Weather

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fxus62 kffc 060115 aaa 
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Area forecast discussion...updated 
National Weather Service Peachtree City Georgia 
815 PM EST Monday Jan 5 2009 


Update... 
with a large area of rain bearing down on north Georgia probability of precipitation were 
raised. 


&& 


Short term /tonight through Wednesday night/... 
Arctic front located just north of the forecast area this 
afternoon...creeping into the far northwest corner of Georgia. Dramatic 
temperature difference noted along the front with 30s as close as 
bna and hun...50s northwest Georgia...and 60s northwest Georgia to near 80 southeast Georgia. Parent 
Arctic high located over MO...shifting east-northeast under strong SW flow with 
deepening upper trough SW U.S. And persistent subtropical ridge 
aloft across the eastern Gulf. This scenario will not allow the 
front to make any further progress southward and it will likely 
remain stationary near its current position through Tuesday... 
perhaps sinking a little further southeastward into northwest Georgia overnight as waves 
of surface low pressure ride along the front. For the first 24 
hours...the majority of the rain will remain along and north of the 
front...with little to no precipitation expected further south across central 
areas deeper into the warm sector and upper-level subsidence. In 
fact...a brief amplification of the upper ridge Tuesday morning/early 
afternoon...will shunt the bulk of the precipitation into far northwest/NC Georgia until after 
18z. By late Tuesday and overnight Wednesday...the upper trough begins to 
shift eastward in response to upstream Pacific energy...finally 
allowing the front to move through the County Warning Area. It is during this period 
that the majority of the County Warning Area will see the effects of this weather 
system. 


The main concern from this system will be heavy rain...especially 
across the northern counties. In general...GFS...ECMWF...NAM show 
heaviest rainfall along and either side of the Tennessee/Georgia border through 
the next 24 hours. Current flash flood advisory well handles situation of heavier 
rain affecting these areas later tonight and through the day 
Tuesday...with lingering river flooding likely into Wednesday 
morning. A situation similar to the 10-11 December event...in terms 
of rainfall...might be possible in the northern counties Tuesday-early 
Wednesday...with 2-4 inches of rainfall during this event. Further 
south...rainfall of significance will likely be confined to the 
frontal passage period late Tuesday- early Wednesday. Duration and total quantitative precipitation forecast 
in these areas likely not sufficient to result in flood concerns 
other than on an isolated basis. Do not plan to make any changes 
at this time to the flash flood advisory area or expiration time. Strong subsidence 
aloft not supportive of much thunder tonight and rain across 
northern areas will largely be isentropic along and north of the 
stalled frontal boundary...so will leave thunder out of the zone 
package through 12z. Second concern is severe threat...which 
overall appears minimal at this time. Instability is marginal at 
best with severe threat should be confined to the 00z-06z time 
frame Tuesday evening. Severe threat hinged primarily on a strong 
50-60kt low level jet jet translating across the area during this time 
frame...as instability is minimal at best with a small tongue of 
lifted indices 0 to -1 and convective available potential energy less than 200 j/kg. Latest outlook from Storm Prediction Center 
has removed Georgia from the slight risk area. However...a squall line 
of rain showers/thunderstorms and rain is likely as the front finally moves through the area 
Tuesday evening. Convective activity will be able to tap into the 
strong low-level winds and could result in some wind gusts near 
severe speeds. After 06z...main concern will be potential river 
flooding across northern zones. By midday Wednesday...strong cold 
advection will be in place from northwest- southeast...as 850mb temperatures quickly 
plummet below zero across the north by afternoon. A non-diurnal temperature trend 
is in order. Wrap around moisture...strong cold advection and 
associated orographic lifting supportive of continuing chance -shsn 
mountains areas Wednesday night into Thursday morning. 


Later shifts will need to monitor winds Tuesday. As the low level jet tracks 
across the state...there is some potential for winds to reach Wind 
Advisory criteria...especially in the north. It is more likely that 
winds will reach Wind Advisory criteria Wednesday behind the cold front. 
Confidence in winds meeting Wind Advisory criteria Tuesday is not high 
enough to warrant a Wind Advisory at this time. 


Long term /Thursday through Monday/... 
the extended offers somewhat of a pattern change. The GFS has been 
hinting at this for a couple of weeks now...and it gradually appears 
to be taking place. The European model (ecmwf) is somewhat more encouraging about a 
pattern change than the GFS is. Most of the extended will be 
dominated by a deep eastern U.S. Trough...hence much colder and 
mostly dry conditions for a change. Expect near or slightly below 
normal temperatures through the period...but the coldest air will be held 
north of 40 degree by the persistent strong westerlies and tendency to 
trend toward zonal flow...even throughout the period. The next precipitation 
system is slated to affect the area by sun as a short wave in 
general northwest flow aloft. There are some discrepancies among the models 
in this period...but they are coming closer to one another. The 12z 
run of the GFS is much weaker and showing considerably less moisture 
return than on the 00z run. I have not seen the 12z European model (ecmwf)...but the 
00z run was not as strong with this system as the 00z GFS. No 
instability and an overall appearance of an Alberta clipper type 
system lead ME to believe that this is just a minimal shower event. 
Dewpoints are only prognosticated to be in the 20s and 30s in advance of 
this system...so little to no appreciable moisture return in the 
wake of the middle-week system. Another Canadian high will follow this 
system with a continuation of an overall cool...dry period. Have 
followed the European model (ecmwf) and HPC beyond this...neither of which are 
depicting a major weather system into the area by early next 
week...as the 12z GFS currently shows. 


&& 


Aviation... 
the break in the low ceilings for atl expected to be short-lived. MVFR 
ceilings just to our west will move in this evening bringing ceilings back 
to around 1500 feet. Ceilings will stay MVFR...going below 1000 feet toward 
after 06z. Ceilings should rise to MVFR late Tuesday morning but at this 
time do not expect anything above MVFR due to abundant moisture over 
the area. Expect rain widespread rain to move in around 06z and hold 
in the area through 12z with visibilities holding in the 2 to 5 mile range. 
There should be a break in the precipitation late Tuesday morning before 
redeveloping after 18z. There is also the risk of thunder after 18z 
most areas until the frontal passage which should be close to 06z in atl and 
by 12z elsewhere. Winds will remain southwest through the forecast. 
Winds could get a bit gusty Tuesday afternoon with the low level jet 
of 35 to 50 kts over the region. 


&& 


Preliminary point temps/pops... 
Athens 56 69 55 55 37 / 50 70 100 100 5 
Atlanta 60 67 50 52 37 / 50 90 100 80 5 
Blairsville 54 56 46 48 32 / 100 100 100 80 20 
Cartersville 57 63 48 50 35 / 70 100 100 60 20 
Columbus 62 74 57 60 38 / 30 50 100 80 5 
Gainesville 56 59 50 52 36 / 70 100 100 80 20 
Macon 58 76 59 60 40 / 30 20 100 100 5 
Rome 56 61 48 50 36 / 90 100 100 60 20 
Peachtree City 59 69 52 54 37 / 50 70 100 80 5 
Vidalia 59 78 62 66 41 / 20 10 100 100 5 


&& 


Ffc watches/warnings/advisories... 
Flood Watch from Tuesday morning through Wednesday afternoon for 
the following zones: Bartow...Catoosa...Chattooga...Cherokee... 
Cobb...Dade...Dawson...Fannin...Floyd...Forsyth...Gilmer... 
Gordon...Haralson...Lumpkin...Murray...north Fulton...Paulding... 
Pickens...Polk...towns...Union...Walker...white...Whitfield. 


&& 


$$ 


19/15 

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