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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
1040 PM EDT Sat Mar 20 2010 


Synopsis... 
high pressure will move off the southeast coast tonight. A cold 
front will move across the western Carolinas and northeast Georgia 
Sunday night...followed by a strong upper low Monday and Monday 
night. The low will move east of the region on Tuesday...followed by 
high pressure through mid-week. Another front will affect the region 
by the end of the week. 


&& 


Near term /through Sunday/... 
current forecast appears to be on track so no update is planned for 
this evening. 


The previous discussion is as follows... 
as the next system approaches the forecast area...increasing high 
level cloudiness will stream in from the west. In the low levels... 
flow backs to south/southeasterly and increases...transporting 
Gulf/Atlantic moisture into the area. Temperatures will drop mainly into 
the 40s to lower 50s. 


Sunday...models in agreement on increasing isentropic lift on the 
295 and 300 k sfcs...as low pressure system moves into the middle 
south. The GFS is moister and a little faster overall in bringing 
the lift into the area. Based on a consensus of the 
GFS/Gem/sref...get categorical probability of precipitation for the western 2/3rds of the forecast area 
for the afternoon. This will be a result of isentropic lift and good 
upslope forcing (with perpendicular 850 mb winds into the Blue Ridge 
Escarpment at 40-50 kts by late aftn). These southeasterly winds may be able 
to mix down to some of the high terrain...as 850 mb-700 mb lapse rates are 
actually rather steep in the BUFKIT soundings. May need some type of 
Wind Advisory for elevations above 3500 feet or higher. Instability will be 
almost nonexistent...except perhaps for the upstate and NE Georgia. Will 
keep a mention of slight chance thunder for now. Quantitative precipitation forecast looks to be 
generally 0.25-0.75" by sunset west of I-26...and just a few 
hundredths to a tenth east. Temperatures will remain in the 50s to upper 
60s under mostly cloudy to overcast skies. 


&& 


Short term /Sunday night through Tuesday night/... 
as of 245 PM Saturday...there are three distinct issues with the 
strong upper low that will work it/S way across the forecast area Sunday night 
through Monday night. 


The first issue is that of severe weather. Differences in the timing 
of the front between the GFS and NAM will play a significant role in 
our severe chances. The GFS is faster with the surface front that lifts 
across the region Sun night. It does this as it lifts a stronger 
initial short wave up the east side of the upper low late Sunday. 
The NAM is more consolidated with the energy...and has a weaker wave 
that is farther to the west. As such...it doesn/T drive the front as 
far east and it takes the next short wave...that comes through late 
Sunday night...to drive the surface front through the region. As the NAM 
is slower...there is more time for moisture to increase ahead of the 
front...and middle level temperatures appear to be a little cooler. As a 
result...it generates 300 to 400 j/kg of cape mainly after midnight 
Sunday night roughly along and north of a line from Greenwood to 
Asheville...with the greatest instability over the NC foothills and 
Piedmont. The sref and European model (ecmwf) favor something in between these 
solutions...which is what I/ll go with. This means enough 
instability for at least the chance of a low topped line of storms. 
I Don/T know if a line will actually be able to form as the 
instability will be so low...but I will keep thunder in the zones 
and play up a small severe threat from the late evening into the 
early morning hours. The strong 500 mb jet and middle level drying that 
work into the region overnight will certainly increase the risk of 
damaging winds should storms develop. 


The second issue is heavy rain. HPC/S quantitative precipitation forecast has been slowly increasing 
over the Escarpment over the past couple days. They now have about 
an inch and a quarter of quantitative precipitation forecast over this area for the sun-sun nght 
period. Looking at a blend of the NAM/GFS/ec/sref qpf/S...this looks 
to line up well. While this is a little more than previously 
forecast...it/S still well below what would be needed to cause any 
river or stream flooding. 


Finally...as far as snowfall potential GOES...there still doesn/T 
appear to be any organized synoptic scale forcing as the upper low 
moves across the region Monday. However...there should be quite a 
bit of shower activity over the region as we heat up and the upper 
low rolls across the mountains in the afternoon hours. Above about 4000 feet 
some of this will fall as bursts of heavy snow. If a persistent area 
of convection were to develop...which is always possible if an area 
of strong positive vorticity advection were to become collocated with an area of 
convection...several inches of wet snow could fall at some ridge-top 
locations. It/S basically impossible to tell where or if this will 
happen...though I have upped probability of precipitation Monday afternoon across the northern mountains as a 
nod to the higher HPC and NAM quantitative precipitation forecast in this area. 


As the upper low moves away Monday night...the GFS develops a little 
better forcing...including good upward vertical velocity/S and higher relative humidity/S that extend 
into the good snow-Crystal growth area on the Asheville BUFKIT 
sounding. The ec has a similar area of forcing...but it/S back over 
KY/TN. This is nothing I/M prepared to jump on at this time...but it/S 
something to watch. 


Tuesday will be cool with gradually decreasing clouds east of the 
mountains...and cloudy skies over the mountains and scattered rain and snow showers 
along the Tennessee line. 


&& 


Long term /Wednesday through Saturday/... 
as of 1 PM Saturday...the medium range models are in descent 
agreement in bringing a low amplitude ridge across the region Wednesday 
and Thursday. Both days should be very pleasant with mainly sunny skies 
with afternoon maximum temperatures rising into the 70s over the foothills and 
Piedmont...and the 60s over most of the mountains the GFS is a little 
flatter with the ridge. This could result in more northwest flow moisture 
affecting the mountains during this time...which would result in a period 
or two of partly to mostly cloudy skies. Another upper low will 
approach the region Thursday nght and Friday. The ec has a more amplified 
trough than the GFS...but they are both in agreement in bringing the 
feature through the area on Friday. Chance probability of precipitation with cooler temperatures 
are in the forecast for that day...followed by improving conditions 
on Saturday. The system looks warm enough that only rain will 
fall...but the relatively low amplitude doesn/T look to support 
severe storms or excessive rainfall. There is some indication that 
the trough could be a little faster...particularly some energy that 
lifts ahead of the main trough late on Thursday. The GFS is the most 
aggressive with this feature...and it would have precipitation moving into 
the region later on Thursday. So confidence on a great day is high for 
Wednesday...but a little lower for Thursday at this time. 


&& 


Aviation /03z Sunday through Thursday/... 
at kclt and elsewhere...VFR conditions overnight with a few altocu 
and cirrus clouds. Wind should go light from the south and then veer 
around to southeast before sunrise. Middle level clouds should thicken 
and move in from the west in the 09z to 12z time frame. After 
that...things go downhill with the approach of the next system. 
Strong forcing and deep moisture will spread across the region from 
the west in the late morning to early afternoon. Southeast flow at 
low levels should develop an MVFR cloud deck across the region 
between 15z and 19z along with a few showers. The steadier precipitation 
and lower MVFR cloudiness should move in during the middle part of 
the afternoon so it was at that point that an MVFR visibility was 
introduced at most sites. The exception is kclt where there is 
enough uncertainty about the lower clouds and precipitation arriving before 
the end of the taf period that it was left out. There is some 
indication that the ceiling could drop into the IFR category late in 
the day but this was also left out for now. Model guidance showed 
significant differences with respect to wind direction Sunday 
afternoon. Have opted to keep wind south of east per the met 
guidance as it does not look like a cold air damming situation. 


Outlook...ahead of an approaching cold front...perhaps a few thunderstorms 
late Sunday afternoon and evening. Scattered showers and gusty 
westerly winds will linger into Monday with MVFR expected. 
Conditions should improve to VFR Monday morning and then persist 
through Thursday. 


&& 


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


&& 


$$ 
Synopsis...McAvoy 
near term...Ark/PM 
short term...McAvoy 
long term...McAvoy 
aviation...PM 

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