Area Forecast Discussion

FXUS62 KGSP 201153
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
653 AM EST Tue Feb 20 2018
High pressure off the East Coast will provide a warm, moist 
southerly flow through the middle of the week. Another cold front 
will descend from Tennessee by Thursday. This front will stall out 
and linger just north of our area next weekend. Chances for rain and 
well above average temperatures will remain through the weekend.
As of 650 AM: Visibilities have been fairly steady, with some 
downward trends within the fog this morning. The dense fog advisory 
still looks good for now thru 11 AM. There is some increase in radar 
returns, as persistent upglide flow is producing light showers 
across the Blue Ridge and adjacent foothills. I tweaked PoPs for the 
next few hours to line up with those trends. The rest of the 
forecast is in good shape.
As of 330 AM: Areas of fog persist across the foothills and piedmont 
early this morning, but dense fog remains patchy for now. I opted to 
add a sliver of zones along the eastern escarpment in NC to the 
Dense Fog Advisory due to low ceiling likely causing obscurations. 
The DFA still looks good to go thru 11 AM, by which point the fog 
should lift. Otherwise, the main story today will be how quickly and 
to what extent the lingering in situ CAD wedge erodes thru the day. 
Guidance generally agrees that it should erode much more efficiently 
than yesterday, as the inversion in the soundings is weaker and 
cloud layer/RH depth is shallow. I took a blend of the CONSRaw 
with the SuperBlend and CONSShort temps, showing temps warming into 
the 60s to lower 70s across the area today, definitely warmer than 
yesterday. It will remain mostly cloudy, however, with patchy 
sprinkles/drizzle still possible, mainly near the escarpment.
Tonight, moist SWLY flow will continue atop the CWFA between a large 
ridge to our east and deep trough to our west. There's just enough 
vestige of a cold pool across the foothills and piedmont such that 
stratus and fog will likely redevelop/expand across the area after 
sunset. Dense fog doesn't look as likely/widespread in the guidance, 
but cannot rule out another DFA needed for some portions of the area 
thru early Wednesday. Temps will remain way above normal under 
clouds and persistent LLVL WAA flow. Min temps will actually stay 
above normal afternoon highs, possibly breaking high minimums for 
Wednesday morning. As for PoPs, guidance really wants to ramp up QPF 
response within the SWLY flow, but other than some weak upglide and 
upslope flow, there isn't much to force precip. Moisture depth in 
the forecast soundings does increase somewhat, so I do allow PoPs to 
ramp up into the CHC range by daybreak Wednesday, favoring the areas 
near the escarpment. Any precip accums should be very light.
As of 310 AM EST Tuesday: A deep-layer southwesterly fetch will 
persist Wednesday through Friday night between upper high pressure 
anchored off the southeast coast and a trough over the western 
CONUS. Weak short waves ripple through the flow and move near the 
forecast area but remain mainly to our north and west. At the 
surface, a similar pattern exists with high pressure over the 
Atlantic and a cold front slowly moving southeast toward the area. A 
moist southerly upslope flow remains in place through the period. 
However, the moisture isn't very deep. Synoptic scale forcing and 
low level isentropic lift are weak as well. The upslope forcing is 
decent and this should help initiate showers from along the Blue 
Ridge north and west across the mountains. Precip chances taper off 
to the east and south where moisture and forcing diminish. There may 
also be a few thunderstorms each afternoon as weak instability does 
develop. QPF is generally light through the period, but some 
moderate amounts are possible across the southwestern mountains 
Wednesday morning. Highs and lows remain very warm with near record 
high temps and lows near record high minimums with readings above 
normal high temps. 
The complicating factor in this forecast is the afore mentioned cold 
front. Guidance agrees that the northern portion of the front will 
sag south toward the area in backdoor fashion. However, they don't 
agree on how far south it moves. The GFS and NAM suggest the front 
moves into the NW piedmont and northern foothills. However, the 
ECMWF and the Canadian show it stalling somewhere near the VA/NC 
border. The models are consistent with themselves from run to run 
but the dichotomy between solutions remains. Have retained some 
features of a backdoor front but don't mainly show a glancing blow 
across the NW Piedmont.
As of 120 AM EST Tuesday: Starting Friday morning, expecting surface 
high pressure over Maine ridging down the East Coast and pushing a 
backdoor cold front to perhaps halfway across NC. The I-77 corridor 
could be affected briefly before the high moves out over the 
Atlantic and this front retreats north as a warm front into 
Saturday. A mostly stationary front extends from the Ohio Valley to 
Texas on Friday. 
The 500mb pattern has a high centered east of the Bahamas with SW 
flow pumping moisture from the Gulf to across the SE States Friday 
into the weekend. The deep 500mb trough over the West on Friday 
begins to move east over the weekend. The Bermuda / Bahamas High 
will be moving east or suppressed south as ridge breaks down with 
this western trough moving toward the East Coast. Moisture gets 
deeper progressively but of course best along the front which will 
progress east from the Mississippi Valley Saturday. 850mb flow on 
both the GFS and EC has SW wind of 40 to 50 kts Saturday night 
crossing our area. POPs increase on Sunday as the front crosses our 
area. Instability rather low Sunday afternoon with CAPE values over 
the Carolina Piedmont around 200 or less. Shortwave energy crosses 
the Great Lakes Sunday with the frontal precip fading as it crosses 
the Carolinas. The GFS is a bit faster in taking the front through 
and EC a bit slower but each have the front out of our area by 
Monday morning. The front stalls out early next week from the 
northern Gulf Coast to northern Florida. General drying but there is 
some potential waves passing along this frontal boundary could 
affect at least southern portions of our forecast area in the first 
half of next week. 
Temps over ten degrees above normal until front passes then closer 
to normal early next week. Max temps a little cooler on Friday for 
the NC areas north of CLT due to the backdoor cold front.
At KCLT and elsewhere: Not much change in thinking from previous TAF 
discussion. LIFR to VLIFR conditions continue thru late morning, as 
a weak in-situ wedge persists. The wedge is expected to erode a 
little more today than yesterday, resulting in more improvement for 
KAVL and KCLT. The Upstate sites and KHKY will be under the low 
stratus the longest, and may not even get to MVFR thru sunset. At 
least, the fog should lift by midday. Tonight, with the air mass 
still very moist, LIFR conditions will likely return with redeveloping 
low stratus and fog after sunset. Winds will be light thru the 
period, variable within the wedge, but favoring a south direction 
Outlook: Unseasonably warm and humid conditions are expected to 
persist through the end of the week. Shower chances will be highest 
across the mtns, while periodic CIG and VSBY restrictions will be 
likely through the week, esp during the late night and morning hours.
Confidence Table...
            12-18Z        18-24Z        00-06Z        06-12Z 
KCLT       Med   66%     Med   77%     Med   65%     Med   77%     
KGSP       Med   67%     Med   71%     Med   70%     High  90%     
KAVL       Med   79%     High  82%     High  85%     High  95%     
KHKY       Med   67%     Med   66%     Med   65%     High  91%     
KGMU       Med   63%     Med   66%     Med   72%     High  90%     
KAND       Med   77%     Med   78%     Med   75%     Med   78%     
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      75 1939     26 1908     50 1951      4 2015    
   KCLT      77 2014     31 2015     59 1939      7 2015    
   KGSP      75 2014     33 2015     57 1939      9 2015    
  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      77 1986     12 1896     54 1997      8 1896    
   KCLT      75 2011     32 1896     56 1997      6 1896    
   KGSP      75 1917     35 1978     55 1997      9 1896    
  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      72 1897     27 1939     54 1897      2 1963    
   KCLT      74 1990     33 1963     62 1897     12 1963    
   KGSP      76 2011     35 1969     55 1980     10 1963    
  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      76 1982     29 1989     54 1922     10 1939    
   KCLT      76 2012     29 1901     56 1944     19 2009    
                1980                                1978    
                1962                                1963    
   KGSP      76 2012     36 1966     57 1922     15 1963    
                1996        1939                            
  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
GA...Dense Fog Advisory until 11 AM EST this morning for GAZ010-017-
NC...Dense Fog Advisory until 11 AM EST this morning for NCZ035>037-
SC...Dense Fog Advisory until 11 AM EST this morning for SCZ001>014-