780
FXUS63 KLOT 221534
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Lincoln IL
1034 AM CDT Tue Apr 22 2025

.KEY MESSAGES...

- Periodic bouts of scattered showers/storms expected through
  Thursday with many dry hours and some areas missing out on
  rain altogether.

- Well above normal temperatures expected inland through week`s
  end, then turning more seasonable for the weekend.

- The stormy and warmer pattern will resume early next week.

&&

.UPDATE...
Issued at 1034 AM CDT Tue Apr 22 2025

Updated PoPs earlier this morning based on radar trends. Recent
trends have shown a noticeable decline in reflectivity over
northern IL and expect this to continue through the rest of the
morning, with generally isolated showers or perhaps a
thunderstorm for noon through mid afternoon. Expect activity to
redevelop in the late afternoon period in the plume of higher
theta-e air pushing northeastward into the area. The SPC has
continued with a Marginal Risk for the NW half of the forecast
area for today where around 1000 J/kg of surface-based CAPE and
20-30 kts of deep shear exist. While the late afternoon and
evening storms could be marginally severe, the threat for severe
storms diminishes later tonight. Still...thunderstorm chances
continue through the night with a boundary lingering in the area
and some weak upper support.

Knutsvig

&&

.DISCUSSION...
Issued at 320 AM CDT Tue Apr 22 2025

Through Wednesday:

Water vapor imagery early this morning shows a vigorous
shortwave trough moving east from the northern High Plains
toward Lake Superior. Scattered showers and storms across
southern Minnesota into Iowa are associated with the southern
extent of this trough. Air mass over our CWA and downstream of
this convection is drier and more stable, so anticipate a
weakening trend as these showers/storms move east into northern
Illinois. How fast this activity weakens and dissipates isn`t
clear, so carry slight chance pops into our eastern CWA, but
higher chances for rain should remain farther west closer to
the Mississippi River.

This afternoon through Wednesday afternoon, forecast confidence
with respect to rain chances is on the lower end of the
spectrum. Our CWA looks to remain on the southern edge of a belt
of brisk, zonal westerly mid-upper level flow. This westerly
flow will advect steep mid level lapse rates associated with an
EML east into the area, aiding in destabilization needed for
convection. At the sfc, an east-west boundary is expected to be
in the area, the exact position potentially augmented by
convection. Aloft, there`s pretty good agreement in the guidance
that low amplitude, subtle waves will traverse the zonal flow
across the region, likely leading to occasional bouts of
increased coverage of scattered showers and t-storms anytime
from this afternoon through Wednesday afternoon.

Unfortunately, the subtle nature of these waves, leads to a
great deal of uncertainty with regards to timing and their
ability to kick off convection given only modest frontal
convergence. There`s a pretty solid signal in CAMs that
convection will develop in the area this afternoon, though it is
possible that the boundary layer could end up drier than progged
and lead to some capping issues that prevent convection from
developing or limits coverage. Tonight, there`s another pretty
noteworthy QPF signal in guidance over northern IL. No
appreciable low level jet to drive storms tonight, so again,
uncertainty is a bit higher than normal, so kept pops in check
tonight too.

Deep layer shear would likely support storm scale updraft
separation and potential for some strong to severe storms given
sufficient instability. Given all of the uncertainties
(shortwave timing, boundary layer moisture and subsequent
destabilization, possible capping, etc) the level 1 of 5 severe
weather risk from SPC is quite reasonable.

Synoptically, guidance suggests there could be enough of a
southwesterly gradient to keep lake breeze at bay over all but
perhaps the IL north shore. Assuming the upstream convection
over IA dissipates as expected and doesn`t alter sfc pressure
pattern significantly remnant cold pool/meso high, then highs
today should be able to climb upper 60s to mid 70s, though
cooler north shore. If IA convection persists longer than
expected, it raises the possibility of a more prominent lake
breeze than is currently forecast. Wednesday should be warmer
with highs in the 70s across most of the CWA, except near Lake
Michigan where lake breeze is expected to keep temps noticeably
cooler.

- Izzi


Wednesday Night through Monday:

The frontal boundary that will serve as the focus for showers
and thunderstorms in the near term is expected to be shifting
northward out of northern IL Wednesday night. However, some
lingering showers and perhaps even an isolated storm or two will
likely be ongoing as the boundary exits. That said, Thursday is
looking to be a mostly dry day as plume of mid-level dry air
should pivot overhead and keep precipitation to our north in WI.
Continued warm advection across the area will allow temperatures
to warm well into the 70s with locations south of I-80 likely
to see readings near 80 degrees Thursday afternoon. The exception,
however, will be those along the lakeshore where temperatures
are forecast to be in the mid to upper 50s due to a lake breeze.

While we enjoy a reprieve from the active weather, our next
system will be getting organized in the southern Plains. The
system will be driven by a shortwave trough that will be
ejecting into the south central Plains on Thursday before
lifting into the Mississippi and Ohio River Valleys on Friday.
As this occurs, a broad area of showers and thunderstorms is
expected to develop in the humid airmass beneath the wave and
gradually work its way northeast into MO Thursday night and then
across our area on Friday. Guidance has generally remained in
good agreement with this evolution over the past couple days,
but there remains some uncertainty as to whether or not a weak
surface low will develop along the aforementioned frontal
boundary that is forecast to still be stretching from southern
WI to eastern NE. If this low does develop as the ECMWF and
Canadian guidance suggest, then the showers and storms in our
area on Friday would be more widespread as opposed to the more
scattered coverage noted in the GFS solution. In the grand
scheme of things the forecast is more or less the same for a
50-80% chance of rain, but whether or not Friday is a washout or
there are some dry hours still needs to be ironed out.

Regardless of which solution comes to fruition for Friday, the
shortwave and associated precipitation will gradually move east
Friday night as the front finally gets forced through the area
by a northern stream shortwave pivoting over the northern Great
Lakes. Therefore, dry conditions are expected behind the system
for Saturday and Sunday. However, modest cold advection will
keep temperatures in line with more typical late April values
with highs in the 60s to around 70 (inland of the lake) and lows
in the 40s to around 50.

Heading into next week, the upper pattern is forecast to become
more amplified once again which will return the chances for
showers and thunderstorms through at least the middle of next
week. On the bright side, temperatures during this time are
forecast to return to their above normal readings with highs
back near 80 for Monday and Tuesday.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 638 AM CDT Tue Apr 22 2025

Forecast concerns for the terminals...

- Line of decaying showers to move through northwest IL and RFD
  this morning.

- Periods of scattered showers and thunderstorms this afternoon
  through tonight.

- Breezy southwest winds this afternoon gusting 20-25 kt.


A line of showers continues to make steady progress towards
northwest IL this morning, but has been showing signs of
gradual weakening. That said, showers are expected to reach RFD
by 1230z and persist for a few hours this morning before
dissipating prior to reaching the Chicago terminals. While
lightning trends have been decreasing as the showers enter the
more stable air in IL, a stray strike or two may be seen near
RFD prior to 13z.

Additional showers and thunderstorms are expected to develop
this afternoon and persist off and on through the night as
several weak disturbances pivot overhead. While confidence in
showers and storms occurring is relatively high, the scattered
nature of the showers/storms makes confidence in the timing and
coverage at the terminals somewhat low. Therefore, have decided
to maintain PROB30s for the afternoon thunder potential for now
but will continue to monitor trends for further refinement. The
second round of showers and thunderstorms expected late tonight
into Wednesday morning has slightly higher confidence from a
rain coverage perspective but the limited instability makes the
thunder coverage still uncertain. Thus have maintained the
PROB30s for this period as well.

Outside of the shower and storms, winds will start off light out
of the south-southeast this morning before turning southwesterly
around midday. As winds turn southwest they will also increase
in speed with gusts of 20-25 kt expected this afternoon before
easing this evening. While winds will remain mostly out of the
west-southwest tonight, they will become southeasterly again
Wednesday morning. Finally, VFR conditions are generally
expected to prevail through the TAF period but there is a chance
for a period of MVFR ceilings with the showers/storms tonight.

Yack

&&

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

&&

$$

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