Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
332 am EDT Tuesday Jul 22 2014
a weak upper low will move away to the west today which should allow
for a brief return to more typical Summer weather for Wednesday.
However...a front will approach and move across the region on
Thursday. The passage of the front will not bring cooler air...but
it should bring drier air for Friday and into the weekend.
Near term /through tonight/...
as of 330 am...recent west/v images indicate that the center of the middle
level low was located near the Panhandle of Florida. Deep moisture has
spread across most of the southeast u... precipitable water up to 2
inches...or 125 percent of normal. Forecast soundings show that deep
skinny cape will exist across the region today...with cape values
around 1000 j/kg. The combination of weak isentropic lift...east 850 mb
winds...and level of free convection between 3-4 kft should result scattered to numerous
showers and thunderstorms today. Storm motions will likely remain
less than 10m/S. Heavy downpours and cloud to ground lightning will
be the primary threats. Overcast to mostly cloudy sky and periods of
rain should limit heating today. High temperatures are forecast to
range from the u70s within the mountain valleys to the low 80s east.
Tonight...near term models indicate that the middle level low will
track west...allowing heights to rise across the forecast area. 850 mb
winds will veer from the SW...resulting in downslope flow across the
foothills and ending low level isentropic lift. Probability of precipitation will quickly
decrease after sunset...falling to schc late tonight. Low
temperatures will remain near normal.
Short term /Wednesday through Thursday night/...
as of 230 am EDT Tuesday...it looks like a transition of sorts will
be complete by Wednesday morning...as the weak upper low will have
retrograded far enough for the cyclonic flow to have moved west of
the forecast area. That will bring the flow around to something more SW
to west for the first part of the short range...with still plenty of
low level moisture remaining to fuel diurnal showers and
thunderstorms. Am a bit surprised with the lack of coverage in the
NAM solution for Wednesday afternoon/evening and think that the
entire forecast area will see scattered storms...so the forecast stays more
in line with the greater coverage seen on the GFS. Expect most of
the activity to weaken/end gradually with loss of heating Wednesday
night east of the mountains...but by then a cold front will approach from
the northwest which warrants keeping the chance pop overnight over the
mountains the new guidance shows a stronger push by a short wave helping
to carve out a new eastern upper trough on Thursday. This feature is
now expected to push the cold front all the way through the forecast
area Thursday afternoon and evening...but the problem now is the
timing. The NAM brings it through earlier and drives the front
farther S than the other guidance. The front appears a bit stronger
which makes ME think we are likely to see showers and storms over
most of the forecast area...however...uncertainty about when the most
likely time of the storms will be actually results in a slight
decrease in the precipitation probability for the day as a whole. Once the
timing is more certain...expect this to creep back toward the likely
category. The precipitation chances should slowly diminish and push off to
the S through early Friday morning. Temperatures will also cool down a bit
Thursday night as dry air comes in behind the front.
Long term /Friday through Monday/...
as of 315 am Tuesday...the new guidance for the start of the medium
range confirms that the surface front will probably be driven south
of the forecast area by Friday morning giving US a cleaner frontal
passage than earlier thought. With this trend established in the
guidance...the precipitation chances were lowered initially on Friday.
However...it is hard to rule out some shower activity behind the
front on Friday afternoon as there does not appear to be enough to
suppress deep convection...so a slight chance was kept over most of
the forecast area per the mosguide...with a chance on the southeast fringe.
Temperatures will not drop much at all behind the front...but hopefully
there will be enough drop in the dewpoint to make the relative humidity fall. No
other changes were made to the medium range. The GFS suggests some
mesoscale convective system-like activity moving in from the west on Sunday but confidence
in that is low. There is decent agreement between the GFS and European model (ecmwf)
with the passage of another front next Monday...so we will continue
to feature the highest precipitation chances in the medium range on that
Aviation /08z Tuesday through Saturday/...
at kclt...latest observation indicated MVFR clouds across the
terminal. Recent tclt scans show a patch of light reflectivity
across Cabarrus drifting west...with another area of light rain
lifting northwest across the midlands. Guidance and trends indicate
that IFR ceilings will develop between 6z to 8z. Once the low
ceiling arrive...conditions should remain until middle morning.
However...MVFR ceilings will be slow to improve under widespread
high cloud cover. Based on favored guidance...VFR clouds are
expected by 19z. I will maintain a prob30 from 19z-24z for MVFR
conditions during thunderstorms and rain.
Elsewhere...the initial group of the 6z tafs will range from VFR at
hky to MVFR elsewhere across the western Carolinas. At 2 am...recent
images indicate an expansion of light reflectivity across the
upstate...with another band lifting northwest across the midlands. The
develop of the -ra will likely signal the development of IFR
ceilings...expected by 9z. Once the low ceiling arrive and
develop...conditions should remain until middle morning. However...MVFR
ceilings will be slow to improve under widespread high cloud cover.
Based on favored guidance...VFR clouds are expected by middle day. I will
maintain a prob30 from 18z-24z for MVFR conditions during thunderstorms and rain.
Outlook...a gradual return to a more typical pattern with diurnal
showers and storms is expected Wednesday. A frontal system
approaching from the northwest will cross the region Thursday
night...bringing stronger convection and possible flight
restrictions through Friday.
07-13z 13-19z 19-01z 01-07z
kclt medium 79% high 88% high 96% high 100%
kgsp high 87% high 89% high 96% high 93%
kavl medium 63% high 85% high 83% high 86%
khky medium 70% high 87% high 93% high 100%
kgmu high 87% high 89% high 96% high 98%
kand high 80% high 85% high 97% high 93%
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: (must be lower case)