635
FXUS63 KLOT 271734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 PM 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.

&&

.UPDATE...
Issued at 949 AM CDT Mon May 27 2024

The cloudy conditions over much of the area are slowly eroding
away with sunny skies already starting to crop up in Rockford
for a brief window before the next round of showers move in.
Currently, monitoring a cluster of showers and a few storms in
Central Iowa that are moving southeast toward Illinois in
addition to some weaker showers moving through southeast
Minnesota toward southern Wisconsin. No major changes to the
forecast in the chance for widely scattered showers and
thunderstorms across the forecast area.

However, with better moisture convergence and theta-e advection
south of the Quad Cities, cells that develop in Iowa would
likely be the focus for any potential severe threat this
afternoon/evening. Most hi-res guidance has been suggesting this
initial round in Iowa would be weak with renewed development
behind it and along the boundary descending southeastward. While
there has been a little decaying of cells along the leading
edge, the bulk of the system is simply maintaining itself a
little bit better than anticipated, with initial showers
potentially arriving as early as 3 PM in Lee and Ogle counties.
The chances for severe conditions to develop still look marginal
and mainly confined over the southwestern portion of the
forecast area. However, even sub-severe cells will have the
potential to still generate lightning, localized stronger wind
gusts as well as some small hail (less than an inch) in the
strongest cells.

DK

&&

.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 /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1234 PM CDT Mon May 27 2024

Forecast concerns for the terminals...

- Scattered showers and thunderstorms late this afternoon
  through early evening

- Period of low-end VFR to MVFR ceilings late tonight into
  Tuesday morning

- Another round of showers and thunderstorms Tuesday afternoon


The MVFR ceilings that have blanketed northern IL and northwest
IN this morning continue to erode and should be fully scattered
out over the next couple of hours. As skies scatter, the
atmosphere should quickly mix into stout winds aloft generating
20-25 kt wind gusts through this afternoon before gusts subside
late this evening.

At the same time, a potent upper-level low pressure system will
begin to pivot into northern IL which should allow for the
development of scattered showers and thunderstorms. However,
instability continues to look rather limited, especially with
eastward extent, which makes confidence on thunder coverage at
the Chicago terminals low. Despite the lower confidence have
still decided to maintain a TEMPO for thunder at the Chicago
terminals for a couple of hours this evening in case sufficient
instability is realized and storms do form overhead. Additionally,
there is still the potential for a few storms to produce
locally gusty winds in excess of 30 kts but the coverage of
these "severe" storms will likely remain along and west of FEP
to IKK line.

Showers and storms will come to an end this evening (by 05z at
the latest) leaving dry and mostly VFR conditions overnight.
Though, there continues to be a signal for some 2000 to 3000 ft
ceilings to develop after midnight and persist into Tuesday
morning. Heading into Tuesday, any MVFR ceilings that develop
are expected to lift through the morning leaving VFR conditions
for the rest of the forecast period. However, another upper-
level disturbance is forecast to pivot through the Great Lakes
Tuesday afternoon which will once again promote the chances for
scattered showers and thunderstorms. While instability does look
notably better on Tuesday than today, coverage is still
somewhat uncertain so have decided to handle with PROB30s at the
30-hour TAF sites for now. Given that the storm timing on
Tuesday is forecast to be after 18z I have not added formal
mentions to RFD, DPA, and GYY yet but will need to consider it
with future issuances.

Yack

&&

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

&&

$$

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