Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1026 PM EDT sun Jul 27 2014
a cold front will approach the Carolinas late tonight and push east
across the region on Monday. In its wake...Canadian high pressure
will dominate the surface pattern through midweek leading to well
below normal temperatures and pleasant conditions. Precipitation
chances will ramp back up again on Thursday and Friday as Atlantic
moisture spreads over the region and low level winds become more
Near term /through Monday/...
evening update...strong/severe convective activity continues across
the southern NC zones and into the eastern upstate. Many reports have been
received of trees blown down and large hail. These storms are
organizing in a prefrontal area of steep mlvl lapse rates and have
tapped into dcape values over 1100 j/kg. There exists a high amount
of effective shear...but the storms have morphed out supercell
Mode...with loss of MLCAPE and increasing low level cin. There is another
line of cold pool convection located over northern Tennessee and this area
should gradually merge with existing activity across eastern Tennessee...then
cross the gsp County warning forecast area through the overnight. Hydro issues are becoming
a concern and will monitor overnight for possible ffg
exceedance...with ratios currently running around 40-50 percent
across the eastern smokies and the balsams. Probability of precipitation have been bumped into
the chance range across the upstate overnight with little changes
needed to the min temperature grid. Quantitative precipitation forecast amounts also have been bumped up
across the current mesoscale convective system track.
730 PM EDT update...severe storms over eastern Tennessee are having a hard time
maintaining organization and supercelluar structure as they approach
the smokies and northern mountain spine counties. Still...there is enough
instability and shear combo to help maintain strong cold pool
outflows as these storms move east across the complex terrain.
Expect downsloping flow east of the fhills to limit storm coverage
and intensity through the evening. Will count on the prefrontal trough
axis to warrant a pop increase after 08z or so. Probability of precipitation were thus adjacent up
east across the NC zones from 08z through daybreak.
430 PM EDT update...organized upstream convec remains vigorous and
severe across eastern Tennessee and southeast/rn Kentucky. Baseball size hail has been
reported in Kentucky and a few tornados warnings have been issued by mrx and
jkl. This activity is aligned a prefrontal trough with a -8 c 500 mb cold
pool...some discreet cells have also been forming...with a long
lasting tornadic supercell now moving into SW Virginia. Low level Theta/east
has increased the most across the western NC zones the past couple hours
and will anticipate increasing instability across the mountains through
the next update. Effective shear remains above 40 kts just west of
the County warning forecast area at this time...however this area of shear will move into the northern
mountains after 22z. Thus...the atmos will be primed for a continued severe
and possibly a tornado threat as the convec activity develops
into our area between 22z-23z. Not many changes were needed to the
forecast grids with minor tweaks made to sky and T/TD. Will still carry
the damage wind and lg hail mention across the northern mountains for later this
afternoon and evening.
Multiple supercells have developed along and ahead of a pre-frontal
trough across eastern Kentucky this afternoon. Expect convection to
continue to develop toward the southwest through the afternoon...
likely consolidating into an organized mesoscale convective system that will approach
western NC by early evening. With that in mind...still expect the
primary threat over our area to be damaging downburst winds...
although would not be surprised to see an isolated tornado...
especially if the pre-frontal/discrete supercell currently southeast
of jkl manages to hold together into our area. Seventy probability of precipitation will be
continued across the northern NC mountains...and have also added a mention of
damaging winds and large hail to the forecast suite in these areas.
The short term models continue to dissipate any convection east of
the mountains this evening...probably due to a developing strong westerly
downslope flow. However...strong/cold pool-driven convection often
/does not care/ about downslope flow...and some of the mesoscale
models are beginning to pick up on the potential for an mesoscale convective system to blast
through the fhills/Piedmont of NC this evening. Therefore...have
begun to step into higher probability of precipitation east of the Blue Ridge in NC this
evening. The severe weather threat is expected to wind down by late
evening...or early in the overnight...although there will be a
continued potential for convection across the western mountains through
the night...as the actual cold front approaches and west rather Stout
west/northwest upslope flow develops. With cold advection holding off until
tomorrow...and the expectation that surface winds will remain
somewhat elevated tonight...expect min temperatures to be about 5 degrees
Monday will see the onset of cold advection over the mountains...where
maxes are expected to top out a little below climatology. Meanwhile...
despite falling thicknesses...downslope warming should yield highs
very close to climatological norms east of the mountains cannot rule out
some northwest upslope flow showers across the western mountains during the
afternoon...but will continue with a pop forecast just a shade below
Short term /Monday night through Wednesday/...
as of 130 PM Sunday...northwesterly flow will prevail in the low- to
middle-levels Monday night with the axis of a deep eastern Continental U.S. Trough
extending from eastern Ontario southward into SC/GA. Cool air will continue to
filter into the region at this time as Continental high pressure
settles in across the inland south. High low level relative humidity is noted on GFS/NAM in
the cold-advective flow into the mountains...reminiscent of the cool
season. The moist layer is so shallow only the most favored upslope
areas look to have any mentionable chance of precipitation.
700 mb heights bottom out around daybreak Tuesday and increase slightly
thereafter. This creates a capping inversion that will restrict deep
convection Tuesday afternoon...but the major models variously still develop
small amounts of quantitative precipitation forecast in the forecast area. By Wednesday there is little real change in
the pattern...but the flow regime becomes very weak in the lower
troposphere and model profiles show the inversion weakening as well.
A greater proportion of guidance produces quantitative precipitation forecast in the higher
terrain...but this may be overdone given recent Post-frontal
performance. Also...given the continued slight height rises I will
currently favor the inversion holding fast. Thus no quantitative precipitation forecast is in the
forecast for Wednesday. Maximum temperatures will be near 10 degrees below climatology...
trending upward slightly from Tuesday to Wednesday. Min temperatures however will be
slightly cooler Wednesday morning with winds being lighter overnight.
Long term /Wednesday night through Sunday/...
as of 140 PM EDT Sunday...the medium range forecast picks up at 00z on
Thursday with broad and deep upper troffing in place over the
eastern Continental U.S. While an equally impressive upper ridge is in place
over the west. The long range guidance continues to suggest that we
will remain in this amplified pattern into the weekend with some
amount of deamplification likely over the weekend. Compared to
previous forecasts...its looking less certain that the northern stream
upper flow will completely cutoff the trough and allow a closed low to
develop over the weekend. The latest GFS ensemble run has about half
of the members completely cutting the trough off and half maintaining
some degree of troffing.
At the surface...Canadian high pressure will continue to slide
southeastward and weaken on Thursday as boundary layer flow remains northerly to
weak and vrb. By early Friday...the models continue to veer the flow
more from the east to NE and move a large plume of deeper layer
moisture over the County warning forecast area from the southwest. This upslope potential
coupled with upper level divergence courtesy of the upper trough and
possibly the closed upper low if it actually develops...will improve
our chances for widespread showers and ts for the rest of the medium
range. Otherwise...no significant changes were made to the
temperature forecast with values expected to remain well below normal
for late July/early August.
Aviation /03z Monday through Friday/...
at kclt...no flight restrictions outside of convective activity. For
now will count on downsloping flow to limit the coverage downstream
of current mountain mesoscale convective system activity and will go go with a tempo thunderstorms and rain after 02z
through 06z. The latest hrrr develops a decent amount of convection
associated with the pre/frontal trough sched to cross the terminal
around 08z or so. Low level winds will remain strong enough to offset a visibility
threat around daybreak. Low end wind gust are anticipated tomorrow
afternoon ahead of the main frontal passage.
Elsewhere...the NC taf sites will experience bouts of thunderstorms in the vicinity and
perhaps prevailing thunderstorms and rain this evening into the early overnight. Winds
could become quite variable and gusty with any nearby outflow.
Expect things to settle down a bit after 03z then pick back up as
prefrontal trough works into the area around 06z through 09z. No early
morning flight restrictions at any taf site as bl winds remain
fairly strong. Low end gusts should develop at all sites by the
middle of Monday afternoon.
Outlook...dry conditions will return by Monday afternoon with VFR
conditions expected to remain through middle week. Moisture will begin
returning by late in the week...with increasing chances for low
clouds and precipitation.
02-08z 08-14z 14-20z 20-02z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 86% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
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: (must be lower case)