130
FXUS63 KLOT 150538
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1238 AM CDT Thu May 15 2025

.KEY MESSAGES...

- Summer-like warmth continues through Friday, with record
  breaking heat possible Thursday.

- Conditional threat of severe weather exists Thursday afternoon
  and evening (Threat level 2 to 3 out of 5), dependent on
  storms forming.

- Threat of rapid fire spread Thursday and especially Friday due
  to a combination of strong winds and dry conditions.

&&

.UPDATE...
Issued at 900 PM CDT Wed May 14 2025

The inherited forecast appears to be on track. Dense fog
continues to slowly march inland off Lake Michigan and into
northeastern Illinois. With continued easterly steering flow,
it`s hard to see reason why fog won`t continue to expand during
the overnight hours. A Special Weather Statement is in effect
for Lake IL, northern and central Cook, and Lake IN counties
through 10 PM. Thereafter, we will have to give consideration
for a Dense Fog Advisory (particularly for Lake IL and perhaps
northern Cook counties).

Shifting focus, an expansive region of ACCAS arcs from
southwestern Iowa through southern Illinois and into central
Kentucky. Per 00Z ROABs, surface observations, and RAP
mesoanalysis data, the ACCAS is rooted along the edge of an EML
plume and atop the surface instability reservoir (surface dew
points in the mid to upper 60s). As both shift east-
northeastward over the Upper Mississippi River Valley and Great
Lakes region tonight, do think a thunderstorm or two may sprout
with an associated threat for damaging hail. The threat is
probably around 5 to 10% at any given location (meaning there
is an 90 to 95% chance any given area will remain dry).
Regardless, will maintain inherited slight chances for isolated
thunderstorms overnight.

We don`t have much to add regarding the threat for severe
weather tomorrow afternoon/evening. We`ll note that the
incoming 00Z HRRR is quite bullish showing widespread
supercell development with attendant threats for both 2" hail
and 75+ mph winds along/east of I-39. Such coverage is much
higher than all prior runs, which curtained the threat to
largely lakeshore counties. We`ll let the midnight crew take a
much closer look, including other incoming 00Z guidance, to
discern if the 00Z HRRR is onto something.

Updated products are in the process of being sent.

Borchardt

&&

.DISCUSSION...
Issued at 308 PM CDT Wed May 14 2025

Through Thursday:

Following the possibility for some isolated showers and
thunderstorms late this afternoon across interior sections of
northern IL into northwestern IN, we are looking at a fairly
quiet and mild evening (particularly for inland areas). However,
similar to yesterday, we will have to keep a close eye on the
dense marine fog, which remains in place across southern Lake
Michigan this afternoon. I suspect that the continued light
onshore easterly flow will again push some of this fog inland
across far northeastern IL after sunset this evening.
Accordingly, we will have to monitor the need for another dense
fog advisory for at least parts of Lake county IL and Cook this
evening and tonight. Any dense fog that moves inland will
quickly improve by mid morning on Thursday.

There also continues to be a low chance (~20%) for a few
showers and thunderstorms overnight. This as isentropic upglide
(warm air advection) kicks up along a modestly increasingly
southerly low-level jet late tonight. It still appears any
activity that develops into our area will remain low in areal
coverage, thus most will remain dry overnight.

Thursday is still shaping up to be an unseasonably hot day
(with near record high temperatures), with a conditional threat
of severe late afternoon and early evening storms. This as a
strong sub-985 mb surface low tracks northeastward while
occluding across eastern South Dakota and the Upper Midwest late
Thursday into Thursday night. Our area will become oriented
solidly within the warm sector of this cyclone following an
early morning warm frontal passage. Breezy south winds (gusting
up around 30 mph) in the wake of this warm front will drive a
thermal ridge northeastward into the Lower Great Lakes, with 925
mb temperatures progged to increase to around +25C. Locally
climatology suggests that this will support lower 90s high
temperatures for the area Thursday, which is right around the
record high`s for the 15th.

The warm sector airmass is expected to remain capped through
most of the day by an EML. Accordingly, the threat for storms
through at least 3 PM Thursday afternoon will remain very low.
However, the chances for at least isolated storm development
will begin to ramp up after 3 PM as a cold frontal/dry line
feature shifts eastward into northern IL. There continues to be
uncertainty with the overall storm coverage with southward
extent into northern IL later in the afternoon, owing to
lingering capping and rather weak large scale forcing. However,
there are signs in the guidance that the exit region of an
upper-level jet will try to nose into northeastern IL towards
the early evening period just as modest mid-level height
falls/cooling spread into the area. This is should thus foster a
gradual uptick in storm coverage along and ahead of the surface
boundary into Thursday evening as cap weakens.

What remains somewhat unclear at this time is how quickly this
uptick in storm development occurs along the frontal boundary
into early Thursday evening. If it takes a bit too long, the
higher storm coverage may end up occurring largely east-
northeast of our area as the front quickly shifts out over Lake
Michigan. Nevertheless, it appears that at least some isolated
to widely scattered storms may break out across parts of
northern IL late in the day. Assuming they do, the thermodynamic
and kinematic environment in which they develop will be very
supportive of organized severe storms, particularly supercells.
The presence of very steep mid-level lapse rates (700-500 mb 8C+
per km) suggests that instances of large to very large hail
(2"+ in diameter) will be possible with these storms, and may
end up be the primary severe threat in our area. However, some
instances of damaging winds and a possible tornado cannot be
ruled out. Expect the the threat of storms to quickly end from
west-to-east following the frontal passage into early Thursday
evening.

KJB


Friday through Wednesday:

In the wake of Thursday`s conditional severe weather risk, a
substantial mid-level dry layer atop a residual EML just above
the surface will advect across the area Thursday night into
Friday. Meanwhile, near-record low MSLP values for May of
potentially sub-980hPa over Minnesota on Friday will generate
anomalously strong synoptic winds over the forecast area. The
combination of RH values lowering to 25% or less and southwest
winds gusting to around 40 mph amid deep diurnal mixing into the
dry air mass will support the potential for critical fire
weather conditions Friday afternoon. Rapid spread of brush fires
is possible with any remaining dry/dead vegetation from prior
seasons. Additionally, fine fuels have started drying out given
the recent lack of rain and persistent days of lower RH values.
A Fire Weather Watch may be needed as early as the next forecast
cycle.

In addition to the windy and dry conditions Friday, have added
patchy blowing dust for much of interior northern Illinois away
from the Chicago/Rockford metros as soils in fields could result
in locally reduced visibility (particularly for NW to SE
oriented roads).

Even with the expected dry weather and brush fire concerns,
impressive lapse rates of 8-8.5C/km through much of the
troposphere will still promote high-based (10kft) cumulus during
the afternoon. A subtle mid-level impulse quickly tracking from
the western Rockies to the mid-Mississippi River Valley could
induce deeper growth of the high-based cumulus and ultimately
result in isolated thunderstorms late in the afternoon and early
evening. Associated DCAPE values approaching 1500 J/kg amid a
deep sub-cloud inverted-V profile would support locally damaging
wind gusts with any thunderstorms.

A strong Hudson Bay high will gradually replace the strong
surface low through the weekend, allowing for cooler conditions
to filter over the area. High temps in the 70s area-wide on
Saturday will settle into the 50s along the shore to 70s well
inland on Sunday as a backdoor cold front crosses the area from
the northeast Saturday night. Next week, overall guidance is
painting another period of active weather as deep troughing over
the western CONUS slows or potentially cuts off over the region
by midweek.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1238 AM CDT Thu May 15 2025

Key Messages:

- 30-40% chance for thunderstorms to affect the terminals late
  this afternoon. IF storms develop, they will likely become
  strong to severe.

- Gusty southerly winds this afternoon, followed by a
  southwesterly wind shift behind a cold front during the late
  afternoon/early evening.

- 2-4 hour period of strong WSW wind gusts >30 kt may develop in
  the wake of thunderstorms early this evening (greatest
  potential at RFD).

Earlier concerns of the marine fog layer drifting as far inland
as ORD/MDW have decreased over the past few hours, with the
expectation that it remains north and east of the terminals.
Will continue to monitor satellite trends closely overnight,
however.

While an isolated shower or storm can`t entirely be ruled out
early this morning (~15% chance), confidence remains too low to
include in the TAFs with this update. There are attempts at
development across central Illinois though conditions become
less favorable with northward extent through the overnight
hours.

Late this afternoon, a cold front will approach the area from
the west, and as it gets close to the terminals, explosive
thunderstorm development is possible during the 22-01Z
timeframe. Confidence in whether a storm will develop over or
near the TAF sites is still only low-medium (about a 30-40%
chance at any particular site). However, wherever storms do
develop conditions are favorable for severe thunderstorms
capable of potentially destructive hail (2"+ diameter) and
damaging winds (>50 kt), in addition to lightning and visibility
reductions in heavy downpours. The threat for storms at the
terminals should end by about 01Z as the cold front pushes
through the area.

Lastly, south-southeasterly winds will become gusty during the
late morning, and pick up in intensity during the afternoon --
possibly gusting to around 30 kts at times. The prevailing wind
direction will likely turn due south or just west of south at
some point during the afternoon before turning southwesterly
after the passage of the cold front during the late
afternoon/early evening. We are also monitoring the potential
for a 2-4 hour period of strong WSW wind gusts >30 kt early in
the evening that may develop in the wake of any storms that
develop, which some of the hi-res guidance is attempting to
resolve. Confidence in this occurring remains very low;
however, there is perhaps a locally higher potential at RFD.

Ogorek/Petr

&&

.MARINE...
Issued at 716 PM CDT Wed May 14 2025

Dense fog continues to plague the southern half of Lake
Michigan in response to humid air pooling over the cool lake
waters. As winds adopt a southerly component overnight, fog may
begin to drift away from the Indiana shoreline. Fog may then
linger along the Illinois shoreline through at least tomorrow
morning but possibly into the afternoon. A Dense Fog Advisory
remains in effect for much of southern Lake Michigan through
Thursday morning.

Thursday afternoon, thunderstorms appear poised to develop just
west of if not directly over Lake Michigan. Any storm may
produce gale to storm force winds and damaging hail. A brief
period of southwesterly gale force winds cannot be ruled out
behind the storms, as well.

Finally, a period of southwesterly gale force winds may
materialize Friday afternoon, particularly northwest of Gary,
Indiana. A Gale Watch may be needed soon.

Borchardt

&&

.CLIMATE...
Issued at 643 PM CDT Wed May 14 2025

Record high temperatures are possible on Thursday May 15.

Here are the current record highs for May 15:

Chicago: 91 in 1962

Rockford: 90 in 1944

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 7 AM CDT this morning for ILZ006-
     ILZ103.

IN...None.
LM...Dense Fog Advisory until noon CDT today for Winthrop Harbor IL
     to Gary IN.

&&

$$

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