-- Discussion containing changed information from previous version are highlighted. --
589
FXUS63 KLOT 260553
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1253 AM CDT Wed Jun 26 2024

.KEY MESSAGES...

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

&&

.UPDATE...
Issued at 957 PM CDT Tue Jun 25 2024

A cold front continues to gradually push southward across
northern IL this evening as an associated upper-level trough
pivots into the northern Great Lakes. While some showers
continue to be noted along the front, the bulk of any shower and
thunderstorm activity has remained west of I-39 where the
combination of higher moisture, better instability, and weaker
capping has established. As the cold front continues its
southern progress overnight, the complex of thunderstorms in IA
and western IL is expected to continue its south-southeast
trajectory and remain outside of our forecast area. Though,
there is a chance (around 20%) that a stray shower or storm may
brush southern Livingston and/or southwestern Ford Counties
prior to 1 AM CDT as the aforementioned complex moves by.
Regardless, the threat for severe weather and flash flooding has
greatly diminished and are no longer a concern for tonight.
Thus, dry conditions can be expected for most with otherwise
mostly cloudy skies and temperatures cooling into the upper 60s
to lower 70s by daybreak.

However, a secondary shortwave disturbance is forecast to
develop from the area of thunderstorms currently developing in
western IA and eastern NE and track eastward along the
aforementioned cold front overnight. Given that the cold front
is forecast to still be working through our southern CWA when
the disturbance arrives Wednesday morning, there continues to be
the potential for some isolated to widely scattered showers and
thunderstorms for areas south of I-80 late Wednesday morning
into early afternoon. Thankfully, it does appear the
instability in our area will be rather modest so the threat for
any strong to severe thunderstorms remains low at this time.


Yack

&&

.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 /06Z TAFS THROUGH 12Z THURSDAY/...
-- Changed Discussion --
Issued at 1252 AM CDT Wed Jun 26 2024

Key Aviation Messages:
- 10% chance of isolated TS early this morning
- Lake breeze expected at MDW and possibly at ORD this
  afternoon, followed by a NE wind shift areawide early this
  evening.

VFR conditions are expected through the period. While dry
conditions are generally favored across northern IL and
northwest IN through tonight, spotty -SHRA cannot be ruled out
through early afternoon. There is also a very low chance (10%)
that isolated elevated TS develop prior to and around sunrise
(08-11Z) over the Chicago metro as ongoing subtle saturation in
the 4-6kft layer may allow the realization of higher elevated
instability currently over the area. If these TS do develop,
they would be capable of +RA and potentially small GR.

W/NW winds under 10 knots will settle NNW around 10 knots by
late morning. A lake breeze is expected to develop, then stall,
in the vicinity of ORD/MDW early this afternoon. The current
forecast depicts a NE wind shift at MDW early/mid afternoon. It
remains about a toss-up on whether the lake breeze reaches ORD
during the afternoon, with a 60/40% chance in favor of no
passage. Either way, a reinforcing backdoor cold front will
ultimately shift winds NE at all sites early this evening.

Kluber
-- End Changed Discussion --

&&

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

&&

$$

Visit us at weather.gov/chicago