504 
fxus62 kgsp 211135 
afdgsp 


Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
635 am EST Friday Nov 21 2008 


Synopsis... 
cold northwest flow in the wake of a cold front will continue over 
the area through the day. A strong area of high pressure will 
then move over the southeast early Saturday...and remain the 
dominant weather feature through the weekend. Another cold front is 
then expected to move over the Carolinas early next week followed by 
another ridge of high pressure building in towards the middle of 
next week. 


&& 


Near term /through tonight/... 
updated 630 am...snow has ended over the western NC Piedmont...so the 
advisory for that area has been cancelled. No other changes. 


Updated 455 am...I/M going to expand the Winter Weather Advisory to 
include Buncombe County...Graham County and the mountain zones of 
McDowell...Burke and Caldwell. Several reports from spotters in 
Buncombe County...and the County itself...reveal that snow has been 
heavier than anticipated this morning. Many roads are snow covered 
and even as far south as the west side of Asheville an inch of snow 
has been reported. This is a strong cold advection event...which 
often does push snow beyond the favored northwest flow areas. As an 
advective shortwave will be moving over the mountains over the next 
couple hours...snow may even increase a little. Hopefully this will 
cover things...though we will need to watch accums in the northern mountains 
as it only takes 3 inches in 12 hours to qualify as a winter storm. 
Previous discussion follows... 


A band of deep layer frontogenesis is acting on moisture pooled 
along the east slopes of the Appalachians...resulting in a band of 
rain/snow/sleet over our NE zones along the I-77 corridor. The 750 
mb level shows the frontogenesis quite well on the GFS and NAM. This 
area of forcing slowly drifts to the south this morning...which can 
also be seen on the BUFKIT sounding for gso. After 12 UTC the models 
really dry out the column...and while some qc forcing will 
remain...it appears the snow Crystal growth area will dry 
out...ending precipitation. That said...a Winter Weather Advisory will be 
issued for the Rowan...Iredell and Davie County areas as snow is 
falling a good clip in this area and the snow band continues to 
become better defined on radar. Only looking for an inch on grassy 
sfc/S...but as our winter weather criteria is 1 inch...that will do it. 


The Winter Weather Advisory for snow for the mountain counties that 
border Tennessee will be left as is. I did up the quantitative precipitation forecast and hence snowfall 
amounts a little. With 800 mb temperatures falling so quickly this 
morning...snow ratios will be pushing 20:1 by the time the precipitation 
starts to taper off in the early afternoon. So a few places may 
approach 3 inches...especially in the northern mountains where lapse rates 
will be steeper and layer relative humidity/S higher to the north of the middle-level 
jet core. 


A Wind Advisory will be issued for most of the rest of the NC 
mountain counties that are not already in a Winter Weather Advisory. 
Winter weather advisories now encompass wind products...which is why 
a Wind Advisory won't be issued for those counties. I/ll issue a 
lake Wind Advisory for the rest of the forecast area. As cae and ffc both have 
wind advisories up for their entire fa/S...it would look a little 
odd not to have one up over the western upstate and NE Georgia where I have 
my weakest winds today. And even in these zones the potential for 
gusts up to 35 miles per hour will be pretty good considering the strong cold air advection 
and pressure rises during the course of the day. 


There are also red flag products and fire danger statements in 
effect. See the fire weather part of the discussion for those details. 


&& 


Short term /Saturday through Monday/... 
as of 210 am Friday...much of the short term will be characterized 
by very cool and dry conditions...as Canadian high pressure 
dominates through most of the period. A rather potent SW trough will 
swing across the area west/in northwest flow aloft Saturday night. 
However...the low levels of the atmosphere will remain extremely 
dry...so increased high cloudiness will be the extent of the effects 
on our sensible weather. 


By Monday...the upper flow transitions to zonal...as the eastern 
trough moves rapidly over the Atlantic...and a significant storm 
system organizes over the central Continental U.S.. the associated cold front 
will race across the tenn valley early Monday...with showers 
possible across the western Carolinas/northeast Georgia by 
afternoon. Although the day will be off to a cool start...any precipitation 
should occur late enough in the day to preclude wintry precipitation 
concerns at onset. 


&& 


Long term /Monday night through Thursday/... 
as of 130 am Friday...the large scale synoptic pattern continues to 
look good for a northwest flow snow event Tuesday. The models agree well with 
each other and have kept good temporal continuity the last few runs. 
See no reason to deviate from going forecast. The gmos and the HPC 
pop guidance seem much too low after 12z Tuesday considering the good 
model confidence and their recent performances. So...will keep probability of precipitation 
in the chance range and perhaps bump up a bit. Quantitative precipitation forecast still will be a 
challenge as we get closer to the event. The models continue with 
the eastward displacement of 850 mb low and good gl moisture flux and 
consistent vectoring into the NC mountains also...upper level short wave looks 
to enhance snowfall amounts around 18z Tuesday...especially on the European model (ecmwf). In 
any regards...will anticipate a lingering event into Tuesday evening and 
perhaps early Wednesday morning...but dissipating before sunrise. The 
progressive pattern continues through the remainder of the ext 
period with small scale ridging building across the Tennessee Valley late 
Wednesday into Thursday. This setup should enable maximum temperatures to slowly rise to 
near normal by weeks end...after maxes of about 10 f below normal on 
Tuesday. Min temperatures look to remain about 10 f below normal throughout the 
ext as surface high settles over the region and allows good 
radiational cooling to develop each night. 


&& 


Aviation /12z Friday through Tuesday/... 
strong cold advection will develop across the region after 0900 UTC. 
Winds at kclt...kgsp and kgmu will be out of the north-northeast for a few more 
hours...but once the mixing becomes deep enough they will turn out 
of the north and then the northwest. A band of light rain/snow many just clip kclt 
from the north as the front moves in. I won't carry that in the taf but 
will keep a close eye on radar trends. Winds later this morning and 
through the afternoon should be quite gusty as strong cold advection 
continues. Gusts over 30kts are possible...especially from kgsp 
northward. Kept the previous winds pretty much in tact..though I did 
raise gusts to 30kts at kclt for the afternoon hours. Kavl will 
probably see a period of snow showers as the front moves 
through...though I Don/T think there will be anything but a very 
brief period of low ceilings or clouds associated with it. Winds should 
diminish quickly around sunset tonight as high pressure builds into 
the region. A while 


Outlook...VFR conditions expected across the region Friday night 
through Sunday as strong dry high pressure settles in. A cold front 
will move through the area Monday with a chance of showers and ceiling/visibility 
restrictions. 


&& 


Fire weather... 
windy conditions are still expected today over the western Carolinas 
and northeast Georgia. Current forecast relative humidity values are a little lower 
than yesterday...satisfying red flag criteria in NE Georgia. Over SC the 
combination of relative humidity and wind meet criteria over about 50 percent of 
the area...enough to upgrade the watch to a fed flag warning. Not as 
large an area hits red flag criteria in NC this afternoon. The 
Piedmont areas around Charlotte still may meet criteria...and while 
the temperatures will be a little cooler in NC this afternoon...the winds 
are expected to be stronger. The fire danger statement will be kept 
running in NC...and an upgrade still may be needed later in the 
morning for parts of the foothills and Piedmont. 


&& 


Gsp watches/warnings/advisories... 
Georgia...Lake Wind Advisory until 6 PM EST this afternoon for gaz010- 
017-018-026-028-029. 
Red flag warning from 10 am this morning to 6 PM EST this 
afternoon for gaz010-017-018-026-028-029. 
NC...Winter Weather Advisory until noon EST today for ncz033- 
048>053-058-501-503-505. 
Wind Advisory until 6 PM EST this afternoon for ncz053-059- 
063>065-501-503-505-507-509. 
Lake Wind Advisory until 6 PM EST this afternoon for 
ncz035>037-056>058-062-068>072-082-502-504-506-508-510. 
SC...lake Wind Advisory until 6 PM EST this afternoon for 
scz001>014-019. 
Red flag warning from 10 am this morning to 6 PM EST this 
afternoon for scz001>014-019. 


&& 


$$ 
Synopsis...jpt 
near term...McAvoy 
short term...jdl 
long term...sbk 
aviation...McAvoy 
fire weather...McAvoy 

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