Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290737
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 AM CDT Fri Mar 29 2024

.KEY MESSAGES...

- Showers and thunderstorms are expected Friday night into
  Saturday morning with a level 1 out of 5 threat for damaging
  hail.

- Another period of showers and thunderstorms is expected Sunday
  afternoon through early Monday morning especially south of
  Interstate 80 with level 1 out of 5 threats for damaging hail
  and flash flooding.

- A third period of showers and thunderstorms may occur on
  Monday, though exactly where and with what vigor are
  opportunities for refinement in future forecasts.

- After a brief period of cool weather Tuesday and Wednesday,
  warming temperatures are expected Thursday and beyond.

&&

.DISCUSSION...
Issued at 237 AM CDT Fri Mar 29 2024

Through Saturday Night:

Weak ridging will keep the forecast for much of Friday dry as
temperatures steadily increase. Seasonal normal values are
expected closer to the lake, but areas west of I-39 and south of
I-80 are expected to be in the 60s, which would be 10 to even
15 degrees above normal for late March.

The next short wave trough will move over the region late
Friday afternoon, bringing with it the next opportunity for
elevated thunderstorms and even the slight chance for some
storms to become severe. Forecast soundings are showing a robust
inversion around 00Z but much steeper lapse rate aloft driving
the expectation for storms to be elevated in nature. MUCAPE
values are not overly impressive, but still plenty to work if
storms have a chance to tap into it. But with that instability
tapping into the colder temperatures aloft, it provides the
potential for hail development. The main inhibiting factor
Friday night is the meager wind shear in the convective layer.
Though thunderstorms remain the main expectation with increasing
chances after 9 PM, the threat of severe storms is more
marginal (level 1 out of 5 threat) with the potential for some
hail stones around an inch in diameter possible around and west
of the I-39 corridor.

Chances for thunderstorms taper off around daybreak Saturday
morning as the front moves east, though lingering showers are
possible into the early afternoon. Temperatures will once again
warm up Saturday afternoon above normal.

DK

Sunday through Thursday:

Sunday into Monday, the upper-level trough currently offshore of
California will eject into the central United States leading to
periods of inclement weather across the central United States. Not
withstanding evolutionary differences in the timing and structure of
the low pressure system within the ensemble envelope (owing to
opportunities for phasing with upper-level short waves/vort maxes
diving south out of Canada), the "big picture" storyline seems
fairly stable and reads as follows:

- An elongated stationary front is expected to develop from
  southern Iowa to southern Ohio on Sunday and serve as a focus
  for training thunderstorms Sunday afternoon through early
  Monday morning. The exact placement of the front will dictate
  the highest coverage of thunderstorms, and the latest
  100-member ensemble mean currently favors areas south of I-80
  and near/along US-24 as being in the center of convective
  activity. The PoPs in our gridded database may be way too
  "smeared out" with >80% chances for precipitation appearing
  way too far north and into southern Wisconsin. In other words,
  a portion of our forecast area (e.g. north of I-80) may end
  up dry Sunday afternoon through early Monday morning.

- With the overlap of the northern edge of an EML plume, a
  seasonably strong 100kt+ 250mb jet stream, warm-season like
  PWATs of 1-1.25", and persistent forcing via low-level
  isentropic ascent, the strongest storms Sunday afternoon
  through early Monday morning will produce soaking downpours
  and marginally damaging hail. Pattern recognition supports a
  narrow swath of 1-3" of rainfall where convection trains the
  longest along and just north of the front. The SPC and WPC
  currently have level 1/5 threat levels for severe weather and
  flash flooding, respectively, for the activity Sunday
  afternoon through early Monday morning (again, focused south
  of Interstate 80).

- Depending on the eventual structure, timing, and track of the
  surface low, another round of showers and thunderstorms may
  occur on Monday including the threat for severe weather. The
  spread in the ensemble envelope for Monday is too vast to say
  much more beyond that at this point.

- Temperatures Sunday and Monday will vary from north to south,
  with onshore flow keeping highs in the mid to upper 40s to in
  northeastern Illinois and near the Wisconsin border, and the
  upper 50s to around 60 near the stationary front along US-24.

The main message for Sunday and Monday is to stay up to date on
the forecast.

Tuesday into Wednesday, a sharp upper-level shortwave is expected to
dive into the Great Lakes and phase with the departing surface low
to our east. The net result will be a push of seasonably cool air
into northern Illinois and northwestern Indiana, with highs possibly
struggling to reach 50. Pattern recognition also supports a threat
for showers within the aggregate "troughiness" aloft, which may
include those of the frozen variety. Toward the end of the week and
beyond, ensemble model guidance is showing an unusually strong
signal for deep ridging to build into the eastern half of the
US, which will support warming temperatures back into the upper
50s to lower 60s, if not warmer.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1148 PM CDT Thu Mar 28 2024

Thanks to a passing surface high pressure system, light and
variable winds will continue through daybreak. Thereafter,
winds will become easterly and gradually increase in magnitude
(to 8 to 12kt) as a surface low pressure system approaches from
the west. After dark, increasing low-level confluence along the
nose of an intensifying LLJ on the eastern side of the
aforementioned approaching system is expected to activate a
pocket of steepening lapse rates, leading to a relatively
prolonged period (6+ hours) of scattered showers and
thunderstorms. With HREF neighborhood probabilities for thunder
peaking from 60 to 80% generally between 04-10Z across the
general area, opted to introduce targeted TEMPO groups for TSRA
at RFD/ORD/MDW (04-06Z at RFD, 06-10Z at ORD/MDW) in the
outgoing TAF package. Targeted TEMPO groups for TSRA will be
introduced at DPA/GYY in the upcoming 12Z TAF package.

Cigs are expected to remain VFR through the entire TAF period.

Borchardt

&&

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

&&

$$

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