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fxus63 klsx 210030 
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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
730 PM CDT Sat Mar 20 2010 


Discussion... 
/318 PM CDT Sat Mar 20 2010/ 


Plan on leaving going advisory as is for this forecast. Models 
remain in relatively good agreement that closed currently over the 
Southern Plains will move slowly eastward into the lower Mississippi River 
valley on Sunday before lifting northeastward in the middle Atlantic. Rain/snow 
over the northwestern half of the County Warning Area currently is being generated by 
decent 800-600mb frontogenesis which is prognosticated to gradually 
diminish through the night. By Sunday morning...greatest 
likelihood of precipitation will shift to the southern County Warning Area where middle 
level frontogenesis/low-mid level moisture convergence will 
markedly increase. Forcing in the northern part of the County Warning Area will 
decrease bringing only slight chance/chance probability of precipitation for the wintry mix. Focus 
for precipitation will then gradually shift into the southeastern County Warning Area by late 
Sunday/early Monday as upper low lifts out. 


This intense system will continue to wrap warm air around it...with 
forecast soundings from both the NAM/GFS indicating that thermal 
profiles in the area currently seeing snow will climb above 
+1c in the 950-850mb level...and eventually above +3c by midnight 
which favors snow changing to sleet and then rain or freezing rain 
late tonight/early Sunday depending of surface temperature. The 
heaviest axis of additional snow/sleet of up to 2" will lie 
northwest of a cou-uin line tonight. Have included the mention of 
-fzra over central/northestern MO and west central Illinois late tonight/early Sunday 
morning as lower atmosphere continues to warm...but do not expect 
much ice accumulation. Precipitation axis will be shifting southward and 
ground temperatures according to MO ag weather observation are in the middle 30s to 
around 40. 


Did back off on high temperatures a bit the next two days based on 
clouds and precipitation. 


Main question for the extended part of the forecast is timing and 
areal extend of precipitation with next closed low moving into the 
central Continental U.S. Late next week. While the GFS/Gem/ECMWF are showing some 
minor continuity issues and disagreement with the strength/timing of 
this system...there is enough agreement to go continue chance of 
rain showers over parts of the County Warning Area from Wednesday night into early Friday. 
The forecast depth of this closed low will not be as deep as the 
one currently moving across the plains and forecast soundings are 
relatively warm...so have not mention of wintry precipitation. 


Britt/Byrd 


&& 


Aviation... 
/646 PM CDT Sat Mar 20 2010/ 


Primary concern this forecast period is ptype at KUIN/kcou as warmer air 
aloft wraps into the low pressure system and eventually causes a 
transition from -sn to -ra. Bufr soundings suggest a few hours of 
ice pellets or -fzra is possible at KUIN/kcou during the 
transition but regional metars over the past few hours have not 
been reporting these ptypes...therefore did not include in the 00z 
tafs. -Ra should end by Sunday morning with IFR to MVFR ceilings and 
gusty northeasterly winds persisting through the 24hr taf period. 


Specifics for kstl...kstl/ksus are far enough south and east that 
precipitation will fall as rain and have continued mention of -ra at 
kstl/ksus given increased radar echoes over the past hour. Solid 
MVFR ceilings will gradually lower to IFR overnight before rising again 
on Sunday as the system departs. Gusty northeasterly winds will continue 
through the 24hr taf period. Models indicate that there may be a brief 
period of -ra near the end of the 30hr taf period as an area of moisture 
convergence develops through parts of central MO/IL. 


Kanofsky 


&& 


Lsx watches/warnings/advisories... 
MO...Winter Weather Advisory until 1 PM CDT Sunday for Audrain MO- 
Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Marion MO- 
Moniteau MO-Monroe MO-Ralls MO-Shelby MO. 


Illinois...Winter Weather Advisory until 1 PM CDT Sunday for Adams Illinois. 


&& 


$$ 


Weather forecast office lsx