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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Monday
• 
• High: 81 °
• Low: 64 °
• T-Storms

• Tuesday
• 
• High: 81 °
• Low: 66 °
• Chance of T-Storms

• Wednesday
• 
• High: 82 °
• Low: 64 °
• Chance of T-Storms

• Thursday
• 
• High: 84 °
• Low: 59 °
• Chance of T-Storms

• Friday
• 
• High: 79 °
• Low: 52 °
• Partly Cloudy

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
121 am EDT Monday may 20 2013 


Synopsis... 
a weak upper level trough will swing east over the region through 
Monday. The forecast area will remain in a moist and rather unstable 
airmass...before a cold front crosses through late in the week. 


&& 


Near term /through today/... 
0515 UTC update...probability of precipitation were adjusted based on radar trends... 
favoring the areas along and east of Interstate 77. Sky cover was 
updated from infrared satellite imagery...capturing breaks over SC. 
Visibility was updated from observations...resulting in a few areas 
of fog. Winds were updated with a blend of the NAM and ajmav... 
favoring the later slightly to capture a little more winds in the 
mountains. 


As of 1000 PM...overall...guidance has been overdone with convection 
this evening. There is only a few showers and an isolated thunderstorm or two 
remaining along an old outflow boundary at this time. There is about 1000 j/kg 
of cape on laps in vicinity of the boundary. And there is a remnant mesoscale convective vortex crossing 
the County warning forecast area at this time. So probably will see some lingering convection for a 
few more hours into the night...mainly NC Piedmont and eastern upstate. 
So I have tried to match up pop trends accordingly. Basically 
cutting back substantially most places. A fair amount of clouds 
overnight...and valley fog. Temperatures will probably only drop a few more 
degree or hold steady through the night. 


As of 800 PM...only had time to address aviation grids with this 
update. However...convection is waning quickly with loss of heating. 
Interestingly...still seeing 1000-1500 j/kg of cape on rap/laps across 
portions of the upstate and southwestern NC Piedmont...with outflow 
boundaries working through the zone. However...convection is just not 
able to get going. Will be adjusting probability of precipitation downward for the 
evening/overnight shortly with another update. 


As of 515 PM...so far the convective cirrus shield...which 
encompasses 75% of the County warning forecast area...is limiting convective initiation for 
the most part. The northern NC mountains are being worked over...while 
convection seems to be losing some steam along a confluence axis 
along I-77 corridor. The 21z laps cape still shows 1500-2000 j/kg to 
the west of the axis...from Caldwell County south through the eastern 
upstate. Sure enough...cells have been back-building into this zone. 
The 18z NAM lines up well with having the best convective coverage 
in this zone as well...with an mesoscale convective vortex crossing the area...I think 
convection will continue in these areas. Outside of this...there is 
not much instability...so have cut back probability of precipitation for this evening...generally 
along and west of I-26. The main threat still appears to be 
localized heavy/excessive rainfall. A non-zero severe hail/wind 
threat still exists as well. 


Previous discussion... 
convective coverage continues to increase as expected across the NC 
Piedmont this afternoon within low level confluence axis acting upon 
a moderately unstable/very moist air mass. Farther west...expansive 
cirrus shield associated with tenn valley convection...as well as 
outflow from this morning/S convection has largely stabilized the 
atmosphere across the upper Savannah River valley. Meanwhile...tenn valley 
convection has mostly been struggling to make its way into the 
southern Appalachians...as it is being fueled by the very unstable 
air to its west/southwest. However...scattered convection has begun 
initiating over the higher terrain...likely assisted by outflow 
boundaries as well as differential heating across the edge of the 
cirrus shield. Will continue to feature likely/Cat probability of precipitation through this 
evening across western areas. Locally heavy/perhaps excessive 
rainfall will be the primary threat...but some pulse severe storms 
cannot be ruled out. The primary threat area for heavy rain may 
prove to be the I-77 corridor...as persistent confluent zone could 
support repeated development/training of cells. 


As the weakening upper trough axis continues to slowly push east 
across the area this evening...drier middle-level air will begin to 
advect into western zones...which should allow the focus for 
convection to shift to eastern areas. An overall downward trend in 
coverage is expected overnight...especially across the eastern 
zones. 


For Monday...drier air should spell more clearing/better insolation 
and therefore improved instability by afternoon...especially across the 
west. As a result...convection should fire across the higher terrain 
during the afternoon. Although coverage should be less than today... 
improved instability should lead to more vigorous updrafts...and a 
greater threat for isolated pulse severe storms. Temperatures will be above 
normal through the period. 


&& 


Short term /tonight through Wednesday/... 
surface/upper ridge remains fixed over the western Carolinas and NE Georgia 
Monday night and Tuesday keeping the region devoid of any significant 
forcing...with weak winds throughout the column. Though the thermal 
gradient is quite weak in the low-middle levels there is some slight warm air advection 
Monday night...which along with subsidence under the ridge looks to 
produce enough capping to preclude any overnight convection. 
Soundings do remain somewhat favorable into Monday evening mainly in the 
eastern zones...but all activity should diminish by midnight or so. 
Skies should clear enough to allow for quick heating on Tuesday and 
breakout of convection. Flow will be very weak through most of the 
cloud layer and storms will be slow moving...and surface temperatures should 
support SBCAPE of 1500-2000 j/kg per both NAM and GFS forecast soundings. 
Heavy rain possibly causing localized flooding will be a concern...as 
will pulse severe storm modes primarily producing hail due to the 
instability. Soundings are only modestly dry...but dcape of 200-700 j 
indicative of some damaging wind risk. Activity should have 
difficulty organizing enough to persist past sunset so probability of precipitation again 
diminish Tuesday night. Highs Tuesday will be quite warm with weak southerly 
flow at the surface and initially mostly sunny skies allowing temperatures to 
rise a few degrees above normal...into the middle to upper 80s across 
much of the area. 


Convective cirrus debris may remain over much of the area 
overnight...with increasing upper moisture ahead of central US trough 
introducing more high cloud cover toward daybreak and continuing into 
Wednesday. This should limit heating and instability making Tuesday of more 
interest for significant storms...with the additional middle-upper 
moisture arriving ahead of the incoming trough also making severe 
less likely. The trough works into the west by afternoon perhaps providing 
some additional lift. With instability already present there have 
included likely probability of precipitation near the tenn border and chance elsewhere...with 
the lklys waning with the instability toward sunset. Temperatures will be a 
bit cooler Wednesday with high cloudiness and slightly lower thicknesses 
having an impact. 


&& 


Long term /Wednesday night through Sunday/... 
as of midday Sunday...GFS/ec have come into better agreement 
regarding middle to late week pattern. Midwest cyclone shifts into the 
NE Continental U.S....dragging a cold front across the Ohio/Tennessee valleys which 
weakly pushes across our County warning forecast area Friday. Eastern trough develops as the 
front pushes south. GFS has come more in line with the ec solution 
which was also shown on previous runs...and GFS ensemble members are 
spread somewhat evenly around the GFS/ec consensus. Most of the upper 
energy associated with the Midwest system remains well to our north 
though some trailing vorticity moves across Wednesday night/Thursday concurrent with 
some very weak low level warm air advection. This may allow some elevated convection Wednesday 
night...in fact that is probably the best case for probability of precipitation during the 
event with relatively dry columns shown on forecast soundings. Fairly 
steady probability of precipitation continue until frontal passage Friday. Largely dry forecast Friday night. 


Over the weekend position of surface high and upper trough are indicative 
of cad...especially on the GFS which also develops a weak subtropical 
system off the southeast coast enhancing the low level Ely flow in The Wedge. Cad 
looks to be low impact being accompanied by little moisture. 
Resulting temperatures are near to just below normal for the weekend. Some 
diurnal convection possible on Saturday with slightly unstable 
profiles seen mainly in the mountains...but by Sat night subsidence 
inversion will inhibit further convection...leaving dry and mclr 
conditions for sun. 


&& 


Aviation /06z Monday through Friday/... 
at kclt...shower activity around the field is expected to diminish 
overnight as instability decreases. LIFR ceiling and MVFR visibility should 
persist...with visibility possibly dipping to IFR around dawn. VFR may be 
slow to return after dawn...but with more breaks in clouds expected 
Monday...heating will eventually erode restrictions. Afternoon 
convective chances look better Monday with greater instability... 
even though the precipitation coverage will be lower as the upper 
system drifts east. Light south winds will prevail. 


Elsewhere...only khky faces the potential for overnight showers as 
instability decreases. Guidance supports MVFR fog at all sites but 
kavl...where IFR is supported...and LIFR ceilings at all sites before 
dawn. Guidance is slow to erode restrictions...but with the upper 
system drifting east...better heating should result in erosion of 
restrictions by late morning. More breaks in clouds should result in 
greater instability...and better afternoon convective chances. Light 
and variable winds will become southerly during the day in the 
foothills...and and briefly northerly in the mountains before 
becoming southerly. 


Outlook...scattered showers and thunderstorms...and associated 
restrictions...are expected to continue during the week...but should 
become more focused during the afternoon and evening. 


Confidence table... 


05-11z 11-17z 17-23z 23-05z 
kclt medium 68% high 94% high 94% high 100% 
kgsp medium 76% high 91% high 100% high 100% 
kavl medium 76% high 90% high 100% high 97% 
khky medium 76% high 91% high 98% high 100% 
kgmu high 84% high 88% high 100% high 100% 
kand medium 60% high 87% 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) 


Www.Weather.Gov/gsp/aviation_tables 


&& 


Gsp watches/warnings/advisories... 
Georgia...none. 
NC...none. 
SC...none. 


&& 


$$ 
Synopsis...sbk 
near term...Ark/jdl 
short term...Wimberley 
long term...Wimberley 
aviation...jat