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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
649 am CDT Thursday Mar 18 2010 


Discussion... 
/459 am CDT Thursday Mar 18 2010/ 


Only forecast concern today is the dense fog which has developed 
mainly in low lying areas and river valleys from jef to uin. This 
fog should dissipate quickly later this morning with full sunshine 
expected today causing temperatures to warm into the 60s this afternoon. Do not 
think there will be as much fog late tgt with plenty of daytime 
heating and mixing later this morning and afternoon although the sky will be 
mostly clear again tgt. There will be a little more cloudiness on 
Friday but with S-southwesterly low level winds the maximum temperatures should be 
a couple of degrees warmer maybe hitting 70 in spots. The weather 
will get more interesting beginning Friday night and continuing through 
the weekend. A cold front will be sagging southeastward through the County Warning Area Friday night 
likely making it down to stl by 12z Sat which is a little quicker 
than the past model runs. Discounted the 00z Thursday NAM model run as 
it is the outlier with its quick progession of the approaching 
middle-uppper level trough and cold front as well as the lack of a surface low 
or wave. Went mainly with a blend of the 00z Thursday GFS and European model (ecmwf) 
models...although it appears that the European model (ecmwf) may be a little slow 
with the movement of the closed middle-upper level low and associated 
surface low. The precipitation Friday night will be mainly Post frontal and 
mainly over the northwestern portion of the County Warning Area. There may be a change 
over to snow late Friday night over the extreme northwestern counties of the 
County Warning Area as much colder air filters into the County Warning Area behind the cold front. 
Precipitation should overspread much of the County Warning Area on Sat as upper 
level divergence increases over the County Warning Area ahead of the approaching 
middle-upper level trough and between the ulj streak over the Great 
Lakes region and the ulj streak over the Southern Plains in the base of 
the upper level trough. The middle-upper level trough will become closed 
and cutoff over eastern OK by late Sat with an associated surface low 
ahead of it over Arkansas. The middle-upper level low will move eastward through Arkansas 
Sat night with the surface low moving east-northeastward through southeastern MO. Although this 
track should put the County Warning Area in a good position for snow the GFS and 
European model (ecmwf) has a split flow regime with the large scale cold air 
remaining well north of the area and only a localized area of colder 
air underneath the middle level low. The models also wrap up the 
system so much that warm air wraps around the northern portion of the 
surface/850 low Sat night and sun which complicates the p-type forecast. 
For now will forecast rain or snow over the northern and western portion of 
the County Warning Area on Sat...then a gradual change over to snow from the northwest 
to southeast Sat night...then rain or snow Sun afternoon. With plenty of 
uncertainty with p-type and precipitation amounts...as well as 
precipitation intensity and surface temperatures to allow the snow to 
accumulate on warm surface/ground it is too early to forecast specific 
total snow accumulation amounts although this storm system bears 
watching as it could produce heavy amounts over a small area. 


Gks 


&& 


Aviation... 
/626 am CDT Thursday Mar 18 2010/ 


High pressure currently dominating the area has resulted in good 
radiational cooling and fog development. LIFR conditions/dense fog 
is confined primarily to portions of west central Missouri through 
northeast Missouri into west central Illinois including KUIN. 
Predominately MVFR exists elsewhere. Fog should persist for 
several more hours then visibilities are expected to improve 
quickly between 14-15z as fog dissipates/Burns off. Some scattered cumulus 
expected in response to heating as the morning progresses. This 
will dissipate by sunset. Westerly winds today will back to more 
southerly tonight as the high pressure center sinks into the Gulf. 


Specifics for kstl...morning satellite low cloud enhancement 
showing some fingers of low stratus just west of kstl protruding 
towards the southeast. Thinking this will erode quickly once the 
sun rises and shouldnt impact the terminal. MVFR visibilities with fog 
should persist for several more hours until mixing commences with 
heating this morning. VFR conditions expected after the morning 
fog dissipates with some scattered cumulus based from 3000-4000 feet during the 
later half of the morning into the afternoon. 


Glass 


&& 


Lsx watches/warnings/advisories... 
MO...dense fog advisory until 10 am CDT this morning for Audrain MO- 
Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Marion MO- 
Moniteau MO-Monroe MO-Osage MO-Pike MO-Ralls MO-Shelby MO. 


Illinois...dense fog advisory until 10 am CDT this morning for Adams Illinois- 
Brown Illinois-Pike Illinois. 


&& 


$$ 


Weather forecast office lsx