131
FXUS63 KLOT 151948
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 PM CDT Sat Jun 15 2024

.KEY MESSAGES...

- There is a low (20-30%) chance for a few thunderstorms
  tomorrow morning mainly across northern Illinois.

- A period of warm and humid conditions will start Sunday and
  last through at least Tuesday. Monday will be the warmest day
  with peak heat indices near or above 100 degrees.

- Chances for showers and storms Monday onward are relatively
  low (20% or less), with expected limited coverage.

&&

.DISCUSSION...
Issued at 248 PM CDT Sat Jun 15 2024

Through Sunday night:

The Western Great Lakes are positioned between a 1020mb surface high
pressure system in southern Ontario and a 1002mb surface low
pressure system over the Northern Plains, both beneath inflection
points of a developing upper-level ridge centered over the Upper
Mississippi River. A series of upper-level shortwave are
propagating northeastward from the Plains toward the Great Lakes on
the southern periphery of the ridge, with the first now lifting into
Minnesota and the second located in southern Kansas. Locally,
filtered sunshine, modest humidity levels, and a southeasterly
breeze are making for a pleasant mid-June day.

Tonight, the second upper-level shortwave will lift into Iowa and
Missouri and induce the development of a broad low-level jet into the
Great Lakes. As a result, we should not decouple in spite of
relatively low humidity levels, and the night will be marked by
breezy southerly winds with lows in the mid to upper 60s.

Tomorrow, the upper-level shortwave is expected to lift into
Wisconsin carrying the majority of any remnant overnight convection
(initially in Iowa) with it. However, ongoing isentropic ascent
along the eastern fringe of the moisture plume shifting eastward
into the Great Lakes and along the northern edge of a plume of steep
mid-level lapse rates may nevertheless allow for isolated to
scattered thunderstorms to develop as far south as northern
Illinois. Model guidance typically struggles with the southward
extent of thunderstorm LLJ/lapse rate-driven convection, and
accordingly, felt introducing 20-30% chances for thunderstorms
across northern Illinois tomorrow morning would be prudent. The far
more likely scenario is that most areas stay dry tomorrow
morning.

Tomorrow afternoon, rising heights in the wake of the shortwave
should lead to low-level capping in spite of rising humidity levels
and temperatures. As a result, we currently do not favor any
(renewed) development of showers and thunderstorms beyond anything
that occurs in the morning. Forecast 925mb temperatures around +25C
should support afternoon highs in the low to mid 90s. A
southwesterly breeze won`t offer much respite from the humidity
levels and partial sunshine, with max heat indices expected to peak
from 95 to 100F during the afternoon. After sunset, winds will relax
but high humidity levels will hold overnight lows to the balmy mid
to upper 70s.

Borchardt


Monday through Saturday:

The upper ridge that is currently building across the southern
CONUS is expected to be lifting into the Mid-Atlantic on Monday
as a broad trough digs into the Pacific northwest. This pattern
will place stout southwest flow across much of the Mississippi
Valley and Great Lakes generating a period of above normal
temperatures and high humidity. The warmest day during this
period continues look like Monday where high temperatures will
likely be in the mid-90s with overnight lows in the mid to upper
70s. Though, there continues to be the possibility for high
temperatures to overperform into the upper 90s (20-30% chance)
if there is more sun than currently forecasted. Regardless, the
combination of these temperatures and dew points in the upper
60s to around 70 should yield peak heat indices in the 95- 105
degree range Monday afternoon. Given that these values are below
our local Heat Advisory criteria (widespread heat indices
greater than 105), no heat headlines are anticipated. With that
said, we still encourage those who are sensitive to heat to
limit time outdoors and remain hydrated.

Furthermore, some guidance (namely the GFS) continues to hint
at the potential for scattered showers and storms to develop
Monday afternoon. However, all guidance continues to show modest
high rises occurring overhead in combination with the mid-
levels looking rather dry, which I suspect will prohibit any
shower/storm activity despite the robust instability. While I do
expect the area to remain dry on Monday, I have maintained a
20% chance for showers/storms Monday just in the off chance
something is able to develop. It should be noted that if a stray
shower or storm does develop, then high temperatures in those
location may verify slightly cooler than forecasted.

Heading into Tuesday, the western extent of the ridge is
progged to get shoved east as a trough ejects into the northern
Plains. While this will allow more moisture to advect into the
southern Great Lakes, the resultant increase in cloud cover
should also keep high temperatures a few degrees cooler, in the
lower 90s, for Tuesday. Though, peak heat indices in the upper
90s to near 100 are still expected. Additionally, the increase
in moisture in conjunction with stout diurnal instability and
modest upper height falls will support the potential for
isolated to scattered showers and thunderstorms to develop
Tuesday afternoon. Though, the lack of more defined forcing may
mean most areas stay dry.

Beyond Tuesday, guidance continues to attempt to break down the
ridge and develop a more zonal (east-west oriented) pattern. As
a result, a frontal boundary is forecasted to be near the
Mississippi River and may move through northern IL during the
later half of the week, breaking the heat and humidity.
However, there continues to be a wide range of timing
differences amongst guidance members on when the front would
arrive and the coverage of any associated showers and storms.
Therefore, I have decided to maintain the advertised 90 degree
temperatures and 30-40% rain chances offered by the NBM through
the end of next week which seem reasonable at this range.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1230 PM CDT Sat Jun 15 2024

Key Aviation Impacts for the current TAF period:

- Gusty southwest winds Sunday

- Chance (30 percent or less) for scattered showers and storms
  around KRFD Sunday morning

A broad swath of high level clouds will continue to move over
the airspace, but VFR skies and southeasterly winds are
expected. The occasional gust up to 20 knots or potentially the
direction flops to 180 or 190 once or twice are possible, but
are not expecting to be prevailing.

Winds will become out of the southwest as a low level jet aloft
increases. Better mixing should keep LLWS out of the TAF and
allow for wind gusts to make it to the surface between 20 to 25
knots through Sunday.

While the bulk of the storms currently in Iowa and Minnesota
will move northeastward into Wisconsin overnight and into
Sunday, there should be just enough instability and moisture
present for the chance for a shower or thunderstorm near KRFD so
a PROB30 group was introduced into the TAF. If the showers over
KRFD do materialize, it would not be surprising if a rogue non-
impactful shower made it to the Chicago terminals, but thunder
is not expected so the the TAF was kept dry for the time being.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert from 1 AM Sunday to 1 AM EDT Monday for
     INZ019.

LM...None.

&&

$$

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