263
FXUS63 KLOT 271942
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
242 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 /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1234 PM CDT Mon May 27 2024

Forecast concerns for the terminals...

- Scattered showers and thunderstorms late this afternoon
  through early evening

- Period of low-end VFR to MVFR ceilings late tonight into
  Tuesday morning

- Another round of showers and thunderstorms Tuesday afternoon


The MVFR ceilings that have blanketed northern IL and northwest
IN this morning continue to erode and should be fully scattered
out over the next couple of hours. As skies scatter, the
atmosphere should quickly mix into stout winds aloft generating
20-25 kt wind gusts through this afternoon before gusts subside
late this evening.

At the same time, a potent upper-level low pressure system will
begin to pivot into northern IL which should allow for the
development of scattered showers and thunderstorms. However,
instability continues to look rather limited, especially with
eastward extent, which makes confidence on thunder coverage at
the Chicago terminals low. Despite the lower confidence have
still decided to maintain a TEMPO for thunder at the Chicago
terminals for a couple of hours this evening in case sufficient
instability is realized and storms do form overhead. Additionally,
there is still the potential for a few storms to produce
locally gusty winds in excess of 30 kts but the coverage of
these "severe" storms will likely remain along and west of FEP
to IKK line.

Showers and storms will come to an end this evening (by 05z at
the latest) leaving dry and mostly VFR conditions overnight.
Though, there continues to be a signal for some 2000 to 3000 ft
ceilings to develop after midnight and persist into Tuesday
morning. Heading into Tuesday, any MVFR ceilings that develop
are expected to lift through the morning leaving VFR conditions
for the rest of the forecast period. However, another upper-
level disturbance is forecast to pivot through the Great Lakes
Tuesday afternoon which will once again promote the chances for
scattered showers and thunderstorms. While instability does look
notably better on Tuesday than today, coverage is still
somewhat uncertain so have decided to handle with PROB30s at the
30-hour TAF sites for now. Given that the storm timing on
Tuesday is forecast to be after 18z I have not added formal
mentions to RFD, DPA, and GYY yet but will need to consider it
with future issuances.

Yack

&&

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

&&

$$

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