936
FXUS63 KLOT 120836
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
336 AM CDT Mon May 12 2025

.KEY MESSAGES...

- Summer-like warmth and increasing humidity expected through
  midweek, though locations near Lake Michigan will remain
  cooler most days. Near-record temperatures are expected on
  Thursday.

- Isolated to scattered showers and storms are expected through
  Thursday, though many hours and areas will remain dry.

- There is a threat for severe weather Thursday afternoon and
  evening, contingent on storms developing.

&&

.DISCUSSION...
Issued at 336 AM CDT Mon May 12 2025

Through Wednesday night:

The closed upper low that has been meandering across the Lower
Mississippi Valley the past few days has begun to lift north
toward the region early this morning. This will lead to a plume
of low-level moisture will advect into the area through the day
today. Guidance has trended slower with the northward
advancement of the higher dewpoints which will likely limit any
diurnal pulse showers and storms to areas mainly south of I-80
through the daytime hours. By the evening these showers and
storms will begin to lift across the rest of the area through
the overnight hours tied to a 700 mb vort lobe as it drifts
northwest across the area. Some of the most recent guidance
seems to have a better handle on this feature, tied to
convection over far northeast Alabama as of this writing. PoPs
only in the slight/20% chance range overnight into Tuesday
morning may ultimately need to be bumped up for our next
issuance. Given increasing low-level moisture over the cold
lake, suspect fog development will commence late tonight through
much of the day on Tuesday, some of which could ooze inland
along the Illinois shore, particularly into Lake County.

Warm and humid conditions will continue Tuesday and Wednesday
as the upper low slowly lifts across the Ohio Valley leading to
additional isolated to scattered showers and storms, with the
greatest coverage expected on Tuesday across northwest Illinois
within the vicinity of the aforementioned vort lobe. Given weak
flow aloft, any storms the next few days should be pulse-like
capable of lightning and localized downpours. Any of the taller
storms would be capable of producing locally gusty winds after
they collapse/pulse back down. Low-level instability paired
with areas of low-level vorticity suggest a few funnel clouds
also can`t be fully ruled out. Highs are forecast to warm into
the mid 70s to mid 80s inland while still notably cooler near
the lake in the 60s to lower 70s.

Petr

Thursday through Sunday:

The summer-like warmth and humidity will then continue into
Thursday, with even warmer temperatures expected due to the
continued solar insolation and increasing warm air advection. In
fact, it`s quite possible that we could see our first 90 degree
readings of the year in our forecast area on Thursday. If that
were to occur, then the daily high temperatures for May 15th in
Chicago and Rockford (91 and 90, respectively) would potentially
be in jeopardy. For once, these warmer temperatures may reach
at least some of our lakeside locales as winds on Thursday
should be breezy and from the south.

Late Wednesday into Thursday, the western CONUS trough is
expected to eject into the northern Plains and Upper Midwest.
The low pressure system that develops in response will help
advect a more pronounced EML into the Midwest and will also
cause kinematic fields across the region to strengthen. This
will augment the pre-existing warmth and moisture to foster a
potential threat for severe weather across the region late
Thursday into early Friday. This severe weather set-up remains
conditional into our area due to uncertainties regarding impacts
of capping, very dry mid-upper levels (dry air entrainment
influence), displaced forcing mainly to the north and west of
the area, etc., which will impact whether storms will be able
to initiate this far south. Thus, while confidence is relatively
high at this point in time that the parameter space that will
be in place over the region late Thursday will largely be
favorable for severe thunderstorms, confidence is still fairly
low in whether everything will come together just right for this
parameter space to be fully utilized, or whether we`ll even see
storms at all. Stay tuned!

Our weather for Friday will largely depend on how things play
out on Thursday with the low pressure system`s evolution. A
strong cold frontal passage Thursday night/Friday morning would
tend to favor dry and breezy (to potentially windy) conditions
across the area on Friday. On the other hand though, the nearby
presence of the aforementioned upper-level trough could still
support shower and thunderstorms chances here if the cold front
arrives too early or too late and/or isn`t robust enough to
sufficiently scour out the warmth and moisture in the area,
particularly south of I-80. While there should be a reprieve
from the warmer and at times stormier conditions this coming
weekend, the overall weather pattern across the central CONUS is
expected to remain fairly active into next week. This could
result in renewed chances for showers and thunderstorms in our
area during next work week.

Ogorek/Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1240 AM CDT Mon May 12 2025

Main Concerns:

- Potential for low CIGs and reduced VSBY late tonight/early
  Tuesday morning.

- Chance for SHRA this evening and overnight.

VFR conditions will prevail through at least mid to late this
(Monday) evening. Expect easterly winds to increase to 10+ kt
this morning and then ease back below 10 kt this evening.

Showers south of the terminals (and perhaps an isolated TS or
two well south) later this afternoon may move into the area on
a weakening trend this evening-overnight. If showers occur,
suspect they`d be non-impactful. Once again withheld any precip
mention from the TAFs and will reassess for the 12z issuance.

Some of the guidance brings in low CIGs (low MVFR to IFR) as
early as mid to late this evening. This may be a bit too
aggressive, so leaned on the slower guidance for now, which
suggests lower MVFR CIGs (or lower) developing after 06z
Tuesday. If sub-1000 ft CIGs occur, they should be in the
300-500 ft range. Finally, the higher humidity air mass and
lighter winds early Tuesday should support modestly reduced
VSBY. Dense fog may develop over the lake and stream onto the
Illinois lakeshore, but likely remain east of MDW and ORD.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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