587
FXUS63 KLOT 150905
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
405 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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1250 AM CDT Mon Jul 15 2024

Forecast concerns include...

Chance of thunderstorms late Monday afternoon/evening.
Chance of fog and mvfr cigs this morning and Tuesday morning.

The current thunderstorm activity will continue to move south
and east and away from the terminals early this morning. Some
vicinity thunder mention is prudent for the next hour or so as
all of this departs.

Dry weather is then expected through early/mid afternoon when
the focus will be for another round of strong/severe
thunderstorms that may affect the area. Did not make any changes
to the tempo timing which still looks on track. However, there
are some models that now show this activity possibly shifting a
bit further south. Its too early to determine if this shift
south in the guidance will pan out and trends will need to be
monitored. These storms this evening will be capable of
producing strong/erratic winds as well as torrential rainfall.

Wind speeds will be light for the next few hours, but during
this time wind directions may range from northeast to southeast
before eventually turning back to a southwesterly direction
which will continue into tonight. There may be a period of gusty
southwest winds during the mid/late afternoon when gusts into
the 20-25kt range will be possible.

There is a large area of mid and high level clouds that will
slowly scatter out overnight. How that impacts fog potential as
well as low cloud potential is uncertain, but both are possible
across the entire area and opted to include tempo mention for
now for early this morning. There is also some potential for
low clouds/fog late Monday night into Tuesday morning. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory from 8 AM this morning to 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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