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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 190752

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
252 AM CDT Mon Aug 19 2019

239 AM CDT

Through Tuesday...

The main story of the short term portion of the forecast is that 
mainly dry conditions will likely be punctuated by occasional
shower and storm chances. We also continue to monitor a potential
for a forward propogating MCS to drop out of Iowa on Tuesday with
an attendant threat for severe weather somewhere across Illinois 
(whether or not impacts are felt well into northeast Illinois 
remains to be seen). 

A weak cold front is ever-so-slowly sagging southward early this 
morning into southern Iowa and just now pushing far southwestern 
Wisconsin. Ahead of this feature, the airmass remains noticeably 
moist, with dewpoint running in the mid and upper 60s across all of 
northeast Illinois and northwest Indiana. Dewpoint depressions are 
essentially near nil in most locales at this time. Given the 
combination of mainly clear skies (south of I-80), favorable 
downward-directed near-surface moisture flux with a notable 
hydrolapse on area soundings, expect that some visibility reductions 
will occur through the mid-morning. Crossover temperatures will 
probably be met mainly near and south of I-80 where stunted OLR due 
to incoming mid-level stratus will be limited. Can't rule out some 
patches of dense fog into sunrise where skies can remain clear. 

We're also watching a subtle mid-level wave (most prominent around 
700 mb) pushing out of southeast Iowa early this morning. A 
pocket of associated modest warm advection is forecast to drift 
out of west-central Iowa after 4 AM, and RAP forecast soundings 
immediately south of Ford County show the 800-700 mb layer within 
a whisper of saturation as a result. Lapse rates are certainly not
steep over a deep layer above this subtle ascent, but thin layers
of lapse rates of 7-8 C/km do have me wondering if we'll be able 
to pop a few elevated showers and maybe a storm from near and 
south of a Pontiac to Valparaiso line. Lower probability of 
occurrence, but have added some 20% PoPs here through daybreak to 
account for this potential. 

With the moist airmass in place down low, we'll destabilize quickly 
this afternoon with heating. That said, notable drying and warming 
in the 500-700 mb layer (a manifestation of subsidence overhead) 
look to cap us off (mostly) to surface-based convection. That said, 
enhanced surface convergence near a weak surface low and ahead of 
the aforementioned cold front may help a few parcels complete the 
journey to their LFCs during peak heating. Think the potential for 
electrification is on the low side today with the warm air aloft and 
poor lapse rates, so we'll indicate 20-30% precip chances but cap 
thunder wording just to isolated. The most over-achieving of cells 
might be able to produce some gusty winds to 50 mph given the well-
mixed boundary layer today. There might be another shot at some very 
patchy fog tonight/Tuesday morning, but will leave this mention out 
of the gridded forecast for now given very low confidence. 

Very late Monday night we'll be turning our attention off to our 
northwest as a robust shortwave--readily apparent this morning on 
all moisture-channel bands crossing into Idaho--dips into the area. 
The associated low-level mass response will result in increasing 
warm advection on the nose of a burgeoning low-level jet, but 
exactly where this occurs remains a bit in question but will have a 
great deal of bearing on where a potential MCS initiates and 
subsequently tracks. This evening's ECMWF came in a bit farther 
southwest, the GFS remains a southern outlier, and the NAM and CMC 
are both slightly farther north. The attendant warm advection will 
intercept a very steep mid-level lapse rate plume, resulting in 
likely robust convective development. Best guess at this point is
CI occurring across north-central to north-west Iowa with 
activity tracking southeast from there. This would take the brunt 
of wind-related impacts from this system mostly south and west of
our CWA, but any northeastward shift in the LLJ would change 
this. A potential for wind damage certainly exists, most notably
south and west of a roughly Rockford to Gibson line. The current 
Day 2 SPC outlook conveys a reasonable demarcation of the location
of the greatest severe threat based on this evening's multi-model
consensus. We'll obviously keep close tabs on this portion of the
forecast as a reservoir of very high instability will be 
available for convective processing into Tuesday afternoon. Most 
likely time for any impacts from this system look to be from 
daybreak Tuesday into early Tuesday afternoon. 



248 AM CDT

Tuesday night through Sunday...

Depending on the mesoscale evolution that takes place on Tuesday
afternoon with any MCS activity, we may have to monitor for a
potential for additional convective development into Tuesday 
night and Wednesday morning. Some guidance indicates an 
approaching jet streak and attendant ageostrophic mass response in
the low-levels (tied to the thermally indirect transverse portion
of the ageostrophic jet circulation) with another LLJ possibly 
driving renewed convection near and atop and remnant outflow 
boundary (ies). Should something like this take place, a threat 
for training convection could develop given the anticipated mid 
and upper-level flow orientation. PoPs will be capped at 30-40% 
through Wednesday due to low confidence nature of this portion of 
the forecast. 

At this time, Thursday and into the weekend is looking rather
pleasant and dry with refreshing temperatures and low humidity as
dewpoints drop back into the 50s. Some low shower and storm
chances will spread back east across the region towards the very
end of this forecast (Sunday and into Monday) as high pressure
builds off to our east. 



For the 06Z TAFs...

Relatively quiet period expected for the TAF sites, and with dry 
conditions likely. Scattered shower and thunderstorm development 
is possible this afternoon into early evening, however, will stay 
just to the south. VFR ceilings likely early this morning, with 
some MVFR ceilings then possibly developing closer to daybreak. 
Don't think these ceilings will get too low, and also don't 
anticipate them to be in place for too long today. A light 
westerly wind expected early this morning, and then turning to the
north northeast later this morning.