800
FXUS63 KLOT 151947
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
247 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 1142 AM CDT Mon Jul 15 2024

Satellite imagery late this morning shows an apparent MCV over
western WI moving eastward. This feature seems likely to remain
north of our CWA this afternoon and probably a non-factor.
Upstream, there is another subtle shortwave apparent over South
Dakota which is progged to move east-southeast into northern IA
later this afternoon.

At the surface, a very warm and humid air mass blankets the
Midwest. There is some evidence of a lingering outflow/moisture
discontinuity boundary roughly along the I-80 corridor in our CWA.
To the south of this boundary, dewpoints are in the mid-upper 70s
with lower 70 dewpoints more common to the north of the boundary.
This boundary should continue drifting north and likely dissipate
with mid to upper 70 degree dewpoints expected to be fairly common
across the area this afternoon.

Morning soundings and SPC objective analysis both show an
expansive EML from the Plains east into the mid-Mississippi
Valley. These steep lapse rates juxtaposed overtop the very
theta-e rich boundary layer will result in extreme instability
with 4-5k+ J/kg of MLCAPE this afternoon in an axis from Iowa east
into northern Illinois.

As ascent from the South Dakota shortwave trough begins to
overtake the extreme instability and weakening inhibition this
afternoon, expect explosive severe thunderstorm development over
Iowa later this afternoon. Belt of moderate westerly mid-level
flow (around 40 kt at 6km) should result in 30-35kt of 0-6km bulk
shear. Given the extreme instability, this should allow for
supercells with the initial development, but given DCAPE over 1000
J/kg would anticipate an evolution into one or two bowing QLCSs by
early evening as the storms approach and move east of the
Mississippi River.

Conditions continue to look favorable for a QLCS with damaging
winds across much of the CWA this evening. Given combination of
extreme instability and moderate shear, cannot rule out some
swaths of significant wind damage with gusts over 75mph in spots.
In addition, strengthening low level jet this evening should
result in favorable environment for QLCS type tornadoes as well.

Finally, PWATs over 2" and very high freezing levels will result
in storms being extreme efficient heavy rainfall producers
capable of hourly rainfall rates of 2-3" per hour. Should any
"warm air advection" wing type cells develop ahead of the
approaching QLCS or upwind propagating storms develop on the tail
end of the QLCS, then they would pose a potentially significant
flash flood threat. Think it is likely that a (flash) flood watch
will eventually be needed for portions of the CWA once the
mesoscale evolution of things becomes clearer this afternoon. Of
particular concern would be areas near and north of I-88, from the
Rockford into the Chicago metro where there have been multiple
rounds of heavy rain over the weekend, making them particularly
susceptible to flash flooding with any heavy rainfall.

- 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 from 6 PM CDT this evening through late tonight
     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 from 6 PM CDT /7 PM EDT/ this evening through late
     tonight for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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