National Weather Service Weather Forecast Office

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

000
FXUS63 KLOT 161751
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 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

&&

.UPDATE...
Issued at 933 AM CDT Tue Apr 16 2024

Now moving into the latter part of the morning, the occluding
surface low responsible for today`s severe potential is centered
over east- central Nebraska. The storm`s warm front is pushing
northward through central IL and will progress across the CWA
into the afternoon. While nothing is currently on the scope,
there is a slight chance that some isolated elevated storms
develop ahead of the front through early afternoon, especially
as the low level density gradient tightens up as the boundary
continues northward. A reservoir of around 1,000 J/Kg of MUCAPE
exists near and behind the front. If anything does manage to go
up ahead of the front, it will likely stay sub-severe but could
support some small hail given the elevated instability in place.

Meanwhile, a line of severe storms currently extends from far
eastern KS northward into southwestern IA. This line actually
resides in the storm`s open warm sector and is being driven by a low
level jet/vort stream. This looks to be our key feature to watch for
severe storms this evening. Efficient warm, moist upglide ahead of
the feature is supporting a curtain of elevated showers and non-
severe thunderstorms occupying central Iowa. This wave of showers
and storms will approach the I-39 corridor around mid-afternoon with
the main line trailing a couple to a few hours behind.

From here, the thinking in the forecast hasn`t changed much. We`re
still looking at late afternoon through the evening for the main
severe window. Severe hail remains the primary concern with any
elevated convection. It`s still uncertain how early storms will be
allowed to go surface-based which will introduce this evening`s
tornado threat and drive up the severe wind potential. Storm`s along
that aforementioned line will almost certainly be surface-based, but
the uncertainty lies in the antecedent activity. Most forecast
soundings suggest that the cap will either break this afternoon
ahead of this linear feature or will be so weak that ambient forcing
is likely to overcome it. The biggest concern for severe weather,
including for tornadoes in particular, remains in our west out near
the I-39 corridor before the convection outruns the higher
instability and better forcing aloft. Nonetheless, the all-hazards
severe potential will be very much present across the CWA this
evening.

Doom

&&

.DISCUSSION...
Issued at 330 AM CDT Tue Apr 16 2024

Through Wednesday:

A well pronounced warm conveyor belt downstream of deepening low
pressure over western NE this morning will continue to foster the
development and eastward movement of an arc of strong convection
across NE into KS early this morning. Thereafter, subsequent
east-northeastward movement into the Corn Belt is expected
through the day as the conveyor belt shifts eastward in advance of
the occluding surface low shifting towards the Mid-Missouri
Valley. It is this activity that will be our main potential
severe weather maker later this afternoon (after 4 pm) into the
evening.

All eyes will certainly be on the evolution of this arc of
convection to our west through the day. However, prior to it`s
arrival, it remains plausible that some widely scattered elevated
storms may develop later this morning across parts of northern IL
to the north of the northward shifting warm front. This activity
is expected to be largely non-severe, though steepening mid- level
lapse rates could support an instance or two of hail with any
stronger and longer lived updrafts. Otherwise, east- southeasterly
surface winds will be ramping-up through the day in response to
strong pressure falls in advance of the occluding surface low over
the Mid-Missouri Valley. This will thus result in a rather windy
afternoon across the area, with the strongest wind gusts (perhaps
at times up to 45 mph) likely to the south of the surface warm
front (areas generally along and south of I-80). These strong
gusty winds will preceded the late day severe convection.

The primary timeline for our severe storm potential will be after
4 PM west of the Fox Valley and Upper Illinois basin, then likely
after 7 PM into eastern IL and northwestern IN. However, the
severe weather threat may tend to wane with eastward extent across
IL through the evening due to the potential for this initial line
of storms to begin to outrun the better instability.
Nevertheless, strong low-level and deep-layer shear will be
concerning for the potential for organized storms, which will be
capable of producing strong winds and some tornadoes.

Perhaps the most concerning area for significant severe weather
later today, including significant hail and tornadoes, appears to
be west of the I-39 corridor, particularly across IA,
northeastern MO and far northwest IL. Redeveloping late day storms
are plausible in this area in the wake of the first line of
storms. Assuming this second area of storms does fire over IA,
shear profiles there will support supercells with an attendant
very large hail and significant tornado risk. Fortunately, it
appears this second area of convection will begin to diurnally
weaken as it approaches the western parts of the NWS LOT forecast
this evening on the heals of the 41initial arc of storms.
However, we will have to keep a close eye on the eastward extent
of these storms prior to weakening.

We will remain in the storm`s warm sector tonight, though a lack
of forcing and a revitalized capping inversion does look to put us
in a general convective lull for a period overnight. While this is
the case, there still is a chance for another round of strong to
severe storms late tonight (after 4 am) through Wednesday
morning. This in response to the approach of the surface cold
front, which will move across the area Wednesday morning. At this
time, the threat for these severe early morning storms remains
largely conditional. The greatest chance of these storms into
Wednesday morning is looking to largely be east of I-55.

In the wake of the cold front, expect a windy afternoon on
Wednesday with westerly winds gusting 40-45 mph.

KJB


Wednesday Night through Monday:

The blustery west wind regime in cold air advection (CAA) in
the wake of Wednesday afternoon`s cold front passage will
persist during the early evening hours. Expect gusts up to 35
mph during this time. Winds will then quickly ease the rest of
the night as a weak surface high pressure shifts overhead. Low
temperatures will be in the mid to upper 40s, still above normal
for mid April, but much cooler than tonight.

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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1251 PM CDT Tue Apr 16 2024

Key Aviation Impacts for the current TAF period:

- Showers and thunderstorms expected at area terminals this
  evening, some of which may become severe

- Gusty southeasterly winds expected through most of the TAF
  period

- MVFR cigs expected overnight

- Another round of storms possible Wednesday morning, but
  confidence remains lower in development and may focus east of
  the terminals

Thunderstorms have already developed over Missouri and Iowa
ahead of the next low pressure system moving north and east
impacting the airspace west of terminals. As the low moves east
through the afternoon, the threat of storms to move over
terminals increases. Winds ahead of the storms will be out of
the southeast with gusts around 30 knots. Thunderstorms could
impact area terminals as early as 22Z, but latest guidance as
the strongest band of storms moving in around and after 00Z.
These storms will have strong wind gusts and frequent
lightning. There is also the potential for these storms to
become severe with the development of one inch hail and/or a
brief tornado near the terminals; though the potential for
tornadoes is better to west (closer to KRFD). While models have
trended earlier with initial arrival, confidence for the
strongest cells maintained the TEMPO from 00Z-04Z.

There is the potential for a dry slot to develop overnight
giving a brief respite from rain, but most guidance was keeping
some scattered activity through 15Z tomorrow, so kept -SHRA
through Wednesday morning. However, as the center of the low
moves into Wisconsin, there is a better chance for MVFR cigs to
remain through the overnight period. Then as daybreak
approaches, there will be enough instability present for the
potential for another round of thunderstorms to develop around
area terminals Wednesday morning, and only adjustments to timing
were made to the previously introduced PROB30 group.

Showers will eventually taper off and move east on Wednesday
morning as winds slowly rotate to the southwest. Gusts are
expected to remain strong up to 30 knots Wednesday morning.
Lower confidence on the exact timing, but VFR cigs are expected
to return Wednesday morning. Winds gusts will slowly diminish by
late afternoon and become more westerly.

DK

&&

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

     Small Craft Advisory until 11 PM CDT Wednesday for the IN
     nearshore waters.

&&

$$

Visit us at weather.gov/chicago