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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1101 PM CST Tuesday Dec 2 2008 


Discussion... 
/348 PM CST Tuesday Dec 2 2008/ 
short range models remain in excellent agreement with their handling of 
moisture...dynamics and thermal profiles with with Wednesday system...so overall 
forecast trends have changed very little in this forecast pkg. Although Wednesday may 
dawn with dry weather...increasing moisture and upward vertical velocity acr area will quickly saturate 
air mass leading to development of precipitation during the day. All guidance suggests that 
north areas will saturate first with a mix of rain/snow developing in the 
cold Post frontal air mass...with cooling certainly aided by evaporative 
processes in the lower tropopause. Models have been consistent forecasting strong upward vertical velocity 
over this area from aprchg shtwv...as well as strong divergence in rrq 
of jet streak acr upper gtlks. Meanwhile in the S...moisture will 
initially be stratified in the lower levels of the air mass...but will deepen 
during the day as shtwv pushes out of S plains. Forecast probability of precipitation for entire 
forecast area remain some 20-30% higher than FWC or mav MOS for Wednesday....with a 
similar upward adjacent in east half of forecast area for Wednesday evening before precipitation finally 
exits County Warning Area bu 06z. 


Cold air on the north side of this system is fairly strong and deep...so as 
it penetrates southward during the day precipitation will mix with and eventually change 
to snow. In most areas the chgovr will occur as the hviest precipitation ends 
so accumulations should be spotty and fairly minor. The subtle 
exception to this may be in the north where snowfall should be sustained 
for the longest period of time...and quantitative precipitation forecast over this area suggests just over 
an inch of accumulate snow. Again...this overall thinking is already 
relfected in current forecasts. 


Cold air plunges acr area Wednesday night and Thursday. With sm residual clouds and 
mixing MOS temperatures dont look too bad for Wednesday night...but they Apr to be 
sm 5 degrees too warm on Thursday with 850 mb temperatures prognosticated to be somewhere 
between -8 amend -12c. Middle clouds ovrsprdg S part of forecast area area during the afternoon 
ahead of secondary shtwv should also put a Dent in the wrmup. 


Ovrall...cold air will remain entrenched in the Friday-sun time frame... 
although clipper system will attempt a brief wrmup on Sat. Still aprs 
that any precipitation with Sat system will remain NE of our forecast area...so forecast remains dry 
for Sat. 


European model (ecmwf) and GFS are maintaining very gd agreement with their handling of 
trough early next week. Although am normally a bit hesitant to intro probability of precipitation 
in the lt periods...consistent handling of this ftr gives enough confidence 
to add precipitation at this rather early juncture. 


Truett 


&& 


Aviation... 
/1100 PM CST Tuesday Dec 2 2008/ 
for the 06z tafs... 


Arctic cold front making steady progress south across upper 
Midwest and plains. Wind remains gusty to around 20kts out of the 
south...with almost 50 kts at 2kft off the deck. Low level wind shear will remain 
in forecast through tomorrow morning. High and middle level clouds 
are on the increase from the west and this will continue overnight. 


Front still on target to reach kcou and KUIN around 18z...and kstl 
and ksus around 19z. Appears there will be two areas of 
precipitation tomorrow. The first will be light rain showers associated 
with warm advection ahead of the front...and the second will be an 
area of snow that will form behind the front in association with 
upper level jet coupling and frontogensis. 


Atmosphere will quickly moisten and ceilings will drop tomorrow 
morning abruptly to MVFR category...and then eventually IFR as the 
front nears. IFR should prevail Post frontal...until drier air 
works into the area tomorrow evening. 


Model cross sections depict a one to three hour window of intense 
lift through the dendritic snow growth region of the 
atmosphere...which is often a signal of a short burst of moderate 
or even heavy snow. Appears KUIN will be subject to this during 
the late afternoon...but would not be surprised to see it occur 
further south into the St. Louis metropolitan area as well during the 
early evening hours. 


Wind will shift to the northwest and gust to at least 25kt Post 
frontal with strong cold advection taking hold of the region 
tomorrow night. 


Cvking 


&& 


Lsx watches/warnings/advisories... 
MO...none. 
Illinois...none. 
&& 


$$ 


Weather forecast office lsx