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fxus63 kdvn 080929 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
300 am CST Thursday Jan 8 2009 


Synopsis... 
deep northwest flow prevails over region early this morning. Weak impulse 
seen in satellite diving down backside of eastern Continental U.S. Trough 
and passing through northern Illinois. Area of low clouds are wrapping in 
behind departing shortwave and dropping south out of WI... accompanied 
by few flurries. 00z subjective upper air analysis shows western ridge 
with core of 130 knots 300 mb jet arcing from Pacific northwest into Southern Plains 
in advance of middle level shortwave approaching the northwest. At the 
surface... ridge axis extends from the northern plains through the Missouri 
River valley. Local area on cyclonic side and still cold advecting with 
temperatures in single digits northwest to the teens south. ..05.. 


&& 


Short term...today and tonight... 
main challenges are temperatures and accumulating snow tonight. 


Today... 
weak shortwave over northern Illinois will continue to depart... with some 
low clouds with few flurries wrapping in around and propagating 
south across the east to northeast 1/3-1/2 this morning. Elsewhere... 
sunny skies... which will develop east by midday as subsidence builds 
in with approaching surface ridge axis. Then by middle to late afternoon into 
early evening expect increasing high cloudiness with 300 mb jet passing through. 
High temperatures a bit of a challenge being that we will still be cold 
advecting this morning and given limited mixing with strengthening low level 
inversion. Given these factors sided with coldest guidance... but still 
could be 1-3 degrees too warm some areas. 


Tonight... 
middle level shortwave approaching Pacific northwest early this morning will 
dive across northern rockies inducing cyclogenesis over the northern 
plains with warm front draped from NE through northern MO. Surface ridge 
axis will shift east in the evening with return flow developing and 
strengthening. This along with thickening clouds to support temperatures 
steady to slowly rising after middle evening lows... and have non diurnal 
trends in grids/zfp. Snow over northern plains to develop/streak east/southeast 
this evening in zone of very impressive warm advection/isentropic lift 
and middle level frontogenesis (h85-h7) which develops across area. This 
should result in rapid top down saturation shortly after 06z northwest 
with snow shifting across north half or so through 12z. Various 
techniques suggest bulk of snow mainly along and northeast of Washington 
Iowa to Galesburg Illinois axis. Deep saturation with favorable dendritic profiles 
with temperatures in range of -13c to -16c near 600 mb within/near maximum vertical 
motion support bursts of moderate to possibly locally heavy snow for 
brief period. Slr ratios on average of 15 to 18:1 with near 0.10 quantitative precipitation forecast 
supports maximum of around 2 inches in some locales north of Highway 30... 
with amounts tapering off to south with up to an inch I-80 corridor. 
Still some uncertainty though with placement of baroclinic zone... which 
could shift axis. ..05.. 


Long term...Friday through next Wednesday... 
initial burst of warm air advection precipitation will exit the County warning forecast area during the morning 
but not before depositing some accumulating snow across the northern 
third. Forcing suggests that the precipitation should really shut 
down or be nothing more than very light precipitation for about 6 hours until 
the next round of forcing arrives late in the afternoon. Going with 
the idea of clouds/very light precipitation for much of the day... 
temperatures should remain somewhat in check. 


Friday night/Saturday the main system precipitation with the clipper system 
moves through the area. May or may not see a brief period of sleet 
during the transition Friday evening over about the south half. 
Highway 30 corridor on north should see another round of 
accumulating snow which may warrant an advisory. However...since 
there will be a lull of nearly 6 hours before the snow resumes...will 
not issue an advisory at this time. Lingering flurries or perhaps some shsn 
should be see Saturday morning. 


Quiet weather is then expected Saturday afternoon through Sunday as the 
next Canadian high moves across the area. 


Sunday night on is getting more interesting. Models suggest another 
clipper system will move through the area bringing some more snow. 
Pattern aloft is becoming more meridional with time and Arctic air 
will follow this clipper system. There are also hints of additional 
clipper systems the middle of next week and toward the end of next 
week. Did not make much if any in the way of changes from Sunday 
night on due to timing issues. Nonetheless...the signal indicates 
temperatures well below normal for most of next week. ..08.. 


&& 


Aviation... 
low clouds will drop south out of WI through northern Illinois this morning. 
Latest satellite trends suggest bulk of clouds to stay just east 
of kdbq and kmli sites. But... lingering cyclonic flow and low level 
relative humidity could produce some stratocu by middle morning kdbq and kmli with 
potential for brief period of MVFR ceilings. Northwest winds to prevail 
this morning at terminals with some gusts possible. Surface ridge axis 
builds in this afternoon with winds becoming light and variable. Increase 
in high cloudiness expected into early evening with upper jet core shifting 
across region. Surface high then slides east this evening with southeast to 
south winds developing by 06z and increasing to around 10 kts overnight. 
Warm advection/isentropic lift to bring snow across mainly northern 
terminals (kdbq...kcid and kmli) after 06z. Ceilings/visibilities likely to quickly 
deteriorate by 08z-10z to IFR with potential for period of LIFR conditions 
especially at kdbq. ..05.. 


&& 


Dvn watches/warnings/advisories... 
Iowa...none. 
Illinois...none. 
MO...none. 


&& 


$$ 


05/08