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000
FXUS63 KLOT 261143
AFDLOT

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

.KEY MESSAGES...

- Rain moves into the area late this morning and afternoon and
  continues into tonight. Isolated to scattered thunderstorms
  may be seen during this time as well, particularly this
  evening.

- The weekend looks warm and breezy with high temperatures in
  the upper 70s to lower 80s and overnight lows in the low to
  mid 60s.

- Multiple rounds of showers and thunderstorms over the weekend
  through Monday, some of which could be severe late Saturday and
  again Sunday night.

- After a brief break, shower/storm potential returns toward the
  middle of next week

&&

.DISCUSSION...
Issued at 412 AM CDT Fri Apr 26 2024

Through Saturday:

GOES infrared satellite imagery has quite the colorful look to it
early this morning across the central Plains with two groups of
widespread deep convection evident. One of these is a large
conglomerate of thunderstorms over eastern Nebraska and western
Iowa, while the other is a newly blossoming swath of isentropically-
induced convection over western Missouri. This second thunderstorm
cluster is of most relevance to our weather forecast for today as,
with time, these storms will propagate towards and eventually into
our forecast area later this morning and afternoon.

As this cluster approaches the area, it is likely to outpace the
instability reservoir that it currently resides in and, hence,
should arrive in a weakened state, with most locales in northern
Illinois and northwest likely to just see a plain rain today as a
result. The best chances for any lightning in our area through this
afternoon will thus likely remain confined to the southwestern half
of our CWA. The eastern periphery of the main instability plume will
eventually creep into the area later tonight and lead to a slightly
better chance for thunderstorms area-wide, though exact storm
coverage will hinge largely upon how this afternoon and evening`s
severe thunderstorms in the central/lower Missouri River Valley
evolve and whether they manage to survive their northeastward trek
across Iowa/Missouri into Illinois. Suppose that there is an outside
chance of a strong to marginally severe thunderstorm sneaking into
our western counties late this evening -- especially if the cold
pools from the upstream convection manage to consolidate and yield a
more organized forward-propagating storm cluster. However, in all
likelihood, the upstream thunderstorms should weaken below severe
limits by the time they reach our forecast area as they encounter
an increasingly stabilized boundary layer with eastward extent.

The 992-994 mb surface low passing to our northwest will be culpable
for a tight surface pressure gradient across the region that will
keep breezy southerly winds in place today through Saturday.
Depending on how deeply we are able to mix Saturday morning, a brief
period of 40-45 mph gusts may be observed for a few hours before the
overhead low-level jet weakens, resulting in gusts diminishing to
closer to 30 mph Saturday afternoon. These winds will also serve to
advect in an unseasonably warm and moist air mass with temperatures
and dew points likely to be on the rise tonight into Saturday
morning. Despite a good deal of cloud cover likely being around on
Saturday, warm air advection on its own will push temperatures into
mid to upper 70s, and with the expectation that there will still be
some sunshine to be seen on Saturday, highs will likely end up near
80 degrees across our forecast area. When paired with dew points
climbing into the 60s, Saturday will certainly have a summer-like
feel to it.

With this relatively muggy air mass in place on Saturday, there will
also be a summer-esque potential for isolated showers and storms to
develop in the afternoon. The general thinking is that height rises
and subsidence on the backside of the departing upper-level trough,
relatively nebulous large scale forcing for ascent, and the presence
of a capping inversion should favor most or all of the area
remaining dry through most of the daytime hours once any lingering
morning showers depart. However, by the late afternoon, forecast
soundings show that the cap should have weakened or eroded entirely,
and isolated attempts at convection may start to see some success
wherever warm air advection and/or confluence in the low-level
flow is maximized. Thus, have kept slight chance PoPs going
during the afternoon to account for this possibility, with the
highest probabilities currently advertised in our southeastern
counties, where the convective signal is greatest in the 00Z/06Z
CAM suite. With the heart of the aforementioned instability
plume expected to be positioned over the area by Saturday
afternoon, MLCAPE values may get as high as 2500-3000 J/kg
during peak heating, which would be more than enough to support
the development of thunderstorms that could grow deep enough to
carry some threat of strong to severe downburst winds and/or
hail. However, with lingering uncertainties about shear profile
quality and the aforementioned dearth of forcing for ascent,
whether an isolated threat for severe weather will materialize
Saturday afternoon remains in question.

Ogorek


Saturday Night through Thursday:

By Saturday evening the lead short-wave trough and attendant
surface low will continue to weaken while lifting northeast into
Ontario. Showers and storms may persist along the trailing cold
front near and north of the WI/IL stateline into Saturday
evening. Meanwhile a secondary and more amplified upper level
trough will eject off the Rockies into the PLains with an
associated deepening surface low setting the stage for
significant severe weather potential near the NE/KS/IA/MO
stateline region. This activity is expected to lift northeast
toward the area by Saturday evening/overnight. With a warm and
moist airmass still in place into the evening (near record warm
low temperatures in the low-mid 60s!), a cluster or line of
strong to severe storms may manage to persist into portions of
the area, currently favoring areas north of a Peru to McHenry,
IL line.

The details become less clear heading into Sunday with the
evolution of the broader system will be dependent on what
transpires Saturday and Saturday night, though there is a signal
that much of the area could be dry for much of the daytime
hours on Sunday with temperatures warming back into the upper
70s to around 80. Showers and storms are anticipated to develop
well to the west across portions of MO/IA which then push east
Sunday night across the area. A strong to potentially severe
storm cannot be ruled out with this activity though it will
likely be on a weakening trend with eastward extent. Showers may
linger into Monday as the system lifts out of the region and
the trailing cold front gradually moves through.

Southwest flow quickly returns in the wake of the cold frontal
passage with highs still in the mid-upper 70s Tuesday and
Wednesday. Several mid-level disturbances moves within the
broader quasi-zonal upper-jet which could bring additional
rounds of showers/storms toward the middle of next week with
temperatures turning more seasonable by the end of the week.

Petr

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 643 AM CDT Fri Apr 26 2024

Key aviation messages:

- Multiple waves of SHRA this afternoon through tonight
- <20% chance of TS this afternoon
- 30-40% chance of TS this evening/overnight

An band of showers, currently across IA and northeast MO will
spread across the area by this afternoon. While it remains
electrically active early this morning, the expectation is for
the lightning coverage to decrease as it approaches the
terminals and accordingly have left TS out of the TAFs for the
afternoon. Nevertheless, a few embedded lightning strikes
cannot be ruled out with this initial round, and a short-fused
addition of VCTS may become warranted. Additional waves of
showers likely continue into the evening and overnight hours
with associated MVFR ceilings. There remains a signal that TS
may accompany these showers this evening into the overnight
hours with no changes made to the PROB30 groups (~2-8Z).

Winds steadily increase this morning with SE gusts to 25kt
expected by late morning. Winds remain gusty through the
evening and overnight hours, gradually veering SSE then SSW
into early Saturday morning. Wind gusts to 30kt become
more frequent by mid morning Saturday out of the SSW.

Petr

&&

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

&&

$$

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