332
FXUS63 KLOT 020844
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
344 AM CDT Thu May 2 2024

.KEY MESSAGES...

- Waves of showers and thunderstorms expected through Friday AM.

- Short window for strong to severe storm development this
  afternoon into early evening (~3-7pm).

- Periodic chances for showers and thunderstorms again this
  weekend into next week, with some potential for severe weather
  on Tuesday.

&&

.DISCUSSION...
Issued at 344 AM CDT Thu May 2 2024

Through Friday:

All is quiet early this morning with light northeast winds and
temperatures in the 50s. Well to our west, an expansive complex
of showers and thunderstorms is ongoing across northern Kansas
and eastern Nebraska with scattered warm advective showers and
embedded thunderstorms extending well ahead of it across much of
Iowa. Coverage of showers farther southeast along the surface
warm front across northeast Missouri and southern Illinois is
much lower than expected just 24 hours ago, likely due to more
persistent dry air in the lower levels, though there are a few
spotty showers attempting to develop near and south of
Springfield, IL that may try to lift into the area toward
daybreak.

While there remains a fair amount of variability in the timing and
coverage of upstream showers and storms through the morning and
early afternoon hours into portions of the local area, the general
expectation is that the NE/KS complex of storms will continue to
drift east northeast toward the area and gradually decay upon
approach with spotty showers and isolated non-severe storms
potentially reaching the Rockford area by mid-morning. Can`t rule
out gusty winds and small hail with any more persistent cores
along/west of I-39 late morning. The afternoon strong to severe
storm potential will likely be dependent upon the evolution of the
morning showers/storms. A faster movement and persistence of
precipitation across far northern Illinois could limit the north and
westward extent of the warming, keeping the better instability east
of the more favorable shear parameters. Meanwhile an earlier
weakening of the line would allow for a further north/northwest
expansion of the warm unstable air toward the WI/IL state line and
west of I-39. In this scenario, the more favorable instability and
shear parameters would be supportive of at least isolated strong to
severe thunderstorm development with damaging hail and wind being
the primary hazards. Will also note that with the location of the
surface warm front hugging the lake, there is a narrow axis where
slight backing of the winds at the surface could be supportive
of a localized brief tornado threat. The timing of the strong to
severe potential still looks to be confined to roughly a 3-7 PM
CDT window as instability diminishes with sunset.

Additionally, with the anomalously high PWATS for this time of
year combined with the antecedent wet soils, there remains the
potential for localized flooding where heaviest rain rates
develop.

As far as temperatures go, as mentioned above, they will be
dependent on the coverage of AM showers and the resultant placement
of the warm front. Locations that manage to stay dry through early
afternoon (most likely south and east of a Mendota to Elgin, IL
line) will be warm and even a bit muggy for this time of year with
highs in the low-mid 80s and dew points in the lower 60s. Where
showers persist through the morning may only warm into the 70s.
One final note on temperatures: those along the immediate Illinois
lakeshore may struggle to warm out of the 60s, especially into
Lake County.

Looking ahead to the evening hours, waves of showers and embedded
non-severe thunderstorms are expected to continue through the
overnight hours as the system and associated cold front very
slowly drift east with time. Expect showers to clear out from
northwest to southeast with the front during the morning hours on
Friday, with perhaps a few showers/storms lingering toward central
Illinois into early afternoon. Temperatures on Friday will be
cooler than today with highs in the upper 60s to lower 70s.

Petr


Friday Night through Wednesday:

An active weather pattern is expected going into next week with
periodic chances for showers and storms. An initial shortwave
impulse arriving in tandem with a cold front at the surface may
bring some showers and storms into the area on Saturday, likely
during the latter half of the afternoon or at night. Another
disturbance will likely pass to our south late Sunday into
Monday, but may still be in close enough proximity for showers
and possibly some storms to spread into our forecast area,
particularly into our southern counties. With both disturbances
being fairly subtle and somewhat detached from the stronger flow
aloft, there is still quite a bit of spread amongst the EPS and
GEFS ensemble members in their evolution and how that will
translate QPF-wise, but enough of a signal for measurable
precipitation exists to continue carrying slight chance and low-
end chance PoPs as output by the NBM during the Saturday
afternoon through Monday time frame. High temperatures during
this three-day stretch look like they`ll end up in the 70s
inland with cooler temperatures closer to Lake Michigan each day
as a result of onshore flow prevailing through much of this
time frame.

A much more potent upper-level trough will eject out of the Rockies
and morph into a closed-off low over the northern Great Plains
Monday into Tuesday, with another follow-up wave likely to then be
thrusted towards the Great Lakes as it navigates a belt of strong,
predominantly westerly flow along the upper low`s southern
periphery. This process will likely culminate in at least two
episodes of cyclogenesis across the central part of the CONUS that
will encourage the poleward advection of warm, moist air of Gulf
origin into the Midwest. The increased warmth and moisture, coupled
with the strong dynamics at play, will likely result in consecutive
days of severe weather from the Great Plains to the Appalachians
during the first half of the upcoming week. It`s still too early to
say with much confidence whether we will get in on the action here
in northern Illinois and northwest Indiana as there are several
moving parts and pieces that still need to be resolved, but it looks
like Tuesday should be the main day for us to monitor for potential
severe thunderstorms and/or heavy rainfall. Though, depending on how
everything evolves, some potential for severe weather may ultimately
present itself on Monday night and/or Wednesday as well.

Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 126 AM CDT Thu May 2 2024

Key Messages:

- Periods of showers likely today and tonight.

- Potential for thunderstorms this afternoon into this evening.

- Low confidence in wind direction/wind speed this afternoon
  into this evening.

Periods of showers are likely to be observed over a good chunk
of the TAF period as a low pressure system and its associated
fronts approach from the southwest. An initial wave of showers
is possible after daybreak this morning out ahead of a warm
front lifting northward into the area. However, confidence in
these showers developing is only medium, and their coverage may
be isolated enough to not affect the terminals much anyways.

There will probably be a break in the shower activity late this
morning into the early afternoon before an additional wave of
showers and scattered thunderstorms approaches from the
southwest ahead of a cold front. While confidence is medium to
high in thunderstorms occurring in northern Illinois today,
particularly along the leading edge of this second wave of
precipitation, confidence in the timing of these storms and
whether the storms will affect any particular TAF site remains
somewhat low due to the somewhat disorganized character of this
convective activity. Have elected to largely retain the
inherited PROB30 groups for TSRA for now and will continue to
reassess. If storms do end up passing over the terminals, they
may produce some stronger winds and will likely reduce
visibility below VFR levels.

Going into tonight, shower coverage is expected to become more
widespread, but thunderstorm coverage should dwindle due to the
loss of instability with time. Ceilings will likely lower as
the showers continue over the course of the night, with MVFR
ceilings possible towards the end of the current TAF period.

Lastly, the wind forecast for this afternoon into this evening
is not an easy one. Initially easterly winds should veer more
southeasterly later this morning into the early afternoon as the
aforementioned warm front approaches. Winds may then become
southerly south of the warm front as it passes by, but some
uncertainty remains as to how far north the front will get as
the influence of onshore flow off of Lake Michigan will probably
cause it to get hung up somewhere in the Chicago metro, which
would tend to favor winds remaining east of south at the Chicago
metro terminals. If thunderstorms end up passing near or over
the terminals this afternoon and evening, then that will likely
cause winds to become quite messy for a time before a general
trend towards a southwesterly wind direction is expected
tonight. Finally, winds should turn westerly towards the end of
the current TAF period as the cold front passes through.

Ogorek

&&

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

&&

$$

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