327
FXUS63 KLOT 150529
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1229 AM CDT Wed May 15 2024

.KEY MESSAGES...

- Showers and embedded thunderstorms will continue through
  around sunset roughly south of a Pontiac to Valparaiso line.

- A narrow axis of heavy rainfall (20-30% chance of >1" of rain)
  resulting in localized flooding remains possible through this
  evening over locations that experienced heavy rainfall across
  Livingston, Ford and Iroquois counties on Monday.

- Chances for showers and thunderstorms will return on Thursday,
  followed by warming temperatures into the weekend.

&&

.UPDATE...
Issued at 823 PM CDT Tue May 14 2024

No significant changes made to the going forecast this evening.
While a gradual diurnal decrease in coverage/intensity of
convective showers and isolated thunderstorms is expected, we`ll
continue to monitor these slow, west-southwestward moving cells
for locally heavy rainfall potential across our east and southeast
counties this evening.

A broad upper-level low was evident in GOES vapor imagery this
evening, with a primary center located over southeast Missouri.
A couple of other smaller vorts were noted within the broader
trough however, including one near KIND. This vort is likely
aiding continued rain development over north-central IN, as well
as scattered convective showers and a few embedded
thunderstorms across northern/northwest IN including the far
east/southeast counties in the WFO LOT cwa. Thankfully, rainfall
rates have been fairly low (<0.10" per hour) since this
afternoon across areas which saw locally heavy amounts yesterday
(roughly between the I-55 and I-57 corridors south of Chicago),
with some of the earlier more widespread shower activity
focused across Lake and Porter counties which saw little
previous rainfall. North-northeast low level flow off of Lake
Michigan has helped to keep temperatures cooler and instability
much weaker than farther east into IWX`s cwa, where deeper
convective cores (and the only recent lightning) have been noted
across Starke and Pulaski counties. As noted above, while a
gradual decrease in coverage and intensity is expected, we`ll
continue to monitor rainfall rates/amounts with these slow-
moving cells as they drift west-southwest at around 10 mph into
our eastern counties.

The going forecast has the general trends handled well, with
showers mainly east of a Pontiac to Gary line gradually becoming
less numerous and gradually shifting off to the southeast
overnight. Thus, no significant changes are needed.

Ratzer

&&

.DISCUSSION...
Issued at 300 PM CDT Tue May 14 2024

Through Tonight:

We remain under the influence of an upper-level low over
central Missouri this afternoon, with the associated surface low
over far western Kentucky. An ongoing light rain band near the
IL/WI state line will gradually dissipate over the next few
hours as mid/upper forcing begins to wane north of the exiting
low. Focus then settles on an active mid-level deformation axis
stretching from near Pontiac ENE toward South Bend this
afternoon. This axis has roughly aligned with an exiting (and
stalling) surface boundary in an area with locally higher low-
level instability after some morning sun. While overall surface-
based instability is modest at best, marginal lapse rates and
low LCLs will support a general axis of broken showers and
embedded storms that should persist through sunset.

The main concern is on localized flooding for areas that
experienced heavy rain across east-central Illinois on Monday.
Had considered a targeted Flash Flood Watch for Livingston, Ford
and Iroquois counties through early evening, but the expected
small footprint of higher convective rates supported more of a
tactical gridded/graphical forecast approach. Would not be
surprised if an areal Flood Advisory is needed at some point
through early evening, but the likelihood of a Flash Flood
Warning remains under 30%. If a Flash Flood Warning were needed,
the most favored area would be from southeast Livingston into
northwest Iroquois counties where the highest axis of rain on
Monday has resulted in a Flash Flood potential with rates as
little as 1"/3hr this afternoon.

Overall activity will only slowly shift southeast through the
night as the upper-level drifts east, with some showers
lingering into Wednesday morning south of a Watseka to
Rensselaer line.

Kluber


Wednesday through Tuesday:

Other than a few spotty showers lingering early Wednesday
morning (as noted above) dry conditions are expected the rest of
the day on Wednesday as mid-level ridging builds overhead in
the wake of the departing low. This should also allow for warmer
temperatures compared to today, especially well inland from the
lake in Illinois where low to mid 70s are expected if clouds can
clear out. Continued northerly flow will keep temperatures
cooler near the lake and further inland across northwest Indiana
where highs likely still struggle to warm out of the upper 50s
to mid 60s.

As mentioned in previous discussions, confidence in the details
decreases with time in the extended forecast period due to a
greater degree of variability in the handling of a cut-off low
across the desert southwest as it drifts east and whether it
remains south of the area or phases with the broader upper jet
near the region late week into the weekend.

There is, however, a continued signal for dissipating
convection (storm clusters/MVC/etc) to to approach the area
heading into the day on Thursday which could result in renewed
convective development during the afternoon/early evening
period. At this range conditions do not appear favorable for
severe weather given generally weaker shear/instability
parameters this far north.

Additional showers/storms may continue into Friday though again
will be dependent up on the southern stream cut-off
low/shortwave. While a dry forecast can`t be guaranteed over the
weekend there is perhaps a bit better signal for increased
precipitation chances early next workweek. Temperatures also
look to trend warmer each day with highs back into the upper
70s to lower 80s by the weekend (with the exception of lake
adjacent areas).

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1229 AM CDT Wed May 15 2024

Forecast concern for the terminals...

- Low-end MVFR to IFR ceilings tonight mainly across northwest
  IN and central IL

A broad area of low pressure will continue to track eastward
across the Ohio River Valley tonight which will allow an area of
low clouds to pivot across northwest IN and adjacent areas of
IL overnight. Latest observations show that the majority of
these low clouds are within the 1500 to 2000 ft range, but some
pockets of 900 ft IFR ceilings have been noted at times. The
expectation is for these MVFR ceilings, and occasional IFR, to
remain primarily across northwest IN (i.e. GYY) overnight, but
I cannot completely rule out a brief MVFR ceiling at ORD or MDW.
Therefore, have maintained a SCT025 mention at ORD and MDW.
Ceilings will improve back to VFR Wednesday morning (around
daybreak at ORD/MDW and 15z at GYY) as the low pulls away with
VFR conditions prevailing through the rest of the period.

Otherwise, expect winds to maintain an east-northeasterly
direction with speeds generally around 10 to 15 kts through
Wednesday afternoon. Though speeds are expected to ease to
around 5 kts Wednesday evening.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 AM CDT this morning for the IL
     and IN nearshore waters.

&&

$$

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