeast Georgia. Most of
the forecast area should have quiet weather conditions right into
the state of the upcoming weekend.

Subtle pattern changes will commence Sunday as the upper level high
shifts off the eastern seaboard, while some low level moisture will
start to enter our forecast area (FA) Sunday afternoon. This influx
of low level moisture will be arriving in a moistening southeast
flow from the Atlantic around H85 mb. Cirrus will likely be entering
the picture, as the upper level flow becomes more southwesterly.

The earlier "faster" GFS has slowed with respect to precipitation
development late Sunday afternoon. It is more in line with the new
European model, which is also slower with any precipitation
development. As a result have pulled back on rainfall chances Sunday
afternoon (with a slight chance south) toward early Sunday evening.

As far as the temperatures, the current guidance is very similar to
our running values. As a result only minor adjustments needed.


As of 200 PM EDT: A pattern change will be underway at the start of
the period, as the East Coast upper ridge will be in the process of
weakening and progressing east in response to major height falls
emanating from the central Conus. Evolution to a bit of a split flow
regime also appears to be in the offing, as global model guidance is
in agreement in closing off an upper low over the Arklatx by the end
of the weekend. How this feature evolves (a detail that global model
guidance does not agree upon) will determine the fine points of the
forecast over much of the Southeast during the early part of the new
work week. Despite this, it is becoming more obvious that it is
going to rain across our area early next week...

The operational GFS remains the wettest deterministic model (even
among the GEFS system, the operational GFS is one of the wetter
outliers), primarily due to keeping the aforementioned upper low
from phasing into the northern stream until very late in the
forecast period. This results in considerably more surface
development across the Southeast, and much more in the way of a warm
conveyor belt precipitation event prior to the arrival of a cold
front on Tuesday. Meanwhile, the ECMWF depicts more of a frontal
band scenario, albeit a wet one. Regardless of the details, likely
to categorical pops appear warranted across much of the area from
early Monday through early Tuesday. Some concerns linger regarding
the potential for organized/perhaps severe convection late Monday
into early Tuesday, but much will depend upon the evolution of the
surface patter. Due to the initially dry air, a warm conveyor belt
precip scenario may tend to establish in situ cold air damming, and
this air mass would have to be dislodged to create more than a very
isolated threat. However, even if this occurs, guidance suggests
that the combination of shear and instability will be adequate for
an isolated threat with the passage of any frontal band on Tuesday.

Much cooler and drier air will begin filtering into the area by late
Tuesday. A moist post-frontal NW flow across the TN Valley and
southern Appalachians could result in some showers across the high
terrain near the TN border through mid-week, with snow possible in
the higher elevations. Otherwise, the remainder of the medium range
will feature dry weather and temperatures approximately 10 degrees
below climo.


At KCLT and Elsewhere: VFR through the period for all but KAVL
where another round of mountain valley fog/low stratus is expected,
though some guidance sources continue to indicate a more optimistic
trend. Will continue with a persistence forecast as this has been
the case the past few nights. For the rest of the period at KAVL and
for all other sites, clear skies and calm to lgt/vrb winds, but
favoring the N side.

Outlook: VFR conditions will persist through the end of the week
with the only possible exceptions being mtn valley fog/stratus
restrictions in the morning. Moisture will begin to return to the
area Saturday in advance of a cold front early next week.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Low   30%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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