National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS63 KLOT 162336
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Tue Apr 16 2024

.KEY MESSAGES...

- Severe thunderstorm threat late this afternoon and evening,
  greatest threat area west of I-39.

- Windy this afternoon, and again Wednesday with peak gusts 40+
  mph.

- Next round of rain/showers on Thursday-Thursday evening.

- Much cooler over the weekend with frost/freeze concerns,
  particularly on Saturday night.

&&

.DISCUSSION...
Issued at 241 PM CDT Tue Apr 16 2024

Through Wednesday:

This afternoon, we find the surface low pressure centered over
eastern NE and its warm front extending eastward across the
heart of the CWA. A long line of showers and thunderstorms
extends from east-central IA down through eastern MO and is just
beginning to move across the Mississippi into portions of
western IL. This line will be the first feature to watch for
severe weather this evening. A second are of showers and storms
is located further the west from central IA through north-
central MO.

Precip with the first band will make it into our far west near
the I-39 corridor around mid-afternoon. The activity along the
first band is expected to gradually dampen on its way to our CWA
this afternoon as it outruns the better instability. In fact,
it looks like this line may already be outrunning the
instability quicker than guidance suggests. Regardless, severe
hail is the primary concern with the first band of convection
early on. If storms go surface-based along this line though, if
not sooner, the scope broadens to an all-hazards potential
including a noteworthy concern for a few to several tornadoes.
The tornado threat stems from an impressive low level shear
profile with as much as 40 kt of veering 0-1 km shear expected
this evening resulting in upwards of 300+ m2s-2 of 0-1 km SRH.

That aforementioned secondary band presents a second
opportunity for severe storms tonight for parts of the CWA. This
second push looks to move across the area between mid-evening
through the early overnight hours. This second round will have
more instability aloft to work with, as much as about 1,500
J/kg. This will keep the severe potential alive along this line
during the evening, though the tornado potential may be lower
with the lesser low level instability (e.g. increasing static
stability) and slightly veered surface winds behind the first
line.

While most of the overnight should be quiet, one last chance for
severe storms exists early tomorrow morning. The storm`s cold
front will be working across the area through the morning and a
line of storms may attempt to go up ahead of the boundary.
There`s still plenty of uncertainty on when precip will get
going ahead of the front and if it will even wait until later in
the morning when it`s east of our area. However, the signal for
precip to develop over the CWA has been on a big upward trend
over the past day. If storm`s do fire up, a brief tornado or two
will again be possible with the low level shear still cause for
concern, even if instability has come down quite a bit. We
could see storms develop as early as the predawn hours east of
I-39. Chances build with eastward extent with the seemingly best
potential for storms tomorrow morning being over northwest
Indiana.

Doom

Wednesday Night through Tuesday:

Quiet, precipitation free conditions will be short-lived as our
next rain producing system arrives Thursday morning. An
unseasonably deep/cold upper low over south central Canada will
tighten the baroclinic zone/thermal gradient across the region.
Modest large scale forcing but fairly robust progged low-mid
level frontogenetical circulation will likely bring an area of
rain/showers shifting eastward across the area through the
afternoon, evening, and early overnight period. Chance for any
embedded thunderstorms appears minimal and confined to south of
the Kankakee River. Highs on Thursday will be in the upper 50s
to 60s inland, and cooler near the lake.

Initial surge of dry and cold advection on Friday will result in
a partly cloudy, breezy and dry, seasonably cool day with highs
in the mid to upper 50s. There has been appreciable run to run
variance regarding the magnitude of the colder air mass aloft
arriving this weekend. In the most recent model cycle (00z
4/16), the pendulum swung back to colder, particularly in the
foreign (ECMWF and CMC) suites, which indicate 850 mb temps
plunging to near -10C Saturday morning.

If the colder solutions pan out, breezy CAA driven lows may
flirt with the freezing mark across parts of interior northern
Illinois Saturday morning, with mid 30s to around 40F elsewhere.
Saturday`s highs are currently forecast to reach the lower to
mid 50s, but the colder side of the spectrum points toward
downside potential being 40s for many locales. High pressure
building overhead amidst forecast sub-freezing dew points
Saturday night looks synoptically favorable for elevated
frost/freeze concerns outside of Chicago if skies clear out
enough. Following Sunday`s chilly start, afternoon temps should
recover some vs. Saturday but still likely below normal. Near
normal temps (lower to perhaps mid 60s highs) will then return
to start next work week, with the next precip. chances probably
holding off until just beyond the current day 7 (Monday).

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 636 PM CDT Tue Apr 16 2024

Forecast concerns include...

Thunderstorms this evening.
Chance of thunderstorms Wednesday morning.
Gusty southeast winds this evening.
Gusty west/southwest winds Wednesday.

Scattered thunderstorms are likely to continue to develop across
northern IL, roughly along a line from RFD/C09/PNT and move
across the terminals over the next few hours. Additional storms
crossing from eastern IA into western IL may also move across
the terminals later this evening, though likely in a weakening
phase. Given the uncertainty for thunder coverage, timing and
duration, opted to continue current tempo mention from 00z-04z
with vicinity through 06z. Refinement to this forecast may be
needed as trends emerge. The overnight hours are expected to be
generally dry and then there will be another chance for
thunderstorms Wednesday morning as a cold front moves across
the area. Its possible this activity may develop just east of
the terminals but prob mention seems on track for now. There may
also be some isolated showers Wednesday afternoon.

Southeast winds this evening, may gust as high as 30kts.
Directions are likely to turn more southerly this evening, but
confidence is low for specific wind directions, which may turn
back southeasterly overnight. Winds will shift to the southwest
behind the cold front Wednesday morning and more westerly
Wednesday afternoon with gusts into the lower 30kt range
possible.

Mvfr cigs will be possible with thunderstorms this evening and
then prevailing mvfr cigs are possible overnight into Wednesday
morning, but confidence is very low for prevailing mvfr cigs and
trends will need to be monitored. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 11 PM CDT Wednesday for the IL and
     IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago