-- Discussion containing changed information from previous version are highlighted. --
415
FXUS63 KLOT 241740
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 PM CDT Mon Jun 24 2024

.KEY MESSAGES...

- Today will be pleasant with light winds, sunshine, and highs
  in the mid to upper 80s (cooler along Lake Michigan).

- Multiple rounds of thunderstorms are possible late tonight
  through Tuesday night, including threats for severe weather
  and flash flooding.

- Another period of active weather with strong to severe
  thunderstorms and flash flooding is possible Friday into
  Saturday.

&&

.DISCUSSION...
Issued at 312 AM CDT Mon Jun 24 2024

Through Tuesday night:

Today will be relatively quiet. The center of a surface pressure
ridge currently near the Mississippi River will drift eastward
across the area throughout the day. Light winds, plenty of sunshine,
and pleasant temperatures will result, with afternoon highs expected
to top off in the mid to upper 80s. The weak surface pressure
gradient will provide no resistance to a lake breeze surging
inland this afternoon, leading to cooler temperatures in the low
to mid 70s along the lakeshore.

The main focus in the short term period is on the threat for several
rounds of severe and heavy-rain producing thunderstorms from tonight
through Tuesday night. This afternoon and evening, a plume of
deep low-level moisture (with surface dew points in the mid to
upper 70s) will arc northeastward into Minnesota beneath an EML
with impressively steep mid-level lapse rates (sampled near 9
K/km on the 00Z GGW RAOB). The net result will be a pool of
extreme instability characterized by MUCAPE >5000 J/kg within an
highly sheared kinematic environment (upper-level jet located
near the US/Canadian border). Thunderstorms appear likely to
explode at some point this evening in Minnesota, though exactly
where, when, and at what coverage remain unclear.

Based on an ensemble of high resolution model guidance and
current observations, there appear to be three main candidates for
forcing mechanisms to initiate convection this afternoon through
the overnight period. The first is a subtle low- level wave
responsible for mid-level clouds currently in northwestern
Nebraska due to arrive in southern Minnesota this evening. The
second is an upper-level shortwave and associated surface low
(currently supporting thunderstorms in western North Dakota) due
to arrive in central Minnesota after sunset. And, the third
will be forcing along the nose of an intensifying 925-850mb low-
level jet expected to lift into northern Iowa and western
Wisconsin after midnight. With the instability axis expected to
be oriented from northwest to southeast, any convection that
develops upstream in Minnesota and Wisconsin will be encouraged
to move southeastward into into northern Illinois or western Lower
Michigan overnight. As a result, to what extent, when, and where
each candidate forcing mechanism initiates convection (and all three
may very well initiate their own area of convection) will ultimately
dictate how the night unfolds for us.

In these kinds of regimes (extreme instability but with
unclear/multiple sources of forcing), the range of forecast outcomes
for areas downstream (e.g. our area) can be incredibly vast and
include no thunderstorms (miss to the north) to destructive
derechoes with a threat for widespread wind damage. At this
point, even among different convective scenarios advertised by
the past 24 hours of CAM guidance, there is a definite signal
that at least part of our area will be impacted by thunderstorms
tonight. Timing may be everything with severe weather potential, as
an arrival too early before midnight (before deep instability
arrives) may encourage thunderstorms to decay as they roll in.
However, if thunderstorms arrive after midnight as instability
arrives, they may very well be severe. In all, the message tonight
is to have multiple ways to get warnings including ways to wake you
up.

The forecast for tomorrow (Tuesday) is, well, unclear. Big-picture
wise, a cold front is expected to drop southward across our area
from late afternoon through early evening and into a pool of extreme
instability and considerable column moisture (MUCAPE >5000 J/kg,
PWATs >2"). Even with modest deep layer shear (the upper-level jet
is still well to our north), environments with extreme
instability can allow for small but highly organized clusters
with destructive wind potential. And, any sort of training in
environments with PWATS >2" would entail a threat for flash
flooding. Now, based on the coverage and intensity of
thunderstorms tomorrow morning, there appear myriad of forecast
scenarios for the afternoon and evening. One scenario is a dry
period between the morning and an eventual evening round of
thunderstorms in part of our area, another is simply no evening
thunderstorms (e.g. eventual frontal convergence is shunted too far
south/west of our area by morning thunderstorms), and third is
thunderstorms backbuild off of morning activity and continuing to
train over our area literally all day. And, that`s not to mention
the scenario where no storms reach our area tonight, leading to a
widespread convective event across our entire area tomorrow
afternoon and evening (e.g. instability axis is pristine). At this
point, the best we can do is simply advertise the threat for
thunderstorms on Tuesday including a threat for severe weather and
flash flooding, and encourage everyone to stay up to date on the
forecast.

Perhaps one thing is becoming more clear... With prospects for
thunderstorms or associated cloud cover around the neighborhood
throughout parts or most of the day tomorrow, chances for dangerous
heat and humidity developing on a widespread basis appear to be
lowering.

Borchardt


Wednesday through Sunday:

A cold front responsible for potential convection Tuesday into
Tuesday night will be shifting SSE across the CWA at sunrise on
Wednesday. Any showers and residual storms Tuesday night should
be exiting the southern CWA by mid-morning Wednesday. However,
with the main mid-level wave trailing the cold front by several
hours, some diurnally based showers and perhaps an isolated
storm may develop across primarily the southeast half of the CWA
late Wednesday morning into early afternoon before dry air
advection and mid-level subsidence end precip changes.
Otherwise, a brief period of quiet conditions is in store late
Wednesday afternoon through Thursday with dry weather and
seasonable temperatures as a broad surface high moves across the
Great Lakes region.

A low-amplitude mid-level ridge will cross the region Thursday
night into Friday morning as broad lee cyclogenesis occurs ahead
of a deep trough digging into the northern Rockies. Strong low-
level moisture transport below a plume of modest mid-level lapse
rates across the mid and Upper-Mississippi River Valley will
set the stage for a conditionally unstable environment by Friday
morning. There is some concern that a cluster of convection
developing across the Missouri River Valley Friday morning will
approach the forecast area before outrunning the better
thermodynamic environment toward the western Great Lakes Friday
afternoon. While this is still pretty far out for pinning down
the exact evolution of convection, the signal of robust
convection in global ensemble guidance combined with a modest
parameter space for severe convection bears close watching over
the next several days. This also includes a notable signal for
flash flooding with PWATS likely around or above 2" and the
potential for a stalling effective outflow boundary in the
vicinity. Needless to say, Friday through Saturday could be
another rather active period for our region.

A dry and seasonably cool airmass is progged to settle over the
region as we turn the calendar from June to July before another
signal for active weather appears during the lead-up to
Independence Day.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
-- Changed Discussion --
Issued at 1240 PM CDT Mon Jun 24 2024

Aviation weather concerns are:

- Increasing thunderstorm chances late tonight/Tuesday morning
  with some threat for strong to severe wind gusts.

A lake breeze has brought a NE/E wind shift to ORD, MDW, and
GYY. Whether it continues to surge at its current pace towards
DPA or slows down remains a bit unclear, but elected to move up
the timing to account for the faster movement. Regardless, winds
will eventually return to a S and SW direction overnight.

The main change was to convert the PROB30 groups to TEMPOs for
TSRA late tonight and Tuesday morning. Expectation is for
explosive thunderstorm development across Minnesota this evening
within a highly unstable environment, with thunderstorms growing
into a surging complex. While uncertainties remain regarding
specific timing of this complex into the c90 (and refinements
to TSRA timing will likely be needed), confidence was high
enough to justify TEMPOs. While this thunderstorm complex should
be in a gradual weakening phase, strong to severe wind gusts
will remain possible as it moves through the region.

A VCTS threat may continue a bit beyond the end time of the
current TEMP groups, but this was not high enough confidence to
justify a mention at this time. Conditions may remain largely
quiet through Tuesday in the wake of this morning activity with
gusty southwesterly winds developing through the morning and
afternoon.

Carlaw
-- End Changed Discussion --

&&

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

&&

$$

Visit us at weather.gov/chicago