Scientific Forecaster Discussion
fxus62 kgsp 231739
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
139 PM EDT Mon Oct 23 2017
a cold front will move through the area today bringing increased
chances for showers and thunderstorms, some of which could be
severe. Drier and cooler air will sweep into the region on Tuesday
leading to possible frosty conditions Wednesday and Thursday
morning. Dry conditions are expected to persist through the
remainder of the work week before another front moves in next
Near term /through tonight/...
as of 140 PM edt: Storm Prediction Center is considering a watch for portions of
the Piedmont. This is reasonable given the continued shear and
increasing surface based instability. Will update again as soon as a
decision on exact location and timing is made.
Otherwise, latest radar, Sat pix, and cam guidance are all
continuing the trend toward a faster onset and end of any
significant convection today. Looks like close to an 18z time frame
for the convective line reaching avl and gsp and a 21z time frame
for clt. Latest guidance and morning soundings are also in agreement
on the high shear Low Cape scenario for any severe convection. The
cam forecast soundings also show lfc's lowering and instability
developing in concurrence with the arrival of the convective line.
There is plenty of low level shear and helicity with the system. So
any instability would allow rotating updrafts to form creating the
potential for severe storms and possibly tornadoes. Winds have
already picked up across the mountains and Wind Advisory still looks
good. There is the potential for additional heavy rainfall as the
convection moves through. The flood threat remains isolated but an
advisory or warning cannot be ruled out, especially in the favored
upslope areas. The current forecast has all these trends in place,
so updates were mainly for current conditions.
The primary hazard requiring our focus will be the potential for
strong to severe convection this afternoon. The convection-allowing
models feature general agreement on timing, with widespread showers
breaking out this morning in upslope flow then the main line just
ahead of the cold front arriving in our southwest zones 15z to 18z
and then blasting northeast of the area around 00z. The system will
be strongly forced - with upper jet divergence and deep layer DPVA
arriving during the warmest period of the day. The low-level jet
could reach 50 to 55 kt by mid to late afternoon and surface wave
development along the frontal zone in or near the southern
Appalachians might serve to keep low level flow a bit more backed.
Thus, anticipate enhanced low-level helicity along with 50 to 60 kt
of surface to 6 km bulk shear to be realized just as the main line
arrives. Despite the lack of insolation, mid-level lapse rates will
adjust up modestly under the passing trough axis this afternoon.
SBCAPE remains reserved on the GFS and rap, but it might not matter
much given rich low level Theta-E values and strong forcing along
with robust wind fields. The most unstable sections should be
generally along and southeast of I-85 this afternoon and then along
the I-77 corridor through the early evening hours, with some 500 to
1000 j/kg of best parcel cape available during peak activity. The
upgrade to slight risk is warranted in Piedmont sections.
The second issue posed by the southerly low level jet will be very
gusty winds over the high terrain, especially over the southern
mountains this morning early this afternoon. A Wind Advisory will be
posted there for some 40 to 50 mph gradient wind gusts, especially
at the higher elevations.
Finally, The Rounds of upslope showers this morning will serve to
moisten the area and then rainfall rates will pick up through the
day in the better convection. Localized 4 to 5 inch rainfall amounts
look quite likely in the southern Escarpment areas. Urban areas
could see street flooding as well given the expected high rates.
However, dry antecedent conditions should preclude any widespread
flooding - so no flood watches are expected at this time.
Mid-level dry slotting will wrap in quickly from the southwest
behind the main convective line and front after 21z. The entire
forecast area should be on the back side of the lifting wave by late
evening, with rapidly drying conditions overnight. Cannot rule out
some continued nocturnal showers near the Tennessee border in westerly
upslope flow. Temps will cool back through the 40s in the mountains
and 50s east.
Short term /Tuesday through Wednesday night/...
as of 230 am EDT monday: the short term fcst period initializes on
Tuesday morning amidst the beginnings of a pattern shift across much
of the eastern Continental U.S.. as for northeast Georgia and the western
Carolinas, the upper trof axis looks to still be just west of the
region at fcst onset, however will quickly transition east through
the period allowing low/mid level flow to veer nwly. With that,
cold advection will commence into Tuesday evening allowing
temperatures to return to below normal levels through the remainder
of the period. Guidance favors some residual moisture behind the
front on Tuesday morning thereby warranting sustained pops along the
Tennessee line where nwly upslope flow is favored. These lingering showers
will cease into the middle part of the day on Tuesday, however only
to have another round of moisture advect into the Tennessee Valley as
shortwave energy dives sharply southward out of the upper Midwest.
Said moisture will once again bank against the western slopes of the
southern Appalachians, lifting orographically. Guidance seems to be
trending downward with regards to qpf response Wednesday morning,
thus pops have been lowered to include only minimal slight chances
along the Tennessee line. By that point thermal profiles will have cooled
enough to support possible wintry precip in the form of snow showers
at the highest elevations, if moisture is sufficient, with no
accumulation expected. Finally, given the aforementioned below
normal temperatures, chances for patchy/widespread frost in the
sheltered mtn valleys are high with some patchy frost also possible
for outlying locales across the fthills on Wednesday morning.
Chances for a more widespread frost will be highest on Thursday
morning as temperatures bottom out and winds calm, with the first
widespread freeze possible in the mtns.
Long term /Thursday through Sunday/...
as of 245 am EDT monday: the medium range fcst period initializes
on Thursday morning as the persistent 500 mb trof finally shifts east
over the western Atlantic allowing for a region of upper confluence
to settle across the southern Appalachians atop building surface
high pressure. The pattern remains rather progressive as said
surface high shifts northeast into Friday and another deep upper
trof digs out west as a series of 500 mb shortwaves deepen in both the
northern/southern streams. The pattern moves further east into
Saturday as the two shortwaves phase into a full latitude trof with
a potent surface cold front beneath marching through the MS River
Valley, while moist sely flow persists across northeast Georgia and the
western Carolinas. Some model discontinuity is present with regards
to the timing of the frontal intrusion into the fcst area at this
range, however it looks to make a run at the region either Saturday
afternoon/evening or Sunday, bringing along with it increased
chances for showers and perhaps thunderstorms. As for the fcst,
pops will remain below mentionable levels through Friday before
increasing slowly into/through Saturday morning from the west with
widespread chance levels highlighted. These pops will remain spread
about the region through Sunday as well given timing uncertainty
with the fropa. Temperatures through the period will start off well
below normal levels then gradually warm back to near normal through
Saturday, before cooling once again behind the front.
Aviation /18z Monday through Saturday/...
at kclt and elsewhere: line of convection is moving through the area
at this time. Ahead of the line is IFR cigs, VFR vsby, and gusty southeast
wind. Thunderstorms and rain and heavy showers are occurring with the line along with
variably gusty winds. Behind the line is improving conditions but
gusty SW winds. Some low level wind shear is also occurring with the line. Have
taken these conditions and timed the movement of the line east
across the County warning forecast area for the afternoon. Expect VFR with diminishing winds
this evening. Some fog is possible overnight, and have included MVFR
vsby in these locations. Any fog Burns off quickly after daybreak
with increasing west-southwest wind and VFR clouds. Kavl the exception with S
wind becoming northwest and remaining there through the rest of the period.
Outlook: drier and cooler conditions will build back into the area
behind a cold front through Thursday. Moisture will slowly return
ahead of another cold front toward next weekend.
17-23z 23-05z 05-11z 11-12z
kclt high 100% Med 72% high 100% high 100%
kgsp Med 71% high 100% high 100% Med 75%
kavl Med 78% high 100% high 100% Med 75%
khky Med 64% Med 66% high 100% Med 75%
kgmu Med 78% high 100% high 100% high 100%
kand Med 71% high 100% high 100% Med 75%
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:
Georgia...Wind Advisory until 2 PM EDT this afternoon for gaz010.
NC...Wind Advisory until 2 PM EDT this afternoon for ncz051-052-058-