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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 201130

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Sat Jul 20 2019

248 AM CDT

Through Monday...

Another hot and very humid day leads off the forecast docket, and
today also has some thunderstorm potential. Scattered to possibly
organized, clustered storms are expected in or near far northern 
Illinois this afternoon and early evening, with another wave of 
more organized storms favored during the overnight. Note specific 
timing is not of particularly high confidence especially this 
evening into overnight. A scattered severe weather threat exists 
in northern Illinois north of I-80, as well as at least localized 
potential for flash flooding.

Early this morning once again finds a decaying MCS to the north 
in Wisconsin, with outflow oozing into far northern Illinois as 
of 230 a.m. This outflow has brought near 10 degree cooling in
just minutes to lakeside areas in far northeast Illinois. This 
outflow boundary should become stationary in far northern Illinois
near daybreak, with an outside chance of popping a few cells 
along it through this morning. The cirrus canopy from last 
night's MCS should erode fairly quickly this morning. Low-level 
jet driven moistening like yesterday may create some 3-6 kft 
clouds in primarily far northern Illinois through the morning. 
Elsewhere the sun should be out and heating locations fast.

The area this morning is at near record daily warm minimum 
temperatures -- and not only daily as Rockford yesterday tied 
their all-time record warm minimum temperature of 80 and is close
to that again! Very high dew points of 75-80 prevail and while 
today's soundings show just the slightest bit more of afternoon 
mixing in the boundary layer, oppressive dew points will rule much
of the day. Highs are likely to be similar to yesterday (92-96) 
with a possible exception being far northeast Illinois where 
there are some early morning onshore winds due to the outflow 
noted above. If those winds hang in there a little longer this 
morning than forecast, that could lead to a tad cooler highs. 
As for the southern CWA, could justify extending the Excessive 
Heat Warning a couple hours into the evening, but felt at this 
juncture there is nothing new to add to the message by doing such.

The cumulative effects of the heat have built thanks to
unfortunately little relief these past two nights, especially in
Chicago. So a main message is to emphasize the checking on those 
without A/C and the elderly.

The upper level ridge had strengthened yesterday up to the CWA,  
with the nearby 00Z ILX sounding sampling 500 mb heights of 594 
dam and very warm temps of -2C. This will start to dampen today 
as a short wave trough associated with an impressive 125 kt upper 
jet translates across the Upper Midwest. With a very unstable 
(MLCAPEs > 3500 J/kg) and a richly moist air mass (PWATs near or 
above 2 inches), the weakening of the cap in the 5-10 kft layer in
the region this afternoon raises concerns for isolated convective
initiation apart from any stronger forcing. Also, the earlier 
mentioned short wave will be riding into Wisconsin during the mid
to late afternoon, and convection-allowing models show a decent
amount of storms with this which is not a surprise given the belt
of mid-level westerlies and low-level stationary/slow-moving cold
frontal focus. This mid-late afternoon north of I-88/I-290 is the
most likely to see some of this or be close to it in Wisconsin. 
There could be festering or redevelopment of activity from 
outflows or the outflow-augmented front throughout the evening, 
though confidence on that time period is low. There currently
looks to be more organized overnight activity moving into more of
the CWA from the west-northwest associated with another short 

There is potential for the northern Illinois convection this 
afternoon to be organized, such as in a form of a loosely- 
defined MCS with clusters of storms. Precipitation loading in the 
presence of a large theta-e difference, along with steep sub cloud
lapse rates, raise the concern for a scattered severe wind 
threat, especially from any storm segments. The mid-level lapse 
rates are forecast to lessen some this afternoon as even higher 
moisture moves in, but still major MUCAPE of 5000 J/kg+ is 
forecast, and some hail would be possible from the deepest, 
sustained updrafts today. It's practically a given there will be 
locally heavy if not torrential rainfall with any deep convection 
this afternoon and tonight. Any of this activity that tracks over 
the same area, or shows backbuilding tendencies which guidance 
Corfidi vectors indicate become more plausible in the region 
overnight, will raise the flooding concern. There is not enough 
consistent signal in convective evolution or in that backbuilding 
will be likely to warrant a flash flood watch for a portion for 
the area (mainly northern/western CWA), but will be something we 
have to watch. Often heat waves tend to end with a deep 
convective heavy rain event, and this has the look of similar.

Sunday and Monday...

As noted in the previous paragraph, overnight Saturday convection
should be continuing into Sunday morning, focusing along/near the
surface boundary that will be slowly sagging southward through the
area. Because of likely augmentation by outflows, this boundary
should be into the southern CWA by afternoon, taking the primary
focus for more robust convection into that area. There will
continue to be an isolated wind threat with any stronger storms,
along with the heavy rainfall. There still could be thunderstorms
north of the front, but the chances will likely be fading and 
activity would probably trend more toward showers. Guidance
solutions depict more prominence with a short wave Sunday night 
that may keep some showers going in the CWA including even into 
Monday morning.

Behind the surface front on Sunday, northerly winds will steadily
increase with a gradual oozing in of drier and cooler air. The
northerly winds will build Lake Michigan waves, though the 
stronger cold advection does not arrive until Sunday night into 
Monday, which is when the waves are more favored to build to 
levels for a high rip current risk. Will need to watch this timing
though as the risk could overlap some of the later daytime hours 
on Sunday.

The Monday pattern and 850-925 mb temperatures look to favor 
highs below normal area-wide, likely staying shy of 80 with 
lakeshore areas probably only in the lower 70s. The signal for sun
is a bit more delayed though, but still looks to arrive by the 



353 AM CDT

Tuesday through Friday...

A period of quiet, dry conditions is expected for the long term 
forecast period.  A large, sprawling area of surface high pressure 
is expected to spread through the Northern and Central Plains and 
across the Upper Mississippi Valley to the Great Lakes region.  In 
the upper levels, a high amplitude long wave pattern, with little 
progression, will set up, with a broad ridging centered over the 
continental Divide and a broad trough over the ern CONUS. The local 
area will be under nwly flow aloft through the period. The center of 
the surface high is expected to slowly slide south and east and 
should be centered over IL/IN by Wednesday night and Thursday. The 
center of the sfc high is expected to shift to the Mid atlantic 
region by Friday. After a relatively cool Monday, temperatures 
should begin to moderate back into the mid to upper 80s by mid week. 
The next chance for pcpn may not be until next Saturday when some
of the long range models are suggesting a cold front may push 
through the area.


For the 12Z TAFs...

Forecast concerns include...

Timing of a wind shift from sely back to swly this morning. 

Timing of wind shift and thunderstorm chances associated with a cold
front passage this afternoon/evening. 

Winds shifted to enely following the passage of an outflow
boundary earlier this morning. Winds have been gradually veering
through sely and should return to the synoptically driven swly
direction over the next couple hours. Given the warm/moist air 
mass in place, some isolated showers are possible in the wake of
the outflow boundary, but coverage will be too low to include in 
the TAFs at this time. Winds should become gusty again by late 
morning and through the afternoon, with the southwest winds 
gusting to 18-20 kt. 

Latest guidance continues to suggest a wind shift and shower/ 
thunderstorm development with a cold front pushing through the 
region Saturday evening. Winds should shift to northeasterly 
following the fropa. Confidence on the timing of the fropa remains
relatively low, with some guidance suggesting an earlier start
time and some suggesting a later timing. While confidence is 
relatively high that there will be terminal impacts from 
thunderstorms late this afternoon by early evening at the latest,
will keep the current timing and adjust tactically as the scenario
begins to unfold.


IL...Excessive Heat Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until 7 PM Saturday.

IN...Excessive Heat Warning...INZ001-INZ002-INZ010-INZ011-INZ019 
     until 7 PM Saturday.