547
FXUS63 KLOT 151756
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1256 PM CDT Mon Jul 15 2024

.KEY MESSAGES...

- Heat Advisory in effect from noon to 7 pm CDT for much of our
  IL forecast area. Hot and humid with peak afternoon heat
  indices of 100- 110F.

- Another round of 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 405 AM CDT Mon Jul 15 2024

Through Tuesday night...

Linear MCS which brought severe weather and heavy rainfall to
the area earlier in the night continues to move off to the east
and southeast of the forecast area early this morning. Trailing
stratiform rain and embedded thunder linger southeast of roughly
a Gibson City to Chebanse to Rensselaer line as of 330 am CDT,
but should exit the area by 5 am CDT. Clearing of extensive
anvil cloud cover from the west may allow for patchy fog
development across northern IL through sunrise given moist
ground and high humidity, but should burn off fairly quickly
after the sun comes up.

Mostly sunny skies are then expected this morning, allowing
temperatures to warm into the low-mid 90s area-wide through this
afternoon. The potential exists for increasing high clouds to
increase by this afternoon from decaying upstream convection,
leading to somewhat lower confidence in widespread mid-90s high
temps. However, combined with surface dew points in the mid-
upper 70s especially across our IL rural farmland, peak
afternoon heat indices between 105-110F are likely at least for
a time. Have elected to issue a Heat Advisory for our IL
counties roughly south of a line from Forreston to Joliet to
Beecher line from noon through 7 pm CDT, where values of 105F
and above are most likely. If cloud cover is thinner or of less
coverage and allows temps to overperform, the advisory could be
expanded farther north and east if necessary.

Attention then turns to the potential for another round of
strong to severe thunderstorms expected to affect the area from
late this afternoon into tonight. The expected evolution of
convection across the region remains complex, and depends upon
how thunderstorm clusters currently ongoing across the eastern
Dakotas evolve through the remainder of the morning hours.
Synoptically, an area of surface low pressure is expected to
deepen across the upper Midwest and northern Great Lakes region
through this evening, in response to a mid-level short wave
transiting the south/southeast periphery of an upper low over
eastern Manitoba/western Ontario. A weak cold front trailing
this low is progged to slowly move southeast across the upper
Mississippi Valley, eventually reaching northern IL later
tonight. Re-intensification or redevelopment of upstream
convection is expected by mid-late afternoon ahead of the cold
front, and within a very warm/humid high Theta-E airmass
characterized by dew points in the low-mid 70s across
IA/WI/northern IL. Latest 06Z CAM guidance suggests clusters of
storms to our west/northwest will congeal into another organized
forward-propagating linear MCS which would move into our area
either very late this afternoon or early this evening. (CAM
trends have been to slow this development and arrival from
earlier runs, though some differences remain in just where/when
convection initiates this afternoon). Higher-end severe winds
appear to be the greatest threat, though forecast soundings do
indicate sufficient low-level shear which would conditionally
support a tornado threat with earlier discreet storms or
embedded QLCS circulations within the evolving linear mode.
Severe threat looks to be mainly focused within the evening
hours, perhaps a bit longer in our far east/southeast (similar
to Sunday night). A locally heavy rainfall/flash flood threat
will exist as well, especially along the trailing portion of the
linear MCS where it may tend to become more west-east oriented
after midnight. Our forecast area remains in SPC`s Day 1
Enhanced (level 3 of 5) and WPC`s slight (level 2 of 5) risks of
severe and excessive rainfall.

Scattered thunderstorm chances appear lower across northern
parts of the area by Tuesday, mainly focused along/south of the
slowly advancing cold front along and south of I-80. A few
storms could linger across our far southern cwa into Tuesday
evening. Between cloud cover south, and cool advection behind
the front to the north, cooler temperatures in the 80s are
expected Tuesday.

Wednesday through Sunday...

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

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

&&

$$

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