829 
fxus63 klot 212031 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
231 PM CST Sat Nov 21 2009 


Discussion... 
230 PM CST 


Not much change in concerns from this morning. Fog potential still 
looms tonight despite passing ac advection northward through western Illinois. Dewpoints 
still near 40 tonight and with calm winds and nearly clear skies...visibilities 
should start lowering after mdngt and progress toward zero near sunrise. 
Ac passage northward through western Illinois will likely keep visibility up above advisory 
criteria for sections west of I-39. 


This passing ac to continue through Monday and prompt partly cloudy 
conditions until Monday night. While maximum temperatures try to warm up early 
in the week under increasing SW flow aloft...increased cloud cover 
and lower sun angle will inhibit the usual temperature jumps associated 
with early autumn warm air advection...but respond readily to any 
upcoming cold air advection. Thus highs early this week will hover 
in the 50s but settle into the 40s after Tuesday. 


Tuesday then is the focus of next concern. The upper level pattern 
associated with this upcoming change hasnt even made it onto the 
West Coast as yet...but should be making landfall late tonight and 
crossing the northern rockies Sunday night. Deepening Monday into a 
strong closed low in the plains...expect to see ac and cirrus advecting 
into the MS valley and precipitation chances increase Thursday Illinois late Monday night 
into Tuesday. 


All models are trending toward occluding this system from the middle 
and upper MS valley to the Great Lakes by midweek. Timing and storm 
center location differences are still apparent among them. The European model (ecmwf) 
is considerably slower than the GFS and NAM with the NAM bearing a 
more southerly track than either of the two. At this point...a blend is 
probably prudent. The general trend though is for rain to begin Tuesday 
with cold air wraparound filtering in Wednesday and mixing with 
snow Wednesday night before diminishing to scattered flurries Thursday. 


GFS and European model (ecmwf) actually resolve two distinct closed upper troughs 
swinging through late in the week. The first trough associated with the 
rain onset on Tuesday should swing through on Wednesday. Weak upper 
ridging may put an end to the rain Wednesday afternoon before a secondary 
clipper-like system swings down from Manitoba to northern Illinois Wednesday night. 
The second shot of cold air advection following this second trough passage will be 
the impulse that pulls down air cold enough to changeover to a snow 
mixture. Upper ridging working in Friday and Saturday will end any 
lingering flurrie chances while returning warm air advection for 
the weekend. 


Rlb 


&& 


Aviation... 


1800 UTC tafs...high pressure will remain anchored over the region 
through the next 24-30 hours. Close proximity of the high will 
keep winds calm to very light and mainly southeasterly. Return 
flow aloft is sending a bit of high based stratus/low based Alto 
cumulus northward into the middle Mississippi Valley this afternoon. 
Some of this cloudiness will spill eastward over the terminals 
tonight...particularly out west toward rfd. 


Despite the cloudiness...light winds and stagnant air mass will 
once again favor the development of patchy ground fog tonight. 
Even with fairly extensive cloudiness over Missouri last 
night...fog was still fairly prevalent...so even with potential 
for scattered-broken VFR deck tonight have continued to included reduced 
visibilities in mainly light/shallow fog. Dpa appears to have highest 
probs of bottoming out with visibility IFR or lower...though a period of 
IFR visibilities are even possible closer to urban heat island near 
Ord/mdw. Much like today...visibilities will likely be somewhat slow to 
climb above MVFR with stagnant airmass allowing for light fog to 
transition to lingering haziness. 


Izzi 


&& 


Marine... 
145 PM CST 


No significant winds/wave expected through the next several days 
with high pressure in control of the weather. Light/variable 
winds will tend to turn more southerly tonight...albeit not very 
strongly...and generally continue in a general southeasterly 
direction through midweek. 


Izzi 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...none. 
&& 


$$