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000
FXUS63 KLOT 210700
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
200 AM CDT Sun Apr 21 2024

.KEY MESSAGES...

- Freeze for areas west/southwest of Chicago this morning with
  frost likely elsewhere outside of Chicago.

- Areas of frost likely late tonight away from Chicago.

- Elevated fire danger Monday.

- A period of inclement weather with periods of showers and
  storms likely towards the end of next week into the weekend.

&&

.DISCUSSION...
Issued at 200 AM CDT Sun Apr 21 2024

Through Monday:

Clouds are slowly clearing from northwest to southeast early
this morning. Where clouds have cleared across northwest IL,
temps have dropped into the upper 20s to around 30 with light
winds. The clouds are still expected to clear out across
northwest IN by daybreak but the slower this process takes, the
less time there will be for temps to drop and frost to form but
not planning any changes to the current headlines, which expire
at 13Z. Temps will steadily increase through the morning with
highs in the upper 50s/around 60 for most locations today, with
mostly sunny skies. A lake breeze will move into northwest IN
and possible the far southern Cook County shore, keeping temps
cooler for those areas.

Clear skies and light winds tonight should allow for a favorable
frost potential and maintained areas of frost outside of
Chicago. There is some weak warm air advection overnight and
much of guidance keeps temps generally in the mid 30s, with the
usual cool spots dipping into the lower 30s. Thus, confidence is
only medium for how much frost will form tonight. Given the
current headlines, opted to hold off on any potential headlines
for tonight.

Monday will be warmer with highs back into the mid 60s, perhaps
a few locations tagging upper 60s. Southwest winds will increase
and gust into the 25-30 mph range, especially northwest of I-57.
Dewpoints may remain in the upper 20s/lower 30s with minimum
relative humidity levels in the mid/upper 20 percent range,
resulting in an elevated fire danger Monday. cms

Monday Night through Saturday:

An initial wave of primarily warm advection-driven showers will
develop on Monday night into Tuesday morning. Instability
continues to look pretty limited, so we`ll continue to withhold
thunder chances from the forecast during this period although
can`t entirely rule out a lightning strike or two. During the day
on Tuesday, while some timing and placement differences remain, it
looks like the greatest shower chances towards midday and early-
afternoon may shift south and east of us as the main ribbon of
low-level warm advection develops into central Illinois and
Indiana. Once this period gets in range of more of the extended
hires guidance, we may be able to refine and chop PoPs a bit
during this time period.

Ahead of the main cold front, which will still be located to our
north Tuesday afternoon, temperatures look to surge into the mid
and upper 60s--possibly near 70 in spots--in the wake of morning
showers and cloud cover. Model guidance is coming into better
agreement now depicting a region of rapidly-steepening low-level
(0-3 km) lapse rates towards dry adiabatic, particularly north and
west of I-55. At the same time, mid and upper-level temperatures
will be cooling as the parent upper vort max/trough axis presses
into northern Illinois. This combination looks like it will
support some degree of destabilization with a few hundred J/kg of
SBCAPE materializing as mid-level lapse rates steepen in response
to cooling 500 mb temperatures. ELs nearing -20 C will support the
chance for some electrification, and continue to advertise
isolated thunder wording in the gridded forecast during the
afternoon as a result, with the expectation that low-topped
convection will materialize ahead of the incoming cold front.
While there`s variability regarding EL heights (which will
modulate effective bulk shear values), there may be enough shear
within the convective cloud layer to support some more organized
updrafts and a threat for hail, as well as gusty winds given the
deeply-mixed nature of the boundary layer.

A strong cold front will arrive through Tuesday evening, shutting
off precipitation chances except for possibly some lingering
drizzle near the lake where enhanced convergence and deeper
moisture (up through 800 mb) may hang on for a time. Cold
advection will push a notably cooler airmass back into the region,
with some near-freezing overnight lows possible across interior
portions of northern Illinois. This doesn`t look like much of a
frost threat given the anticipated winds, but something we`ll
monitor given the threat for some sub-freezing temps.

Wednesday will feature highs which will struggle to make much
headway through the 40s near the lake, with low to mid 50s
farther inland. The threat for frost will build Wednesday
night/Thursday morning as winds ease.

Thereafter, guidance is in general agreement of another active
stretch of weather in the vicinity as a series of disturbances
pinwheel across the central CONUS towards the end of the week and
through next weekend. While significant timing and placement
differences remain, a handful of model solutions suggest we`ll
need to keep a close eye on a threat for severe weather.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1208 AM CDT Sun Apr 21 2024

The only aviation weather concerns are:

- Timing the cessation of VFR cigs
- Potential lake breeze at GYY this afternoon

VFR cigs will gradually clear all of the Chicago-area terminals
tonight with W-NW winds prevailing with intermittent gusts to
15-20 knots developing into the afternoon. A lake breeze looks
like it may push through GYY, although confidence remains a bit
too low to introduce a NE wind shift at this point. If this were
to occur, main timing looks to be around 20z.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Freeze Warning until 8 AM CDT this morning for ILZ003-ILZ004-
     ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-ILZ021-
     ILZ032.

     Frost Advisory until 8 AM CDT this morning for ILZ006-ILZ013-
     ILZ023-ILZ033-ILZ039-ILZ103-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Frost Advisory until 8 AM CDT /9 AM EDT/ this morning for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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