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Area forecast discussion...updated 
National Weather Service Peachtree City Georgia 
530 am EST Thursday Nov 20 2008 


Short term /today through Saturday/... 
while the overall flow tends to become slightly more progressive 
over the next seven days...the dominate feature for the eastern U.S. 
Will continue to be the NE U.S./Se Canada polar vortex. This will 
result in a continued succession of mostly dry...Canadian cold 
fronts through Thanksgiving as northwest flow aloft continues. The 
exception will be Monday...when there is a slight chance of showers. 


The Canadian high that brought the near record cold to the area 
early Wednesday...has drifted south of the region this morning and has 
weakened considerably. Although 850mb temperatures have warmed considerably 
over the past 24 hours...to around +8c in West Georgia...clear skies and 
light winds resulting in excellent radiational cooling...with many 
of the usual cold spots well below freezing this morning. 
Meanwhile...a strong short wave was evident in southern Alberta this 
morning with an associated large 1045mb Canadian high. The short 
wave will dive southeast into the mean polar vortex...allowing the 
Canadian high to spread southeast into the middle and eventually southeast U.S. By 
Friday. The leading edge of this next cold air mass should reach north Georgia 
by late afternoon...then surge rapidly southward through the state 
by Friday morning. In advance of this front...westerly downslope winds 
will result in a continued warm up and a rather mild day. Plan to 
use a mav/met/FWC MOS blend for maximum temperatures today. A few clouds may 
drift across north Georgia during the afternoon and evening in association 
with the front...but that will be about it. Orographic lifting and 
strong cold advection will likely result in SC development on the 
west facing slopes of the NE Georgia mountains overnight and early Friday. 
An increase in bl moisture and even some vertical velocity indicated in these areas 
overnight...but the deepest moisture remains north of Georgia...so will 
not introduce any -shsn at this time. For Friday...significant cold 
advection expected. This is a very large and cold high and mav MOS 
has had a poor handle on the intensity of these cold air masses this 
week. Will lean heavily toward the cold met/FWC MOS...which in some 
cases is nearly 10 degrees colder than the mav MOS. Most of north Georgia 
will not get above 45 Friday...similar to what was seen this past 
Tuesday. Very cold min temperatures are expected again early Sat...again 
near...but perhaps not quite as cold as what was seen early Wednesday. The 
large surface high will drift slowly from the middle-south into the southeast 
through the weekend. This will allow for only slow moderation of the 
well below normal temperatures with below freezing temperatures expected area wide 
Sat-sun. Have leaned toward the cold side of guidance through Sat. 


The upper flow becomes slightly more progressive by Saturday as a 
strong short wave moves from the Pacific northwest into British 
Columbia...and then toward central Canada. In response...the deep NE 
U.S. Polar vortex shifts a little further east...allowing a short 
wave ridge to progress into the eastern states. A dry short wave 
dives southeast from Alberta toward the southeast U.S. And dampens out within the 
ridge traversing the region. No sensible weather is expected with 
this system. Clear skies are expected to continue through the 
weekend. 


Long term /Saturday night through Wednesday/... 
the European model (ecmwf)/GFS are generally in good agreement at the beginning of 
the extended period...but become significantly different by the end 
of the extended. A rapid intensification of the polar vortex is 
expected again early in the extended period as the aforementioned 
western Canada upper low drifts into the mean eastern U.S. Trough. 
This will result in yet another significant area of Canadian high 
pressure spreading toward the southeast U.S. During the early part of the 
week. In contrast to the past couple of fronts...however...enough 
moisture return...albeit limited...is expected around the departing 
weekend high...combined with strong upper dynamics to continue to 
support low chance probability of precipitation for Monday-early Tuesday from NW-se. The GFS is 
slower and more aggressive with probability of precipitation than the European model (ecmwf). Prefer to stick 
with model consensus...which favors the European model (ecmwf). Thus...will continue 
with slight chance -shara for Monday-early Tuesday. Quantitative precipitation forecast expected to be less 
than 0.25 inch. Air mass is entirely too dry and stable to support 
any thunderstorms and rain with this system. 


Another large Canadian high spreads across the region behind this 
system and remains in place through Thanksgiving. The GFS continues 
to be much more aggressive in breaking down the polar vortex and 
bringing a deep upper low into the SW U.S. The European model (ecmwf)...on the other 
hand...holds onto a deep polar vortex over New England through much 
of the week as it dumbbells around through that region. The European model (ecmwf) 
holds any low pressure area in the SW U.S. There...with yet another 
significant upper low diving southeast out of Alberta after Thanksgiving 
and a continued reinforcement of the current pattern. Both models 
show another system...almost identical to the Monday system...affecting 
the region by the Sat after Thanksgiving...so will introduce low 
probability of precipitation for the last period of this package. 


Clouds will hold temperatures down somewhat Monday...but it will be warmer 
then much of the remainder of the extended when cold high pressure 
dominates the area again and 850mb temperatures drop well below zero by 
Tue-Wed. Slowly moderating temperatures are expected Thanksgiving and Friday 
in advance of the next cold front. Overall...the weather for 
Thanksgiving looks good for travel...dry with seasonable temperatures. 


&& 


Fire weather... 
relative humidity and winds do not meet rfw criteria for today...so am canceling 
the fww for today with this package. However...conditions will need 
to be closely monitored after 00z when winds pick up to near 20 miles per hour in 
the north combined with dry fuels. Will let day shift decide if rfw 
is needed for tonight. By Friday afternoon...very low teen dewpoints back in 
place again leading to critical low relative humidity values. The exception will be 
in the far north/NE mountains where wrap around moisture holds relative humidity values 
above 30 percent. Winds will still be in the 10-15kt range much of 
the day as well. Will issue a fww for all but the far north for Friday 
afternoon. For Sat...very low dewpoints/relative humidity values remain in place...and 
with 5-7 degree f warmer maximum temperatures...critical relative humidity values met for five hours 
or more across much of the County Warning Area. Will issue a fww for the entire County Warning Area 
for Sat afternoon. 


&& 


Aviation... 
high pressure centered over the northern Gulf will continue to sink 
southward today. Another strong Canadian high will push into the 
southeast beginning late this afternoon and continue overnight. 
Winds will be southwest today 10 to 15kt..becoming more west to 
northwest by late afternoon and overnight tonight...sustained 15kt 
and gusty. No visibility restrictions expected. Skies should remain 
clear today...but scattered to broken clouds 4 to 5thsd feet should 
accompany the cold front into north Georgia around 23z or later. 


&& 


Preliminary point temps/pops... 
Athens 62 31 48 22 52 / 0 0 0 0 0 
Atlanta 60 28 45 26 52 / 0 0 0 0 0 
Blairsville 52 24 36 20 45 / 0 10 5 0 0 
Cartersville 58 26 43 20 50 / 0 0 0 0 0 
Columbus 64 33 49 28 56 / 0 0 0 0 0 
Gainesville 58 27 43 25 49 / 0 0 0 0 0 
Macon 63 34 51 23 54 / 0 0 0 0 0 
Rome 59 26 43 21 52 / 0 0 0 0 0 
Peachtree City 60 29 46 19 52 / 0 0 0 0 0 
Vidalia 64 36 54 29 55 / 0 0 0 0 0 


&& 


Ffc watches/warnings/advisories... 
Fire Weather Watch Friday afternoon for the following zones: 
Baldwin...banks...Barrow...Bartow...Bibb...Bleckley...Butts... 
Carroll...Chattahoochee...Chattooga...Cherokee...Clarke... 
Clayton...Cobb...Coweta...Crawford...crisp...DeKalb...Dodge... 
Dooly...Douglas...Emanuel...Fayette...Floyd...Forsyth... 
Glascock...Gordon...Greene...Gwinnett...Hall...Hancock... 
Haralson...Harris...heard...Henry...Houston...Jackson...Jasper... 
Jefferson...Johnson...Jones...Lamar...Laurens...Macon... 
Madison...Marion...Meriwether...Monroe...Montgomery...Morgan... 
Muscogee...Newton...north Fulton...Oconee...Oglethorpe... 
Paulding...Peach...Pickens...Pike...Polk...Pulaski...Putnam... 
Rockdale...Schley...South Fulton...Spalding...Stewart...Sumter... 
Talbot...Taliaferro...Taylor...Telfair...Toombs...Treutlen... 
Troup...Twiggs...Upson...Walton...Warren...Washington... 
Webster...Wheeler...Wilcox...Wilkes...Wilkinson. 


Fire Weather Watch from Saturday afternoon through Saturday 
evening for the following zones: Baldwin...banks...Barrow... 
Bartow...Bibb...Bleckley...Butts...Carroll...Catoosa... 
Chattahoochee...Chattooga...Cherokee...Clarke...Clayton...Cobb... 
Coweta...Crawford...crisp...Dade...Dawson...DeKalb...Dodge... 
Dooly...Douglas...Emanuel...Fannin...Fayette...Floyd...Forsyth... 
Gilmer...Glascock...Gordon...Greene...Gwinnett...Hall... 
Hancock...Haralson...Harris...heard...Henry...Houston... 
Jackson...Jasper...Jefferson...Johnson...Jones...Lamar... 
Laurens...Lumpkin...Macon...Madison...Marion...Meriwether... 
Monroe...Montgomery...Morgan...Murray...Muscogee...Newton... 
north Fulton...Oconee...Oglethorpe...Paulding...Peach... 
Pickens...Pike...Polk...Pulaski...Putnam...Rockdale...Schley... 
South Fulton...Spalding...Stewart...Sumter...Talbot... 
Taliaferro...Taylor...Telfair...Toombs...towns...Treutlen... 
Troup...Twiggs...Union...Upson...Walker...Walton...Warren... 
Washington...Webster...Wheeler...white...Whitfield...Wilcox... 
Wilkes...Wilkinson. 


&& 


$$ 


19/41 

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