194
FXUS63 KLOT 221928
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
228 PM CDT Wed May 22 2024

.KEY MESSAGES...

- Tomorrow will be quiet with highs in the upper 70s to lower
  80s. A lake breeze will turn lakeshore locations cooler.

- Scattered thunderstorms are likely on Friday afternoon/early
  evening, a few of which may be strong to perhaps severe.

- For the holiday weekend, a dry and pleasant Saturday, waves of
  showers and scattered t-storms (some severe threat) on Sunday,
  then cool, breezy, and possibly showery conditions are
  expected on Memorial Day.

&&

.DISCUSSION...
Issued at 228 PM CDT Wed May 22 2024

Through Thursday night:

There are no weather concerns through the short-term period.
Breezy west winds this afternoon will ease after sunset and even
become calm by daybreak Thursday. The passing of a surface high
pressure system will keep winds weak throughout the day on
Thursday, and allow for a lake breeze to surge inland during
the afternoon. Sunny skies will boost highs into the upper 70s
to lower 80s. Tomorrow night will similarly be quiet with light
winds and clear skies.

Borchardt

Friday through Wednesday:

With dry high pressure overhead through Thursday night/early
Friday, there are question marks on how quickly an elevated
instability plume can advance eastward across the MS River and
into our area. Some of the faster guidance brings decaying
convection into the western half of the CWA or so as early as
the mid to late morning hours, while the slower (ie. most recent
ECMWF and Canadian) guidance hangs onto dry conditions through
the early afternoon.

A relatively narrow plume of 60s dew points will be drawn
northeastward ahead of a cold front. Along with temps in the
low-mid 80s, and mid-level lapse rates around 7-7.5 C/km,
minimally capped MLCAPE will build up to around 1500 J/kg. As
large scale ascent overlaps the front, paired with frontal
convergence, scattered mainly pulse-type thunderstorms are
probable in the afternoon into the evening before shifting out
of the area along with weakening with post-sunset stabilization.
SPC included our whole area in a level 1 (marginal risk) severe
threat, mainly for isolated damaging winds (downbursts) and
marginally severe hail. Aside from the convection onset
uncertainty, the other item to monitor will be for any potential
MCV to enhance wind fields and possibly increase the threat
somewhat for organized severe storms, including supercellular
structures.

For the all important Memorial Day Weekend forecast, the
unofficial start of summer 2024, there`s some good news and some
bad news. First, the good news: underneath weak surface high
pressure, Saturday looks pleasant, dry, and seasonable with
highs in the 70s, except cooler near the lake due to lake breeze
associated cooling. The pleasant conditions should persist
through the evening for holiday weekend outdoor activities.

Now, onto the bad news: Sunday is looking inclement as an
unseasonably strong synoptic system and surface low affect the
region. Lead showers and scattered thunderstorms spreading
northeast in the pre-dawn hours of Sunday will overspread the
rest of the area through the morning hours. Warm/moist advection
ahead of the surface low, as a warm front lifts north, will
boost dew points back to the lower to mid 60s south of I-88,
with the northward extent of the warm front and moist sector
dependent on the exact storm track. While cloud cover and
ongoing rain will limit destabilization, especially the farther
north you go, there may be enough heating for temps to reach the
70s. Thus, near and south of I-80, despite, again, instability
appearing to be a limiting factor, the moist adiabatic lapse
rates could still support pockets of 1000-1500 J/kg of uncapped
MLCAPE during the afternoon.

SPC already included areas south of I-80 within our CWA in day
5% severe probs (level 1), which appears reasonable given the
current forecast thinking on instability. Potential hazards
include damaging winds and brief tornadoes, whether from low-
topped mini-supercells, which appear possible per forecast
hodograph shape (small but with strong low-level shear), or from
a QLCS mode (considering aforementioned strong low-level
shear). A farther north track of the low-mid 990s mb surface low
would also entail the brief tornado threat correspondingly
shifting northward near the warm frontal zone, if sufficient 0-3
km CAPE can materialize.

Aside from the somewhat conditional severe weather threat,
confidence was high enough to feel comfortable with the likely
(60-70%) PoPs offered by the NBM. Outdoor plans could certainly
be in jeopardy given the current forecast. Finally, seasonably
high PWATs and fairly deep warm cloud depths may also support a
localized flooding threat (WPC already has us included in its
day 5 level 1 (marginal risk) ERO. Shower and thunderstorm
coverage will wind down Sunday night as the surface low tracks
into the central Great Lakes, though deformation type showers
may hang on longer across the far northern CWA.

Monday (Memorial Day) currently looks to have less than ideal
conditions for holiday BBQs and pool/beachgoers, albeit with a
much lower, if any chance for thunderstorms. Pattern wise, it
appears unlikely to be a wash-out of a day, but on and off
showers are plausible, with a bit higher coverage (up to 50%)
perhaps focused across the northeast half or so of the CWA. In
addition to the at times showery conditions, it will be mostly
cloudy and cool, with highs in the upper 60s-lower 70s amidst
northwesterly winds gusting up to 25-30 mph, if not stronger.
Another short-wave wrapping around the mid-upper trough over the
region may bring another bout of showers Tuesday into Tuesday
night, possibly followed by a quieter period on Wednesday as
high pressure builds in. Cooler than normal temperatures and
breezy conditions will likely continue both days.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1230 PM CDT Wed May 22 2024

Main Concern:

- Timing of likely lake breeze wind shift on Thursday afternoon

Quiet VFR conditions are in store through this TAF cycle. Gusty
west winds will ease with sunset and become light southwest to
variable overnight. Weak winds aloft on Thursday are expected to
allow a lake breeze to push inland across ORD and MDW (and GYY
beyond its current TAF period), shifting winds to easterly near
10 kt. Indicated a 20z timing of the wind shift at ORD and MDW,
which will be refined as needed in subsequent updates.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 6 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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