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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
1133 am EDT sun Mar 21 2010 


Synopsis... 
a cold front will move across the western Carolinas and northeast 
Georgia tonight...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 tonight/... 
I/ve accelerated the precipitation timing over the southern NC Piedmont and the 
northern upstate as showers have already pushed that far north. The 
latest NAM and 6 UTC GFS hint at better elevated instability 
tonight. Considering that surface dewpoints are verifying higher than 
forecast...this seems reasonable. In fact...an axis of u50 dewpoints 
has developed across east central Georgia...down to the southern upstate of SC. 
Despite limited heating...these signs imply an increasing chance 
that some kind of low topped linear mesoscale convective system will move across the forecast area 
starting late this evening. Storm Prediction Center has gone this Route...too...as they 
now have a 5 percent severe wind and 2 percent tornado threat over 
the forecast area along and east of a Greenville to Hickory line. 


Previous discussion follows... 
lead impulse will continue to lift north-northeast from the Gulf today...and in 
concert with an increasingly strong south-southeast low level upglide/upslope 
flow...showers expected to become numerous to 
widespread...especially across the SW half of the County warning forecast area. Given 
thickening and lowering overcast conds...instability should be 
limited enough to preclude afternoon thunder 
possibilities...although heavy showers could be seen across favored 
south-southwest Escarpment areas. 21 UTC sref 850 mb wind flow could support high 
elevation Wind Advisory level conditions across the SW NC and north Georgia mountains 


For tonight...short range model timing of cold frontal passage is 
close enough that even the slower NAM is progging dry slot and 
the onset of the advection of lower 100-850 mb thickness values 
across the western County warning forecast area around midnight...and then across the 
NE County warning forecast area before the pre-dawn hours. Coincident with frontal 
zone passage...aforementioned 45-50 knots south-southeasterly 850 mb jet will 
translate northeast. This period of enhanced low to middle level 
shear...0-3km sreh values of greater than 400 m2/s2...is 
prognosticated to be tempered by a questionable amount of instability. 
NAM MUCAPE values of 500 or more are prognosticated to be limited to the 
coastal plain...where the greater wind damage threat...a 5 percent 
risk on new day 1 Storm Prediction Center outlook...will be limited. Given at least 
some positive values of MUCAPE and Stout middle-level jet translating 
across the Piedmont overnight...there still remains a non-zero 
chance of damaging wind gusts...although anything more than 
isolated covering remains a low probability occurrence. 


So...sensible weather remains limited to widespread showers 
and slight chances for embedded thunderstorms within the potential 
low-topped line of deep convection. A period of heavy showers 
is probable as well just about anywhere...although greatest 
total rainfall amounts are expected across the upslope areas... 
where a local 1.5 inch total is possible. 


&& 


Short term /Monday through Tuesday night/... 
as of 350 am Sunday...the cold front will be east of the area at the 
start of the period...while the upper low will still be to our west. 
There is still some discrepancy with the exact track and timing of 
the center of the low. However...a significant cold pool is still 
expected to move across the County warning forecast area with the low Monday. Any lingering 
showers with the front will quickly end early in the morning...while 
scattered showers continue across the mountains weak instability develops as 
the cold pool moves over. This combined with the lingering low level 
moisture will create widespread showers over the mountains and scattered 
showers elsewhere. Isolated thunder cannot be ruled out...nor can 
small hail with wet bulb zero levels below 5k feet. Snow levels across 
the mountains will be tricky as low levels will be warm...but middle level 
thicknesses and 850 mb temperatures will be cold. Right now...have kept snow 
limited to the higher elevations of the smokies...balsams and 
cherohala Skyway. That said...precipitation could be high enough for 
significant accums above 4k feet. That said...confidence in accums is 
low due to the variable nature in upper lows. 


The upper low pulls away from the area Monday night. However...temperatures 
over the mountains will remain cold with the low levels cooling off as 
well. This will allow snow levels to drop to many of the valley 
floors. Although precipitation coverage over the mountains will remain 
numerous...quantitative precipitation forecast will be low. Therefore...only light additional accums 
are expected in the higher elevations and the northern mountains...with very 
little to no accums in the central and Southern Mountain valleys. The showers 
should come to an end outside of the mountains by midnight. 


Scattered showers remain over the mountains as northwest flow and good low level 
moisture remain in place. However...temperatures and thicknesses rapidly 
warm as the cold pool continues to move northeast of the area. Any 
lingering snow showers should change to rain by noon...with only very 
light additional accums in the higher elevations. Some isolated 
showers are possible outside of the mountains during the morning. The northwest 
flow showers come to an end Tuesday night as the moisture dries up. Dry 
and clearing expected elsewhere. Highs Tuesday below normal with lows 
Tuesday night near to slightly below. 


&& 


Long term /Wednesday through Saturday/... 
as of 250 am Sunday...the medium range models remain in decent 
agreement bringing a low amplitude ridge across the region Wednesday and 
Thursday. However...the GFS/Canadian bring a mainly dry back door front 
into the area Wednesday night and stalling it over the area Thursday. The European model (ecmwf) 
keeps the front to our north. The guidance keeps temperatures above normal 
both days. An upper low opens up and moves across the area Friday. 
This pushes another low pressure system across the area Friday. Kept 
the increasing probability of precipitation Thursday night and Friday and lows above normal with 
highs near normal. With lingering moisture and Ely upslope flow as 
high pressure noses in from the north...kept the slight chance pop Friday 
night and Sat. Lows still above normal with highs near normal. 


&& 


Aviation /16z Sunday through Thursday/... 
at kclt...southeast winds will gradually increase through midday as VFR level 
clouds thicken. Showers will be scattered about the terminal this 
afternoon as MVFR ceilings develop. A round of gusty showers and perhaps 
IFR ceilings looks probable at some point from middle-evening into the 
overnight hours. A cold frontal passage...with gusty winds shifting 
to the SW looks probable during the pre-dawn hours Monday. 


Elsewhere...flight conditions still expected progressively 
deteriorate as the day wears on as showers and lowering ceilings 
develop... especially this afternoon. A line of gusty showers and 
perhaps a thunderstorm could be seen this evening into the night ahead of a 
cold front which will pass through during the late night hours. 


Outlook...scattered showers and gusty westerly winds will linger 
into Monday with MVFR ceilings expected. Conditions should improve to 
VFR Tuesday morning and then persist through Thursday. 


&& 


Fire weather... 
as of 830 am Sunday...skies are already overcast across the region 
and a band of showers is advancing into the upstate from the south. 
The previous mixing heights look to be too high...and I/ve lowered 
them quite a bit...especially over the upstate and NE Georgia. This is 
closer to the 06 UTC NAM...though even these values may prove a 
little high. Temperatures/rh/pcpn/transport winds all still look 
good...though the lower mixing heights will affect some of the 
stability parameters as well. 


&& 


Gsp watches/warnings/advisories... 
Georgia...none. 
NC...lake Wind Advisory until 4 am EDT Monday for ncz033-048>053- 
058-059-062-063. 
SC...none. 


&& 


$$ 
Synopsis...rwh 
near term...csh/McAvoy 
short term...rwh 
long term...rwh 
aviation...csh 
fire weather... 

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