719
FXUS63 KLOT 242113
AFDLOT

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

.KEY MESSAGES...

- It is increasingly likely that a line of thunderstorms will
  march through area late overnight into early tomorrow
  (Tuesday) morning. Damaging winds will be possible with these
  storms, particularly as you get closer to the Illinois-
  Wisconsin state line.

- Another round of thunderstorms appears possible tomorrow
  (Tuesday) afternoon into Tuesday night, with some potential
  for severe weather and/or flash flooding, though confidence in
  this round of storms is low.

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

&&

.DISCUSSION...
Issued at 413 PM CDT Mon Jun 24 2024

Through Tuesday Night:

The forecast through Tuesday night remains a challenging one,
though there have been some signals that have emerged in the
latest hi-res guidance that have increased confidence in some
aspects of our forecast.

First, convection is expected to erupt late this afternoon/early
this evening near a surface triple point in the vicinity of the
Twin Cities. Upscale growth of this convection into a bowing
MCS appears quite likely late this evening, with Corfidi
downshear propagation vectors and a northwest-southeast oriented
thermal gradient across Wisconsin suggesting that the apex of
this MCS will propagate southeastward into southeast Wisconsin
overnight. West/southwest of the bow`s apex, convection should
have a tendency to propagate in a more southerly direction, and
the expectation is that this southward- propagating, quasi-
zonally oriented portion of the MCS will track through our
forecast area late overnight/early tomorrow morning.

Almost ubiquitously westerly flow above 850 mb should
concentrate the greatest damaging wind potential in the
eastward-surging segments of the MCS, and these line segments
would also likely carry some QLCS tornado threat as well. The
current thinking is that these more intense line segments will
probably remain to our north in Wisconsin, but it could at least
be a close call for our northern counties. Regardless, the
substantial degree of buoyancy aloft will still support the
potential for damaging downburst winds within any deep
convection associated with the MCS, though the near-surface
stable layer will get increasingly stout with time and southward
extent, putting into question how late into the night/morning
the damaging wind threat will persist.

The forecast for Tuesday remains shrouded in uncertainty and
hinges heavily upon what the MCS does in the morning. With that
being said, two general scenarios appear plausible. The first,
and most likely, scenario is that the MCS plows through the
entirety of our CWA and shunts its outflow to our south and
west. This outflow would then serve as an effective frontal zone
and would be the main focus for convective redevelopment
Tuesday into Tuesday night, which would accordingly largely
remain concentrated to our south and west. In such a scenario,
most or all of our forecast area would find itself capped and
sans precipitation through at least the early-mid afternoon.
However, it`s still conceivable that isolated to scattered
convection could redevelop thereafter -- either within a narrow
ribbon of isentropic ascent or along any surface boundaries in
the area, be it the lagging true cold front and/or a potential
differential heating boundary along the northern periphery of
the cloud shield associated with the MCS.

The other, less likely, scenario is that tonight`s MCS largely
misses us or weakens faster than expected. Either of these
outcomes would likely result in either the MCS`s outflow
boundary getting laid out across our forecast area during the
daytime on Tuesday, or the outflow boundary becoming
diffuse/getting washed out and allowing for the lagging true
cold front to become the main focus for convective development
in the afternoon and evening. Regardless of which boundary would
become the dominant feature, it would likely get lit up with
convection, much of which would become severe with damaging
winds and large hail as the main threats given the strong to
extreme degree of instability but somewhat lacking shear that
would be expected to be present in this scenario. Perhaps even
more concerning is that this setup would conceptually resemble a
Maddox flash flood setup, with a low-level jet overriding and
nearly paralleling the initiating boundary, allowing for
convection to continuously regenerate and train over the same
areas, which with precipitable water values near and in excess
of 2" would almost certainly lead to flash flooding occurring
somewhere. Though again, this scenario is the least likely of
the two general scenarios for Tuesday, and if the first scenario
were to verify, then the greatest potential for flash flooding
should remain to the south/southwest of our forecast area.

Last but not least, there is still unsurprisingly some
uncertainty for our high temperature forecast tomorrow with the
first scenario generally favoring cooler (but still fairly warm)
temperatures than the second scenario. Fortunately, the heat
and humidity shouldn`t be too oppressive in either scenario with
100F looking like a reasonable higher bound for peak heat
indices tomorrow.

Ogorek


Wednesday through Monday:

A cold front will press across the region on Wednesday. Depending
on how things evolve Tuesday evening and overnight, some
convection may be ongoing across the area Wednesday morning,
mainly south of I-80, but the trend should be towards a drier one
through the day. Some guidance does show some fairly deep boundary
layer mixing developing through the afternoon, perhaps enough to
pop a few high-based showers, but this doesn`t seem like a
significant enough signal to justify PoPs at this point. A brief
period of dry and quiet conditions will then prevail through
Thursday as a transient region of high pressure drifts overhead.

By Thursday night into Friday morning, the surface high will be
departing and shuttling across the northeast, yielding a return of
moist southerly trajectories in the region. A series of low-
amplitude/subtle shortwaves emanating from a robust upper low
pinwheeling across Montana and into Saskatchewan will likely get
swept up within freshening mid and upper level west-southwesterly
flow. This in turn will lead a commensurate increase in low-level
warm advection across Iowa, which will eventually spill east of
the Mississippi River during the day on Friday.

Latest model guidance suggests that warm advection/isentropic
upglide-induced convection will probably develop late Thursday
night into Friday morning across Iowa within the plume of
returning elevated instability. However, latest indications are
that this activity may outpace the slowly-returning elevated mixed
layer leading to a gradual weakening trend through the day.
However, Friday night continues to look like a period we`ll have
to watch closely, as another area of low pressure and associated
cold front will be impinging on the region from the north. Ahead
of this, intensifying warm advection acting on any remnant outflow
from morning activity will also finally be able to impinge on the
returning warm/moist sector as the kinematic parameter space also
becomes more supportive of storm organization. Still pretty far
out with plenty of spread and uncertainties, but this is a period
we`ll continue to closely eye for a severe and also flash flood
potential with PWATs forecast to surge into the 2 to 2.5 inch
range.

We`ll get into another brief quiet period over the weekend as high
pressure returns. From a synoptic perspective, winds may surge a
bit down the lake as the high arrives which could yield a building
wave/rip current potential over the weekend.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
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

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 AM to 1 PM CDT Tuesday for the IL
     and IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago