Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
122 am EDT Sat Oct 10 2015
a cold front will cross the mountains tonight. Low pressure is
forecast to form along the front early Saturday over Georgia or
South Carolina. This front will move off the Carolina coast Saturday
night with high pressure in control Sunday and Monday. Another
cold front arrives Monday night and reaches the Carolina coast
Near term /through today/...
0515 UTC update...probability of precipitation and sky cover were updated based on satellite
and radar trends...with increases early on in the mountains...upper
savannha River Valley...and SC Piedmont. Winds were updated with a
blend of the latest NAM and adjmav.
As of 1030 PM EDT Friday...as expected...frontal convection has
weakened substantially as it encountered the terrain...and a more
stable airmass over western NC. In addition...starting to see
some reinitiation of showers over the northern NC Piedmont where
the front is sliding in from the north. All in all...forecast looks
good and thus no sig changes needed at this time.
As of 215 PM...surface front extends from low pressure over the
St Lawrence Valley southwest into the lower Ohio Valley. Moist
southerly flow continues across our County warning forecast area ahead of this front...which
will push across the southern Appalachians tonight and lay out in an
east-west fashion across the Carolinas. This will allow Canadian
high pressure to build in from the north tomorrow. The initial
impact of the front in our area will be to drive convection into
this evening. Scattered cells are beginning to develop over the
Cumberland Plateau just ahead of the front. Sufficient shear is
present that I expect these cells will be able to develop into
a linear feature which will make a run at western NC. Seasonably
modest cape values of 500 to 1000 j/kg are seen in that area on
the Storm Prediction Center mesoanalysis...which also shows our mountains in a relatively
stable pocket likely due to the cloud cover which has been there
through the day. That will be the limiting factor for today/S tstorm
threats. Indeed mesoscale guidance suggests the coverage of convection
remains quite isolated until the front enters the area. Hrrr
trends have been to fizzle the east Tennessee cells as front crosses the
mountains...and redevelop activity over the NC foothills. Have delayed
most of the thunder chances until tonight as a result. Min temperatures
should be above climatology under cloud cover.
Overnight the building high...diminishing instability...and eastward
progression of a shortwave atop the front suggests a transition to
dynamically forced precipitation. Low level forcing is not exciting...with
only weak upglide developing over the cool side of the front as a
surface wave develops along it. Some convection could continue mainly
over GA/SC. This regime lasts through the rest of the period...with
latest round of guidance again having slowed down their forecasts of
the departure of the surface/upper wave. A consensus of MOS values
was used for maximum temperatures...with raw values looking perhaps a bit
too cold. Nonetheless they will be below climatology.
Precipitable water values...while not exceptional...will support
higher than normal rainfall rates. However the setup for heavy
rain is not nearly as ideal as it was in The Wedge events of
the previous couple of weeks. Best rates will likely be on the
warm side of the front tomorrow where destabilization...and thus
convective enhancement...are most likely to occur.
Short term /tonight through Monday/...
as of 230 PM EDT Friday...still hoping to salvage the back half of the
weekend...as the GFS/NAM drop the cold front S of the area Saturday
evening and move the upper system to our east by sunrise on Sunday.
Assuming that timing does not slip...we could still have some
wraparound moisture during the day...but all the forcing should be
well to the east. Not really buying the NAM solution of developing
Ridgetop convection Sunday afternoon...as the atmosphere should be
fairly inhospitable to deep convecton with dnva aloft and a
subsidence inversion roughly between 750mb and 850mb. The forecast will
be kept dry. From that point Onward...the weather remains benign and
fair across the region...with upper ridging stuck between the old
upper low off the East Coast and the next wave digging down across the
plains. That should support weak high pressure across the area from
Sunday night through Monday...which should keep temperatures below normal
on Sunday and near normal on Monday.
Long term /Monday night through Friday/...
as of 300 PM EDT Friday...main issue for the long term is the
passage of a frontal system on Tuesday. GFS/ec are in fairly good
agreement with the frontal passage timing being sometime early
Tuesday morning along with a coincident period of light to moderate
rain showers. Amounts of total precipitation with passage of this system
are expected to be light in the trace to 0.25 inch range due to a
lack of antecedent in-land moisture transport. Scant moisture also
keeps the boundary layer convectively stable with only an outside
chance for any thunder. Models also agree that the frontal passage
on Tuesday is followed by the area being on the west and dry side of
a broad long wave upper trough with low chances of precipitation Wednesday
through Friday and temperatures near seasonal normal.
Aviation /06z Saturday through Wednesday/...
at kclt...guidance favors lowering ceilings and visibility as a cold front and
associated moisture move into the area. Moisture persisting behind
the front will support continued restrictions through the day and
into Saturday night. Winds will favor the NE behind the front...with
modest gusts. Precipitation chances ramp up toward midday...then
slowly decrease as the front and an associated upper system drift
off to the east.
Elsewhere...fairly good agreement exists in models data...supporting
lowering ceilings and visibilities as moisture increases with an arriving
front...then persists behind the front. Winds will favor the NE in
the foothills...and the north-northwest at kavl. Precipitation chances will
initially favor the mountains...then the foothills by morning...with
a gradual decrease thereafter.
Outlook...dry weather is expected for Sunday and Monday...with
another weak front arriving from the west on Tuesday...and more
drying on Wednesday.
05-11z 11-17z 17-23z 23-00z
kclt high 100% medium 61% medium 66% medium 78%
kgsp high 100% medium 61% medium 78% high 85%
kavl high 100% high 83% medium 61% high 85%
khky high 100% medium 61% medium 66% medium 78%
kgmu high 100% medium 66% medium 72% high 100%
kand high 100% medium 72% 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: (must be lower case)