Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180732

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
332 AM EDT Tue Jul 18 2017

A stalled frontal boundary will slowly dissipate over the next
few days as a strong upper ridge gradually builds into the region.
This will bring hot temperatures and a reduction in the coverage
of diurnal showers and thunderstorms.


The latest water vapor imagery depicts the axis of the upper trough
currently bisecting the forecast area. This feature is forecast to
push east of the forecast area by afternoon, likely closing off in a
weak cyclonic circulation as it does. Therefore, an extended period
of steadily rising heights is beginning. With the forecast area
becoming increasingly placed on the "wrong" side of the trough, and
weakening mid-level lapse rates resulting from the downstream
progression of the mid-level thermal trough, conditions are expected
to be a bit less favorable for convection this afternoon/evening. In
fact, the latest convection-allowing model guidance generally
depicts paltry coverage of diurnal convection, although it
should be said that the CAMs have been too conservative with the
coverage the past two days. That being the case, opted to advertise
more or less climo pops this afternoon, generally 30-50% across the
high terrain (the highest amounts favoring the high terrain near the
Blue Ridge), to 20-40 percent across the foothills and Piedmont, as
steering currents should take convection and attendant outflows SE
from the Blue Ridge late this afternoon into the evening. SBCAPE
should be adequate for a few strong updrafts, while mid-level drying
should yield strong dCAPE. Thus, at least a couple of pulse severe/
microburst-producing cells appear likely. Convection should diminish
fairly quickly after early evening. Temps are expected to run right
around normal through the period, albeit a degree or two warmer than
on Monday.


As of 310 AM EDT Tuesday: the short-term fcst picks up at 12z on
Wednesday with weak upper trofing over the East Coast and very broad
upper ridging in place over the rest of the CONUS. The latest model
guidance now has the upper trof linger over the coast thru Thursday
before becoming separated from the mean flow and spinning off a
weak H5 low early Friday to end the period. At the sfc, broad high
pressure will be centered well to our north with the Bermuda High
in place offshore. The overall synoptic pattern is not expected to
change much thru the short-term period with weak high pressure
persisting to our north and relatively steep low-lvl lapse rates
remaining over the fcst area each day. I kept the near climo PoPs
for Wednesday with Thursday expected to be drier as we`ll likely
see more of a downslope component across the CWFA. Temps will start
out just above normal and warm another 2 to 4 degrees on Thursday
as thicknesses increase.


As of 225 AM EDT Tuesday: the extended fcst picks up at 12z on
Friday with very broad upper ridging over most of the CONUS and
what`s left of a weak H5 low that tries to spin off from the upper
trof that was lingering over the Atlantic Coast the previous day.
The long range models now have the weak H5 low separating from
the main flow as it drifts SW and along the northern Gulf Coast
on Fri and Sat. At the same time, the ridge flattens as upper
trofing amplifies over Eastern Canada. By early next week, heights
will start to lower again as the upper trof digs farther southward
and the ridge amplifies again to our west. At the sfc, the Bermuda
High will be in place to our east keeping light SLY flow over the
region thru most of the period. The models also keep some degree
of weak lee trofing over the fcst area on Fri and into the weekend,
although the GFS is less bullish on the lee trof persisting beyond
Fri compared to the ECMWF. The ECMWF also develops a fairly strong
low over the Great Lakes on Sunday and moves its associated cold
front thru the CWFA on Monday, while the GFS is considerably weaker
with the low and the front appears to stall out just to our NW. As
for the sensible fcst, we can expect diurnally-driven climo PoPs
for Fri and Sat with higher PoPs for Sunday and Monday as the front
could impact the CWFA. Temps will be a good 6 to 8 degrees above
climatology with highs approaching 100 degrees on Fri and Sat and
heat index values approaching advisory criteria.


At KCLT and elsewhere: Development of fog/low stratus this morning
appears to be more likely than it was Monday morning, while timing
should be at least an hour or two earlier than on Monday. The best
chances for restrictions will be at KAVL and KHKY, where heavy rain
fell Monday pm, and dewpoint depressions were observed at 0-1
degree as of TAF issuance time. Categorical LIFR conditions will be
advertised at those two sites by 10Z, with tempos for IFR/LIFR for a
couple of hours prior. Otherwise, KCLT is expected to see at least
periods of IFR cigs, with perhaps some reduced visby btw 10-13Z.
Restrictions are a bit more uncertain at the Upstate SC terminals,
if for no other reason than the lack of rainfall Monday, and have
opted to include tempos for MVFR and/or SCT IFR/LIFR layers btw
11-13Z. Otherwise, winds will remain light at less than 5 kts
through the period, likely favoring a SW direction by Tue afternoon.
Isolated/widely scattered convection is expected to develop Tue
afternoon, although coverage is anticipated to be a bit less than in
previous days. Nevertheless, a prob30 for TSRA is warranted at all
terminals during the afternoon and early evening.

Outlook: Scattered to afternoon and evening thunderstorms are
expected into Wednesday across the area, with better coverage
over the mountains. Morning fog development can be expected
in the mountain valleys and in areas that receive rainfall the
previous afternoon/evening. Drier air will likely limit convective
coverage for Thursday, with typical mid-summer conditions returning
by Saturday.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       Med   75%     High  91%     High 100%     High 100%
KGSP       High  82%     High  96%     High 100%     High 100%
KAVL       Med   64%     High 100%     High 100%     High 100%
KHKY       Med   71%     High  96%     High 100%     High 100%
KGMU       High  85%     High  96%     High 100%     High 100%
KAND       High  86%     High  96%     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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