000
FXUS63 KLOT 110827
AFDLOT

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

.KEY MESSAGES...

- Moderate to locally heavy rainfall expected over portions of
  northwest Indiana through this morning.

- Showers, possibly with some gusty winds and a slight chance of
  a t-storm, will develop western CWA early-mid afternoon, then
  spread southeast late this afternoon into tonight.

- 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.

&&

.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 /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1245 AM CDT Thu Apr 11 2024

The primary aviation weather concerns for the 06Z TAF period
include:

* Rain showers in the area through the whole of the period

* Periods of MVFR cigs possible

* Gusty NNE winds on Thursday going NNW in the evening

A large system of rain is closing in on the Chicago metro from
the south. Through the night and into Thursday afternoon, the
western periphery of this body of rain will hang out somewhere
in the vicinity of the Chicagoland terminals. At least light
rain is expected at all Chicagoland sites at times through the
morning. Like with the rain, it`s difficult to gauge the western
extent of any MVFR cigs or vsbys. While GYY will likely go MVFR
tonight, there`s more support for the Chicago sites to remain
VFR through the morning. RFD is not expected to see any rain and
will remain VFR through the morning.

There is a growing signal for a few to several hour break from
the rain in the afternoon over the Chicago metro. A second
push of rain is expected to begin near RFD during the afternoon
and move into the metro in the evening. Scattered showers should
then persist across all sites into the night. The signal for
MVFR cigs is stronger with this evening push, although no MVFR
has been introduced at the Chicago sites for now.

Meanwhile, NE winds under 10 kt tonight 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 early in the
evening. Winds will stay gusty during the evening and overnight.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 10 AM this morning to 7 PM CDT
     Friday for Winthrop Harbor to Wilmette Harbor IL.

     Small Craft Advisory from 7 AM this morning to 7 PM CDT Friday
     for Wilmette Harbor IL to Michigan City IN.

&&

$$

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