614 
fxus63 klot 060000 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
700 PM CDT Sat Jul 5 2008 


Discussion...regarding afternoon zones/grids... 
300 PM CDT 


Not much change to going forecast...especially through near term...as area 
remains dominated by sprawling hi pressure over Great Lakes. Result will 
be a continuation of pleasant...low humidity conditions with cool 
nights and warm days tonight and Sunday. Sunday will be about 5 f degree 
warmer on highs as residual Continental air mass warms and weak warm air advection 
begins as surface hi moves slowly off to east. Fairly deep low pressure 
center will pass well to north...into northern Ontario...on Sunday...allowing 
somewhat warmer air to be drawn northward across Midwest. By late 
Sunday-Sunday night...low level advection of tropical air is expected to 
increases ahead of frontal boundary trailing west-southwestward from Canadian low. 
This will focus best Theta-E forcing heading into Sun night over eastern 
Nebraska-western Iowa (big surprise there). Some of the resulting convection 
may develop into western/extreme northern Illinois by daybreak Monday. Have fashioned 
pop grids to reflect this scenario based on posn of trailing front 
to north...focus of Theta-E convergence...and persistence...which would 
dictate higher probability of precipitation over northern tier of Illinois counties in closer proximity to 
better forcing...and over southwestern counties where instability may be 
greater...with least probability of precipitation along I-80 corridor. Thunderstorm chances to increases Monday 
as both GFS and NAM forecast soundings show instability becoming rather high 
by Monday afternoon with GFS showing convective available potential energy above 3000 j/kg. Again...favored 
area for thunderstorm coverage looks like extreme northern Illinois where low level forcing 
will focus development. Have also bumped up temperatures a bit Sun night-Monday as 
per MOS numbers...and ditto on dew points. Result will be a very 
warm and muggy period beginning Sun night and peaking Monday-Monday night. Of 
course...this could be tempered by any convective debris. Models 
diverge somewhat Monday night-Tuesday regarding surface low development along western 
extension of trailing boundary with GFS being more vigorous with 
development...and a little faster and further S with surface low. 
Nonetheless...looks like best chances for thunderstorms will shift off to east on 
Tuesday...and exit forecast area by Wednesday morning...with fairly good model consistency 
in bringing in clearing/cooler/less humid regime middle-late week as 
another short wave is forecast to dig southeastward from central can to upper Great Lakes 
during this period. 


Merzlock 


&& 


Aviation... 


700 PM CDT 


For 00z tafs...surface high pressure over Great Lakes continues to 
control weather across forecast area this evening with quiet VFR 
conditions to continue into Sunday. Main foci are short term winds 
and decay of lake breeze circulation this evening...then small but 
increasing threat of convection close to the forecast area by the 
end of the 24 hour taf period. 


Radar indicates lake breeze boundary has pushed well west of 
kord/kmdw terminals...and was nearing kdpa at 2335z. This boundary 
separates easterly winds at kord/kmdw from south winds across 
remainder of northern Illinois. Kgyy with typical light gradient Northeast 
Lake breeze component. Lake breeze circulation should weaken and 
veer more southeast with diurnal loss of insolation next couple of 
hours...with light south/southeast gradient winds less than 10 knots 
expected for overnight hours as surface ridge slowly drifts off to 
the east. Southerly winds expected Sunday as gradient tightens in 
response to low pressure moving across southern Canada...and middle 
level short wave trough which propagates across upper Midwest and 
northern lakes. Forecast soundings from both NAM/GFS indicate some 
gustiness developing as boundary mixes out to about 850 hpa Sunday 
afternoon...supporting gusts into the 15-20 knot range especially 
across western parts of forecast area...where surface pressure 
gradient a little stronger closer to pressure trough across plains. 
Magnitude of south gradient winds should prevent lake breeze from 
reaching any of terminals Sunday afternoon. 


As for convection...GOES vapor loop shows middle level short wave 
trough across northern plains this afternoon...with low level jet 
across Upper/Middle Missouri Valley. RUC/NAM develop modest low level 
jet axis into upper Mississippi Valley Sunday as short wave 
continues to move east across northern lakes region...with 850 hpa 
Theta/East Ridge and zone of moderate moisture advection/convergence 
developing from Iowa/southeast Minnesota/western Wisconsin. This indicates 
some potential for convective development across those areas. 
850-300 hpa thickness field does suggest that propagation of 
convection from Iowa/MN/WI should be off to the south...potentially 
into northwest/north central Illinois. Airmass across forecast area not 
expected to be favorable for continued development however...with 
retreating surface ridge and drier/more stable low levels. NAM 
forecast soundings do indicate some moistening of middle levels 
suggesting some convective debris cloudiness may spill into northern 
Illinois later in the day. Based on these factors will indicate a little 
more cloudiness into krfd during the afternoon though will maintain 
dry forecast at this time. 


Ratzer 


&& 


Marine... 
221 PM CDT 


Surface ridge axis across the Great Lakes will continue eastward 
tonight into tomorrow with southerly winds across Lake Michigan. 
An upper trough moving across southern Canada will drag a cold 
front across the northern Great Lakes Sunday night into early 
Monday before stalling out across Northern Lake Michigan. Another 
area of low pressure will ride along this front and will track 
across Central Lake Michigan on Tuesday resulting in a wind shift 
to the northwest. In terms of speeds...wind gusts to around 20 
knots are possible across northern half of Lake Michigan tonight 
into tomorrow...and northwest winds behind the low pressure area 
are expected to gust around 20 knots Tuesday evening through 
early Wednesday. A ridge of high pressure will settle back into 
the western Great Lakes for Thursday. 


Marsili 


&& 


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


&& 


$$