fxus63 klot 200836 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
336 am CDT Fri Jul 20 2018 

Short term... 
335 am CDT 

Through tonight... 

Main focus for the short term forecast period will continue to be 
impacts of an unseasonable upper low slowly tracking across the upper 
Mississippi Valley and upper Midwest. 

Early this morning, the center of the upper low was located over 
sern Minnesota. The system is vertically stacked with the associated 
sfc low right under the upper low, with a warm front extending from 
nwrn Illinois nr Rockford to the Champaign area. A series of vort maxs 
rotating around the srn periphery of the upper low will keep chances 
for shra/tsra across the region until the system moves off to the 
east. Exact timing and coverage of ts will be low confidence 
through the period, with scattered low-topped convection and 
isolated stronger storms possible. Given pwats in excess of 1.6 
inches expected to remain in place, locally heavy rainfall will be 
likely with some of the heavier showers and storms. Temperatures 
will be well below normal today, under mostly cloudy-cloudy skies 
with highs only in the lower to middle 70s across much of the County Warning Area. 
The far sern portions of the cwa, generally south an east of a ling 
from Gary, in to Pontiac, Illinois could remain in low level sly-swly flow 
today long enough to allow temps into the upper 70s, but as the 
system slowly moves ewd, low level flow across the region will 
gradually veer to wly-wnwly. 


Long term... 
335 am CDT 

Saturday through Thursday... 

The unseasonably cool and cloudy conditions will continue through 
the weekend as the upper low very slowly tracks ewd. The longer 
range guidance is in relatively good agreement on the general track 
and ewd progression of the system. As the upper low tracks to 
Southern Lake Michigan by Saturday morning, upper level flow will 
veer from wly to nwly aloft, setting up deep layer cold advection 
and a track for weak impulses to cross the County Warning Area. This pattern will 
continue into early next week as the upper low settles over the 
upper Ohio Valley from Sunday afternoon through Monday, with 
temperatures remaining below normal, only in the middle 70s on 
Saturday. A ridge axis extending from an expansive upper high 
centered over The Four Corners region will begin to approach the 
region Sunday, bringing minor height rises and, consequently, 
slightly higher temps for Sunday. However, with highs generally in 
the upper 70s to around 80 f, and cooler near the lake in onshore 
flow, temperatures will still be 5 to 10 degrees below normal with 
continued chances for scattered showers and isolated thunderstorms. 
By late Sunday or early Monday, the center of the upper low should 
be far enough east, with upper ridging continuing to build across 
the region, that temperatures should trend back to near seasonal 
normal levels for Tuesday into mid-week, with minimal chances for 


for the 06z tafs... 

A slow moving upper low traversing the region will bring multiple 
chances for showers and thunderstorms through the period, along 
with lowering ceiling heights. 

Precipitation: scattered showers and thunderstorms are expected 
through the period, though there do appear to be a couple windows of 
better chances for precipitation at the terminals. The first 
window will be 06z through 08-09z overnight. A line of 
thunderstorms is currently lifting across north central Illinois 
and a few isolated showers/storms are possible farther south over 
or near the Chicago area terminals. Meanwhile, a shortwave trough 
evident on water vapor imagery digging across north central Iowa 
late this evening is expected to push into northern Illinois mid 
to late Friday morning and will help to increase precip coverage. 
Instability appears best during the late morning hours as this 
initial wave moves over the area, and will introduce a thunderstorms in the vicinity into 
the forecast this issuance. If confidence in scattered storms 
increases further, may have to add a tempo for thunderstorms and rain. 

Ceilings: VFR conditions are in place across most of northern 
Illinois and Northwest Indiana late this evening outside the 
influence of precip, but closer to the low center, more widespread 
IFR and lower MVFR ceilings are present across northeast Iowa 
and southern Minnesota. Guidance suggests these lower ceilings 
will gradually build southeast across the terminals. Conceptually, 
not favoring the lowest solutions given the low center track to 
our north with moderate southwest flow locally. 

Winds: south to south-southeast winds are in place late this 
evening but will veer to the south-southwest by the pre-dawn 
hours Friday. Winds will continue to gradually veer through the 
day, becoming southwest through the afternoon and eventually 
northwest by late evening or overnight. Winds will be gusty at 
times, particularly during the afternoon hours Friday when gusts 
into the low 20 kt range are expected. 



329 am CDT 

Moderately strong south to southeast flow with gusts to 30 kt is 
occurring early this morning across portions of Lake Michigan 
ahead of an approaching low. The low, over far southeast Minnesota 
early this morning, will slowly track east across southern 
Wisconsin this afternoon and across the southern third of Lake 
Michigan overnight. Split the open waters (glflm) forecast into 
three zones to try to better detail the winds across the lake 
during the low passage. 

Several models are hinting at winds over the far southern end of 
Lake Michigan staying moderately strong as they veer to the 
southwest late this morning into the afternoon. With Michigan City 
already gusting mid 20kt range and expectations for winds to stay 
elevated through mid/late afternoon, will hoist a Small Craft 
Advisory for the Indiana nearshore zones through late afternoon. 
By this afternoon, expect the Illinois zones to be slightly 
weaker with respect to wind speeds later today, so will maintain 
the advisory currently in place which expires mid morning. The 
passage of the low overnight into the weekend will support 
additional chances of showers and thunderstorms as well as the 
chance for waterspouts. 



Lot watches/warnings/advisories... 
Illinois...beach hazards statement...ilz006-ilz014 until 10 am Friday. 

Lm...Small Craft Advisory...Illinois nearshore waters until 10 am Friday. 

Small Craft Advisory...in nearshore waters until 5 PM Friday. 



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