Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 290220

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1020 PM EDT SUN AUG 28 2016

A tropical system will move toward the North Carolina Outer Banks
through Tuesday before likely moving back out to sea by mid week. A
cold front will approach the region from the northwest Wednesday
night into Thursday, with another tropical system likely emerging
from the Gulf of Mexico. There remains a great deal of uncertainty
with the Gulf system but there are no imminent threats to our area
at this point.


At 1020 PM Sunday: An upper ridge was centered near the Delmarva
Peninsula, extending inland to the central appalachians. Meanwhile
an upper low associated with a surface tropical system was off the
SC/GA coasts. By monday the upper ridge nearly splits in two, with
center over KY and NW of Bermuda, while the upper low reaches the
SC/GA coasts.

Convection has ended, but convective debris is moving over from the
west while clouds rotating around the upper low are moving in from
the east. Expect patchy dense mountain valley fog to develop
overnight, with some low clouds possible across the I-77 corridor.
Although instability is present on Monday, model soundings suggest
significant drying in the afternoon, ostensibly related to
subsidence on the west side of a coastal tropical system. This would
limit convective chances over much of our area. Temperatures will
run near normal.


As of 200 PM EDT Sunday, Tropical Depression Eight (or a named
system) will likely be positioned just off the NC Outer Banks
Tuesday morning - situated in a weakness in the upper level ridge
along the East Coast. The main question for our forecast area will
be whether or not any tropical moisture from the system can work
west across the piedmont. Consensus keeps the better moisture
confined to the coast, with very low end precipitation chances
reaching our lower piedmont, and a secondary and better maximum of
PoP over the mountain ridges in scattered diurnal convection.

Falling heights from the north on Wednesday should permit the
coastal tropical circulation to begin moving back out over the open
water. Meanwhile, a surface cold front will slowly approach from the
northwest but remain NW of the area through late day. Again, the
bifurcated PoP forecast will feature slight chance eastern piedmont
PoPs but with the best focus for continued diurnal thunderstorm
chances over the mountains. Temperatures will continue slightly
above climo.

Any development in the Gulf with the current 99L system, and
associated impacts on the western Carolinas and NE GA, should become
much more apparent by Tue/Wed.  No adjustments to sensible weather
from this system will be needed for our area through late Wed at
this point.


As of 215 PM EDT Sunday, the medium range fcst picks up at 00Z on
Thursday with an upper trof digging down over the Great Lakes and
a very steep upper ridge spreading over the central CONUS. In
addition, a tropical disturbance that is currently located just
north of Cuba is expected to be located off the west coast of
Florida by the start of the medium range. As we move into Friday,
the trof axis will move NE of the fcst area and eventually move
offshore while the upper ridge spreads farther east eventually
encompassing most of the CONUS. At the sfc, a fairly complex
synoptic pattern will likely exist to start the period early
Thursday. The tropical low corresponding to the above mentioned
upper low is expected to be located a couple hundred miles off the
west coast of Florida. The long range models continue to differ
with respect to the strengthening of the system and to a lesser
extent the track, over the next few days. As has been the case
for days now, the ECMWF has a deeper low and moves the system
faster and farther north than the GFS. The CMC is also deeper
than the GFS and moves the low even farther westward and then
inland than the ECMWF. The main discrepancy lies in the strength
of the Canadian sfc high that settles over the Great Lakes on
Thurs and Fri. If the high is strong enough and able to move far
enough south early in the period, it will keep the low from
advancing as far north on Fri and Sat. By Sunday, the models agree
that the high will be sliding off the New England Coast and will
have less of an impact steering the tropical low. As for the
sensible fcst, if the current 12Z GFS solution verifies our CWFA
would remain mostly dry. If the older 00Z ECMWF verifies, we would
see more deep moisture over our SE zones but drier air over NW
half of the CWFA. Only the Canadian solution brings deep tropical
moisture over the entire fcst area, and thus widespread precipitation.
I have opted for a blend of the GFS and ECMWF solutions with solid
chance POPs on Thurs, Fri, and Sat and mostly dry conditions for
Sunday. Temps will start out above climatology and cool to near
or even slightly below climo by the weekend.


At KCLT and elsewhere: Convection will diminish with sundown this
evening. Cumulus will diminish as well. An upper low off shore will
spread cirrus west across the area through the period. KAVL has the
best chance of seeing any morning restrictions as valley fog
develops. Guidance showing MVFR stratocu may develop with sunrise
near KCLT. Cannot rule out a brief ceiling before clouds scatter and
lift through the morning. Have limited to SCT for now. Otherwise...
scattered CU and broken CI expected through the day. N to NW winds
diminish overnight then increase in speed as mixing begins by mid
morning then continue through the afternoon.

Outlook: Ridging will persist across the region through mid week.
Convective chances may be limited by sinking air to the west of a
tropical system at the coast. Patchy fog will be most favored in
mountain valleys.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High  98%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     Low   38%     Med   66%     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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