000
FXUS63 KLOT 270006
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
706 PM CDT Fri Apr 26 2024

.KEY MESSAGES...

- Showers and a few thunderstorms will move over the area late
  this evening into the overnight hours.

- Saturday looks summer-like with highs in the upper 70s to
  lower 80s, increasing humidity levels, and eventually partly
  cloudy skies.

- Showers and thunderstorms will develop toward and after sunset
  Saturday night and continue through Sunday morning. A threat
  for flash flooding will exist where thunderstorms train.

- Periods of showers and storms will continue through midweek,
  a few of which could be strong to possibly severe on Sunday.

&&

.DISCUSSION...
Issued at 248 PM CDT Fri Apr 26 2024

Through Saturday Night:

Regional water vapor and visible satellite imagery, surface
observations, and radar data depict a surface low pressure
system centered along the Kansas and Nebraska borders underneath
an upper level shortwave trough lifting northeastward into the
Upper Mississippi River Valley. Bands of showers and a few
thunderstorms are evident east of the center of the low,
including severe convection across the Plains and warm-air
advection driven showers along and east of the Mississippi
River. A diffuse surface warm front is draped across southern
Illinois, and delineates summer-like moisture to the south and
more seasonable (and wet) conditions to the north.

Over the next 18 hours or so, waves of showers will parade over
northern Illinois and northwestern Indiana as broad warm air
advection continues atop the northward-surging warm front. While
coverage of thunderstorms will initially be isolated this
afternoon and evening, gradually increasing instability (by
virtual of low- level moistening and steepening mid-level lapse
rates) will support embedded thunderstorms particularly after
sunset and especially midnight. While the threat for severe
weather remains low, a localized threat for pea to nickel size
hail or gusty winds exists across much of the area through the
overnight hours, particularly if any upscale growth of any
cluster originating out of Missouri occurs overnight. The SPC
Level 1/5 threat level for the overnight hours covers the
potential well.

After daybreak, the warm front is expected to stall across
central Wisconsin as the low-level jet wanes. At the same time,
a secondary upper-level trough moving into the Southern Plains
will induce gradual mid-level height rises across the Great
Lakes. The net result should be the erosion of any remaining
shower or thunderstorm after daybreak affording a dry period
from mid-morning to mid- afternoon. As low-level stratus thins
from mid-morning onward, a "pop" of southwesterly winds gusting
40-45 mph may occur before noon as mixing builds into the waning
low-level jet. Thereafter, southwesterly gusts of 30-35 mph
will continue through the afternoon. With 850mb temperatures
climbing to +13 to +14C and prospects for even filtered
sunshine, temperatures tomorrow are poised to shoot into the
upper 70s to even lower 80s. When combined with a humid airmass
(surface dew points in the low to mid 60s and PWATs nearing
1.5"), it will feel more like late June than late April by
tomorrow afternoon!

Tomorrow afternoon, neutral height tendencies and boundary
layer heating may prove sufficient to erode capping. As a
result, isolated showers and thunderstorms may develop after 2
or 3 PM, though confidence in anything developing so early is
pretty low (equating to a 20% chance for thunderstorms).
Tomorrow evening, coverage of showers and storms will increase
markedly first across Missouri and Iowa, and later across
northern Illinois, as the next upper-level shortwave pivots
northeastward and supports the redevelopment of an expansive
low-level jet. With a steady feed of low-level moisture and
convergence along the nose of the low-level jet overnight,
showers and storms may continue more or less continuously
through daybreak Sunday along a southwest to northeast, or even
west to east, oriented axis.

While the pattern doesn`t conceptually match classic flash-
flood set- ups (as the upper-level jet will be displaced too far
to our north and west, and cells may be more broken than within
a continuous, training line), the ability for the low-level jet
to continuously feed replenishing instability and moisture into
a relatively confined band of thunderstorms over a prolonged
period of time does raise concern for a localized threat for
flash flooding mainly in urban areas. At this point, CAM
guidance (including an experimental FV3 core extended CAM
ensemble) appears to favor the axis aligning near the Illinois
and Wisconsin state line which is reflected in our forecast.
However, we would be remiss to ignore the deterministic HRRR,
RAP, and experimental RRFS, which all favor the thunderstorms
aligning somewhere near I-80 or I-88. For now, felt the course
of least regret was to collaborate a southeastward expansion of
the D2 WPC Flash Flooding Threat level 1/5 area to encompass
much of northern Illinois.

Not to be forgotten, overnight lows Saturday night will be
incredibly (record?) warm and in the low to mid 60s.

Borchardt

Sunday through Friday:

The secondary upper trough, currently over the Great Basin, is
expected to be traversing the central Plains into the upper
Midwest as a closed upper low on Sunday. This trajectory will
place the Mississippi River Valley and western Great Lakes in
the diffluent region of the trough and therefore continue to
support periods of showers and thunderstorms through Sunday
night. However, there is a notable signal amongst the mid-range
forecast guidance that a modest cap may develop Sunday morning
which could limit (if not inhibit) shower and storm development
until better forcing and destabilization arrives Sunday evening
into the overnight hours as the aforementioned trough is closest
and a low-level jet forms overhead. While it continues to look
as if the better forcing and subsequent instability should
remain near and west of I-39, 30-35 kts of effective shear are
forecast to be in place which could allow a few storms to become
better organized and be capable of producing severe weather,
primarily during the late afternoon and evening hours Sunday.
Therefore, a marginal (level 1 of 5) severe weather risk remains
for the western half of our forecast area.

As the aforementioned upper low lifts into the upper Midwest
Sunday night into Monday, a cold front is expected to move
through northeast IL and northwest IN bringing yet another
period of showers and thunderstorms. Though, the instability on
Monday does look to be notably weaker than on Sunday which
should keep the threat for any strong to severe storms to a
minimum. While temperatures are expected to cool a bit in the
wake of the front, high temperatures are forecast to remain in
the mid to upper 70s through the middle of next week.

A brief break in the active weather is expected for Monday
night and Tuesday as the upper-level pattern transitions into a
more zonal (east-west oriented) pattern. However, additional
shortwave troughs are forecast to traverse through the pattern
Tuesday night through Thursday which looks to return the threat
for periodic showers and thunderstorms. It does look as if high
pressure may try to establish over the central CONUS towards the
end of the week and into next weekend with more seasonable
temperatures as well.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 705 PM CDT Fri Apr 26 2024

Key Aviation Messages:
- Isolated to scattered elevated TS late this evening and early
  overnight.
- SW wind gusts over 30 knots on Saturday.
- Periods of MVFR ceilings through the period.

SE winds gusting to 20 knots early this evening will increase
to around 25 knots late this evening, then veer SSW overnight.
An area of SHRA with embedded TS across eastern Missouri early
this evening is expected to lift northeast toward the Chicago
terminals late this evening into the overnight hours. Steepening
lapse rates combined with renewed forcing ahead of trough over
western Missouri should support at least isolated to scattered
TS at ORD/MDW in the 04-08Z window. Have therefore included
TEMPO TS for this period.

Beyond roughly 08-09Z, dry conditions are expected to prevail
through the daytime hours Saturday. Winds will veer SW after
sunrise, with gusts quickly increasing to over 30 knots by mid-
morning. MVFR ceilings in the morning may lift into lower end
VFR levels or become SCT in the afternoon.

Additional isolated SHRA are expected to develop Saturday
evening, so have included VCSH at this time.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT Saturday for Winthrop
     Harbor to Northerly Is. IL.

     Small Craft Advisory until 10 PM CDT Saturday for Northerly
     Is. IL to Michigan City IN.

&&

$$

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