416
FXUS63 KLOT 151220
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
720 AM 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.

&&

.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 at 720 AM CDT Mon Jul 15 2024

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

* A localized area of MVFR/IFR cigs and vsbys early this morning.

* Another round of thunderstorms, possibly severe, this evening.

Early this morning, a swath of fog and low stratus rides the I-90
corridor from northern Cook County out toward RFD. It`s recently
extended its reach over RFD where we find MVFR vsbys as of 12Z. It
possible that the fog/stratus propagates over some more of the
airfields this morning, ORD has the seemingly best chance as the fog
sits right on its northern doorstep. In the heart of the plume, it
looks like there are IFR cigs and vsbys to be found, although it`s
tough to know for sure with a lack of observations. Thinking this
will burn away by around 13Z.

Conditions will be mostly quiet this afternoon. SW winds will gust
to around or just over 20 kt this afternoon, especially toward late
afternoon.

A complex of thunderstorms is expected to charge across northern IL
this evening from west to east. Timing is rather unclear and model
guidance has recently been trending a bit later in the evening than
previously thought. At the moment, looking like around 00 to 01Z to
reach RFD and closer to 01 or 02Z at the Chicago terminals. In
addition to heavy downpours, these storms may feature strong,
damaging winds with large hail and tornado also possibilities. Storms
should move off to the east late this evening or early overnight
with quiet conditions to follow through Tuesday morning.

Doom

&&

.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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