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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 191818

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

1135 AM CDT

One minor change with this afternoon's forecast was to inch the 
chance for isolated showers and storms northward up to I-88/I-290.
Anything that far north, including into Chicago, should be spotty
though. Also inched up high temperatures into the upper 80s across
the south.

As noted in previous AFD, moist conditions in the low-levels exist
across the CWA, with low 70s dew points and quick cumulus
development. The frontal boundary is draped from just north of
Chicago through the Dixon area and ever so slowly sagging 
southward, with a lake breeze also oozing inland. These are 
sources of some boundary layer convergent flow albeit not very 
strong. A short wave noticed on satellite channels is located over
the Quad Cities and drifting eastward, and looks to be 
responsible for some agitated cumulus already. While RAP soundings
still indicate quite a bit of dryness above 700 mb, observed 12Z 
DVN and ILX soundings were not as dry possibly owing to this 
subtle wave approaching. Also RAP and NAM soundings now both show 
slightly overcoming the cap with mid 80s temperatures. There will 
likely be some dry air entrainment in the mid-levels with any 
cells that do sprout. Because of that and deep layer wind shear 
only around 20-25 kt, feel that the potential for well organized 
storms remain low. 

In the southern CWA have increased temperatures into the upper
80s, which combined with the dew points does result in some spotty
mid 90s heat indices forecast.



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 into far 
southwestern Wisconsin. Ahead of this feature, the airmass remains
noticeably moist, with dewpoints 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 fluxes 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 18Z TAFs...

Main concerns for the next 24-30 hours:
* Chance for isolated to widely scattered SHRA and TSRA this
  afternoon over northeast IL and northwest IN
* BR/FG potential late tonight into Tuesday AM, with IFR VSBY
  likely at times outside of Chicago
* A period of SHRA and embedded TSRA in the area likely mid-late
  Tuesday AM through early afternoon
* Chance for isolated-widely scattered SHRA/TSRA mid-late Tuesday
* Wind directions in wake of 1st round of SHRA/TSRA on Tuesday

A weak cold front has drifted south across the area, with lake
enhanced east and northeast speeds of 7-10 kt at ORD/MDW and GYY.
A moist and weakly unstable air mass remains behind the front, so
isolated to widely scattered SHRA and a few TSRA remain possible
over northeast Illinois and northwest Indiana this afternoon.
Maintained VCTS mention at ORD/MDW/DPA/GYY, but confidence is low
in actual TS occurrence in 10 mile vicinity of the terminals. Will
continue to monitor trends.

Light to calm winds tonight and moist low levels support
development of BR/FG overnight and persistence a few hours past
sunrise Tuesday. Added TEMPO IFR VSBY mention at RFD/DPA/GYY and
TEMPO MVFR at ORD/MDW Possibly precluding more widespread dense 
fog will be some cloud cover, but if skies are clearer, then LIFR 
VSBY will be possible outside of Chicago, especially at RFD and

Attention Tuesday then turns to likelihood of an organized complex
of SHRA/TSRA surging southeast across the region Tuesday AM 
through early afternoon. Latest trends suggest the TAF sites will 
be on the northern fringe of the complex, but progged strength of
system and forecast soundings indicating moderate instability 
support VCTS inclusion in the TAFs. Wind variability will be 
possible during and behind the activity tracking across the area,
but strong to severe wind gusts are likely to be safely south of 
the terminals. Large scale winds will be coming around to 
southerly and then southwest, but concern is there for southeast 
or east- southeast winds in the wake of the SHRA/TSRA complex, 
possibly over 10 kt. Isolated to widely scattered SHRA/TSRA may 
linger over the area through Tuesday afternoon, but confidence is 
low in this scenario and suspect it's more likely that mainly dry 
conditions will be in place after the exit of the organized