718
FXUS63 KLOT 151125
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
625 AM CDT Wed May 15 2024

.KEY MESSAGES...

- Today will be pleasant with clearing skies and highs in the
  lower 60s (lakeside) to lower 70s (inland). A few showers
  cannot be ruled out east of I-57 this afternoon.

- Chances for showers and thunderstorms will return on Thursday,
  and peak between 40 and 50% areawide during the afternoon and
  evening hours.

- Periods of showers and storms may occur Friday into Saturday,
  but looking mostly dry for the weekend.

- Active pattern looks to develop next week bringing several
  opportunities for showers and storms.

&&

.DISCUSSION...
Issued at 302 AM CDT Wed May 15 2024

Through Thursday night:

Regional radar imagery and a recent hand surface analysis depict
several areas of showers rotating counter-clockwise around a
broad region of surface low pressure centered on the Ohio River.
Seasonably cool air moving southwestward atop the relatively
moist waters of Lake Michigan continues to support areas of
stratus across northeastern Illinois and northwestern Indiana.
With all of that said, it`s a nice and quiet morning.

Today, the vertically-stacked low pressure system across the Lower
Great Lakes will continue drifting eastward away from our area,
allowing for gradually clearing skies. A few showers rotating around
the broad cyclonic circulation may graze areas east of I-57 this
afternoon, though today looks dry for most of our area.
Continued northeasterly winds will maintain a lake shadow in
temperatures, with highs generally expected to top off in the
low to mid 60s east of I-355 and north of the Kankakee River,
compared to readings in the low to mid 70s further inland.
Tonight will be similarly quiet as a short wave-length upper-
level ridge and associated area of relative surface high
pressure slides overhead. Overnight lows will fall into the
upper 40s to lower 50s, which is close to average for this time
of year.

Our break in waves of showers and thunderstorms today will be short-
lived, as a pair of upper-level vort maxes (seen readily in water
vapor imagery this morning over central Idaho and Wyoming) merge
together and propagate overhead on Thursday. Surface pressure falls
across the Upper Mississippi River Valley (associated with a surface
low beneath the aforementioned merging upper-level vort maxes) will
cause winds to gradually turn southeasterly and then
southwesterly throughout the morning, allowing for a reservoir
of relatively humid air with surface dew points in the low to
mid 60s positioned across eastern Kansas and central Missouri to
slosh northeastward into northern Illinois and northwestern
Indiana by mid-afternoon. With forecast soundings depicting
minimal capping and 500-750 J/kg of MLCAPE by mid-afternoon,
widely scattered showers and thunderstorms appear poised to
develop across the area in "popcorn" fashion.

Now, the vigor and coverage of showers and thunderstorms Thursday
afternoon and evening is not overly clear. For one, upper-level
cloud cover "debris" from expansive convection in the Plains tonight
will be poised to spread northeastward over northern Illinois
and northwestern Indiana during "peak heating" tomorrow
afternoon, and may limit the amount the instability that
develops at least across parts of our area. At the same time,
any convective-induced MCV that develops tonight will be poised
to lift northeastward into Illinois tomorrow afternoon, which
may provide enough localized forcing to offset limited
instability. For now, felt that broad-brushing 40-50% chances
for scattered thunderstorms from tomorrow afternoon into early
Friday morning was the safest bet in favor of making refinements
in later forecast packages.

Borchardt

Friday through Tuesday:

The upper trough currently trying to organize off the coast of
southern California is forecast to be traversing across the mid-
Mississippi and Ohio River Valleys Friday into the day on
Saturday. While there continues to be some timing differences
amongst the 00z guidance suite, there is a decent signal that
south-southeasterly low-level flow should allow for warm-moist
advection to persist. Thus, temperatures and dew points in the
mid to upper 70s and lower 60s respectively should promote
destabilization Friday afternoon and allow for the development
of shower and thunderstorm activity across at least the southern
half to 2/3rds of our forecast area. Depending on which timing
solution comes to fruition, Saturday could see either another
period of showers and storms (primarily in our south and
southeastern counties) or be mostly dry as mid-level height
rises start to move into the region. However, a more robust
upper trough is expected to be pivoting across the Upper Midwest
on Saturday which looks to force a cold front through our area
Saturday night. Guidance is not overly impressive with rain
occurring along the front as the anticipated timing (after
sunset) should reduce the effective instability available, but a
few isolated to scattered showers and perhaps a rumble of thunder
cannot be ruled out especially for areas near the IL-WI line.

Regardless of how Saturday pans out, Sunday should be dry as
the aforementioned height rises are expected to be overhead and
we will be in the presence of northwest winds behind the front.
Though, temperatures are still forecast to be in the seasonably
warm category with highs in the upper 70s to lower 80s Sunday
afternoon. The exception, however, will be for those along the
lake where a lake breeze is progged to keep temperatures notably
cooler in the 60s to around 70.

Heading into the beginning of next week, guidance continues to
show that a more active pattern will take shape across the
CONUS. During this period the northern jet stream is forecast to
dig into the Pacific Northwest and phase with a developing sub-
tropical jet across the southwestern US. In doing so, a broad
trough looks to develop across the western CONUS which will
allow the aforementioned sub-tropical jet to intensify and nose
into the southern Plains and mid-Mississippi Valley. Not only
will the sub-tropical jet aid in enhancing the southwest low-
level flow over the Plains and Midwest supporting seasonably
warm and humid conditions, but it will also provide an avenue
for shortwaves to break off the trough and eject into the Great
Lakes virtually unabated. This combination of synoptic forcing,
warm-humid air, and potential wind shear looks to make for a
period of rather stormy weather across our area which may
include the threat for severe weather at times next week. While
this signal seems fairly consistent amongst guidance, it still
should be noted that specifics on timing, location, and
intensity of any potential storms (and severe weather) still
remains very uncertain. Therefore, we recommend keeping an eye
on next week with subsequent forecasts and in the meantime
review those severe weather plans just in case.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 625 AM CDT Wed May 15 2024

Forecast concerns for the terminals...

- Lingering MVFR ceilings near Lake Michigan and northwest IN
  this morning

- Chances for showers increase Thursday morning

An area of MVFR clouds continues to pivot across northwest IN
and adjacent areas of eastern IL and southern Lake Michigan as a
broad surface low tracks over the Ohio River. Recent satellite
trends have shown a hole developing in the clouds over Chicago
which is expected to continue allowing clouds to scatter at ORD
and MDW. However, the clouds over northwest IN are notably
thicker than those in IL which should allow MVFR conditions to
persist through 15z at GYY. Once the clouds scatter later this
morning, VFR conditions are expected to prevail through the rest
of the forecast period.

Otherwise, expect winds to remain northeasterly today with
speeds generally around 10-12 kts before speeds diminish this
evening to around 5 kts. Directions will also begin to turn
southeasterly by daybreak on Thursday before becoming
southwesterly late Thursday morning as speeds increase.

Additionally, an upper-level disturbance will begin to pivot
into northern IL late tonight through the day on Thursday which
will increase chances for showers and thunderstorms. While some
guidance is hinting at isolated showers developing near RFD
after 09z, the better coverage (and thus higher confidence) in
showers and storms is not expected to materialize until after
12z at RFD and 18z at the Chicago terminals. Given that these
times are after the conclusion of the 24 and 36-hour TAF
periods respectively, I have maintained dry forecasts for now.
However, a formal shower and thunderstorm mention will likely be
added with the 18z TAF issuance.

Yack

&&

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

&&

$$

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