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000
FXUS63 KLOT 160855
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
355 AM 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 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. While this
is the case, the most concerning area for significant severe
weather including significant hail and tornadoes appears to be
west of the I-39 corridor, especially across IA, northeastern MO
and far northwest IL. Late day redeveloping storms appears
plausible, and assuming they develop shear profiles will support
supercells with an attendant very large hail and significant
tornado risk. Fortunately, it appears this second area of
convection will diurnally weaken as it approaches the NWS LOT
forecast area after dark, though we will have to keep a close eye
on its eastward extent.

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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 100 AM CDT Tue Apr 16 2024

Main Concerns:

- A few rounds of SHRA and TS, highest confidence in the PM when
  strong to severe TS remain possible

- Gusty east to southeast winds today and tonight

- Lower CIGs possible overnight into early Wednesday

A large and strong system will affect the Midwest through
Wednesday morning. Confidence remains on the lower side for the
potential first round of scattered SHRA and TSRA in the late
morning through early afternoon. That said, forecast soundings
suggest that if convection gets going, it will likely be capable
of producing lightning. Converted the VCSH over to VCTS for this
and maintained the PROB30 TS groups with this issuance.

Confidence continues to be high in SHRA/TSRA moving across the
terminals late this afternoon through this evening, with only
small timing tweaks vs. the previous issuance. The next possible
round after the evening activity exits east will arrive from the
west early Wednesday morning.

Steady east winds will increase and become gusty by the mid to
late morning. East-southeast to southeast wind gusts peaking in
the 30-35 kt range appear probable this afternoon through this
evening outside of TS. Finally, most of the guidance indicates
the development of lower CIGs overnight, so indicated prevailing
lower MVFR in the ORD and MDW TAFs and hinted at IFR potential
with scattered IFR cloud mention.

Castro

&&

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

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

&&

$$

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