629
FXUS63 KLOT 152345
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
645 PM CDT Wed May 15 2024

.KEY MESSAGES...

- Scattered thunderstorms are expected area-wide Thursday
  afternoon and evening. A few of these storms could become
  strong to severe and contain hail and gusty winds late
  afternoon and early evening northwest of a Mendota to Chicago
  line.

- Periods of showers and storms may occur Friday into Saturday,
  but looking mostly dry for the weekend.

- Active pattern looks to develop next week bringing several
  opportunities for showers and thunderstorms.

&&

.DISCUSSION...
Issued at 250 PM CDT Wed May 15 2024

Through Thursday Night:

With the recent upper-low around our region finally drifting
southeast this afternoon, dry conditions are expected through
tonight amid passing mid to upper-level clouds.

Focus then turns to developing convection over northeast NM and
southeast CO this afternoon as it grows upscale across the
central Great Plains overnight. A resulting convectively-
enhanced wave should traverse somewhere across central Illinois
late Thursday morning into mid-afternoon. Potential for the
coverage of convection into our area will hinge both on the
degree of low-level moistening as well as any changes in the
expected timing/location of the wave. The expected late morning
arrival of the wave will align with gradually increasing
surface-based instability, so any delay in arrival of the wave
would further support increasing coverage of convection. As for
the low-level moisture profile, surface high pressure still
anchored over LA this afternoon has continued to suppress low-
level Gulf moisture return. Therefore, most of the increase in
low-level moisture into the CWA will be highly driven by ET and
from recent rainfall. Given only modest mid-level lapse rates
and dew points struggling to rise much above 60F, CAPE profiles
support more of a scattered pulse-like set-up during the
afternoon hours.

If the convective wave remains well south or does not fully
materialize during the morning, the thermodynamic and kinematic
field will become increasingly favorable for a few strong to
possibly severe storms with small hail and gusty winds across
the northwest third of the CWA ahead of a late-arriving cold
front. This scenario remains conditional on several of the above
factors as well as the ability for any storms to stave off mid-
level entrainment.

Chances of convection will wane after sunset and with the
passage of the slowing cold front from northwest to southeast
Thursday night.

Kluber


Friday through Wednesday:

In the wake of Thursday`s showers and thunderstorms a separate
disturbance moving across the southern CONUS may be expansive
enough on the north side for renewed shower and thunderstorm
development into portions of the area on Friday, particularly
south of I-80. Depending on how how slowly this feature drifts
east, precipitation may linger into Saturday morning. The GFS
continues to be most favorable for precipitation lingering into
Friday with a broader extent to precipitation north of the low.
Meanwhile higher-res guidance starting to capture this time
range has precipitation shifting east a bit more quickly and
keeps much of the precipitation south of the area. Accordingly,
PoPs have been lowered a bit with this update to account for
this trend. Nevertheless, there remains a window for a few
thunderstorms to develop in our south in the afternoon on
Friday.

Other than a few showers potentially lingering into Saturday,
dry weather is forecast for the weekend with surface high
pressure settling over the region in the wake of a dry cold
frontal passage late Saturday. This will set up what looks to be
a great late-spring weekend with temperatures warming into the
upper 70s to lower 80s inland from the lake (expect upper 60s to
near 70 near the shore).

Confidence is increasing in a return to a more active pattern
early to mid next week as the upper jet begins to orient
southwest to northeast across the western half of the CONUS.
This would allow for multiple disturbances to potentially lift
across the region. Accordingly, long-range ensemble meteograms
highlight this as a period favorable for accumulating
precipitation paired with a signal for an unstable air mass
building across the region. While there is still plenty of time
for the details to be ironed out, this will be a time to watch
for severe weather and heavy rainfall somewhere in the region.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 634 PM CDT Wed May 15 2024

Key Aviation Messages:

- Scattered TSRA possible Thursday afternoon into early evening.

Quiet, VFR aviation weather is in place across the terminals
this evening, and will remain so into Thursday morning. Surface
high pressure centered northeast of the Great Lakes will supply
light mainly northeast surface winds tonight (though winds may
go light/variable for a time), before turning southeast Thursday
morning and eventually south by midday.

A mid-level disturbance currently across the Plains will
approach then approach the area by midday/early afternoon
Thursday, with an increasing potential for scattered
thunderstorms to affect the terminals. A few spotty elevated
showers could work into parts of northern IL during the morning,
though they will be fighting drier air and a lack of low-level
instability. Better potential looks to arrive early to mid
afternoon. Have moved timing for the Chicago terminals up an
hour or so to 19Z, based on latest high-res CAM trends, with the
best window roughly 19-23Z for KORD/KMDW. Isolated to widely
scattered SHRA/TSRA may linger into the early evening as well,
though confidence/coverage appears too low for a TAF mention
beyond a VCSH at this distance. Surface wind direction may get a
bit messy behind the afternoon convection as well, with a couple
of high-res models suggesting a shift back to southeast by early
evening. Majority of other guidance suggests a south-southwest
direction, and have maintained a 180 deg wind into the evening
for KORD/KMDW, though will have to monitor trends in wind
details tomorrow afternoon/evening.

Ratzer

&&

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

&&

$$

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