Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS62 KGSP 230905
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
505 AM EDT Mon Oct 23 2017

.SYNOPSIS...
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 
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 500 AM EDT: Minor adjustments were made in the latest 
update to hasten the coverage of widespread showers this morning. 
Otherwise, the forecast remains on track as water vapor imagery 
shows the phasing closed low center lifting northeast over the 
middle MS River Valley this morning, with the deep moisture plume 
ahead of the system moving rapidly eastward across GA. Surface high 
pressure continues to retreat offshore, with just a lingering nose 
of the ridge stubbornly holding on across the western Carolinas. 
Very little thermal or moisture contrast is evident now under the 
surface high, so any shallow, in-situ cold air damming should erode 
very quickly today and permit gusty south to southeast winds to 
develop. Dewpoints have already recovered to mid 60s values across 
most of the southern tier, and the improving southerly component to 
the flow should permit mid to upper 60s values across most of the 
area east of the mountains by midday. 

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 TN 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 CONUS.  As for northeast GA 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 
TN 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 TN 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 TN 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 H5 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 H5 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 GA 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 /09Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Showers are breaking out rapidly across 
the region this morning as Atlantic and Gulf moisture begins pouring 
in ahead of the rapidly lifting wave and associated frontal zone to 
the west. Expect a quick transition down into IFR and LIFR 
conditions as the rain fills in through the morning hours, with 
thunder chances then picking up from the southwest late this morning 
through the afternoon hours. Gradient winds will increase and become 
gusty from the southeast through the day, and LLWS conditions cannot 
be ruled out from KHKY to near KCLT this afternoon as the southerly 
low level jet rapidly increases to 50 to 60 kt. Will generally time 
thunder from 17Z to 21Z over the upstate TAFs and KAVL, and run TS 
chances later through 00Z from KHKY to KCLT. Any recovery to MVFR at 
times through the day should be short-lived, but rapid clearing and 
drying is expected with the fropa through the evening hours. 

Outlook:  Drier and cooler conditions will build back into the area 
behind a cold front Tuesday through Thursday. Moisture will slowly 
return ahead of another cold front toward next weekend.

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z 
KCLT       High  81%     High  92%     High  81%     High 100%     
KGSP       High  87%     High  80%     High  88%     High 100%     
KAVL       Med   78%     High  80%     Low   54%     High 100%     
KHKY       High  87%     High  85%     Med   76%     High 100%     
KGMU       High  89%     High  81%     High  90%     High 100%     
KAND       High  84%     Med   78%     High  98%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Wind Advisory until 2 PM EDT this afternoon for GAZ010.
NC...Wind Advisory until 2 PM EDT this afternoon for NCZ051-052-058-
     059-062>064.
SC...None.

&&

$$

National Weather Service Glossary of Abbreviations