Weather

895 
fxus63 klot 080811 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
311 am CDT Monday Sep 8 2008 


Discussion... 
251 PM CDT 


Little or no change to going forecast through tonight. Diurnal cumulus field 
should dissipate Erly this evening ahead of cloud band assd with 
front now moving into upper MS valley. Focus of concern this forecast 
issuance is with upcoming rain event Monday into Monday night. 
Current water vapor imagery shows short wave over western Ontario 
lifting east-northeastward. This will continue to push front southeastward across upper 
Great Lakes...and across forecast area Monday morning. Meanwhile...of greater 
importance...is another jet maximum indicated in water vapor imagery 
emerging eastward from southern Montana/northern Wyoming. This feature is forecast to amplify 
as it heads eastward tonight...causing low level flow to back and increase 
overnight...with deeper low level moisture from Ozarks...southward starting 
to advect northward into MO valley by 12z Monday. As surface wave moves east-northeastward along 
frontal boundary...frontogenetic forcing to increase over 
forecast area...particularly over north central and extreme northestern Illinois...with strong moisture 
convergence overriding this boundary during afternoon and evening. As a 
result...will go with categorical probability of precipitation rfd-chi areas. Forecast 
soundings show little or no instability...but given increase in low level flow 
into frontal zone...will at least mention so isolate thunder over 
southern/eastern portions of forecast area where there may be enough limited sunshine to 
allow surface temperatures to rise to around 80. As short wave continues to 
amplify and become negative tilted as it passes across western Great Lakes on 
Monday...decent pressure couplet expected to develop that will bring a 
period of rapid surface pressure rises across area Monday night along with 
rather breezy conditions and drying by Tuesday morning. Building surface 
hi pressure then dominates our weather Tuesday afternoon-Wednesday with clearing skies 
and cool...almost fall like conditions. 


A little less confidence in Thursday-sun forecast. Another cold front will be 
moving eastward into upper MS valley on Thursday...bringing at least a chance 
of some showers. This is followed by some clearing Friday-Friday night before 
middle level ridging develops from Ozarks southeastward across southeastern U.S. As this 
ridging develops...low level flow should back and allow more tropical air 
mass to develop northward across Midwest...thus incrsg precipitation chances over next 
weekend. Combination of some model inconsistency...and potl 
interaction with Hurricane Ike...forecast by GFS/European to be approaching Texas 
coast as a major hurricane on Sat...make extended forecast a bit 
problematic. 


Merzlock 


&& 


Aviation... 


0600 UTC tafs...not much change from previous set of tafs. Still 
appears as though middle-level deck will overspread the terminals 
overnight. A few echos showing up on radar over Iowa...but not 
expecting much more than a sprinkle or two overnight so kept -ra 
mention out of tafs overnight. The greater aviation concern will 
be on the storm system scheduled to arrive Monday. 
Rain...initially light...will overspread the terminals from west 
to east during the late morning/afternoon Monday. Expect VFR 
ceilings/visibility initially with the rain as low levels start off rather 
dry...but with time as rain picks up and low levels saturate 
should see ceilings/visibility drop to MVFR during the middle/late afternoon. 
Rain should taper off some Monday evening...however as very moist 
boundary layer cools would expect ceilings to build downward. For now 
only taking ceilings to bottom of MVFR in tafs...though IFR ceilings are a 
distinct possibility with sref probs of cigs<1000ft up to nearly 
50 percent Monday evening. 


Izzi 


&& 


Marine... 


300 am CDT...high pressure over the Ohio Valley... will move off 
the middle Atlantic coast tonight. Low pressure near the southern 
tip of James Bay...will move northeast to far northern Quebec this 
afternoon. A trough of low pressure extending south from this low 
and an associated cold front...will move across Lake Michigan this 
morning. Another area of low pressure will develop over the 
Central Plains today...and move northeast across central Illinois 
and Indiana tonight. The low will then move into the eastern lakes 
on Tuesday. High pressure over the central and northern plains 
will build east across the middle Mississippi Valley Tuesday and 
into the western Great Lakes Tuesday night. 


&& 


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


$$