899
FXUS63 KLOT 272344
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
644 PM CDT Mon May 27 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

&&

.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 /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 628 PM CDT Mon May 27 2024

Forecast concerns for the terminals...

- Outflow boundary from storm NE of ORD likely shifting winds to
  NE at ORD toward 0030z; lesser chance at MDW

- Showers and isolated thunderstorms crossing the area early to
  mid evening from the west; may bring northwest wind gusts of
  30-40 kt, especially toward RFD

- Period of MVFR ceilings late tonight into Tuesday morning

- Showers with embedded thunderstorms expected Tuesday afternoon

Initial focus is on corridor of isolated thunderstorms moving
southeast to the east of ORD and MDW with an outflow boundary
being enhanced by Lake Michigan and tracking southward towards
ORD. Looks like it will make it to ORD around or not long after
0030z so have added a shift to northeast winds at that time.
It`s less clear if the boundary will make it to MDW so have kept
northwesterly winds there for now.

The other focus is on an approaching upper wave moving into NW
IL with an associated band of showers and thunderstorms moving
eastward with it. This activity will reach RFD 0030-0100z and
the Chicago area terminals towards 02z or so. However, lightning
is expected to be on a downward trend with sunset. Have left
TEMPO TS in the TAFs for now. In addition, west to northwest
wind gusts of 30-40 kt may linger as the activity arrives so
have maintained TEMPO gusts as well. The spotty shower activity
that trails the main band looks to dissipate with sunset and may
be holding onto the shower mention a bit long at each terminal.

Scattered to broken stratus is expected overnight in the
2000-3000 ft range. Another trough will approach Tuesday morning
and cross the area during the afternoon bringing more shower
activity with some embedded thunderstorms. Have hit this a
little harder with this forecast update as activity looks
likely. Activity will shift off to the east by or into the
early evening.

MDB

&&

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

&&

$$

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