863 
fxus62 kgsp 211901 
afdgsp 


Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
201 PM EST Sat Nov 21 2009 


Synopsis... 
high pressure over the region will shift north over the weekend. 
This will allow a low pressure system to move east along the Gulf 
Coast then north along the Carolina coast Sunday through Monday. 
The low is expected to reach the New England coast by Tuesday as 
weak high pressure remains over the southeast states. Another cold 
front or coastal low could move into the area by the middle of next 
week. 


&& 


Near term /through Sunday/... 
confluent flow off the New England coast will gradually strengthen 
surface high pressure over the Middle Atlantic States through Sunday. The 
morning upper air showed very dry air down along the Gulf Coast. For 
example...the Tallahassee radiosonde observation had a 40 degree dewpoint depression 
at 850 mb. With this much dry air to the south...and with a 
developing classical cold air damming event...it/S not surprising 
that the models have been delaying the onset of precipitation over the past 
several runs. At least the GFS has toned down The Pot vorticity maxes and 
heavy precipitation that it had been driving across The Heart of The Wedge 
ridge. This seems more realistic as quantitative precipitation forecast should be fairly light 
with the low level dry air and the weakening southern stream system. The best 
chances for heavy rain will be along the coastal front...well to our 
south. 


Overall...the sref seems to have done the best job of handling the 
evolution of the Gulf Coast low and precipitation distribution so far. I/ve 
followed it fairly closely for the timing of precipitation. This means a dry 
forecast tonight. Precipitation should overspread the region from the 
southwest on Sunday...though I Don/T see anything falling north of 
Greenville until the afternoon hours. I raised maximum temperatures a couple 
categories over the NE zones where thinner cloud cover early in the 
day will result in a little more warming than originally expected. I 
followed a met/mav blend and we match up well with the neighbors. 


&& 


Short term /Sunday night through Monday night/... 
as of 130 PM Saturday...not that there is a great deal of confidence 
with regards to the specifics of the Sunday night forecast...but likely 
some degree of deep relative humidity and east-southeast flow above lingering wedge will remain 
all night. Sensible weather is still expected to be damp with rainy and 
drizzly periods...although precipitation totals will be moderate at worst. 
Confidence increasing that damming regime will remain through 
Monday...although forcing for additional precipitation will wane...eventually 
just being forced by some degree of warm air advection/easterly flow just above surface. 
Even the milder 12 UTC met numbers...which will probably be too warm 
anyway...support notable lowering of going maximum temperatures. Given the 
weakening of the low level flow Monday night and residual weak surface 
ridging...there seems to be Little Hope of scouring...so will plan 
on leaving overcast conds with spotty areas of light rain/drizzle... 
especially along and east of the Escarpment where weak low level 
easterly flow will continue to lift shallow moisture. 


&& 


Long term /Tuesday through Friday/... 
as of 145 PM Saturday...not much agreement is seen in the op models 
with respect to amount of deepening of the middle week cut off low and 
the level of moisture available for the occluded front to work with 
once it reaches the County warning forecast area Wednesday morning. Even less agreement is seen 
between the GFS and the European model (ecmwf) on Thursday/Friday with the polar vortex 
reinforcing the glakes low and the eventual track and speed of the 
large scale pattern. The GFS 500 mb ens members show the highest spread 
early Friday morning with about 8 dm of uncertainty associated with 
the height field...lending low confidence toward the setup for a 
possible minimal northwest flow event. 


Not many changes have Ben made to the going forecast with little 
confidence had both in the op models and HPC guidance. Some decent 
agreement is seen Tuesday with the slower breakdown of the surface wedge. 
Model soundings indicate rather thick stratus hanging on throughout 
the day and very low winds in the bl. So...maximum temperature guidance has 
been cut back a Cat or so outside the mountains to account for this 
possible scenario. Probability of precipitation still seem suspect with little in the way of 
organized flow interacting over The Wedge...so the lingering slight 
chances were changed little. 


The GFS is more aggressive in bringing a moist occluded front to the 
mountains by 12z Wednesday than the European model (ecmwf). The ec does eventually bring the 
front to the area late in the day...but both models cut off the gom 
moisture flux with low level ridging from the west. With the main large 
scale forcing shifted north...will count on very little impact as 
far a precipitation coverage GOES. For now...slight chances will 
remain...but this could be increased once better data is gleaned on 
this system. 


The next system to think about will be Thu/Fri...as the GFS insists 
on a moist northwest flow event...yet the ec remains dry and Low Key on the 
kinematic setup. The GFS as mentioned earlier is bringing in a much 
stronger reinforcing polar vortex into the glakes low...about twice 
as strong pvu/wise than the European model (ecmwf). This seems a little too strong 
for this time of year...and for now will side with the more 
conservative European model (ecmwf) on this one. Thus...minimal probability of precipitation and an all rain 
p/type are advertised for that period. 


Other than the lower than normal probability of precipitation expected Tuesday....Max temperatures 
should rebound rather nicely Wednesday through Friday. Increased insolation 
and a warm air advection flow will enable maxes at or perhaps a couple degrees 
above normal. The NC mountains will be trickery Friday...as it/S probable 
that cold air advection associated with the passing vortex to the north lowers 
temperatures throughout the day. Yet for now...will only go a couple 
degrees below normal with so much uncertainty and poor continuity 
displayed by the guidance. 


&& 


Aviation /19z Saturday through Thursday/... 
at kclt and elsewhere...the latest computer models have slowed down 
the progression of low clouds and precipitation associated with the 
low coming out of the Gulf. While the ceilings will gradually fall 
during the forecast period...only VFR ceilings are expected through 18 
UTC sun. Winds will be light out of the east to east-northeast this afternoon...backing 
a little more to the north tonight and Sun morning. The exception is 
kavl where the winds will be light out of the southeast today...nearly calm 
tonight...and southeast again on sun. 


Outlook...cloud ceilings will lower more quickly Sunday afternoon as 
rain spreads across the area. MVFR conditions expected most of the 
afternoon. IFR ceilings and MVFR fog are still expected to develop Sunday 
night...lasting into Tuesday. Conditions may not improve much on 
Wednesday as a wedge of cool moist air persists over the region. 


&& 


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


&& 


$$ 
Synopsis...pattern 
near term...McAvoy 
short term...csh 
long term...sbk 
aviation...McAvoy 

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