629
FXUS63 KLOT 271130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Mon May 27 2024

.KEY MESSAGES...

- Threat for a few strong to locally damaging wind gusts with
  scattered thunderstorms tomorrow afternoon, after about 3 PM.

- Additional showers and thunderstorms Tuesday afternoon.

&&

.DISCUSSION...
Issued at 248 AM CDT Mon May 27 2024

Through Tuesday:

A cold front is making its way out of the CWA early this morning
shunting all of the precip off to our east. Cool air advecting
in behind the front is spreading a low stratus deck across the
area with even a few instances of light fog being reported,
mostly up in southern WI so far through 2 AM. The low clouds
will likely hang around well into the morning before burning
off, but additional mid level clouds will fill in during the
afternoon. The cloudiness and cold advection will keep
conditions just a bit cooler today with highs forecast in the
lower to middle 70s. A tight pressure gradient behind the
departing storm system will also bring breezy conditions to the
area today.

During the day today, a low amplitude mid level vort max will
dive from the northern Plains southeastward into the Midwest.
Low level lapse rates are anticipated to steepen up with diurnal
heating through the afternoon while mid level cooling will
steepen up lapse rates upstairs a bit. The result will be a
cluster of southeastward-moving showers and thunderstorms
expected to develop today ahead of the vort max and move across
the CWA during the latter part of the afternoon through about
mid-evening. With CAPE and shear both on the awfully marginal
side when considering severe potential, most thunderstorms are
expected to be sub-severe. However, steep low level lapse rates
with a modest boundary layer hydrolapse could be enough to
produce strong, if not marginally severe, thunderstorm winds.
Models are really favoring the southwestern half of the CWA to
see the strongest storms, following a distinct corridor of
maximized PVA. However, the strong wind threat should wane with
southeastward extent as low level stability builds into the
evening. Instability should fizzle away by the latter part of
the evening, though a few showers may linger past midnight.

Behind the departing trough, a second wave will follow closely
behind and will impact the region on Tuesday with another vort
max diving into northern Illinois during the day. As a result,
another swath of scattered showers and storms is expected to
move across the CWA during the afternoon. Guidance is unsure how
much instability will build through the day with models ranging
from no more than ~500 joules of MUCAPE to over 1,500. Models
hone in on the Chicago metro as having the highest instability
in any case being nearer to the cold core of the passing wave.
Understandably, this is also where most are resolving the
deeper convection. The signal appeared strong enough to warrant
introducing a swath of likely PoPs through the metro during the
afternoon. Weak cloud-layer shear should keep storms from
organizing all too well, but the possibility of higher CAPE
values keeps the potential for a stronger storm or two on the
table. Didn`t think the potential was quite high enough to
justify coordinating an outlook for severe weather in this
morning`s Day 2, but this is something that we`ll continue to
monitor closely. Instability looks to give out early in the
evening which should do away with the thunder potential by
around sundown, though a few showers may persist through the
evening.

Doom


Tuesday Night through Sunday:

The mid/upper-level ridge axis centered across the Rockies early
this week will build eastward across the Midwest into the western
Great Lakes for the second half of the week. As it does so, the
active belt of enhanced northwesterly flow aloft across the Great
Lakes region in the near term will get shunted east of our area.
This will set the stage for a few day period of dry and quieter
weather through the end of the week as surface high pressure sets
up shop over the Great Lakes region.

While the weather will be quieting down by mid-week, below
average temperatures will persist into Thursday before conditions
begin to moderate back above normal for the weekend. Therefore,
expect high temperatures for Wednesday and Thursday to only be in
the 60s to low 70s (coolest lakeside). Overnight low temperatures
also look to get downright chilly for the end of May, with
readings likely to drop into the mid 40s outside of urban areas.
Temperatures then will warm back to around 80 degree highs for the
upcoming weekend.

Unfortunately, with the warming temperatures, comes increasing
chances for showers and thunderstorms. It appears the area will
gradually transition back into a more active weather pattern
sometime this weekend into early next week. Accordingly, our
current forecast is carrying some low end chances (20-30%) for
showers and storms for the weekend. However, it is worth noting
that there is a considerable amount of ensemble spread with how
quickly this transition will occur. For this reason, the chances
currently highlighted in the forecast are likely to broad-brushed
and hence will need refinement as forecast certainty grows.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 630 AM CDT Mon May 27 2024

Main forecast messages/concerns:

- Deck of low MVFR stratus persists through mid morning, then
  improves.

- Another threat for late afternoon and evening thunderstorms.


A deck of low MVFR stratus (bases of 1,000 to 1,500 ft AGL)
will continue for a few hours this morning before gradual
diurnal improvement back to a scattered to broken VFR deck occurs
around midday. Thereafter, the main concern shifts towards
pinning down the most favorable timing for scattered wind
producing thunderstorms early this evening.

A well defined weather disturbance across the Dakotas early
this morning will track southeastward into our area by this
evening. This feature is expected to foster the development of
organized storm clusters capable of producing locally strong
winds gusts to our northwest this afternoon. These storms are
then likely to shift southeastward into northern IL by early
this evening. Confidence with the occurrence of these into this
evening continues to be on the high side. However, with the
coverage potentially remaining more widely scattered in nature
across our area, we have opted to continue with the going
PROB30 mention for the 12z TAFs. However, a couple hour tempo
group will need to be considered for later TAF amendments.

Otherwise, west-northwesterly winds will persist through the
period as surface low pressure tracks northeastward across the
Upper Great Lakes.

KJB

&&

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

&&

$$

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