652
FXUS63 KLOT 090820
AFDLOT

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

.KEY MESSAGES...

- A few instances of penny to quarter size hail, and 50+ mph
  winds may accompany some of the strongest storms in
  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.

&&

.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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1250 AM CDT Mon Jun 9 2025

Forecast concerns for the terminals...

- Scattered showers and thunderstorms late Monday morning
  through the afternoon. A few storms could be severe with gusty
  winds and hail.

- Breezy non-thunderstorm wind gusts Monday afternoon peaking
  around 25 kts.


The cold front that brought us the showers and storms earlier
has moved southeast of the area resulting some sinking air in
its wake and thus tranquil weather overnight. However, the quiet
weather will be short lived as an upper low and a couple
embedded disturbances are advancing across the Upper Midwest
towards the Great Lakes. While the upper low will be centering
into the region Monday morning, the aforementioned disturbances
are not expected to arrive until late Monday morning (16-17z).
Once the disturbances arrive scattered showers and thunderstorms
should develop and move through the terminals through mid-
afternoon. Given the rather cold temperatures aloft, a few
storms could become strong to severe and produce instances of
gusty winds (in excess of 30-35 kts) and hail (possibly up to
nickel to quarter size). Overall the prevailing TAFs seem to
have a good handle on the expected trends so only change was to
convert PROB30s to TEMPO groups given confidence on
showers/storms developing is relatively high.

Outside of the showers/storms, winds will increase as diurnal
mixing commences with non-thunderstorm wind gusts in the 25-28
kt range expected through Monday afternoon. Wind gusts should
decrease Monday evening as the atmosphere decouples, but a
modest 25-30 kt low-level jet may allow for occasional gusts
through the end of the TAF period particularly at the Chicago
terminals. Otherwise, expect VFR conditions to prevail through
the forecast period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for the IL and IN nearshore waters.

&&

$$

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