000
FXUS63 KLOT 152326
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 PM CDT Mon Apr 15 2024

.KEY MESSAGES...

- Periods of showers and storms Tuesday and Wednesday (90-100%
  at times) with a potential for some severe thunderstorms,
  especially Tuesday evening

- Gusty winds to 25 to 35+ mph Tuesday and Wednesday

- Another chance for showers (50-60%) and a slight chance for
  thunderstorms Thursday into Friday

- Drier, but cooler conditions expected over the weekend

&&

.DISCUSSION...
Issued at 217 PM CDT Mon Apr 15 2024

Through Wednesday:

A surface cyclone is beginning to spin up this afternoon on the
lee of the Rockies as an upper low tracks across the
Intermountain West. The low-mid level wind field will really
ramp up around the system tonight into tomorrow and we`ll find
the surface low deepening quickly as it works across the Plains.
The storm`s warm front will push northward into the CWA
tomorrow morning into the afternoon with instability building
atop the region behind the front. Models like the idea of an
initial push of showers and a few thunderstorms beginning as
early as the morning, but chances become notably better after
noon. It looks as though we should remained capped to surface-
based convection through most of the afternoon and this elevated
activity does not appear to pose a big severe threat. If
anything elevated does go severe with this initial wave, it will
likely be closer to mid afternoon when we`ll have more elevated
instability to work with. The primary threat with any earlier
severe convection would be large hail.

The severe potential will increase approaching the evening.
Continued warm and moist advection will erode the cap and allow
for surface-based storms to go up in our west as early as the
late afternoon. Roughly 1,000-1,500 J/kg of MUCAPE will have
built over the area by then. Perhaps the most eye-catching
component of this environment is an awfully impressive low level
shear profile featuring as much as 40 kt of veering 0-1 km
shear. This will support the potential for organized, severe
convection with any surface-based storms and carry with it a
threat for some tornadoes later in the day. High res guidance is
honing in on a cluster of thunderstorms forming ahead of a low-
mid level speed max and closing in on the western CWA early in
the evening. This may be our key feature to watch for severe
weather tomorrow. The tornado potential as well as the overall
severe threat will be greatest in the western CWA out near I-39
where we`ll find the better instability and kinematics.
Nonetheless, the threat will certainly remain present across the
CWA through the remainder of the evening.

The brunt of the severe potential will wrap up close to midnight
after this primary cluster of storms moves off to our east.
While we will remain in the storm`s warm sector tomorrow
night, a lack of forcing and a revitalized capping inversion
should put us in a lull overnight. There is a chance for a
second round of strong to marginally severe storms Wednesday
morning. The surface cold front will charge across the CWA
through the morning which could help parcels tap into the still-
present instability aloft. Guidance is inconsistent on resolving
precip ahead of this front, but can mostly agree that the
greatest chances of seeing additional storms Wednesday morning
will be east of I-55 where we`re seemingly most likely to break
through the stable air. A few scattered showers may then develop
around the center of the low as it passes to our north and fall
on parts of the area through the rest of the day, especially
north of I-80.

Meanwhile, there is quite a bit of spread in temperature trends
for tomorrow. It will be heavily reliant on how the warm front
behaves during the day and the magnitude of the warm advection
behind it. In the forecast, went with the favorite and most
consistent signal which is lower 70s in our north and upper 70s
south of I-80. A potent low level jet will move atop the area
during the day with upwards of 45 kt expected at 925 mb and
surface winds will respond accordingly. The better part of the
day should see gusts out of the southeast between 25 and 35 mph
with a few gusts to 40 mph expected. Winds may want to stay
rather gusty overnight tomorrow. While we`re expected to mostly
decouple and restabilize the boundary layer overnight, such a
tight vertical wind gradient means any even shallow mixing will
result in some gusty winds. Relatively deeper mixing on
Wednesday will cause strong winds to return during the day,
similar in magnitude to what we`re expecting for Tuesday.

Doom


Wednesday Night through Monday:

As the previous low drifts eastward, drier conditions will
slowly move in overnight into Thursday. Winds will become more
west-northwesterly providing a bit of cooler air advection
helping overnight temperatures back down into the 40s, with
Thursday afternoon high temperatures in the upper 50s and lower
60s.

As an upper level low deepens over Manitoba, an associated mid-
level trough will swing over the northern Plains and Great
Lakes pulling in additional moisture from the south with
stronger vorticity. Models are currently projecting stronger
instability to stay a little farther to the south, but kept the
slight chance for thunderstorms to develop for areas south of
I-88 as a line of showers moves through Thursday afternoon into
Friday.

The previously mentioned upper level low will gradually move
east toward Quebec and allow steering flow to become zonal
aloft. While subsidence aloft keeps the forecast dry through the
weekend, cold air advection will help drive temperatures
cooler. Most guidance is suggesting that lows could drop back
down into the 30s over the weekend. However, there is some
disagreement in high temperatures. The Euro has slightly lower
cloud cover so it suggests that highs could be back in the low
50s, while the GEFS and its greater cloud cover caps high
temperatures in the upper 40s. A lot will have to depend on how
quickly that low can move east and take some of the cloud cover
with it.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 611 PM CDT Mon Apr 15 2024

Aviation Forecast Concerns:

- SHRA/TSRA chances increase Tuesday morning after 14-15Z,
  though confidence is fairly low in timing/coverage.

- Higher confidence in occurrence/coverage of perhaps strong to
  severe TSRA late in the day/evening hours.

- East to east-southeast winds strengthen Tuesday, with gusts
  25-30 kt possible.

Quiet VFR aviation weather expected through early Tuesday
morning, as area remains under influence of surface high
pressure across the upper Midwest/western Great Lakes. Winds
will be light east-northeast this evening, though will
eventually start to gradually increase overnight.

Surface low pressure currently across the High Plains deepen
overnight in response to a strong upper level disturbance. The
low is forecast to track northeast toward KFSD on Tuesday, with
an east-southeastward extending warm front lifting into the
forecast area late in the day/evening. Increasing forcing and
moisture transport will likely develop scattered elevated
showers across northern IL Tuesday morning, and due to steep
mid-level lapse rates some scattered thunderstorms can`t be
ruled out. Coverage and to some extent timing are of somewhat
low confidence with some spread in model guidance, though
14Z-19Z window looks to encompass the main low-coverage TSRA
threat. A bit of a lull is then currently expected through mid-
afternoon, before thunderstorm chances ramp up late
afternoon/evening ahead of the approaching warm front. Currently
it looks like the best timing would be from 22Z on for KRFD,
and after 22Z-23Z for the Chicago terminals. TSRA threat looks
to persist for much of the evening, and some storms could be
strong to severe.

As noted above, east-southeast will increase Tuesday as the
surface low continues to deepen. Easterly flow of cooler/more
stable air off of Lake Michigan may limit mixing somewhat,
though guidance supports sustained winds 15-20 kts with gusts
approaching 30 kts by afternoon. Winds should gradually veer
more southeast Tuesday evening as the warm front approaches and
lifts across the area.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 PM Tuesday to 7 AM CDT Wednesday
     for the IL nearshore waters.

     Small Craft Advisory from 4 PM Tuesday to 4 AM CDT Wednesday
     for the IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago