fxus63 klot 211924 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
224 PM CDT Thu Sep 21 2017 

Short term... 
224 PM CDT 

Through Friday... 

Late September heat remains the main focus of the forecast in the 
near term. New record high temp already set today at o'hare with 
a 94 degree reading there. 

Midwest remains along the periphery of highly amplified upper 
level ridge axis, downstream of a deep long wave trough across 
western noam. 12z radiosonde observation from dvn indicated 850 mb temp of +20c, and 
a 925 mb temp of +26c, resulting in low-mid 90 degree temps across 
the region. Residual high cloud cover from decaying mesoscale convective system over 
southeast Iowa/northeast MO has kept temps slightly lower across 
western parts of the cwa, though a decrease in the denser cirrus 
has temps approaching 90 there as well this afternoon. Of course 
the weak surface pressure gradient in the vicinity of the 
stationary front just north of the cwa has allowed a lake breeze 
to push inland across far northeastern IL, keeping temps in the 
mid-upper 80's for areas near the lake. 

Recent runs of the hrrr indicate the potential for isolated 
thunderstorm development across mainly the southern parts of the 
cwa late this afternoon. Experimental esrl hrrr also hints at this 
though with much lower coverage. It appears these models may be 
focusing on a remnant mesoscale convective vortex from the Iowa/MO convection, though no 
distinct circulation is noted in water vapor imagery. While 
current rap mesoanalysis depicts 3000 j/kg of MLCAPE and little 
cin across the southern tier of cwa counties, weak flow and 
fairly shallow cu field per GOES-16 vis imagery suggests these 
models may be quite overdone with thunderstorm potential. 
Maintained less than 20 percent pops through sunset, but may have 
to add an isolated mention if cu field becomes more impressive or 
something pops. 

Another mild night is in store for the region with persistent 
light south winds and unseasonably high dew points. Overnight mins 
should remain in the 70's in most locations, which will also give 
a warm head start for temps on Friday. Less cloud cover and 
similar low level thermal field should allow highs in the low 
90's in most spots again Friday afternoon, again at or near record 
levels for September 22nd. (Records for tomorrow are 92 for 
Chicago, 93 for rockford) weak southeasterly lake breeze should 
again keep temps in the 80's right along the lake. 



Long term... 

Friday night through Thursday... 

218 PM...the models continue to show slight thermal cooling this 
weekend but this will likely have little if any noticeable 
affects as overnight lows remain in the 60s...low 70s 
downtown...and highs in the upper 80s/near 90 Saturday/Sunday. 
Bumped temps up a few degrees to near current guidance values. 
Lake breezes are possible both days which will keep the immediate 
Lakeshore areas cooler but confidence is low regarding how strong 
these lake breezes may be and how far inland they will move. 
Forecast highs are cooler near the lake but trends will need to be 
monitored as these time periods approach. Highs may still reach 
the upper 80s before temps cool and winds shift off the lake. 

There is still some potential that the models are cooling the 
airmass too fast and if that were to occur...highs could still be 
a few degrees warmer than currently expected this weekend. 
However by early next week it does appear that the entire airmass 
will cool enough for highs to remain in the 80s. Additional cloud 
cover...perhaps a more broken afternoon cu field...may also help 
put a lid on high temps...keeping them in the 80s. 

A cold front will move across the area Tuesday night into 
Wednesday next week. While an isolated shower is possible any 
afternoon from Sunday Onward next week...this front will be the 
next best chance of rain for the entire area. However...not 
confident on how widespread precip will become and its possible 
just scattered precip in a narrow time window moves across the 
area ahead of the front. Though a slower front...that arrives 
later into Wednesday could allow for more daytime heating and 
thus better precip coverage. This front will allow much cooler air 
to spread across the area with highs back into the 70s. A second 
push of colder air is possible late next week. Cms 


for the 18z tafs... 

Weak surface pressure gradient across the area this afternoon will 
allow a lake breeze to move close to Ord/mdw and go through gyy, 
with current variable winds expected to become east-southeast less 
than 10 kt. Direction becomes a bit uncertain with little push to 
the lake breeze, though southeast appears to be most likely for 
Ord/mdw. Winds will become more southerly again this evening with 
loss of lake breeze circulation. Slightly stronger low level flow 
Friday should keep lake breeze east of the Chicago Metro 

Otherwise, VFR conditions will prevail with gradually decreasing 
high cloud cover associated with dissipating thunderstorms over 
southeast Iowa/northeast MO. Some patchy shallow MVFR fog is 
possible toward sunrise primarily for dpa in the Fox River valley. 




218 PM...a weak cold front has stalled over south/central 
portions of Lake Michigan this afternoon. This front will lift 
back north as a warm front tonight...shifting light winds back 
southerly. A large trough of low pressure will remain nearly 
stationary over the plains through early next week maintaining 
southerly winds over the lake. This trough/front will move across 
the Great Lakes region Tuesday night into Wednesday shifting winds 
northwest with a period of 15-25 kts expected. Cms 


Lot watches/warnings/advisories... 


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