651
FXUS63 KLOT 152246
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
546 PM CDT Mon Jul 15 2024

.KEY MESSAGES...

- Strong to severe thunderstorms expected late this afternoon
  into tonight. Locally heavy rainfall possible, which could
  lead to additional flooding.

- Additional thunderstorm chances continue Tuesday, mainly
  south of I-80/east of I-55.

- Pattern breaks by Wednesday, with dry weather and seasonable
  temperature and humidity through the weekend.

&&

.MESOSCALE...
Issued at 546 PM CDT Mon Jul 15 2024

Explosive severe thunderstorm development has taken place across
Iowa, with storm tops exceeding 65kft. Special soundings from
DVN and ILX both show extreme instability and moderate deep
layer shear, though sufficient shear to support storm scale
organization in light of the extreme instability.

Already seeing signs of amalgamating cold pools/downdrafts with
convection over Iowa with radar showing some developing rear
inflow jets. The evolution of the convection so far continues to
fit the expectations for today with evolution to QLCS well
underway. As cold pool strengthens, would anticipate a QLCS to
begin accelerating eastward with increasing damaging wind threat
as storms approach and cross the Mississippi River.

Strengthening low level jet this evening over northern Illinois
should result in a ramping up of low level shear and coincident
increasing threat of QLCS tornadoes with bowing segments and/or
line embedded supercells. Given the shear/instability combo,
threat is certainly there for a large number of QLCS tornadoes
this evening over northern IL.

Recent satellite/radar imagery shows some cells developing over
far northern Winnebago County near the Wisconsin line. This
could be the beginnings of a "warm air advection" wind ahead of
the Iowa QLCS, which if it is and continues to fill in this
could result in a potential significant/high end flash flood
threat in our CWA if storms drift south of the IL/WI border.
Given the vulnerability of the Rockford area due to nearly 10"
of rain over the past 48 hours, this will be something that we
will be monitoring extremely closely.

- Izzi

&&

.DISCUSSION...
Issued at 247 PM CDT Mon Jul 15 2024

Through Tuesday Night:

Showers and storms may linger into the morning hours, especially
across the southern portion of the CWA. There is a chance that
the cold front associated with the storms may set up in a west
to east fashion and stall out across the southern portion of the
area, which could lead to storms persisting into the evening
over this area. However, most of the 12Z CAMs show this scenario
playing out south of the area. Although the chances of this
seem to be on the lower side, did include chance PoPs across the
southern CWA through Tuesday afternoon to account for this
possibility.

With the cold front moving through the area, Tuesday will be
cooler than today with temperatures in the mid-to-upper 80s.
Dew points will still be around 70 degrees, so it may still feel
a bit on the humid side, but peak heat indices will only be in
the 90-95 degree range.

Carothers


Wednesday through Monday:

The main upper trough axis is expected to shift east across the
forecast area Wednesday. Despite steep low-level lapse rates
depicted in forecast soundings, warm mid-level temps (700-600
mb) look to provide a capping inversion with respect to surface
based convective development. Therefore, forecast for Wednesday
is dry at this point. Broad surface high pressure then builds
across the area through the end of the week, setting the stage
for a well-deserved period of dry and seasonably mild but less-
humid weather conditions, downstream of a high-amplitude upper
ridge which develops over western North America.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 231 PM CDT Mon Jul 15 2024

Issued by National Weather Service Gaylord, MI
1256 PM CDT Mon Jul 15 2024

The primary aviation weather concerns for the 18Z TAF period include:

* Severe squall line in the 00z-05z time frame

* Period of MVFR ceilings (at least?) ahead of and behind a cold
  front expected to pass through in the 12z-18z time frame..

Main concern heading into the evening hours is the development of a
strong line of thunderstorms over eastern Iowa/northwest Illinois
probably after 21z...expected to organize into a squall line that is
expected to race across northern Illinois in the 00z-05z time frame.
Wind gusts in excess of 40kts will be possible as this line moves
through.  Behind this and ahead of a cold front moving in from the
northwest...some signals for at least MVFR ceilings (and perhaps
some drizzle) after 04z...which would persist into Tuesday morning
before ceilings thin out after 15z.  Cold front wind shift should
move into far northwest Illinois around 12z...and into northwest
Indiana by 18z.

JPB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 3 AM CDT Tuesday for ILZ003-ILZ004-ILZ005-
     ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
     ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

     Heat Advisory until 7 PM CDT this evening for ILZ008-ILZ010-
     ILZ011-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-
     ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 3 AM CDT /4 AM EDT/ Tuesday for INZ001-
     INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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