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Area Forecast Discussion

FXUS62 KGSP 270025

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
825 PM EDT Mon Sep 26 2016

A cold front will cross our area tonight and be near the Carolina
coast by Tuesday evening.  An upper level low will drop south into
the Ohio Valley by Wednesday and linger over the Central
Appalachians through the end of the week.


As of 8 PM, line of convection producing very heavy rainfall extends
from near Forest City, to Lincolnton to near Salisbury NC this
evening. Although its movement is deliberate, it has been
progressive, so rainfall amounts have generally been limited to
around an inch per hours. However, there have been locally higher
amounts of 2-3 inches. This activity will have to be monitored
closely as it moves across metro Charlotte between 9PM and midnight,
owing to the poor antecedent conditions that exist there after this
morning`s heavy rain event. Based upon the upstream rainfall rates,
it`s certainly not a given that additional flash flooding will
develop there, but it`s likely to get close in the areas that were
hit hard this morning. Otherwise, the part of the convective band
pushing off the SC Blue Ridge is less cohesive/more spotty in
nature, but there are signs that it may be trying to reorganize
along the I-85 corridor, which is a good thing since this area is so
desperate for rain. In fact, other than immediate Charlotte area,
the potential for hydro problems is quite low, with most locations
along and south of the I-85 corridor expected to just see beneficial
rain (although some quite a bit more so than others).

Otherwise, over the next 24 hours, big-picture-wise a large
mid/upper low is expected to slowly drop down from western Ontario
to the upper Great Lakes, which will bring mid/upper troffing down
across the western Carolinas. This should drive a surface cold front
from the TN valley eastward across the Srn Appalachians tonight and
Tuesday morning. The front will probably get hung up across the mtns
thru most of the day on Tuesday. Weak upper divergence will get
strung out across the region tonight and Tuesday as the right
entrance region of the upper jet moves overhead, but mid-level
forcing is not that impressive. Most of what the front does will be
the result of deep convection firing along and ahead of the feature
this afternoon and evening, and then the weak forcing overnight. The
guidance suggests holding onto a likely precip prob mainly out
across the NC foothills and wrn Piedmont into the overnight hours.
Min temps tonight should be mild. On Tuesday, with the front in our
midst, precip chances should ramp up especially over the eastern
zones where a likely was included. Temps will be at or above normal


As of 245 PM Tuesday: Showers and thunderstorms will be scattered
along and southeast of Interstate 85 Tuesday evening as the front
slowly drops south. This activity will slowly diminish but may
linger into Wednesday morning. Most of Wednesday should be dry as
cooler air moves into the region.

A fairly potent upper level low pressure system will drop southeast
across the Ohio Valley and central Appalachians impacting the area
Wednesday night and Thursday. Several disturbances will swing around
the parent low bringing rainfall to the area. The highest chance of
rain will be in the upslope areas of western North Carolina while
the lowest chance will be in northeast Georgia and upstate South
Carolina where downsloping low level flow will exist. All locations
will be cooler as the impacts of the low translate across the area.


As of 315 PM EDT Monday...An upper low is expected to be over
Bristol TN Thursday evening then lift north to eastern OH Friday
evening.  The GFS forecast has been accepted as the better
performing operational model. The ECMWF is better today but still
slower in its evolution of the cut off closed low lifting out and
departing New England.  With the lower atmospheric thickness values
associated with this upper low and more cloud cover Thursday night
into Friday, this will keep temperatures several degrees below
normal. Very light rain amounts are possible across the NC Mtns and
north of I-40 Thursday evening then becoming increasingly limited to
the TN border areas until Friday evening when it goes away to the
north of our forecast area.

The closed and cut off upper low will begin to open to the westerly
flow at the north end of the Great Lakes on Saturday. The low will
fill and become absorbed near Montreal Sunday night. We will be at
the southern end of a large area of surface high pressure Sunday
into Monday. The GFS has the high centered over north central Quebec
at 12Z Tuesday ridging down across our area. The orientation of the
low level wind flow with this high may produce upslope wind by
Monday night into Tuesday which may result in light rain in the Lake
Jocassee to Tryon to Lenoir areas.

Temperatures will certainly be several degrees below normal Friday
then slowly rise to near normal as the low moves away from our
region over the weekend. By Monday, temps may even be a little above
normal since the high may not be strong enough and far enough away
to have minimum influence toward wedge conditions.


At KCLT: A significant line of convection producing very heavy
rainfall extends from about 50 miles west through 30 miles north of
the airfield as of 2345Z. Expect this to be on the terminal between
01Z and 02Z, and should produce MVFR conditions, initially gusty
winds, and periodic IFR visby. A tempo has been included from 01-04
to account for the lower conditions. Stronger convection should move
east of the terminal btw 03-04Z, but at least VCSH will persist for
a couple of hours after that. With the expectation of heavy rain
developing over the terminal this evening, chances for development
of fog and/or low stratus are increasing for the early morning
hours, but opted to simply hint at this with SCT IFR/MVFR clouds and
MVFR visby for now, and will re-evaluate at 06Z. Otherwise,
prevailing winds should turn toward the SW through the period. While
additional shower activity cannot be ruled out Tue morning, the best
chances for convection will exist during the afternoon, with
VCSH/PROB30 warranted during this time.

Elsewhere: Stronger convection has moved east of KAVL/KHKY, although
SHRA will persist for another couple of hours, possibly resulting in
periods of MVFR visby. The Upstate SC terminals are also expected to
get in on the convective action very shortly, but convection is
expected to be considerably more tame than that affecting KCLT, so
only expect MVFR conditions, with perhaps very brief IFR possible.
Otherwise, IFR conditions are expected to develop at KAVL/KHKY by
around 06-07Z, if not sooner (this potential handled with a tempo)
as the low level air is saturated due to rainfall. Chances will also
exist for restrictions early Tue morning at the Upstate terminals.
This has been handled with SCT MVFR clouds at some MVFR visby at
some terminals, but this will be readdressed at 06Z. While
additional shower activity cannot be ruled out Tue morning, the best
chances for convection will exist during the afternoon, with
VCSH/PROB30 warranted during this time.

Outlook: Cold front will slowly cross the region through Tuesday.
Dry high pressure builds in slowly behind the front on Wednesday,
which should bring VFR conditions in most places through the end of
the week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  88%     High  94%     High  90%     High 100%
KGSP       High  88%     High  89%     High  85%     High 100%
KAVL       High  82%     Med   76%     High  86%     High  98%
KHKY       High  81%     Med   62%     Med   70%     High 100%
KGMU       High  88%     High  89%     Med   78%     High 100%
KAND       High 100%     High  86%     High  85%     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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