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000
FXUS63 KLOT 111632
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1132 AM CDT Thu Apr 11 2024

.KEY MESSAGES...

- Rain will continue through early afternoon generally along-
  east of I-57.

- Showers develop near/west of I-39 this afternoon and expand
  across the area into this evening and overnight. A few
  thunderstorms will also be possible (30% chance) mainly
  along/near I-39.

- Strong northwest winds, gusting over 40 mph at times, are
  expected late tonight into Friday afternoon.

- Unseasonably mild weather this weekend into early next week.

&&

.UPDATE...
Issued at 1130 AM CDT Thu Apr 11 2024

Overall the forecast remains on track with the morning update.
Only changes made were to clean up PoP timing trends through
this evening and nudge up the thunderstorm chances to 30% along
and near the I-39 corridor.

Rain continues across areas generally along/east of I-57 and
across Chicago late this morning. The back edge of this rain
will gradually shift east with time through early afternoon.
Meanwhile a narrow ribbon of showers has since developed late
this morning currently extending from the Keweenaw peninsula of
Michigan south to the Quad Cities. This axis will gradually
shift east and expand in coverage southward this afternoon
focused initially along the I-39 corridor, likely enhanced
initially by the first of a pair of mid-level vorts in eastern
Iowa as it moves toward the area. The band of showers and
embedded thunderstorms will then shift east with time into early
evening with thunderstorm potential diminishing as showers
develop areawide overnight. The trailing wave, currently in
southeast South Dakota will support showers persisting through
early Friday morning.

Also worth mentioning that we will be monitoring closely for
the potential for funnel clouds along this narrow band of
showers this afternoon into early evening. If the surface
convergence and low-level instability axis aligns a bit better
than currently expected these funnels may be able to briefly
reach the ground.

Petr

&&

.DISCUSSION...
Issued at 327 AM CDT Thu Apr 11 2024

Through Friday:

From a meteorological perspective, some rather interesting
weather on tap over the next 36 hours.

Deformation band of the southern stream shortwave continues to
produce rain over about the SE half of our CWA early this
morning, with some pockets of moderate to rather heavy rain
over portions of northwest Indiana. MRMS estimates show max
rainfall amounts approaching an inch over portions of Benton and
southern Jasper Counties. Rainfall amounts taper off rather
quickly to the northwest with just a trace to perhaps a
hundredth of an inch near the I-55 corridor as defo band battles
low level dry air and subsidence on the cool side of the low-mid
level f-gen circulation.

Generally expect pretty close to status quo through at least the
early to mid morning hours with the heaviest rainfall confined
to northwest Indiana with rain likely struggling to make much
headway much farther northwest than roughly the I-55 corridor
into IL. The low-mid level f-gen circulation should weaken and
shift eastward late this morning into this afternoon likely
taking the steadier rain with it out of our CWA. Max rainfall
totals from Porter south-southwest to Benton County still look
likely to end up in the 1-1.5" range with isolated 2" totals
possible. This will likely be enough to result in some renewed
rises on mainly the Iroquois and Kankakee Rivers, with perhaps
some ponding of water or minor (areal flood advisory type)
flooding.

Meanwhile, upstream over the northern Plains lurks a compact,
but vigorous shortwave with water vapor imagery showing a well
defined closed mid-level circulation. As this shortwave plows
east-southeast toward the area with strong (~100m 500mb) height
falls, most guidance depicts a strengthening north-south
inverted trough developing on the backside of the southern
stream cyclone. Strong synoptic ascent combined with
strengthening low level convergence in the vicinity of the
inverted trough should result in showers developing this
afternoon and likely persisting into tonight.

If mid-high cloudiness associated with the southern stream wave
clears out a bit, it would allow for weak destabilization to
take place in the vicinity of this sfc trough. HREF max SBCAPE
values are in the 500-750mb range, but this is certainly going
to hinge on some clearing and heating in advance of the cold air
aloft associated with the shortwave arriving. Generally,
equilibrium levels in forecast soundings look too low to support
lightning, but close enough to warrant inclusion of a slight
chance of thunderstorms in the forecast late this afternoon into
early this evening. Given the strong synoptic forcing, would
anticipate this shower activity to last well into the evening
and perhaps into the overnight hours tonight, especially south
of I-80 as the shortwave digs southeastward. Showers should end
by around or just after sunrise Friday morning over far east and
southeast CWA.

The developing inverted trough today will likely result in a
weaker/baggier pressure gradient than earlier model runs had
depicted. It will likely still get breezy over eastern CWA
today, but winds should be lighter in and near the sfc trough
which should be over our western CWA this afternoon. As the
upper trough continues east and eventually phases with the
southern stream wave, look for the inverted trough to washout
tonight. As the trough dissipates, pressure gradient should
tighten up with strong enough cold air advection tonight to keep
the boundary layer somewhat mixed allowing winds to ramp up
tonight. Strongest winds are expected Friday morning into early
Friday afternoon when gusts over 40 mph are certainly possible.
Current gridded forecast has winds flirting with wind advisory
criteria and will be something later shift will need to take
another look at and possibly consider.

- Izzi


Friday Night through Wednesday:

Friday night, the upper trough responsible for today`s storm
system will exit the region to the east. Rapid height rises
behind the trough will lead to efficient warm advection aloft
heading into Saturday. As a result, high temperatures are
progged to reach the upper 60s and lower 70s to kick off what
will be a beautiful spring weekend. Surface high pressure
between today`s storm system and the next will also lead to
plenty of sunshine.

A low level wave will track a broad, meager center of surface
low pressure from the northern Plains into the upper Midwest
on Saturday into Sunday. A few models are resolving isolated
pockets of QPF ahead of the weak cold front/low level trough
axis on Saturday night. Precip chances appear rather low at this
point with low level stable air in place and little forcing to
overcome it aside from modest low level upglide in the warm
advection. However, this low level warm advection will cause an
impressive EML to develop atop the region this weekend. If
parcels can manage to punch through the cap and tap into those
steep mid level lapse rates, any shower that goes up will have
the potential of building into a thunderstorm. In the forecast,
stuck with equal slight chances for rain and thunder during the
night.

Models continue to trend warmer for Sunday. 850 mb temps now
forecast near 15C and likely more sunny skies should promote
highs tapping 80, or at least close to it, for the first time
this year across much of the area. A lake breeze may attempt to
push inland and keep lakeside locales on the cooler side.
However, breezy offshore winds may limit the westward extent of
the cooler air. Monday will see similar highs in the upper 70s
to possibly lower 80s. This time though, easterly winds may keep
communities right along the IL lakeshore in the 60s with lower
70s a bit more downwind.

A fast moving upper low looks to dig across the central Plains
Monday into Tuesday and spin up a sizable center of surface low
pressure out to our west. We may see a couple of showers pop up
Monday afternoon and evening, but we may struggle given a lack
of both forcing and especially moisture. Some models suggest the
storm`s warm front may set up over the CWA on Monday which
would promote a bump in precip chances if we can see some
appreciable moisture convergence in the otherwise dry
environment. Rain chances build through the night and into
Tuesday as the mature cyclone closes in on the Midwest. While
there`s still a lot to be resolved with this system, medium
range probabilistic guidance is beginning to highlight Tuesday
as a day to watch for the potential for severe weather in the
region.

Doom

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 648 AM CDT Thu Apr 11 2024

The primary aviation weather concerns for the 12Z TAF period
include:

* Rain showers in the area through this morning

* Additional showers expected this evening and overnight

* Periods of MVFR cigs possible

* Gusty NNE winds today going NNW this evening. Continued
  gustiness on Friday.

A system of rain showers continues to just clip the Chicago
sites with light rain this morning. Rain will hang out in the
vicinity through about mid-late morning before it begins to
retreat off to the east. Conditions should remain VFR through
the morning with the exception of GYY where MVFR cigs may
develop for a few hour period later this morning.

The area will be rain-free for just about the entire afternoon.
A second push of rain is expected to develop out near RFD
during the late afternoon and spread into the metro in the
evening. Scattered showers should then persist across all sites
through the night, possibly stretching into early Friday
morning. The signal for MVFR cigs is stronger late tonight
carrying through much of Friday morning.

Meanwhile, NE winds will build through the morning with gusts to
20-25 kt expected for the better part of the day while GYY may
see gusts push 30 kt. Direction will back to NNE in the
afternoon before flopping back to NW in the evening. A period of
lighter gusts can be expected late this afternoon and evening
in between the two bouts of rain. Winds will build again through
the night achieving gusts to 30- 35+kt during the morning on
Friday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 AM CDT Friday for Winthrop Harbor
     IL to Gary IN.

     Gale Watch from late tonight through Friday afternoon for
     Winthrop Harbor IL to Gary IN.

     Small Craft Advisory until 10 PM CDT Friday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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