000
FXUS63 KLOT 181940
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
240 PM CDT Thu Apr 18 2024

.KEY MESSAGES...

- Showers will spread across the area this afternoon and last
  through around midnight.

- Thunderstorms will spread from central Illinois toward or
  just north of US-24 this evening (between 4 and 10 PM) with a
  threat for damaging winds

- Much cooler temperatures are expected over the weekend with a
  threat for frost or freezes particularly on Saturday night.

- The next opportunity for rain will arrive Monday night into
  Tuesday.

&&

.MESOSCALE DISCUSSION...
Issued at 233 PM CDT Thu Apr 18 2024

A cluster of thunderstorms recently developed in Hancock, Mcdonough,
and Adams county, Illinois, and is rapidly lifting northeastward at
about 50 mph. Radar data from KILX indicate a developing RIJ, and a
wind gust of 40 mph was reported along the southern end of the
cluster in spite of being located well north of the warm front
located further to the south in southern Illinois. If the cluster
holds together, it will move into Livingston, Ford, and Iroquois
counties in approximately 2 to 3 hours. However, with the cluster so
far removed from the instability axis and present much earlier
previously advertised in model guidance, confidence in longevity
is lower than average.

&&

.DISCUSSION...
Issued at 147 PM CDT Thu Apr 18 2024

Through Friday:

Regional radar, water vapor, and satellite imagery augmented by a
hand surface analysis depicts a surface low pressure system centered
in central Missouri along a warm frontal boundary extending eastward
across far southern Illinois and Indiana and ahead of a strong cold
front arcing from central Wisconsin through eastern Oklahoma.
The arrival of a DCVA ahead of a subtle upper-level shortwave
within upper-level southwesterly flow atop the cold front and
surface low has recently caused thunderstorms to erupt across
Missouri.

Over the next 12 hours, the surface low will lift east-northeastward
across central Illinois and eventually northern Indiana as the
cold front surges southeastward into northwestern Illinois. As
the surface low and cold front approach in the next few hours,
the local surface pressure gradient across northeastern Illinois
will "kink" and allow for a lake breeze to surge inland and
merge with the cold front. The net result will be a stark
temperature gradient across the region by late afternoon, with
readings in the mid 50s expected near the Wisconsin state line
and Lake Michigan shoreline to the mid to upper 70s along US-24.

In addition, increasing linear low-level convergence along
combining frontal boundaries as well as ahead of the
approaching surface low will lead to increasing coverage of
thunderstorms across northeastern Missouri and central Illinois
over the next few hours. In fact, the southern edge of a 500mb
jet streak across the central Great Lakes will afford some
45-55kt of convective-layer shear by early evening, allowing
for thunderstorms to quickly evolve into clusters or just one
continuous squall line as they lift northeastward. Even as
thunderstorms lift into relatively cooler and more stable air, a
dry low-level airmass ("onion" sounding) and tight packing of
the surface pressure gradient along the immediate northwestern
edge of the surface low will allow for convective severe surface
wind production with the most organized clusters. With that said,
the northern extent of any eventual thunderstorm clusters
remains challenging to pinpoint, and will depend on the eventual
track of the surface low down to the county scale. Based on how
convection is already evolving across northern Missouri, our
gut feel is the northern edge of any eventual cluster may be
near or just north of US-24, between 4 PM and 10 PM this
evening. However, a reasonable worst case scenario is the
convection lifts as far north as I-80, particularly once it
reaches northwestern Indiana. Northwest of the most active
thunderstorms, persistent showers and a rouge lightning strike
or two will prevail this evening within increasingly breezy
north to northwesterly winds (gust of 20-30 mph).

Tonight, showers will end from west to east as the surface low pulls
away. A quick shot of low-level cold air advection will keep
northwesterly winds breezy and lead to surface temperatures dropping
to the mid to upper 30s (northwest) to lower 40s (southeast). Breezy
winds should curtail the threat for frost development tonight at
least on a widespread basis.

Tomorrow, a 1028mb+ surface high pressure system currently centered
along the northwestern US and southwestern Canadian border will
expand southeastward into the Great Lakes. The tightening of the
surface pressure gradient will cause westerly winds to become breezy
with gusts to 30 mph during the afternoon hours, especially north of
I-88. Finally, an initially sunny morning should turn partly cloudy
by mid-afternoon as a pocket of seasonably cool low-level air
overspreads the warm ground leading to shallow instability. No
precipitation is expected tomorrow.

Borchardt

Friday Night through Thursday:

The upcoming weekend`s weather will be mostly uneventful aside from
much cooler conditions than we`ve seen recently. For Saturday
morning, continued cold advection behind the departing storm
system from today will allow for morning lows to drop into the
30s area-wide with lower to middle 30s likely north of I-80 and
outside of the city. Such areas may wake up to some patchy
instances of frost. Winds staying up a bit through the night,
around 10 to 15 mph, will likely inhibit the development of
widespread frost. During the day, expect highs in the 50s and a
cool breeze out of the northwest. A large surface high
approaching from the west will keep conditions calm and clear
Saturday night. This makes Saturday night/Sunday morning look
more favorable for widespread frost and perhaps freezing
temperatures, especially north of I-80. The high will scoot to
our southwest on Sunday and pump a little bit of milder air into
the region pulling highs into the upper 50s to near 60 degrees.

This weekend, a fast-moving, low amplitude trough will move onshore
in the Pacific Northwest and zoom across the northern Plains and
into the upper Midwest early next week. The surface response will be
a clipper system that is progged to bring us our next solid chance
for rain and thunderstorms following today late Monday and Tuesday.
A big favorite among medium range deterministic and ensemble
guidance is for the low to pass to our north across Wisconsin and
the cold front to cut across northern Illinois. The deterministic
Euro and a handful of ensemble members track it a bit farther south
across northern portions of our CWA. Regardless, models are not
excited about a severe potential with this storm at the moment as
they show a pretty meager thermodynamic environment, but it`s one
that could get a few thunderstorms going. Models are hinting that
general thunder probs may be maximized on Tuesday around the storm`s
triple point near the low pressure center, which again is favored to
pass just to our north. But of course, that is very susceptible to
change over the next several days. Ridging behind the storm system
looks to keep Wednesday quiet before an active pattern may be
kicking off in the region late next week.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1236 PM CDT Thu Apr 18 2024

Key Aviation Impacts for the current TAF period:

- Showers and VFR clouds through mid afternoon at area
  terminals

- A line of storms moves in this evening with lower cigs and
  vis, gustier northeast winds, and steadier rain rates.
  Isolated lightning is possible, but confidence was too low to
  add to the TAFs

- VFR and westerly winds after midnight

The first round of rain is moving over northern Illinois and
Northwest Indiana at the time this discussion was posted. Cigs
have remained above 6000 feet and all the lightning remains
closer to St. Louis. There may be breaks in the rain through
22Z, so switched the TEMPO to prevailing -SHRA with the rain in
eastern Iowa moving west.

By 22Z, a cold front will through the area. This front will
provide an opportunity for steadier rain rates, gustier
northeasterly winds and cigs/vis down to MVFR levels. There is a
chance for IFR conditions to develop but confidence was not
there to add it to the TAF currently. As the system transpires,
if stronger cells develop, amends will be needed to reduce
conditions. Lastly, there will be instability present such that
isolated lightning strikes are possible as the front moves
through. Strong consideration was given to adding thunder to
KGYY`s TAF, but kept it out for now. The stronger confidence in
thunder remains south of area terminals where stronger
instability will be located in Central Illinois. But if there is
more consistent strikes developing, it will be added to TAFs
tactically.

Recent model runs have sped up the front so that the strongest
cells might be east of terminals by 03Z. -SHRA was added for the
chance for lingering precipitation behind the front.
Additionally, as the front passes, winds will slowly switch from
the northeast to the north west and weaken. Clouds will drift
east. There is a chance clouds between 2000 to 3000 feet linger
a through midnight, but there was moderate confidence in skies
clearing out so prevailed VFR through the end of the TAF.
Quieter conditions are expected on Friday with stronger wind
gusts around 25 knots in the afternoon.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 10 AM to 7 PM CDT Friday for the IL
     nearshore waters.

     Small Craft Advisory from 4 AM to 7 PM CDT Friday for the IN
     nearshore waters.

&&

$$

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