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Area forecast discussion...updated 
National Weather Service Peachtree City Georgia 
1022 am EST Wednesday Nov 19 2008 


Update... 


Temperatures running a little below forecast...especially in 
central Georgia. Am not entirely confident that temperatures will warm up 
to forecast levels today...but then again with the dry air it is 
entirely possible. Have opted for a compromise and have gone ahead 
and lowered temperatures by a degree or two in those areas. Will go ahead 
and send a zone update since I have to remove the freeze warning 
wording and send the rfw update...but the changes to maximum temperatures are 
negligible as far as the zone wording is concerned. Dewpoints a 
little drier than forecast but this is not totally surprising with 
the Arctic high. All other grids look okay for now...zones will be 
out shortly. 


Tdp 


&& 


Previous discussion... /issued 530 am EST Wednesday Nov 19 2008/ 


Short term /today through Friday/... 
models in good agreement that the NE U.S./Se Canada polar vortex 
will remain the dominant feature through the short term...with a 
series of reinforcing short waves/Alberta clippers...keeping pattern 
in place. Second cold Canadian high of the week settling over the 
area now. Winds diminished to less than 5 knots and all areas as of 
4 am were below freezing with most spots in the 22 to 27 range...but 
some spots approaching 20 in the north. All areas will see a 
widespread freeze this morning...and in many cases a hard freeze of 
25 or less. No more freeze warnings needed for the fall season after 
this morning. 


The surface high will shift south of the County Warning Area by afternoon...allowing 
moderating westerly downslope flow to begin. Although it will be 
considerably warmer than what we saw Tuesday with temperatures some 20 degree f 
below normal...I do not foresee the kind of warmup we saw between 
cold fronts Monday...owing mostly to the very cold start this morning. 
850mb temperatures rise above zero by evening and to near +8c in the SW...but 
even a 30 degree surface temperature rise would still only bring maximum temperatures to 
near 50 in the NE to near 60 in the SW. Mav/met/FWC MOS in good 
agreement with this thinking. 


The next in the series of Alberta clippers and attendant large 
Canadian surface highs will begin to spread southeast into the U.S. Early Thursday 
as an impressive 1047mb surface high. The associated cold front should 
move into north Georgia early Thursday...then through the state during the day 
Thursday. Warm advection and increasing low-level westerly surface winds will 
result in a much warmer night tonight...but clear skies and 
radiational cooling the first half of the night will allow the usual 
spots...E.G...ffc...to decouple and still drop into the 20s. Some 
areas will remain at or above freezing tonight...however. Prefer the 
warmer met MOS for Thursday mins given synoptic pattern. Cold advection 
does not begin in earnest until after 18z...which should allow central 
areas to warm into the 60s Thursday...similar to what occurred Monday. 
Northern areas will struggle through the 50s as cold advection 
intensifies by afternoon. GFS now in good agreement with the European model (ecmwf) that 
another significant Canadian air mass will settle over the southeast U.S. 
By Friday...as indicated by the European model (ecmwf) all along. Although the upper 
trough is not quite as deep into the southern latitudes as the 
current trough...the mere strength of the surface high and associated 
low-level cold pool provide ME with ample support to lean toward the 
much colder met MOS and undercut the mav MOS by a good 5-7 degrees. 
The met MOS has done remarkably well this week on the cold side of 
these systems. 850mb temperatures near -10c north to 0c south support 
another well below normal day...within a few degrees of maximum temperatures 
observed Tuesday. 


Strong northwest low-level flow at a minimum supports orographic/cold 
advection induced SC NE mountains late Thursday/early Friday...but do not 
feel that moisture is deep enough for -shsn at this point. A few 
middle clouds may skirt northern areas early Friday otherwise...skies 
remain sky clear with a very dry air mass remaining over the region. Precipitable water 
generally less than 0.20 all short term periods. Winds will become 
gusty again Thursday afternoon as the front passes...but not supportive of a 
Wind Advisory at this time. 


Long term /Friday night through Tuesday/... 
the GFS has remarkably come into agreement with what the European model (ecmwf) 
indicated all along regarding the Monday-Tuesday weather system to affect 
the southeast U.S. A positively tilted upper trough will deepen rapidly as 
it moves southeast from Alberta Monday-Tuesday into the mean eastern U.S. Trough 
position. Surface/Gulf moisture return is expected to be quite 
limited...if much at all...as the large Canadian anticyclone that 
spreads into the area Friday slowly retreats eastward by Monday. Dewpoints 
generally only recover to the 30s or low 40s at best. 
All-in-all...the synoptic pattern is not favorable for much 
precipitation...but upper trough dynamics should provide enough support to 
continue the currently advertised slight chance probability of precipitation. Timing in the 
Monday night-Tuesday period has not changed from earlier thinking...so will 
not make any changes to this portion of the forecast at this time. 
Beyond this and into Thanksgiving day...the European model (ecmwf) is emphatic that 
the NE U.S. Polar vortex will remain the predominate 
feature...keeping a deep northwest flow aloft over the southeast U.S. Another 
surge of Canadian air will spread southeast from central Canada into the southeast 
U.S. By Thanksgiving day. After a slightly milder period early in 
the week...temperatures are expected to be well below normal again by 
Thanksgiving. However...the very dry air mass will result in clear 
skies and otherwise excellent travel conditions for the Holiday. Mex 
temperature data is too warm and trending toward climatology for Wednesday into 
Thanksgiving day and not supported by advertised synoptic pattern. 


Fire weather... 
rfw will remain in place for today. Relative humidity values will drop into the 
teens many areas by midday with temperatures rising into the 50s and 
dewpoints in the teens...approaching 10 in some areas of the north. 
Wind will not be a factor today. For Thursday...critical relative humidity values not 
forecast as dewpoints rise into the low 30s. However...with dry 
fuels in place...will need to watch winds as by afternoon some areas in 
the north could be 15kts or higher. Since rfw/fww criteria not met 
at this point...will not issue any watch/warning for Thursday...but it 
needs to be watched. For Friday...however...the next surge of dry 
Canadian air will potentially result in critical relative humidity values. Winds 
may still be in the 15kt range...so will issue a fww watch for Friday 
afternoon with this package. Rfw criteria appears to be amply met for 
Sat-sun...but too early to issue any watch/warning for those periods. 


Aviation... 
high pressure centered over the southeast states will drift south of 
the area during the day. Skies will remain clear and visibility 
unrestricted. Light and variable winds this morning will become west 
to southwest 10kt or less by afternoon. 


&& 


Preliminary point temps/pops... 
Athens 54 33 61 30 47 / 0 0 0 0 0 
Atlanta 52 36 59 28 44 / 0 0 0 0 0 
Blairsville 47 28 51 20 36 / 0 0 5 10 5 
Cartersville 51 27 55 25 42 / 0 0 0 0 0 
Columbus 55 33 63 31 49 / 0 0 0 0 0 
Gainesville 51 33 55 26 42 / 0 0 0 0 0 
Macon 55 31 64 31 50 / 0 0 0 0 0 
Rome 53 30 54 25 43 / 0 0 0 0 0 
Peachtree City 52 28 60 28 45 / 0 0 0 0 0 
Vidalia 56 32 65 34 55 / 0 0 0 0 0 


&& 


Ffc watches/warnings/advisories... 
red flag warning until 6 PM EST this afternoon for all of north 
and central Georgia. 


Fire Weather Watch Friday afternoon for all of north and central 
Georgia. 


&& 


$$ 

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