Scientific Forecaster Discussion
fxus62 kgsp 190656
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
256 am EDT Mon Aug 19 2019
a trough of low pressure and elevated moisture and instability will
persist over the western Carolinas and northeast Georgia today
through Wednesday. A cold front will approach the region from the
northwest on Thursday and then stall in the vicinity on Friday, with
abundant moisture remaining in place. The front will gradually
settle just south of the area through the weekend as high pressure
builds over to the north. Cooler temperatures will develop but
moisture will linger.
Near term /through tonight/...
as of 250 am: there will be a couple of vort maxes that will meander
over the Carolinas and Georgia today between two ridges of high pressure
at 500 mb---one over the Southern Plains and the other over the
Atlantic. They will not provide much upper forcing due to their slow
motions. However, guidance is in good agreement on a pocket of
slightly cooler temps within this weakness aloft, which should
result in strong destabilization this aftn across the region. With
SBCAPE in the 2000-3000 j/kg range today and plenty of mid-lvl dry
air along with 0-3 km shear less than 10 kts, the atmosphere will be
supportive of microbursts. So I expect scattered storms to develop,
starting in the high terrain due to typical mountain-top
differential heating, then expand east with help from lingering
outflow boundaries. Pops will feature mainly mid to high-end chc
this aftn into early evening. Temps will be similar to yesterday's
readings, which are a couple categories above normal. Dewpts in the
upper 60s to lower 70s will result in heat index values in the 95-
100 deg range, with a few spots slightly over 100 across the
Convection should steadily wane mid to late evening, but a few
isolated showers could linger into the overnight. A weak low over
southern Georgia may start to drift slowly north overnight, bringing a
very light moist upslope southeasterly flow atop the forecast area. This flow may also
help some stratus to develop and expand across much of the Piedmont
to the Escarpment. The clouds will hold min temps slightly above
Short term /Tuesday through Wednesday night/...
as of 2 am EDT monday: a weak 500 mb trough axis will persist atop
the forecast area on Tuesday between offshore Bermuda high pressure
and stronger higher pressure centered over the Southern Plains. The
best plume of above climo precipitable water will stretch across
Piedmont areas. Moisture filling in at mid-levels, slightly weaker
lapse rates, and more clouds keeping temperatures in check should
reduce the severe weather potential a touch below Monday values.
The weakness in the height field will persist on Wednesday. Moisture
will continue to increase in profiles, but lapse rates will start to
steepen back up a bit to permit better afternoon instability and a
modest severe thunderstorm/heavy rain threat. Confidence, however,
is low on this given the uncertain forcing.
Long term /Thursday through Sunday/...
as of 230 am EDT monday: more prominent height falls will develop
from the northwest on Thursday. Deep moisture and better forcing
should arrive during peak heating Thursday afternoon and evening as
a surface cold front begins to approach from the northwest.
Anticipate an uptick in pop, with high end likely thunderstorm
coverage in the northwest half by late Thursday, and solid scattered
coverage southeast. Wind shear will be limited despite the falling
heights, and moistening profiles will make the instability, and
associated severe thunderstorm threat, a bit uncertain.
The frontal boundary will slow or stall across the region on Friday,
with a very moist axis of precipitable water values at or above two inches draped
over the region. Showers and thunderstorms will become increasingly
widespread given the moisture, forcing, and instability. With high
pressure building to the north of the area into the weekend, cold
air damming conditions may set up across the western Carolinas east
of the Blue Ridge mountains by Saturday as the boundary slowly
settles southward. Moist cold air damming could persist into Sunday
as the boundary moves just south of the area. Anticipate smaller
diurnal ranges on temperatures over the weekend with the cold air
damming in place.
Aviation /07z Monday through Friday/...
at kclt and elsewhere: mostly clear skies overnight with light
winds and high dewpts should result in Mountain Valley fog
development again thru daybreak. Will go with a 4-hr tempo at kavl,
since conditions are similar to last night. Elsewhere, patches of
saturated ground following heavy rain earlier today may result in a
few areas of reduced vsby thru the pre-dawn hours, though it is
difficult to say how much. Later today, it will be another
seasonably unstable day. Though a weak upper low will begin to take
shape over the region, there is little forcing aside from the
diurnal heating, so prob30s are maintained at all sites for aftn
thunderstorms and rain and associated vsby restrictions.
Outlook: moisture and instability will gradually increase through
the week, resulting in a steady uptick of afternoon/evening
convective coverage each day, especially mid to late week as a
cold front approaches from the northwest. Morning fog and low
stratus will be possible each day across the mountain valleys.
07-13z 13-19z 19-01z 01-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl Med 75% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 96%
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: