fxus63 klot 240459 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
1059 PM CST Thu Nov 23 2017 

807 PM CST 

For evening update... 

Clear skies and light winds have allowed a quick fall off in temps 
this evening, with readings already dipping to around 30 degrees 
in some of the typical cooler spots across northern Illinois. Have 
therefore lowered mins into the upper 20's across parts of the Fox 
Valley and west into the Rochelle area for tonight. Surface 
pressure falls developing across the upper Midwest in response to 
low pressure deepening north of the Canadian border may help slow 
or halt the temp fall in locations where a light south wind will 
persist overnight, so many locations are already at/near their 
overnight mins. 

Otherwise, windy and much warmer weather in going forecast appears 
on track for Friday, with some 60 degree temperatures likely 
especially across the Illinois portion of the forecast area. Models 
typically under-forecast the depth of the mixed boundary layer in 
these situations, so would not be too surprised to see 60 degree 
weather spread all the way into the Chicago area. Of course winds 
gusting to 30-35 mph may make if feel cooler. 



Short term... 
235 PM CST 

Through Friday night... 

A quiet Thanksgiving day of weather with temperatures right around 
normal for late November. High pressure will depart eastward 
tonight with the area in turn under influence of the next deep 
low pressure across southern Canada. South to southwest winds will 
increase sharply during the morning as a tight pressure gradient 
and added isallobaric component from strong pressure falls 
envelop the region. Afternoon wind gusts of 25-35 mph are forecast 
depending on just how much the near-surface layer warms and mixes. 

This regime will steer a pronounced low-level thermal ridge into 
the area, with 925mb temperatures of 11c-15c. These values in 
late November correlate to surface temperatures in the low-mid 
60s based on reanalysis data. While there will be a strong 
inversion probably below 925mb, additional support for a warmer 
Friday are also the facts that today over achieved with mid-upper 
40s, as well as upstream observed temperatures in western Iowa 
into Nebraska this afternoon in the 60s. So have bumped Friday 
highs up quite a bit from guidance, as has been the trend for a 
few days, with forecast highs around 60. 

The system cold front is expected to move through early in the 
evening. With some recent northwest flow cold fronts, model 
guidance has been too dry with passage, and there are some signs 
this may be the case with this fropa (increasing moisture, 
700-925mb f-gen, additional jet support). So we have added a 
slight chance of some light rain/sprinkles, mainly between 5-9 PM 
on Friday. Behind the front, winds will turn northwest and gust 
again to around 30 mph. Temperatures will be on the fall but not 
overly sharp, especially considering what cold fronts can do this 
time of year. 



Long term... 
235 PM CST 

Saturday through Thursday... 

A quiet weekend will unfold with high pressure in control with 
temperatures right around normal under ample sunshine. The upper 
air pattern will remain similar through the first part of next 
week with an active jet to the north. The next low will pass 
north of the area on Monday night with again breezy mild 
conditions in advance on Monday, and a cold front with possible 
precipitation on Tuesday. The warmth on Monday could again be well 
into the 50s or even 60+ given the forecast pattern, however will 
depend on just how quickly the thermal ridge moves in. The 12z GFS 
is quite a bit more robust than the ec run. Global solutions also 
try to show a closed upper low across the Southern Plains 
advancing into the Ohio River valley during midweek. Confidence 
is low on how/if this will impact the area. 



for the 06z tafs... 

Low pressure is over southern Manitoba late this evening and will 
move to James Bay Friday evening while a trailing cold front 
pushes across northern Illinois and Northwest Indiana. Through the 
day Friday, strong south-southwest winds will be in place ahead 
of the approaching cold front. Warm air advection into the region 
will result in poor low level lapse rates which will inhibit 
effective mixing of 50-60kt winds between 015-020. Still, 
anticipate being able to mix down frequent gusts into the mid to 
high 20 kt range and would expect at least a few sporadic 30+ kt 
wind gusts. Cold front will move across the terminals late in the 
evening and overnight with winds turning northwest. There may be a 
brief lull in gustiness immediately ahead of the front within the 
frontal trough, but expect winds to become gusty again behind the 
cold front as steep low level lapse rates develop. No 
precipitation is expected with the front. There is still an off 
chance for MVFR stratocu to develop behind the front but 
confidence remains higher for conditions remaining VFR at this 



235 PM CST 

The active marine pattern continues with a gale event on Friday as 
another deep low pressure moves eastward just north of the Great 
Lakes. South-southwest winds will be on the increase tonight with 
gales likely by sunrise over the open water. There is some 
uncertainty over the nearshore areas, though gusts at and a 
little above 30 kt are likely much of the afternoon. A cold front 
will pass early Friday evening, and as winds turn northwest, there 
could be a few hour period of occasional gale force gusts. 
Otherwise 30 kt winds are likely. 

The next system will influence the lake on Monday again as low 
pressure passing north of the lakes. Southwest winds on Monday 
look to potentially reach gales later in the day or into Monday 
night. The cold front is likely to pass sometime early Tuesday at 
this point. 



Lot watches/warnings/advisories... 
Lm...Gale Warning...lmz777-lmz779...6 am Friday to 9 PM Friday. 

Gale Warning...lmz743-lmz744-lmz745...9 am Friday to 6 PM Friday. 

Small Craft Advisory...lmz740-lmz741-lmz742...3 am Friday to 6 
PM Friday. 



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