739
FXUS63 KLOT 141121
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
621 AM CDT Sun Jul 14 2024

.KEY MESSAGES...

- Additional clusters of storms are expected today, with
  continued threats for severe weather and flash flooding.

- Today and tomorrow will be hot and humid with peak heat index
  values above 100 degrees. Tomorrow (Monday) looks to be the
  hottest day of the pair, and will likely need a Heat Advisory.

- Additional rounds of thunderstorms are expected Monday night
  through Tuesday with continued threats for severe weather and
  flash flooding.

- The pattern finally breaks Tuesday night with a return of
  tranquil conditions Wednesday onward.

&&

.MESOSCALE...
Issued at 538 AM CDT Sun Jul 14 2024

A mesoscale convective system continues to roll down the
Mississippi River across far northeastern Iowa and southwestern
Wisconsin along the eastern fringe of an EML plume. Over the
past few hours, lightning production and reflectivity cores have
been on a downward trend, likely as a result of less
instability across southeastern Wisconsin and within the
effective inflow layer of the system in the wake of an earlier-
morning/overnight MCS. However, recent 1-min GOES water vapor
imagery depicts a broad region of ACCAS lifting into the
southwestern flank of the MCS as recent KLOT radar imagery
depicts blossoming "speckled" radar returns west of the
Mississippi river. Both features suggest the nose of the 30 to
35kt southwesterly low-level jet (sampled nicely by the KDVN
VWP) is now supporting a renewed feed of instability into the
MCS, which may allow for regenerating convection along the
southwestern flank of the MCS over the next 1 to 2 hours. With
the current speed and evolution of the MCS, such regenerating
convection may develop in close proximity to where 2 to 7 inches
of rain fell last night across far northern Illinois,
potentially reinvigorating a threat for flash flooding. A Flood
Watch remains in effect generally along and north of Interstate
88 until 1 PM.

Borchardt

&&

.DISCUSSION...
Issued at 322 AM CDT Sun Jul 14 2024

Through Monday:

On the heels of a robust cluster of overnight thunderstorms, yet
another cluster of storms is surging down the Mississippi River
along the edge of the EML plume. While the next cluster of storms is
moving into a relative minima of instability, a steady feed of
instability via a 925/850mb low-level jet should allow it to reach
at least the Wisconsin state line and likely into the northern-most
row or two of counties of northern Illinois around or after
daybreak. Given the areas of heavy rainfall that fell
overnight, including significant flash flooding that continues
around Rockford at press time, opted to issue a Flood Watch for
counties along and north of I-88 through 1 PM.

Behind the morning round of storms, a period of dry weather is
anticipated into the early afternoon. Attention turns firmly toward
yet another round of thunderstorms this evening. Regional radar
imagery shows a derecho racing across southeastern South Dakota. The
latest radar trends indicate that the gust front is now well ahead
of the reflectivity, which is not surprising given the system is
well south of the upper-level jet and source of deep-layer shear
displaced to the north. As a result, the derecho should begin to
weaken as it surges into northwestern Iowa later this morning. With
that said, there appears to be a highly organized MCV within the
core of the system, which will approach northwestern Illinois by mid-
afternoon and ignite the next round of storms. With low- to mid-
level shear augmented in the vicinity of the circulation, any storm
may be severe with damaging winds, damaging hail, and even a
tornado. In addition, flash flooding may occur as well especially if
thunderstorms favor far northern Illinois once again.

With the threat for two rounds of storms today, confidence in the
temperature and heat index forecast is quite low. In scenarios
with minimal convective coverage and a footprint of outflow,
highs should climb into the lower 90s. When combined with
dew points in the low to mid 70s, afternoon heat indices would
top off in the 100-105F range. I gave serious consideration to
issuing a Heat Advisory, but simply didn`t have high enough
confidence in widespread 105F+ readings today. Now, it does
appear that there will be a break in the waves of thunderstorms
during the day on Monday, which gives higher confidence in heat-
headline criteria being archived. Indeed, highs in the low to
mid 90s and dew points in the mid to upper 70s will support peak
heat indices of 105 to 110 areawide. Additional rounds of
storms are expected Monday night through Tuesday, which will
include the threats for severe weather and flash flooding.

Borchardt

Tuesday night through Sunday...

Following the cold front clearing our southern CWA Tuesday night
into Wednesday morning, mean troughing will set up over northeastern
North America while powerhouse 500 mb ridging rebuilds over the
western US. This should bring an extended period of dry, cooler and
less humid conditions Wednesday PM onward through next weekend.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 620 AM CDT Sun Jul 14 2024

The primary aviation weather concerns for the 12Z TAF period
include:

* A period of showers and storms this morning.

* Additional thunderstorms expected this evening.

Another wave of showers and storms is quickly approaching far north-
central IL this morning. It will continue its trek toward the
southeast and is expected to reach RFD a little before 12Z and the
Chicago terminals closer to around 1230Z. In addition to heavy
downpours and a brief period of gusty winds, MVFR cigs and vsbys
will also be possible underneath the rain. While the evolution of
this system is rather uncertain, rain is expected to clear the
terminals by late morning.

The afternoon currently looks dry. Another push of thunderstorms is
expected to impact the area this evening with more MVFR potential.
These storms could be on the stronger side with heavy downpours and
strong, damaging winds possible. The overnight period looks quiet
while Monday morning is a little more uncertain, although best guess
is that additional rain will hold off until the afternoon.

Meanwhile, expect SW winds mostly under 10 kt through the morning
gusting to near or just over 20 kt for the afternoon. Winds will
subside to under 10 kt while remaining SW for this evening through
Monday morning.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 1 PM CDT this afternoon for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103-
     ILZ104-ILZ105.

IN...None.
LM...None.

&&

$$

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