Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300535

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 AM CDT Thu Mar 30 2023

Issued at 312 PM CDT Wed Mar 29 2023

Through Thursday night...

The main forecast concern for the short term period is the threat
of elevated thunderstorms late Thursday night which may produce
hail and possibly some gusty winds.

A surface high continues to build into the area this afternoon
which is continuing to clear out the earlier cloud cover. The
clear skies have allowed the atmosphere to tap into some stronger
winds aloft which is causing gusts in the 15 to 20 mph range to be
observed. Winds will gradually ease through this evening as they
turn easterly and then southeasterly overnight. However, the
lingering cold advection in combination with the clear skies
overnight will allow temperatures to dip into the low to mid-20s
with our colder spots possibly seeing upper teens.

The aforementioned surface high will begin to move out of the
area Thursday afternoon as our next storm system begins to take
shape across the central CONUS. As the system approaches a warm
front is expected to lift northward which will allow temperatures
to rebound and top out in the low to mid-50s areawide. Winds will
also be increasing Thursday afternoon as the pressure gradient
tightens which is expected to hold temperatures in the 50s
Thursday night. While the warm advection looks to stabilize the
lower atmosphere during this period, the presence of a strong low-
level jet just above the surface should allow for some 30 to 35
mph gusts to be realized late Thursday night into early Friday

Additionally, guidance is continuing to develop showers and
thunderstorms along the Mississippi River Thursday evening in the
warm advection wing. These showers and storms are then progged to
traverse across northeastern Illinois and northwest Indiana after
midnight Thursday night. Forecast soundings do show steep mid-
level lapse rates in excess of 7 C/km in addition to some elevated
instability which should allow storms to maintain themselves as
they move through. Furthermore, the presence of the aforementioned
strong low-level jet and strong upper-level flow looks to
generate 50+ kts of bulk shear overhead Thursday night which may
allow for a few storms to become strong to possibly severe.
However, guidance has been showing that some of the storms may
outrun the better instability which may limit their severe
potential. Regardless, any more robust cores may be capable of
producing at least some hail and possibly some gusty winds if
downdrafts can overcome the stable surface layer.

Given that there is some uncertainty on if all these parameters
will overlap, we agree with the Storm Prediction Center`s (SPC)
decision to hold off on introducing a marginal (level 1 of 5) risk
for severe storms Thursday night. But we will have to closely
monitor trends to see if that addition is needed with future



Issued at 320 PM CDT Wed Mar 29 2023

Friday through Wednesday...

An impressive dynamic set-up will be underway across the region
on Friday as an increasingly negative tilt trough shifts into the
mid-Mississippi River Valley ahead of a strong 160kt upper jet
streak. A multi-hazard severe weather threat remains in place west
of and into the forecast area late Friday afternoon into the mid-

However, the potential for a widespread significant severe
weather outbreak in our area continues to be limited to the
availability of moisture and associated instability profiles.
Favorable low-level theta-e profiles will quickly narrow into
northern Illinois as a low-level occlusion accelerates eastward.
That said, the impressive kinematic environment highlighted by the
nose of a deep-layer jet streak and rapidly dynamic cooling of
the remaining low/mid-level EML just ahead of the front will
compensate for any deficiencies in instability.

Latest expectations of the convective evolution continue to focus
on a strongly forced line of storms quickly crossing the forecast
area from west to east late afternoon into early/mid evening.
Conditional on sufficient instability with eastward extent, the
very strong low-level wind profiles will support severe winds,
possibly significant, with the line as it moves through the area.
Given increasingly favorable separation of deep shear vectors with
the occlusion wrapping NE, then N, into northern IL, concerns for
a broken line with more discrete characteristics and an attendant
higher tornado risk exists into the western CWA.

After Friday evening, focus then turns toward strong non-
convective winds through Saturday and perhaps a quick shot of
light snow Saturday morning. A strengthening surface high
impinging on the back side of the exiting strong low combined with
strong CAA mixing into 50kt+ low-flow will support a non-
convective strong wind threat through the area Friday night into
Saturday morning, with the greatest risk into the southern CWA.
Wind Advisory conditions will likely be met for a large portion of
the CWA, with the potential for conditions nearing or meeting
High Wind Warning criteria in our south.

Dynamic cooling on the backside of a deformation band behind the
exiting mid-level low will likely bring a period of light precip
across the area, but especially for northern IL into southern WI,
Saturday morning. Thermal profiles support wet snow mixed with
rain, though impacts should be minimal with surface temps
remaining above freezing. Still, the combination of blustery
conditions with falling precip will lead to a raw start to the

Skies should clear late Saturday ahead of sunny conditions and
increasing WAA on Sunday. A west to east low-level baroclinic zone
will support a west to east axis of rain across the area late
Monday into Monday night. Additional attention will then turn to
another active system with strong storms late Tuesday into



For the 06Z TAFs...

The primary aviation weather concerns through the 06Z TAF period are
as follows:

* Rain showers Thursday night into early Friday morning with an
  accompanied thunderstorm potential

* LLWS setting in late Thursday night

* Gusty SSE winds Thursday evening turning SSW overnight

Conditions will be quiet through the night with VFR conditions and
light and variable winds. Winds will establish a SE direction as
they pick up to near or over 10 kt by mid-morning. Winds will become
near southerly in the evening, likely predominantly SSE, and build
to feature near 25 to 30 kt gusts. Winds are then expected to veer
to SSW overnight. Additionally, a strong low level jet moving
overhead will provide LLWS near FL020 to around 55 kt also from the
SSW. Such wind conditions, both at the surface and aloft, will
persist through the end of the period.

Near 06Z Thursday night, a swath of scattered showers is expected to
progress into the area from the west. These showers will bring with
them MVFR cigs and likely intermittent vsby reductions to MVFR. A
few embedded thunderstorms will also be possible, especially as we
get closer to Friday morning.





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