297
FXUS63 KLOT 091740
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 PM CDT Mon Jun 9 2025

.KEY MESSAGES...

- Isolated to widely scattered thunderstorms possible this
  afternoon. A few instances of penny to quarter size hail, and
  50+ mph winds may accompany some of the strongest storms in
  far northeastern IL and northwestern IN this afternoon.

- A period of dry and warmer weather is expected midweek.

- Shower and thunderstorm chances return late in the week into
  next weekend.

&&

.UPDATE...
Issued at 1116 AM CDT Mon Jun 9 2025

Made a few tweaks to our going forecast this morning with the
main change being to constrain the spatiotemporal footprint of
thunder chances in our forecast area. The main driver for more
robust convective development today will be a compact, but
vigorous mid/upper-level vort max, evident on GOES water vapor
imagery over northeastern Iowa at press time. Thus far, have not
seen much in the way of radar echoes along this vorticity
maxima`s southern and eastern periphery, but do still anticipate
that convection should start to blossom out ahead of it late
this morning/early this afternoon as it impinges upon an
air mass that will be increasingly unstable as a result of
diurnal diabatic heating. This vort max will also catch up to a
slower eastward-moving cold front, where some focused surface
convergence will provide an extra boost to the vertical
acceleration of air parcels and increase the likelihood of
convection deepening enough to support lightning.

Thus, the thinking is that the potential for and coverage of
thunderstorms this afternoon will be greatest near and along
the cold front. As of 1600Z, the cold front has already passed
through a good chunk of our northern CWA and is on track to
clear the remainder of our forecast area by mid-afternoon.
Therefore, there appears to be a fairly narrow spatiotemporal
window today for the better thunderstorm potential that appears
to be confined between 1800Z and 2200Z and east of a line from
Woodstock, IL to Rensselaer, IN -- roughly in line with the 12Z
HREF calibrated thunder probabilities. Have constrained our
"chance" thunder probabilities to this time window and area, but
will note that based on the less convectively-bullish 12Z CAM
suite and the relatively limited surface convergence along the
cold front, it`s possible that this might still be a bit
generous.

If storms manage to develop here this afternoon, then the cold
mid-level temperatures and strong deep-layer shear could allow
for maturing storms to develop hail cores and pose a threat to
produce strong downbursts. A storm or two could approach or even
reach severe caliber in northwest Indiana, where a Marginal Risk
delineation remains in SPC`s Day One Convective Outlook, but
the better likelihood for severe thunderstorms should remain to
our east and northeast, where storms will have more time to
mature and reach peak intensity. The lower degree of
instability today should also preclude any downbursts as
significant as yesterday`s microburst in Batavia from occurring
within the confines of our CWA.

Ogorek

&&

.DISCUSSION...
Issued at 320 AM CDT Mon Jun 9 2025

Through Tuesday:

Recent GOES-19 water vapor imagery depicts a couple of small scale
impulses across MN rotating around the western and southern
periphery of an upper low centered over the Arrowhead region of
MN early this morning. The first, is currently shifting across
southwestern MN, while the second is noted digging southward
across western parts of MN at the time of this writing. Our
interest today lies with the northernmost impulse digging across
western MN. This feature, along with a secondary surface cold
front, is expected to be the focus for scattered showers and
thunderstorms as it swings southeastward right across northern IL
and northwestern IN during the early to mid afternoon hours today
(roughly 12-3pm).

An unseasonably cold mid-level airmass is associated with this
robust mid/upper low. In fact, the 00Z RAOBs from yesterday
evening across the Dakotas into MN sampled 500 mb temperatures of
-18 to -22C, which based on the SPC`s sounding climatology rank
 within the coldest 10% for this time of year. Our main concern
 with this cold airmass is that as it shifts overhead during peak
 heating today steepening low to mid-level lapse rates will
 support an uptick in thunderstorm chances along the front with
 eastward extent across northeastern IL into IN and Lower MI. The
 presence of relatively low freezing levels around 7,000 ft AGL
 and a strengthening low to mid-level wind field will allow the
 strongest storms to produce some instances of hail (3/4"+ in
 diameter) along with localized strong wind gusts up to 60 mph.

The main threat area for these stronger storms will primarily be
across northwestern IN and points east-northeast into Lower MI
this afternoon. However, the threat for strong storms as far west
as the Fox Valley in northern IL cannot be ruled out. Expect the
primary storm threat to end later this afternoon as the front
shifts east of the area. Some isolated showers and possibly a
storm could linger behind the front late this afternoon across far
northern IL, but this activity would also end prior to sunset
early this evening.

Outside of showers and storms today, highs will range from the
near 70 (northwest) to the mid 70s (southeast). Winds will also be
breezy through the afternoon, with west winds expected to gust up
around 30 mph. These breezy conditions will abate around sunset
early this evening.

The upper low will shift across southeastern Ontario on Tuesday.
in its wake, we are expecting a rather quiet and dry weather day.
High temperatures will rebound a few degrees, with readings
generally in the mid to upper 70s. Skies during the day look to be
partly cloudy, and with more wildfire smoke potentially moving
back into the area, we could end up with some hazy conditions
once again.

KJB


Tuesday Night through Sunday:

Mid-level ridging will be building into the Mississippi and
Ohio River Valleys Tuesday night into Wednesday which will
promote a period of dry weather. Beneath the ridging skies look
to remain mostly sunny with stout south-southwesterly flow both
at the surface and in the mid-levels which will set up a period
of above normal warmth as well with highs in the mid to upper
80s. In fact, with 850 mb temperatures forecast to be in the +16
to +20C range, there is a good chance for temperatures to flirt
with the lower 90s Wednesday afternoon.

From Wednesday night/Thursday onward the forecast becomes much
trickier due to a complicated upper pattern overhead.
Essentially an area of zonal (west-east oriented) upper-level
flow is forecast to set up across the central CONUS with modest
ridging remaining to the south and a more amplified pattern
(characterized with several shortwaves pivoting through it)
across the northern CONUS and southern Canada. With northern IL
and northwest IN currently forecast to be smack in the middle of
these patterns, which side we will ultimately be under is very
uncertain. As if that wasn`t enough, a low-level baroclinic zone
is also forecast to set up somewhere in the Mississippi Valley
and Western Great Lakes region serving as the focus for periods
of showers and thunderstorms. That said, there are a couple of
note worthy scenarios that are worth discussing.

For the Wednesday night through Thursday night timeframe: There
is a rather pronounced signal in the 00z ensemble guidance
(nearly 60% of the members) that the aforementioned baroclinic
zone may stay just north of our area and keep the bulk of any
shower and thunderstorm activity into WI. If this scenario plays
out, then Thursday could very well be similar to Wednesday with
highs once again in the upper 80s to near 90, depending on the
degree of cloud cover, and mostly dry conditions. However, if
the zone sets up farther south (as 40% of members suggest) then
at least the northern third to half of our area could see
showers and thunderstorms along with cooler temperatures due to
more easterly winds. Additionally, there could also be a
conditional flood threat with this baroclinic zone given that
mid-level flow (which steers showers and storms) will parallel
the boundary resulting in training activity over the same areas
for an extended period of time. For now, have maintained the
POPs offered by the NBM which has 20-40% chances for showers and
storms across northern IL and northwest IN (highest near the
IL-WI line) which seems reasonable based on the described
uncertainties.

For the Friday through Sunday timeframe: This period of the
forecast will likely be dependent on how far north the
baroclinic zone is on Thursday and whether or not it gets pushed
southward by the showers/storms forecast to develop upon it.
Furthermore, there is also an upper low that guidance is
developing across the southern Plains and lifting into the Ohio
River Valley and southern Great Lakes late Friday into Saturday.
That said, the ensemble consensus currently favors (60-70% of
the members) the baroclinic zone getting pushed into our area on
Friday brining with it showers and thunderstorms for Friday
afternoon. Then another round of showers and possible storms
looks to move into the area with the aforementioned upper low
Friday night with wet weather lingering through Sunday. Now
there is still a chance (around 30%) that the upper low stays
far enough south that the baroclinic zone is able to push
further south on Saturday and focus the showers south of our
area on Sunday. So at this point the weekend does not look like
a complete washout just yet, but something we will be watching
closely over the coming days. Regardless of how the rain plays
out, Friday through the weekend does look to have more
seasonable temperatures with highs in the mid-70s to lower 80s
inland and cooler conditions near the lake due to onshore winds.

Finally, rain does look to get pushed out of our area Sunday
night into Monday as broad ridging begins to build into the
western and central CONUS for next week. While this pattern will
favor another period of above normal temperatures for mid-June,
current consensus is for the ridge axis to be centered more
into the central Plains which should keep our area on its
eastern periphery. This means that we could see periods of
showers and storms if shortwaves round the ridge next week.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1240 PM CDT Mon Jun 9 2025

Main Aviation Concerns:

- PROB30 for chance of showers and storms this afternoon, best
  chance across the metro terminals.

- Gusty winds this afternoon will diminish overnight, however
  another increase mid-morning Tuesday.

A low pressure core will continue tracking eastward through the
northern Great Lakes, where local terminals remain on the
southern fetch. This brings a chance for showers and storms
this afternoon, with any development anticipated over the next
few hours...lasting only a couple of hours. Best chance will be
across the metro terminals, as the current cu field across
northern IL continues to march eastward into slightly more
unstable air. Drier air filters back into the Midwest Tuesday,
as higher pressure arrives from the central Plains into the
Ohio River Valley.

Gusty westerly winds this afternoon, in excess of ~25kts, will
help the rapid movement of any precipitation...with potential of
30-32kts in a storm. Gusts diminish after 01-02z, however winds
(6-8kts) will remain westerly overnight. Another increase of
westerly winds is forecast mid- morning Tuesday, with gusts
during the afternoon around 20kts.

Baker/Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 8 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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