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FXUS62 KGSP 240835

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
335 AM EST Sat Feb 24 2018

Very warm conditions will continue before a cold front slowly 
crosses the area Sunday into Monday increasing the chances of 
showers and thunderstorms. Dry and relatively cool high pressure 
will settle over the region Tuesday.


Current Sat and obs indicate a very moist llvl atmos in place across 
the region. Some stcu is developing over the wrn zones and a good fg 
potential exists east as sfc tdd/s are running btw 0-4 degrees. This 
area will be monitored for dense fg near daybreak and a possible 
SPS/NPW issuance. 

The persistent synoptic pattern finally begins to break down thru 
the near-term as a strong wrn trof is reinforced with energy diving 
south out of wrn Canada. This trof will propagate east and begin to 
weaken and displace the ST high which has been the dominant factor 
in the weather over the area the past several days. Model soundings 
show PWAT values increasing arnd 70 percent over the next 24-36 hrs 
as moisture increases thru a deepening layer this afternoon. The 00z 
3-km NAM holds on to a dry stable layer aloft more than the other op 
models and therefore has less of a chance of precip and thunder. Even 
the moister and less capped GFS only produces arnd 300 J/kg of sbCAPE 
this afternoon mainly over the higher terrain. So...PoPs will remain 
low-end except across the SW facing srn mtns where mech lift is 
aided by an increase in mlvl destabilization and difl aloft ahead of 
a cold front. An isol tstm or two is possible across the NC mtns but 
these will be short-lived and non/organized. 

Max temps will once again reach abt 20 degrees abv normal in good 
sw/ly flow with weak WAA noted in the h92-h85 layer. With high 
moisture in the low-levels and sw/ly flow maintaining a mixed sfc 
layer...mins will also be held abt 20 degrees abv normal.


As of 310 AM EST Saturday: The model guidance suggests that we
can tighten up the time of arrival of the main precip associated
with the approach and passage of a cold front on Sunday. Still
plenty of support for bringing high precip probability across the
fcst area with excellent forcing and deep moisture, but all the
guidance has the front to the west of the mtns at 12Z Sunday. That
means the eastern zones will have a very low chance at daybreak,
although it will still ramp up into the likely range by the early
afternoon. The main concern is still the possibility of severe
thunderstorms with the front on Sunday, given what should be an
active boundary well to our west through Saturday evening. The
model guidance shows excellent low level convergence with the
front approaching from the west Sunday morning, supporting the
idea that some sort of weakening QLCS might be just to the west
at daybreak, suggesting that the western mountains might have
a wind damage threat early in the day as the front arrives. The
weakening trend continues into Sunday as the front crosses the
mtns though, with sfc-based CAPE perhaps only as high as 300 J/kg,
and shear perhaps only on the order of 20-30 kt. As such, not even
the Marginal risk translates eastward into the western Carolinas
for Day 2, and at this point this seems reasonable unless/until
we see either better instability or better shear. Have included
some thunder, but the severe possibilities are too low to mention
in the HWO right now. High temps will be at least 15 degrees or
so above normal. The guidance also agrees that the frontal precip
band will push off to the east late in the day, perhaps resulting
in brief lull Sunday evening before another shot of mid/upper
forcing moves in overnight to redevelop the precip along and west
of the front. Precip prob ramps right back up to at least likely
early Monday morning in response to the excellent agreement seen
in the model guidance. The jet streak lifts out to the northeast
during the day, along with most of the vorticity, taking the
surface boundary off to the east, so a steady tapering off of
precip is still expected Monday afternoon. Those developments
should help to keep temps merely ten degrees above normal. The
upper trof axis should pass Monday evening, followed by a flat
upper ridge supporting sfc high pressure moving in from the west
to dry us out on Tuesday. Temps will be another category cooler,
but still above normal.


As of 1210 AM EST Saturday: Dry wx is still anticipated on Tuesday 
as low amplitude upper ridging builds atop the SE CONUS and sfc 
ridge axis translates from the mountains to the coast by evening.
Maximum temperatures remain progged to be about 2 categories above 
climo.  Medium model solutions are in decent enough agreement with 
respect to the sensible wx for Wednesday, with the consensus 
developing waa pcpn eastward into the cwfa.  Based on this, will 
plan on creeping chances upward to categorical in favored locales, 
and with the clouds and pcpn, a smaller diurnal temperature range 
should result, maxes topping out near the late February normal. 
There is still plenty of time for model timing differences to work 
themselves out with respect to the passage of a moderately forced 
cold front Thursday and/or Thursday night.  Even at this point, it 
is hard to believe there won't be categorical shower coverage at 
some point during this period followed by a dry punch on Friday 
within a gusty post-frontal airmass.


At KCLT and elsewhere: Another overnight period of developing stcu 
and lowering VSBY. Expect a mix of MVFR/IFR conds to develop close 
to daybreak as good rad cooling continues and moisture remains 
trapped within weak sfc/based inversions. Hires models favor the 
lower elevation piedmont area including KCLT for the higher 
h95-h85 layered moisture adv...thus have lowering CIGS/VSBY to IFR 
at KCLT and the Upstate sites. KAVL and KHKY have a better chance  
remaining MVFR/VFR. All sites will see a breakup of cloud cover this 
afternoon within good isol.    

Outlook: Moist conditions will keep chances for restrictions
elevated through the period, with the best chances for low cigs/vsby
to occur each morning.  Precipitation chances are also slightly
elevated, mainly across the high terrain.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z 
KCLT       High  97%     High  89%     High  94%     High 100%     
KGSP       High  97%     High  81%     High  94%     High 100%     
KAVL       High 100%     High 100%     High 100%     High  94%     
KHKY       High 100%     High  84%     High  94%     High  92%     
KGMU       High  98%     High  87%     High 100%     High 100%     
KAND       High  93%     High  89%     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:





  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      77 1930     26 1947     50 1985      6 1967    
   KCLT      80 1982     26 1889     56 1992     16 1967    
   KGSP      79 1982     33 1901     57 1909     15 1967    


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      78 1930     17 1967     51 1985     -2 1967    
   KCLT      82 1930     27 1967     58 1890      7 1967    
   KGSP      79 1930     30 1967     57 1985      8 1967    





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