826
FXUS63 KLOT 081757
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1257 PM CDT Sat Jun 8 2024

.KEY MESSAGES...

- Scattered showers with a few rumbles of thunder today.

- Northeast wind shift with a cold front this afternoon,
  bringing cooler temps and a short period of choppy 2-4 foot
  waves to Lake Michigan beaches.

- Northwest winds gusting to 30 mph Sunday.

- A shift toward warmer temperatures and higher humidity remains
  on track for the third week of June

&&

.UPDATE...
Issued at 1004 AM CDT Sat Jun 8 2024

A morning surface analysis depicts an expansive plume of low-
level moisture (and implied instability) across the central
Plains and lower Mississippi River Valley ahead of a cold front
currently extending from northern Wisconsin to northwestern
Kansas. Several areas of low pressure were noted along and ahead
of the front, including across southern Iowa and within a
compact MCV now crossing into northwestern Illinois. With
respect to the latter, a weak area of mid-level warm-air
advection continues to support percolating radar returns across
west-central Illinois. However, a pocket of dry air extending
up to about 5000-6000 feet (re:12Z ILX/DVN soundings and
observations of cloud bases) continues to prove too hostile for
rain, with pretty much all observations of such in close
proximity to the MCV. With the circulation located well north of
the instability/moisture axis, it appears it should gradually
weaken while moving just south of the Wisconsin border
throughout the remainder of the morning. With that said, an hour
or two of rain may nevertheless occur along and north of I-88
as the circulation moves through.

Generally speaking, the forecast for this afternoon appears to
be on track. The cold front to our north is poised to surge
south across northern Illinois and northwestern Indiana this
afternoon, causing winds to shift northerly and temperatures to
fall this evening. With broken (not solid) cloud cover across
western Illinois and southern Iowa, it appears that high
temperatures ahead of the front will be a smidge warmer than
previously thought and in the mid to upper 70s. When combined
with pooling of low-level moisture into northern Illinois this
afternoon (dew points in the low to mid 60s), it appears
prospects for the development of a narrow ribbon of low-level
instability along and ahead of the front are increasing. While
temperatures may not quite reach convective temperatures (which
based on forecast soundings will be somewhere near 80-81F this
afternoon), a glancing blow from a low-amplitude shortwave
trough (evident on water vapor imagery extending from southern
South Dakota into northern Iowa) may be just the oomph needed to
kick off convection within the narrow instability axis ahead of
the front this afternoon. Based on the arrival timing of the
front, such an axis may be near I-88 corridor (including in the
Chicago metropolitan area) in the 2 to 5 pm time window, and the
I-80 corridor sometime in the 3 to 6 pm window.

Now, with the upper-level jet positioned over the northern Great
Lakes and an abrupt change in wind direction expected along the
front, low-to mid-level "convective layer" shear will be
supportive of rotating updrafts, however shallow they may be.
And, with pooled low-level moisture and low LCLs, any sustained
updraft may be able to sustain a funnel cloud or a brief tornado
even with little to no lightning or precipitation production.
Now, this does not look like a tornado outbreak set-up but
rather a subtle, "gotchya" scenario with a typical probability
of occurrence of 2% or lower. In other words, it is far more
likely that no severe weather occurs. Again, the time window
during which we`ll be watching will be from 2 to 6 PM in the
I-88 and I-80 corridors.

Other than the threat for convection this afternoon, a quick
increase in waves to 2-4 feet along Lake Michigan beaches is on
track along/behind the front (moreso this evening). With the
brief hit of winds, forecast wave heights remain in the 2 to 4
foot range and indeed the "Moderate" swim risk category.

Updated products have been sent.

Borchardt

&&

.DISCUSSION...
Issued at 230 AM CDT Sat Jun 8 2024

Through Sunday:

Scattered showers are expected later this morning and into the
afternoon and there has been an overall trend upward in coverage
with the 00z guidance. Given current precip coverage with this
wave across MN, this upward shift looks reasonable and have
raised pops to likely for areas along and north of I-80. Still
some timing uncertainty which may need later refinement. There
will likely still be some dry hours, but after mid/late morning,
difficult to pin down when those would be. Areas south of I-80
may stay mainly dry this morning and then as the activity across
the northern cwa shifts southeast, best precip chances will be
in the late afternoon and early evening. Instability remains
quite limited with perhaps the best chance for thunder southeast
of I-57 later this afternoon into this evening but maintained
slight chance (15-20%) for thunder areawide this afternoon.

This expected precipitation and mainly cloudy skies will likely
keep high temps only in the lower to mid 70s and once the
showers begin, temps may drop back into the upper 60s. A cold
front will be moving south down Lake Michigan today and all of
the guidance has sped up the arrival of this front, likely
moving through northeast IL during the mid afternoon hours and
then into northwest IN by late afternoon. Winds will shift to
the northeast and depending on where temps are at, they will
likely fall further into the lower/mid 60s behind the front.
While it may be brief, an increase in winds over southern Lake
Michigan will likely lead to a period of increased waves of 2-4
feet and a moderate risk for rip currents.

Lows tonight are expected to dip into the mid 50s for most of
the area with clouds slowly clearing. Mostly sunny to start
Sunday with increasing clouds later in the afternoon. Winds
will steadily increase during the morning with gusts into the
30 mph range during the afternoon, especially north of I-80.
cms

Sunday Night through Friday:

Pleasant conditions are forecast through the first half of the
upcoming week. An expansive surface high will remain to our
northwest at the start of the workweek resulting in northerly
surface flow across the area. This will result in notably cooler
temperatures on Monday with highs only in the 60s to lower 70s.
Overnight lows Monday night could even dip into the 40s outside
of Chicago.

Long range guidance is beginning to come into a bit better
agreement with the handling of the upper level pattern though
there remain timing differences. The general trend is toward
drier conditions through midweek due to dry low levels amidst
an expansive area of surface high pressure limiting precip
potential as weak disturbances move within the upper jet. It is
possible portions of northern Illinois are grazed by showers
moving across Wisconsin toward midweek.

Temperatures very gradually through midweek with highs back
into the 80s by Wednesday and near 90 on Thursday. A frontal
passage Thursday night into Friday could bring our next chance
of showers and storms (currently at 20%). There continues to be
a longer range signal for above normal temperatures and higher
humidity across the region for the third full week of June as
upper level ridging builds across the eastern half of the CONUS.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1256 PM CDT Sat Jun 8 2024

Forecast concerns for the terminals...

- Scattered showers through early evening with associated MVFR
  ceilings and visibilities

- Low confidence threat for isolated thunderstorms (around 20%)
  this afternoon

- Gusty northeast winds behind cold front late afternoon

- Breezy northwest winds Sunday afternoon


Scattered showers will continue to develop ahead of the cold
front as it drifts southward out of Wisconsin this afternoon.
While most of these showers will be of the lighter variety, a
few may become more robust and result in MVFR (if not high-end
IFR) visibilities. Additionally, ceilings will lower to MVFR
with the showers before conditions begin to improve behind the
cold front this evening.

There also continues to be a threat for isolated thunderstorms
this afternoon as well. However, confidence on coverage and
placement of thunderstorms remains low due to uncertainties on
available instability ahead of the front. Therefore, have
decided to forego a formal thunder mention in the TAFs with this
issuance but did modify the TEMPO groups to encompass the
expected window any storms would occur. If storms do develop we
will tactically amend and include thunder within these
aforementioned TEMPO groups as necessary. Showers and any storms
will come to a conclusion behind the front this evening with dry
for the rest of the period.

A period of gusty northeast winds is expected to develop behind
the cold front this afternoon with gusts topping out in the
lower 20 kt range. While the anticipated timing of the wind
shift at the terminals has not changed, the timing was shifted
back an hour in the TAFs to account for the aforementioned TEMPO
groups. That said, expect the northeast wind shift to occur +/-
one hour of the noted time in the TAFs. Regardless, the gusty
winds will ease this evening as directions become northwesterly
overnight. However, wind speeds will increase Sunday morning
with gusts in the 20-25 kt range expected Sunday afternoon.

Yack

&&

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

&&

$$

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