Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS62 KGSP 201034

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
634 AM EDT Thu Jul 20 2017

An upper level ridge will build over our region from the west 
through the end of the work week bringing hot temperatures and 
isolated showers and thunderstorms each day. A broad upper level 
trough develops over the weekend and into early next week and 
is expected to bring cooler temperatures.


As of 630 am: The flow aloft will become increasingly anticyclonic, 
as ridge associated with expansive subtropical high over the central 
Conus continues to nudge its way into the forecast area. Resultant 
warm mid-level temps of -5 to -6C will yield very poor mid-level 
lapse rates, but forecast soundings are generally not capped, and 
still reveal some modest buoyancy, especially across the southeast 
third or so of the forecast area. Convection-allowing models depict 
a smattering of small convective cells across the southern Piedmont, 
and to a lesser extent across the high terrain. The concern with 
that is that the CAMs have been too reserved with diurnal convection 
for the past 2-3 days, especially over the Piedmont. 

Instability is such that I would expect isolated convection to fire 
along the buoyancy gradient that is forecast to set up between the 
Midlands and the northern Piedmont this afternoon (the HRRR features 
this to some extent), so 20 pops will be advertised roughly along 
the Hwy 72 corridor in GA/SC. The environment is expected to become 
increasingly hostile to convective development as one travels 
farther northwest across the area, but again, forecast soundings 
across the mtns are uncapped and feature sbCAPE in the 1000-1500 
J/kg range. Even with such meager ingredients, it's hard not to get 
at least isolated convection across the southern (i.e., srn NC/GA/ 
SC) mtns in late July, so a 20 pop will also be forecast from the 
Balsams southwest into our GA mtn zones. Considering the forecast 
degree of instability and downdraft CAPE, would not be surprised to 
see a microburst or two across the Piedmont. Another two degree-ish 
increase in max temps is expected this afternoon, with the vast 
majority of the Piedmont and foothills expected to reach the 
mid-90s. Nevertheless, dewpoints mixing out in the mid-to-upper 60s 
should keep heat indices contained within the 95-100 range this 
afternoon. Min temps tonight will average a couple of degrees above 


As of 310 AM EDT Thursday: the short-term fcst picks up at 12z on 
Friday with very broad upper ridging in place over most of the CONUS 
and a weak H5 low, that has separated from the mean flow, just to our 
south. The low proceeds to drift SW towards the Gulf of Mexico and 
by early Sat, what's left of the low is centered over the northern 
gulf coast with flat upper ridging in place over the fcst area. By 
early Sun, another shortwave upper trof begins to dive down over the 
Western Great Lakes as the upper ridge amplifies over the western
CONUS. At the sfc, the Bermuda High will be in place offshore with
warm SLY flow over the region. The overall synoptic pattern is not 
expected to change much thru the period with weak high pressure 
persisting to our north into Sat and some degree of weak lee trofing 
expected over the CWFA each day. By the end of the period early Sun, 
a low will deepen over the Great Lakes and its associated cold front
will begin to approach the fcst area. As for the sensible fcst, no
significant changes were needed with near climo PoPs for Fri and
slightly above climo PoPs for Sat where lapse rates should be greater.       
Temps will be well above normal thru the period with heat index 
values reaching triple digits over the lower elevations each day,
however values are not expected to reach heat advisory criteria.


As of 245 AM EDT Thursday: the extended fcst picks up at 12z on 
Sunday with flat upper ridging over the southeast and an upper trof 
beginning to amplify and dig down over the Great Lakes. The long 
range models move the trof axis to our north by late Monday or early 
Tuesday depending on which model you believe. Most of the energy 
associated with the system is expected to remain to our north as 
height falls over the fcst area are minimal. The trof begins to 
move off the New England Coast by mid-week as broad upper ridging 
persists to our west. The ECMWF remains more progressive with the 
pattern compared to the GFS or Canadian models. At the sfc, the 
Bermuda High will be in place to our east as the models deepen a 
low over the Eastern Great Lakes and move its associated cold front 
towards the fcst area. For the rest of the period, the sfc pattern
is looking more ambiguous and complex than previously depicted. 
The models no longer show a clean fropa on Mon or Tues, but instead
have the front stall out over the fcst area and keep deep-lyr 
moisture over the CWFA well into Wed. There also appears to be 
less drying behind the front than previously anticipated, with 
the ECMWF keeping deep moisture over the CWFA into Thurs. As for 
the sensible fcst, above climo PoPs persist thru the period as 
the front impacts the CWFA with Mon and Tues still having the
best chances for widespread convection. Temps will start out a 
good 4 to 6 degrees above climatology on Sunday and then steadily 
cool thru the rest of the period. By early next week, they are 
fcst to be back near normal.


At KCLT and elsewhere: Fog/low stratus remain possible this morning 
at KAVL, but anything that does develop should dissipate there by 
13Z. Otherwise, VFR is expected to persist at the terminals through 
the period. Light/vrbl or light W/NW winds are expected through the 
morning, becoming light SW during the afternoon. Isolated convection 
is expected to develop mainly south of the terminals this afternoon. 
I can't completely rule out something straying near KCLT this 
afternoon/evening, but the probability still appears too low to 
include in the TAF at this point. However, this will need some 
further investigation for the 15Z update.   

Outlook: Drier air will likely continue to limit convective coverage 
and the extent of low clouds and fog through Friday. Chances for 
diurnal convection begin a steady increase on Saturday, with this 
trend continuing into early next week ahead of an approaching cold 
front on Monday.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z 
KCLT       High 100%     High 100%     High 100%     High 100%     
KGSP       High 100%     High 100%     High 100%     High 100%     
KAVL       Med   61%     High 100%     High 100%     High 100%     
KHKY       High  91%     High 100%     High 100%     High 100%     
KGMU       High 100%     High 100%     High 100%     High 100%     
KAND       High  83%     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:





National Weather Service Glossary of Abbreviations