630
FXUS63 KLOT 040602
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
102 AM CDT Tue Jun 4 2024

.KEY MESSAGES...

- Isolated showers and storms across northern Illinois into
  this evening.

- Unsettled weather pattern continues through mid-week as
  showers and storms are expected Tuesday into Wednesday
  morning.

- Warm, moist air continues to bring the warmer temperatures
  across northern Illinois Tuesday, but cooler temperatures are
  expected Thursday and Friday behind frontal passage.

&&

.DISCUSSION...
Issued at 309 PM CDT Mon Jun 3 2024

Showers and storms have developed this afternoon across
northwestern Illinois, as a north-south oriented boundary sits
along the mid and upper Mississippi River Valley. Additional
development is expected to occur this afternoon through the
evening, however it is expected to remain near and west of the
I-39 corridor over the next hour or 2, before beginning to
shift ne across northern counties of the LOT CWA...north of
roughly the I-88 corridor and then I-90 corridor heading into
the overnight. Isolated strong to severe storms are possible,
however most of the activity is forecast to remain general
showers and storms.

Heading into the overnight and Tuesday, a low pressure system
will deepen across the southern Plains before moving into the
Ohio River Valley late Tuesday into early Wednesday. This will
lift moisture from the Gulf into the area Tuesday morning into
the afternoon, bringing another chance for shower and storm
development as instability once again increases across the
Midwest. With warm southerly air flowing into the region ahead
of this system, expecting showers and storms to rapidly develop
Tuesday afternoon (20-22z) and track eastward through evening. A
larger upper-level low pressure core situated across central
Canada will track eastward Tuesday, trailing an elongated
frontal boundary into the central Plains and deep south. As this
boundary tracks to the east Tuesday night into Wednesday
morning, the chance for a second round of showers and storms
will move through the region. The question at hand today was
timing of this frontal system, as well as the early convection
Tuesday afternoon, this is expected to keep the potential of
strong to severe storms at bay overnight Tuesday into Wednesday.

Drier weather is forecast to return across the Midwest, as the
unsettled weather pattern is finally shifted out of the region
during Wednesday. Along with the dry weather arriving Thursday
and Friday, models are indicating a break in the summer-like
temperatures with highs back in the 70s. The 70s then look to
linger into and through the upcoming weekend across much of the
area, with some cooler temperatures expected near and along the
lakeshore. Chances this weekend for isolated to scattered
showers and storms at times could be possible this weekend,
however the overall probability remains less than 25% at this
time.

Baker

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 102 AM CDT Tue Jun 4 2024

Main Concerns:

- Scattered TS probable near the terminals in the late afternoon.

- SHRA with another round or two of TS possible in the evening
  and overnight.

- Wind direction in the late afternoon and early evening.

- Lower CIGs likely overnight/early Wednesday.

Another challenging convective forecast through this TAF cycle,
though confidence is higher in at least one round of TS causing
some impacts. Quiet VFR conditions with southerly winds (15-20+
kt gusts by the early afternoon) will persist until 19-20z.
Instability will build out ahead of a subtle upper level
disturbance lifting northward, which should be enough to kick
off widely scattered TS in the late afternoon (~20-24z) in the
vicinity of the terminals. While on station impacts can`t be
ruled out, TS coverage with this first round appears likely to
be sparse enough to warrant current VCTS mention.

A stronger upper disturbance will pivot northeastward this
evening and early overnight. Expecting TS to be fairly
widespread with this in a portion of the ZAU airspace, with the
main question being if the higher coverage ends up a bit south
of the TAF sites. Went with a PROB30 for this possible second TS
window. Finally, a cold front will approach overnight, likely
preceded by SHRA and isolated TS (confidence too low for another
PROB30 mention). CIG guidance indicates low MVFR to patchy IFR
CIGs should develop during this time.

Regarding wind direction uncertainty, it`s a bit unclear if
south-southeast or south-southwest will be favored at ORD and
MDW during the strongest winds this afternoon, so the TAF split
the difference. In addition, some guidance indicates a close
approach of the lake breeze toward 00z this evening, which will
need monitoring.

Castro

&&

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

&&

$$

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