Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 142352
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
652 PM CDT Thu Mar 14 2024

.KEY MESSAGES...

- A severe hailstorm or two may develop south of a line from
  Pontiac, IL to Rensselaer, IN this afternoon between 3 and 7
 PM CDT.

- Temperatures will fall from north to south this afternoon as a
  cold front races through the area. Overnight lows will fall
  into the low 30s to low 40s, coldest near the Wisconsin state
  line.

- Mild temperatures but breezy winds in excess of 30 mph on Saturday

- Colder shot of air late Sunday through Tuesday morning with
  the chance for periods of scattered snow showers.

&&

.DISCUSSION...
Issued at 139 PM CDT Thu Mar 14 2024

Through Friday:

A recent hand surface analysis augmented by 1-minute GOES-satellite
imagery depicts a complex multi-centered low pressure system lifting
across northern Illinois and an outflow-reinforced warm front
stretching from near Quincy to Lincoln. Meanwhile, a cold front
stretches from near the Wisconsin state line to Fort Madison, Iowa. A
broad region of low-level confluence exists between both fronts
across central Illinois. Over the next 3-6 hours, the cold front
will surge south and merge with the aforementioned warm front,
allowing for thunderstorms to rapidly develop and mature across
central Illinois. In fact, recent 1-minute GOES-16 imagery depicts
the first signs of the initiation of convection in the confluence
zone to the west of Lincoln, so we are likely not far away from
thunderstorms erupting.

The northern terminus of the convective threat this afternoon
will be sharp and near our southern forecast area, say, no
further north than a line from Pontiac, IL to Rensselaer, IN.
And, with the southward progression of the front, the time
window during which thunderstorms may impact our area appears
limited, from about 3 to 7 PM CDT. With that said, some 1500
J/kg of MUCAPE largely centered in the hail growth zone, veered
low- level flow leading to relative weak low-level storm
relative inflow, and strong mid- to deep-layer shear (re:
passing 100kt 250mb jet streak) all point toward a favored storm
mode of supercells with a threat for large to even giant hail
(greater than 2 inches in diameter). Again, the threat for
storms this afternoon will be south of a line from Pontiac, IL
to Rensselaer, IN between 3 and 7 pm CDT (and the most intense
storms may very well stay just south of our forecast area).

Elsewhere across our area, the afternoon will be marked by an abrupt
northerly wind shift and falling temperatures as the cold front
races south. In fact, temperatures by sunset should range from the
upper 30s along the Lake Michigan shoreline to the mid 40s along
Interstate 88. Continued robust cold air advection, bolstered by
increasingly breezy northwest winds gusting 30-35 mph, will
cause overnight lows to bottom out in the lower 30s near the
Wisconsin state line to lower 40s near US-14.

Tomorrow looks quiet with clearing skies, gradually slackening winds,
and highs in the lower 50s. The exception will be near the Lake
Michigan shoreline where onshore flow will keep temperatures in the
lower 40s (especially in northwestern Indiana).

Borchardt


Saturday Morning through Thursday:

A longer wave trough grows on Saturday and drops farther south as it
phases early next week. While an area of stronger vorticity will
pass over the region Saturday afternoon, profiles look too dry to
squeeze out any precipitation. However behind the surface cold
front, a strengthening pressure gradient will increase winds
out of the north through the day. Wind gusts are expected to be
between 25 to 35 mph, though isolated locally higher values
cannot be ruled out. Winds ahead of the front are expected out
of the southwest which will help advect warmer air to allow high
temperatures Saturday be in the mid to upper 50s (low 60s south
of I- 80). Behind the cold front will usher in colder air,
dropping overnight temperatures Sunday morning to below
freezing in northwest IL, though the temperatures will remain
just above freezing closer to the lake.

Models are showing better consistency in projecting a short wave to
drop south on Sunday as the broader long wave trough deepens.
With strong vorticity associated with the wave, decent lapse
rates, and sub freezing wet bulbs there is a chance for
convective snow showers late Sunday into Monday morning. The
main inhibiting factor will be the lack of robust moisture.
Monday morning low temperatures are expected in the 20s (wind
chills between 10-20), with highs Monday afternoon struggling
to reach the upper 30s.

There is still some uncertainty with the chance for precip on
Tuesday. While the Euro and GFS models show the longer wave
aloft weakening and moving east, the GFS has another very quick,
weak wave dropping down on Tuesday. It appears that the NBM is
picking up on that perturbation which keeps low PoPs in the
forecast for another chance for light snow accums closer to the
lake. However, the Euro does not have that have as of yet, so
there is still some uncertainty which lowers confidence.

Behind Tuesday, weak ridging aloft will likely drive surface higher
pressure to help bring temperatures back to or just above seasonal
average. Both the Euro and GFS ensembles are suggesting another long
wave trough develops in Eastern Canada, but current projections have
it farther to the east in Quebec than what is expected this weekend.
So if that were to hold, northwest flow will remain over the region,
with a stronger ridge growing to the west keeping it dry through the
end of the next work week.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 652 PM CDT Thu Mar 14 2024

Forecast concerns for the terminals...

- MVFR and IFR ceilings to persist into early Friday morning,
  especially for terminals near Lake Michigan

- Breezy northeast winds this evening gradually subsiding
  overnight

- Weak lake breeze to pass through GYY, MDW, and ORD late Friday
  afternoon

An upper-level wave continues to pivot through the area this
evening in the wake of a cold front that moved through earlier.
While most precipitation has come to an end at the terminals, a
band of light rain and drizzle is being observed over ORD and is
expected to drift southeast into Northwest Indiana within the
next 1 to 2 hours. Once this band of precipitation clears the
terminals dry conditions are expected for the remainder of the
forecast period.

However, the presence of low-level moisture and the
aforementioned wave will continue to promote IFR and low-end
MVFR ceilings through tonight and into early Friday morning.
Though, the lowest ceilings are expected to reside closer to
Lake Michigan where some patchy fog will also be possible
through this evening. Ceilings are expected to scatter out
Friday afternoon and prevail through the remainder of the
period.

Finally, breezy northeast winds will persist through the rest of
the evening as cold advection continues to allow the atmosphere
to tap into a strong jet core aloft. Winds will gradually wane
after midnight as the jet exits the area with speeds generally
in the 10 to 12 kt range. Directions are expected to remain
generally out of the north-northeast tonight and Friday, but
there is a developing signal in guidance that winds could wiggle
just west of north (350 to 360 degrees) early Friday afternoon.
Given uncertainty in the longevity of this wind shift have
decided to maintain a 360 mention in the TAFs for now.

Additionally, guidance is also hinting at a weak (around 6 to 7
kt) lake breeze developing late Friday afternoon (after 21z)
and surging through GYY, MDW, and ORD turning winds easterly in
its wake. While guidance is in good agreement right now, we know
how difficult these lake breezes can be especially with more
modest flow and temperatures like we are forecasting for Friday.
Therefore, don`t be surprised if timing shifts by an hour or
two with future forecasts.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 7 PM CDT this evening for the IL
     nearshore waters.

     Small Craft Advisory until 7 PM CDT Friday for the IL
     nearshore waters.

     Small Craft Advisory until 10 PM CDT Friday for the IN
     nearshore waters.

&&

$$

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