fxus63 klot 200808 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
308 am CDT Thu Sep 20 2018 

Short term... 
300 am CDT 

Through Friday... 

Main forecast concerns revolve around the chances for showers and 
thunderstorms both today and tonight, along with the gusty winds 
expected to develop over the area by this evening and continue on 

Convection continues to develop early this morning within a 
corridor of enhanced lower-level warm air advection across much 
of Iowa and into southern WI. While the primary focus has largely 
been to our north and west in closer proximate to the nose of the 
low-level jet, it appears that some of these storms will push 
east-southeast across far northern Illinois over the next few hours. 
There is even signs of outflow shifting southeastward out of these 
storms now entering far northwestern Illinois. For this reason it 
appears likely that there will be storms over the area early this 
morning, especially along and north of I-88, though storms could 
impact areas as far south as I-80. The main threat from these 
storms will be some locally heavy rainfall. These storms should 
begin abate across the area by mid to late morning. 

The next question is how quickly can the warm front lift north of 
the area today, especially given the outflow from the the storms 
entering the area acting to reinforce the front south of the 
area. Overall, I think the front may end up being several hours 
slower than the model guidance suggests. With this in mind, it 
appears that the front may not move north of far northeastern Illinois 
until sometime this afternoon. I don't necessarily think this 
will impact the near record high temperatures much, as once the 
warm front does lift north temperatures should warm quickly with 
the onset of south- southwesterly winds. Instead what could happen 
is that some additional isolated or widely scattered 
thunderstorms could try to develop over portions of northeastern 
Illinois in the vicinity of the front. Little to no capping is forecast 
for this area, so any influence with the surface boundary near the 
lake could spark off some activity. Confidence in any storms 
occurring over northeast Illinois this afternoon is low, but we will 
continue a slight chance for storms through the afternoon. 

The warm front will eventually shift north of the area later today 
as the parent low pressure quickly shifts northeast from the 
plains to the upper Midwest. Once this occurs, expect a mild a 
windy night across the area tonight. The wind field across the 
area will increase significantly by early this evening as pressure 
falls (associated with the fast moving surface low moving into 
the upper midwest) overspread the upper Great Lakes. This will 
quickly tighten the pressure gradient across much of the region 
supporting stronger winds. Winds at 950 mb (about 1000 ft agl) 
are progged to increase to around 40 kt (~45 mph) this evening. 
For this reason it appears likely that wind gusts up around 35 to 
possible up to 40 mph will be possible across the area tonight. 

Finally as the surface low tracks into western Quebec early 
Friday morning, an associated cold front will push eastward across 
the area. With the approach of this front there will be another 
small chance for some widely scattered storms, mainly later 
tonight. However, it appears the main threat for storms will 
remain north of the area in closer proximity to the better 
atmospheric dynamics. In our area, it appears the poor diurnal 
timing will result in weakening storms as they shift into Illinois late 
tonight. Winds will turn west-northwesterly in the wake of this 
cold front, however, and wind speeds will likely remain gusty 
through much of the day on Friday. 



Long term... 
238 am CDT 

Friday night through Wednesday... 

Surface high pressure builds across the Great Lakes region Friday 
night as the cold front settles into the Ohio Valley. Cooler, drier 
low level air spreads into the forecast area on north-northeasterly 
winds. Steep low-level lapse rates develop as the cooler air crosses 
still mild Lake Michigan waters, and forecast soundings suggest a 
period of lake-induced stratocu into northeast Illinois Friday night into 
Saturday morning, which along with a breezy northeast wind will 
likely keep temps from dipping too low close to the lake. Some model 
guidance even tries to develop a Little Lake-effect precip, though 
forecast soundings suggest inversion heights around 5000-6000 ft 
which would likely limit anything to just sprinkles. Further inland 
however, across north central IL, where skies remain clear/mostly 
clear and winds decouple, temps may drop into the low-mid 40s by 
sunrise Saturday. 

Once lake effect clouds decay Saturday, pleasant weather is in store 
for the first weekend of autumn. High pressure will continue to 
drift slowly east across the Great Lakes, providing dry, seasonably 
cool weather with onshore lake winds. Daytime highs should range 
from the low-mid 60s near the lake, to the upper 60s/low 70s farther 
inland and south Saturday. Widespread lows in the 40s are expected 
Saturday night away from the immediate Lakeshore and city, when 
light winds and mainly clear skies will provide the strongest 
radiational cooling conditions. Moderating low level temps Sunday 
should support low-mid 70s away from the lake. 

An upper level trough will propagate from the West Coast into The 
Rockies Sunday night, inducing low pressure development across the 
plains, and a gradual veering of the low level flow into the Midwest 
into Monday. Warm/moist advection associated with this return flow, 
and continued height falls forced by the approaching upper trough, 
will lead to the return of shower chances early next week. While the 
European model (ecmwf) has been developing precip as early as late Sunday night, the 
greatest potential for rain and a few thunderstorms appears to be 
Monday night into Tuesday, with the arrival of the amplifying upper 
trough, and the approach of a cold front. While clouds and precip 
may have an impact, temps should warm in the mid-upper 70s in many 
areas ahead of the front Monday/Tuesday. Cooler weather then returns 
behind the front Tuesday night and Wednesday. Presence of the deep 
upper trough aloft may support lingering clouds and perhaps a few 




Record high temps could be jeopardy Thursday. Here are the 
current records: 

Chicago 92 (2017) 
Rockford 92 (1920) 


for the 06z tafs... 

The main concern through the period will be the timing of wind 
direction changes, and increasing southerly winds today. 
Additionally, we will once again be on the lookout for storms 
possibly impacting some of the terminals overnight, and then 
possibly again this afternoon. 

Winds are likely to remain easterly a bit longer early today as it 
appears that the warm front may be slower to move northward across 
the area into this afternoon. Therefore, it appears that the area 
will remain in an area favorable for low cigs through at least 
around daybreak. Current satellite imagery indicates that the 
eastern edge of these IFR cigs are right near Ord. So, the main 
Chicago area terminals maybe in and out of low cigs through the 
night. Lowering visibility will also be possible at the terminals as dew 
point depressions lower. 

The other concern is for a round of thunderstorms overnight 
towards daybreak. For the most part the focus for storms has 
remained north of the area into southern WI. However, current 
trends with the thunderstorms in Iowa indicate outflow pushing the 
storms a bit more to the south. As a result, it appears probable 
that a round of thunderstorms will push across far northern Illinois 
after 09z. It appears krfd stands the best chance of getting in on 
this activity, though I am concerned the Ord and dpa could also 
see some of this activity move into the area just prior to 
daybreak. For now I have added a thunderstorms in the vicinity at both sites to cover this 
threat, but I have left it out of mdw and gyy for now. Trends will 
need to continue to be watched overnight. 

With the possibly for a slower northward moving warm front, it is 
possible that some additional isolated to widely scattered storms 
could develop in the vicinity of the Chicago area terminals this 
afternoon. However, confidence is very low at this time on if this 
will occur, so I have left the mention out of the taf at this 

Otherwise, expect winds to turn southerly and begin to increase by 
late this afternoon/early this evening as the warm front finally 
shifts north of the area. It appears that winds will gust up to 30 
kt at times tonight from the south-southwest. 



307 am CDT 

The main concern for the near shore waters will be the increasing 
southerly winds later today and especially this evening. A rather 
potent and fast moving area of low pressure will shift 
northeastward from the plains to the upper Midwest today. As this 
occurs, a warm front will north over the lake later today. In the 
wake of this front, expect strong southerly winds to develop and 
continue tonight. It appears that wind speeds will easily reach 30 
kt and we cant rule out some gale force gusts. The winds will then 
turn west northwesterly early Friday following a cold front, then 
turn northerly late Friday into Friday evening as a strong area of 
high pressure builds into the Midwest. Wind speeds will remain up 
in the 25 to 30 kt range Friday before gradually abating into 

Given this rather active period of strong winds and high waves, we 
have issued a Small Craft Advisory for the Southern Lake Michigan 
near shore waters from early this evening through Saturday 



Lot watches/warnings/advisories... 


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