943
FXUS63 KLOT 280537
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 AM CDT Tue May 28 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms are expected this
  afternoon, with a low (~5%) chance for locally damaging winds
  southwest of a Rockford to a Pontiac line

- Better chances for numerous showers and thunderstorms Tuesday
  afternoon

&&

.UPDATE...
Issued at 1008 PM CDT Mon May 27 2024

Isolated to widely scattered thunderstorms will continue to
decrease in coverage through midnight, with a continued gradual
decrease in diurnal instability.

As of 10 pm, scattered showers and isolated thunderstorms
continue to decrease in coverage, with most along and northeast
of a Madison WI to Valparaiso line, as well as south of a
Galesburg to Kankakee and Rensselaer line. Some of these may
persist through midnight as a mid-level short wave and its
associated mid-level cold pool (-20C at 500 mb across SW WI and
NW IL) move across the area, though diurnal cooling and
decreasing low-level instability will allow these to continue to
weaken and dissipate overnight. Some gusty surface winds to 40
mph or so may linger for another hour or so south of the
Kankakee River, however no strong or severe thunderstorms are
expected.

Trimmed pops/cloud cover from the west per current radar and
satellite trends, and adjusted winds per obs trends to be more
northerly across the Chicago area/lakeshore in association with
earlier outflow boundaries. Otherwise, no other significant
changes made to the going forecast through Tuesday AM.

Ratzer

&&

.DISCUSSION...
Issued at 242 PM CDT Mon May 27 2024

Through Tuesday Night:

Visible satellite imagery shows one wave over western Michigan
exiting the region to the east, while another wave over
southwestern Minnesota is digging southeastward into Iowa. A
weak line of showers out a head of the circulation has exited
Iowa and steadily moving into the forecast area, but with
recent AMDAR soundings showing a strong cap around 850, there is
lower confidence in thunderstorms developing in the next few
hours.

However, as the circulation moves southeast into the Illinois,
better instability will advect into Illinois allowing for
showers and thunderstorms to develop. There has already been an
uptick in lightning in Northern Iowa. Model soundings are
suggesting fairly weak lapse rates with CAPE values less than
500 kg keeping the severe threat fairly marginal. While small
hail less than an inch is possible, the main threat for the
strongest cells that develop will likely be locally stronger
wind gusts, which could locally reach up to 60 mph. Away from
the instability to the north and east including the Chicago
metro, isolated cells may develop, but the probability remains
around 30 percent or less. Once the sun sets, the chance for
convection diminishes quickly there after.

Another wave is projected to drop south out of Canada on
Tuesday. With a upper jet moving the left exit region over
northern Illinois, it could allow for better forcing. Wind
shear profiles and mid level lapse rates once again do not look
overly impressive to merit a strong concern for severe weather
specifically; however, models have trended stronger with
instability suggesting a higher coverage of showers and
thunderstorms. Chances for convection diminish after sunset as
the wave moves east. Cold air advection behind the system will
allow low temperatures on Wednesday morning to drop down into
the low 50s and isolated upper 40s.

DK

Wednesday through Sunday:

The mid/upper-level ridge axis centered across the Rockies early
this week will build eastward across the Midwest into the western
Great Lakes for the second half of the week. As it does so, the
active belt of enhanced northwesterly flow aloft across the Great
Lakes region in the near term will get shunted east of our area.
This will set the stage for a few day period of dry and quieter
weather through the end of the week as surface high pressure sets
up shop over the Great Lakes region.

While the weather will be quieting down by mid-week, below
average temperatures will persist into Thursday before conditions
begin to moderate back above normal for the weekend. Therefore,
expect high temperatures for Wednesday and Thursday to only be in
the 60s to low 70s (coolest lakeside). Overnight low temperatures
also look to get downright chilly for the end of May, with
readings likely to drop into the mid 40s outside of urban areas.
Temperatures then will warm back to around 80 degree highs for the
upcoming weekend.

With the warming temperatures, comes increasing chances for
showers and thunderstorms. It appears the area will gradually
transition back into a more active weather pattern sometime this
weekend into early next week. Accordingly, our current forecast is
carrying some low end chances (20-30%) for showers and storms for
the weekend. However, it is worth noting that there is a
considerable amount of ensemble spread with how quickly this
transition will occur. For this reason, the chances currently
highlighted in the forecast are likely to broad-brushed and hence
will need refinement as forecast certainty grows.

KJB/GI

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1236 AM CDT Tue May 28 2024

The primary aviation weather concerns through the 06Z TAF period
include:

* Showers and thunderstorms Tuesday afternoon into the evening
  with a corresponding potential for MVFR cigs/vsbys.

* A chance for additional MVFR cigs late Tuesday night into
  Wednesday morning.

Light westerly winds, at times variable, will take us through
the night underneath a SCT 3-5 kft deck moving into northern
IL from the northwest. Most of the morning will see WNW winds
below 10 kt before ramping up to around 10-12 kt for the better
part of the day.

Scattered showers and thunderstorms will move over the airfields
from the north and northwest Tuesday afternoon. The onset time
continues to trend later in model guidance, now looking like 18Z
at the earliest across all sites but could wind up being closer
to 19 or 20Z before we see rain. There is a solid potential for
some embedded thunderstorms as well, primarily through only
about 00Z. While we should remain predominantly VFR, we could
see periods of MVFR cigs or vsbys while storms are in the area.
Scattered rain showers will persist likely only through about
mid-evening, although there is a growing signal for light, non-
impactful showers to continue later into the night, especially
at the Chicagoland sites.

With the arrival of the rain, winds are expected to veer to NNW.
It`s very possible that we see brief, if not prolonged, periods
of a NNE direction during the afternoon, but overall confidence
in a NNE at any given time is shaky. Following the rain, near-
northerly winds below 10 kt can be expected through the rest of
the period. There is a moderate signal for MVFR cigs to develop
late Tuesday night into Wednesday morning, although best bet
looks like continued VFR.

Doom

&&

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

&&

$$

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