001
FXUS63 KLOT 051826
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
126 PM CDT Sun May 5 2024

.KEY MESSAGES...

- No impactful weather is expected over the next 36 hours.

- Waves of showers and thunderstorms are expected Tuesday
  morning through Sunday, though there will be plenty of dry
  hours.

- Confidence in severe weather in our area on Tuesday is
  lowering, but remains for Wednesday especially along and south
  of I-80.

&&

.DISCUSSION...
Issued at 126 PM CDT Sun May 5 2024

Through Monday:

No impactful weather is expected over the next 36 hours as the
center of a surface high passes through Wisconsin and Michigan.
Lows tonight will fall into the mid to upper 40s beneath broken
to overcast upper-level clouds and with nearly calm winds.
Tomorrow will be partly sunny with highs in the low to mid 70s,
except the low to mid 60s near Lake Michigan where easterly
onshore flow will prevail. A shower or two cannot be ruled out
during the afternoon mainly south of I-80 near a weakening
upper-level wave passing through the Ohio River Valley (only a 10-15%
chance).

Borchardt

Monday Night through Sunday:

A strong upper level low will strengthen overnight Monday into
Tuesday, with a deepening surface low over the Northern Plains.
A strong negatively tilted short wave on the eastern side of the
upper-level low is expected to propagate northeastward into the
mid-upper Mississippi Valley and western Great Lakes region
Tuesday morning with a strong 80-90 kt jet aloft. With increased
forcing and isentropic ascent over an northward propagating
warm front, showers and thunderstorms are expected to move into
Illinois from daybreak through mid Tuesday morning. The risk for
severe weather with this first wave of storms is low given
model soundings projecting only modest instability.

The forecast for Tuesday afternoon continues to evolve. The
main uncertainty with Tuesday afternoon will become how much
clearing happens behind the first line of showers to destabilize
and allow for storm redevelopment associated with a weak
secondary area of lower pressure developing over southern
Wisconsin. Of note, ensemble models have trended weaker and
weaker with that low developing since Friday of last week,
leading to continued southeastern trend to the instability axis
and a lower moisture return. That may be what the 12Z run of the
NAM is keying in on. It has a slightly later arrival of the
morning wave of showers (mentioned in the previous paragraph)
with almost no low developing in Wisconsin. If the NAM scenario
plays out, the area of afternoon redevelopment could be as far
away as Central and Southern Indiana and have limited to almost
no storms in the afternoon/evening for Northern Illinois. And
yet, the 12Z GFS & Euro is still suggesting a slightly (relative
to the NAM) stronger surface low that can assist with better
destabilization that could provide a second round of storms in
the afternoon. Should that play out and some clearing in the
cloud cover behind the morning line, over 1500 J/kg CAPE over
the region with upwards of 50 knots of deep layer shear can be
realized and there could be a second round of showers and storms
that may potentially involve severe weather. In short, the
Tuesday forecast has higher confidence in showers and non-severe
storms in the morning, with now lower confidence in the
potential for showers and thunderstorms developing in the
afternoon given the disparate model projections.

Models have still remained consistent with showing the upper
level low start to weaken, wobble and transition into a
positively tilted trough through Wednesday. The surface low
over the Northern Plains occludes and weakens as a secondary low
over Oklahoma will strengthen and lift with an associated wave
aloft to the northeast. As it moves, southwesterly winds will
allow better moisture to move over Illinois with a warm front
pushing northward. Where that boundary sets up will determine
the overall storm threat for the day, with models suggesting
that it could set up somewhere around or just south of I-80.
With model soundings suggesting another potential for 1500-2000
J/kg of CAPE and deep layer shear around 60 knots, storms could
be strong to even severe, including some localized flooding
potential along and south of the boundary.

The upper level wave will broaden through the end of the week
becoming a positively tilted long wave trough over eastern
Canada. Persistent cool air from northwest flow will allow for
temperatures to fall back into the 60s for the end of the week.
Weaker and smaller waves may pass around the longer wave
Thursday and Friday providing the ability to perturb the
environment along with diurnal heating to create some scattered
showers in the afternoon through at least Saturday, though exact
time and location of showers at this scale would not be
understood until higher res guidance can have a go at trying to
resolve it. Looking at the extended long range, models are
suggesting that that upper level wave moves east and sets up
along the Atlantic coastline. Meanwhile, a long wave ridge will
slowly grow and park itself over the Rockies, thereby placing
Chicago underneath weaker northwest flow. With the exception of
some smaller scale features changing it, days with general
showers are certainly still possible, but diminishes any larger
severe signal for most of the rest of the month.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1218 PM CDT Sun May 5 2024

There are no major aviation weather concerns through the 18Z TAF
period.

The MVFR cigs have cleared the terminals and we`re now looking
at VFR throughout the period. Expect easterly or northeasterly
winds mostly under 10 kt through the period, possibly gusting
upwards of 15 kt on occasion during the day on Monday.

Doom

&&

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

&&

$$

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