767
FXUS63 KLOT 101107
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
607 AM CDT Fri May 10 2024

.KEY MESSAGES...

- Showers with isolated embedded thunderstorms are likely
  tonight.

- Breezy northwest winds are expected on Saturday, but otherwise
  dry and seasonable conditions will prevail.

- Swinging temperatures will continue next week with additional
  opportunities for showers and a few thunderstorms.

&&

.DISCUSSION...
Issued at 220 AM CDT Fri May 10 2024

Through Saturday:

A positively tilted shortwave trough continues to pivot
overhead this morning and has generated some isolated showers
along the Mississippi River. These showers have been dwindling
in coverage and should dissipate over the next couple hours.
Therefore, dry conditions are expected for our Friday with
partly cloudy skies and seasonable temperatures in the mid to
upper 60s. While some guidance does show a weak lake breeze
developing this afternoon, the elevated westerly synoptic winds
should keep the lake breeze confined to the immediate lakeshore
in IL and within the nearest mile or so in northwest IN. If the
lake breeze is able to penetrate inland, those nearest the
lakeshore may see temperatures remain a few degrees cooler
especially in northwest IN where further inland progress is
forecast.

Our next chance for rain will arrive tonight as another upper-
level shortwave dives into the western Great Lakes and forces a
cold front across northern IL and northwest IN. While the threat
for thunderstorms continues to look meager at best, forecast
soundings do show some steepening mid-level lapse rates as the
trough passes overhead which could support an isolated rumble or
two of thunder. Thus, I have decided to keep a slight chance
(around 20%) mention for thunder in the forecast primarily prior
to midnight. Regardless, no severe weather is expected but
there is a non-zero chance that more robust showers and/or
storms could generate localized wind gusts in excess of 25 to 30
mph.

Showers and any thunderstorms are forecast to exit into
northern IN prior to daybreak on Saturday which will once again
leave us with another dry and seasonable May afternoon. However,
a lingering pressure gradient in combination with deeper
atmospheric mixing Saturday afternoon does look to promote
breezy northwest winds gusting in the 20 to 30 mph range.

Yack


Saturday Night through Thursday:

Saturday night into Sunday, the aggregate upper-level troughing
responsible for the recent stretch of cooler temperatures and waves
of showers will finally drift eastward, opening the door for a
return of warm (and above-average) temperatures. Sunday accordingly
looks like a splendid spring day with highs in the upper 70s, a
southwesterly breeze, and plenty of sunshine.

Monday into Tuesday, the upper-level wave currently in the process
of becoming "cut-off" in the southwestern United States is expected
to become "re-absorbed" into the upper-level flow while drifting
eastward toward the mid Mississippi River Valley. Owing to
differences in phasing opportunities with upper-level shortwaves
propagating around the perimeter of an upper-level low settling over
southeastern Ontario, ensemble model guidance offers varying
evolutions of the early week system. With that said, there remains a
reasonable signal (>50% chance) for a wave or two of showers and
perhaps a few thunderstorms sometime in the Monday to Tuesday
timeframe. Prospects for cloud cover suggest highs on Monday should
be a few degrees cooler than on Sunday and in the low to mid 70s. By
Tuesday, the Great Lakes should be on the backside of the system,
leading to pronounced northeasterly onshore flow (10 to 15 mph) and
cooler temperatures in the mid 60s (upper 50s near the Lake Michigan
shoreline).

Wednesday and Thursday, a surface high pressure system and upper-
level ridge will move over the Great Lakes allowing for a rebound in
temperatures into the mid to upper 70s. With the core of the surface
high pressure system expected to pass north of our area, the surface
wind will maintain an onshore component. As a result, temperatures
will continue to be cooler near the Lake Michigan shoreline (it`s
that time of year, after all). Ensemble model guidance favors the
next opportunity for showers and thunderstorms to arrive sometime
toward the end of next week.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1238 AM CDT Fri May 10 2024

Aviation Key Messages:

* Light northwesterly winds (around 5kt) this morning will back
  westerly and increase in magnitude (to 10-12kt) this
  afternoon.

* VFR clouds based above 5kft will prevail through the TAF
  period.

* A band of showers with northwesterly wind gusts as high as
  30kt will swing through the terminals this evening (generally
  between 03-06Z).

Discussion:

Surface winds will gradually back westerly throughout the
morning and increase in magnitude to 10-12kt by early afternoon.
A few gusts of 15-20kt appear likely this afternoon, though
opted to withhold mention of gusts for brevity and the
expectation for limited impacts to aviators.

Broken mid-level clouds based between 5-10kft will transition
into a cumulus field based near 5kft this afternoon. With
forecast soundings showing appreciable depth and instability
within the cumulus layer, a stray shower or two cannot be ruled
out particularly between 20 and 00Z. For now, will withhold any
formal mention of showers this afternoon in the outgoing TAF
package given the expectation for a relatively limited coverage
(not to mention the probability of occurrence being less than
20%).

A vigorous upper-level shortwave and associated cold front will
race southeastward across the Great Lakes this evening
accompanied by a strongly forced band of showers. Chances for
thunder appear to be less than 10% at the terminals owing to a
loss of diurnal instability, though thunder may be observed
earlier in the day further to the north in Wisconsin. With a
pocket of dry air expected to be in place beneath cloud bases
this evening, the band of showers may produce gusty
northwesterly winds perhaps as high as 30 kt in spite of the
development of a shallow near-surface stable layer. A few
showers may continue behind the front as mid-level lapse rates
steepen within the core of the passing upper-level wave.

After daybreak Saturday, winds will turn northwesterly and
increase in magnitude with gusts of 25 to 30 kt a few hours
after sunrise.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CDT early this morning for the
     IL and IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago