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afdgsp 


Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
714 am EDT Monday Mar 22 2010 


Synopsis... 
a strong upper low will move over the area today and tonight as the 
surface cold front pushes off the Carolina coast. The low is 
expected to move east of the region by Tuesday...followed by high 
pressure settling in through mid-week. Another weak front will 
approach the region on Friday...followed by cool high pressure over 
the weekend. 


&& 


Near term /through tonight/... 
a cold front will move east of the area early this morning taking 
the line of showers and thunderstorms with it. Gusty winds along the 
front will move east as well. Clear skies in the wake of the front 
will fill back in as low level moisture moves back in from the west 
ahead of the upper low currently centered over Alabama. Model guidance is 
in general agreement that the low will move up The Spine of the 
Appalachians today. The middle levels will be quite cold creating 
instability across the area and setting the stage for snow over the 
higher elevations of the NC mountains expect numerous showers and 
isolated storms over the mountains by afternoon...with scattered showers and 
isolated storms developing elsewhere. Snow levels still expect the 
highest snowfall amounts to occur above 4000 feet...with up to 4 
inches over the smokies and balsams. 1 to 3 inches possible above 
3500 feet elsewhere. This is our current best estimate...as upper lows 
are notorious for creating havoc with erratic movement and variable 
snow levels and amounts. Outside of the mountains...there could be some 
small hail in any storms that develop given the very low freezing 
levels. Gusty winds will develop by afternoon...but should remain 
below even lake wind advection levels. Highs will be below normal. 


The upper low moves slowly east near the NC/Virginia border tonight. 
Expect numerous showers to remain over the mountains with snow levels 
falling...possibly even to the valley floors before ending. That 
said...the guidance is still relatively warm in the valleys...so no 
significant accums expected below 3500 feet. Scattered showers will 
continue outside of the mountains during the evening with precipitation tapering 
off after midnight. Lows will be quite tricky as the guidance has 
come in colder for most areas. Winds will diminish overnight...but 
remain above 5 kts...with clouds lingering most areas. Given the 
wind and clouds...do not expect frost to be a problem overnight... 
even with temperatures in the middle 30s. 


&& 


Short term /Tuesday through Wednesday/... 
as of 340 am Monday...deamplifying flow on tap for Tuesday as 
cyclonic flow and surface pressure gradient loosens. Generally weak 
west-northwesterly upslope flow and associated low level relative humidity will decrease across the 
western NC mountains throughout the day...leading to numerous rn/snow 
showers early on to become increasingly scattered in nature. 
Considerable clouds to linger across the western NC mountain counties... 
but a mix of sun and clouds expected from the Blue Ridge eastward. 
Under breezy conditions...maximum temperatures will rebound to around climatology. 
Progressively warmer temperatures and dry weather are still expected 
on Wednesday within developing zonal flow aloft...with maximum temperatures 
in the 70s across the lower elevations. 


&& 


Long term /Wednesday night through Sunday/... 
as of 200 PM Sunday...the extended period will begin with a split 
flow upper pattern in western noam...with confluent downstream flow over 
the Great Lakes region Wednesday night into Thursday. Surface hi pressure will weaken 
in vicinity of the Ohio Valley...as the system sags southward. A leading backdoor 
cold front looks to be moisture starved as it pushes into the middle 
Atlantic and central Appalachians. The envelope of guidance is in 
disagreement on how far south this front will go before stalling 
out. The 12z GFS and Gem push completely through the forecast area by Thursday evening. The 
00z European model (ecmwf) stalls the boundary to the north of the forecast area. Based on 
climatology of backdoor fronts in the Spring...think the GFS solution is 
best (and this is where the 12z HPC places the front). Meanwhile...a 
southern stream short wave will propagate from The Four Corners region to the 
Southern Plains states by Thursday evening. A surface wave will develop in vicinity of the 
arklatex and begin to approach the Appalachians on Friday. Guidance 
still at odds with how the system will evolve. The consensus for now 
is for the stalled boundary to begin to push northward...placing the southwestern 
half of the forecast area in the warm sector. The European model (ecmwf) and Gem both have 
SBCAPE Friday afternoon along the boundary. So will add mention of slight chance 
thunder there...while keeping probability of precipitation in the chance range. 


This upcoming weekend/S weather is definitely a low confidence forecast 
(even for days 6-8)...as quasi-zonal upper flow with a possible 
prolonged cad event setting up. For now...went close to HPC for 
temperatures and placed a low-end chance pop for sun and Sun night as low pressure 
system approaches from the west. 


&& 


Aviation /11z Monday through Friday/... 
at kclt and elsewhere except kavl...low VFR ceilings will develop by 
noon as low level moisture returns from the west ahead of a strong 
upper low. Southwesterly winds become gusty for the afternoon as scattered showers 
and possibly a thunderstorm develop. Brief MVFR conditions cannot be 
ruled out in any showers. Low VFR ceilings pull out during the 
evening...showers dissipate and gusts come to an end. However...middle 
level ceilings will continue with winds becoming west-southwest. 


At kavl...low VFR ceilings develop during the morning as low level 
moisture returns from the west ahead of a strong upper low. Southerly 
winds become south-southwest and gusty for the afternoon as numerous showers and 
possibly a thunderstorm develop. Brief MVFR conditions are possible 
in any showers. Winds become northerly during the evening with low VFR 
ceilings continuing. Some snow could mix with the rain and brief MVFR 
again possible numerous showers continue. 


Outlook...generally VFR conditions are expected Tuesday through 
Thursday. Ceiling and visibility restrictions are possible Thursday night and Friday 
as a cold front moves into the region. 


&& 


Gsp watches/warnings/advisories... 
Georgia...none. 
NC...Winter Weather Advisory until midnight EDT tonight for ncz033- 
048>053-058-059-062>064. 
SC...none. 


&& 


$$ 
Synopsis...jpt 
near term...rwh 
short term...csh 
long term...Arkansas 
aviation...rwh 

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