-- Discussion containing changed information from previous version are highlighted. --
636
FXUS63 KLOT 252332
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
632 PM CDT Tue Jun 25 2024

.KEY MESSAGES...

- Another potential round of thunderstorms late this afternoon
  into tonight with a level 2/5 risk for severe weather and
  level 1/5 risk for flash flooding.

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

&&

.DISCUSSION...
Issued at 341 PM CDT Tue Jun 25 2024

Through Wednesday Night:

It`s another complex, relatively low confidence forecast for
the rest of today with several mesoscale subtleties expected to
play a key role in how things play out over the next several
hours.

A recent mesoscale analysis shows our MCS from this morning is
still chugging along well to our south with its outflow boundary
extending from the central Ohio River Valley back towards
northeast Missouri. An extension of this effective front bends
back into southwest Iowa, where it meets up with a true cold
(now quasi-stationary) front that is laid out across the state
and stretches eastward into the northern third of our forecast
area. South of this front in our CWA, the air mass -- still
feeling the effects of the robust cold pool left behind by the
morning MCS -- remains stabilized beneath extensive mid- to
high-level clouds emanating from the MCS. Near and north of the
front, cumulus continues to bubble as sunshine continues to
destabilize the boundary layer, which still appears to be
slightly capped, per the 18Z DVN RAOB.

Confidence is relatively high that the aforementioned outflow
boundary/effective front will continue to serve as a focus for
extensive convective development going into this evening and
keep the bulk of tonight`s convective activity and the greater
potential for excessive rainfall and flash flooding displaced to
the south and southwest of our forecast area. While that may be
the case, it seems probable that we`ll still see some additional
thunderstorms move into or develop somewhere in our CWA later
today, though exactly where, when, and how remains somewhat
uncertain as there are a couple of different ways that things
could play out.

First, thunderstorms could initiate along the aforementioned
slow-moving cold front in our forecast area if low-mid 90s
convective temperatures are breached before sunset and enough
convergence is realized along the front. It`s also possible
that convection initiates along the front later this evening as
synoptic-scale forcing increases with the approach of a mid-level
shortwave. However, by that point, some degree of nocturnal
stabilization may have set in, limiting the likelihood of true
surface-based convective initiation and instead perhaps favoring
the development of more restrained splotchy convective showers
that would have a relatively low chance of producing lightning.
If any surface-based convection does manage to sprout along the
cold front either late this afternoon or this evening, then a
narrow ribbon of substantial pre-frontal instability (up to
around 4000 J/kg of MLCAPE) would support the potential for
explosive thunderstorm development and a corresponding threat
for large hail and damaging winds with any deep convective
updraft cores. A landspout tornado also couldn`t entirely be
ruled out if a storm managed to rapidly develop where surface
vorticity is maximized along the cold front. If storms
materialize, then this severe weather threat generally appears
that it would be focused near the I-90 and I-88 corridors before
potentially spreading southward if the initial convection
congeals into clusters.

Some of the convection that develops later today along the cold
front in Iowa or the aforementioned outflow boundary will
likely be carried off to the east by the westerly cloud-layer
flow, and if any of this convection develops close enough to
home, then it could end up propagating into our CWA. The timing
of these storms` arrival would likely come late -- potentially
even after midnight, and convective coverage would likely be
greatest across the southwestern or southern half of our
forecast area. The severe weather threat would generally be
lower with these storms, but at least isolated instances of
large hail and/or damaging winds would still be possible given
the degree of instability in play. With precipitable water
values between 1.5" and 2", torrential rainfall is also likely
with any storms that develop late this afternoon or tonight.
While they will be efficient rainfall producers, it will likely
take some training for flash flooding to become an appreciable
threat, and at this time, confidence is low as to whether that
will be a problem in our forecast area.

Regardless of how things play out tonight, scattered showers
and storms will either continue into tomorrow morning or will
redevelop after sunrise as a more pronounced upper-level trough
swings into the Great Lakes. By that time, our reservoir of
instability will have largely been depleted, and with lackluster
lapse rates and subpar shear, any storms tomorrow are unlikely
to become severe. Precipitation should then come to end by the
mid-late afternoon. Cold air advection will also bring cooler
temperatures and lower dew points into the area.

Ogorek


Thursday through Tuesday:

Post cold frontal passage, a brief window of dry conditions will be
the result on Thursday as sprawling high pressure will setup across
the upper Great Lakes. Cool northerly flow ahead of the high will
linger into Thursday, thus highs will peak from the low to mid 70s
closer to the lake, to around 80 well inland.

With zonal flow aloft, the dry period will be short lived as the
high will get shunted to eastern Great Lakes Thursday night into
Friday. A fairly potent upper trough will translate across the
northern tier of states Friday into Saturday. Corresponding low
level mass response under a strong southwesterly low level jet will
feed in a warm and very moist airmass (with near record level
PWATs).

Several periods of showers and thunderstorms will occur ahead of
this system, the first in the warm advective regime on Friday
afternoon, and then additional forcing Friday night with a
strengthening upper jet (left entrance region) and with the approach
of a cold front. Their is a severe threat Friday afternoon, which
may be focused west into Iowa and into western Illinois, though some
threat does exist into northern IL as the warm sector of the system
will move in. The threat will continue into the overnight hours as
well.

The heavy rain/flood threat is certainly a concern as the low level
jet continued to feed moisture into a more east-west frontal
boundary. This coupled with westerly flow aloft and the
aforementioned near record PWATs in excess of 2",, some intense
rainfall rates will be possible along with some training storms.

There is some disagreement as to whether the cold front may get held
up a bit on Saturday, so while the bulk of the precip will shift
SE, some lingering showers could occur Saturday morning, less likely
in the afternoon, before the front gets a better push Saturday
night.

Pleasant weather will greet the region as we close out the first
month of meteorological summer on Sunday though with onshore flow,
we could see some rip current/high swim risk conditions into Sunday
as well. Mid summer like temperatures will then return as we move
into early, as the summer ridge builds across the southern states.
This pattern could also bring several potential periods of
thunderstorms depending on the how robust the ridge builds.

KMD

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
-- Changed Discussion --
Issued at 632 PM CDT Tue Jun 25 2024

Forecast concerns include...

Chance of thunderstorms this evening and again overnight.
Wind directions this evening.
Wind shift to northeast Wednesday afternoon.

A frontal boundary is slowly moving south across far northern IL
late this afternoon and has served as the focus for scattered
thunderstorm development. Models handled this development fairly
well and suggest additional thunderstorm development will remain
generally west of the Chicago terminals this evening. While
confidence is low, opted to include vicinity thunder mention at
DPA/ORD as activity just northwest has continued to fester and
trends will need to be monitored through mid evening. Additional
thunderstorm development is possible overnight, mainly south of
the terminals. Confidence is low for coverage and location
later tonight/early Tuesday morning and pulled mention from all
the terminals, though there may still be a broken line of
convection affecting the airspace near the terminals. Whatever
activity does develop overnight will move southeast of the
terminals Wednesday morning with the rest of the period expected
to be dry.

Southwest winds may still gust into the 15-20kt early this
evening. Wind directions are likely to remain light westerly
tonight, becoming northwesterly Wednesday morning. Though there
is low confidence for wind directions, which could be impacted
by any nearby convection. Winds will become more northerly on
Wednesday with speeds increasing to 10-12kts and expect winds to
shift to the northeast during the afternoon. Refinement to the
timing for the northeast winds may be needed with later
forecasts. cms
-- End Changed Discussion --

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
IL...Flood Watch through Wednesday morning for ILZ003-ILZ004-ILZ005-
     ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-ILZ021.

IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     and IN nearshore waters.
-- End Changed Discussion --

&&

$$

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