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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 210508
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1208 AM CDT Sun Apr 21 2024

.KEY MESSAGES...

- Freeze expected for areas west/southwest of Chicago late
  tonight/early Sunday morning. Frost likely elsewhere outside
  of Chicago.

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

&&

.UPDATE...
Issued at 920 PM CDT Sat Apr 20 2024

Have elected to hoist a Frost Advisory for the remainder of the
CWA not already in the Freeze Warning, save for central Cook
County. Observational trends suggest that the overhead
stratocumulus deck should clear out over most of the area by
about 09-10Z, and recent HRRR runs have trended towards a little
more boundary layer decoupling occurring, at least in our
southern counties, which will be more closely positioned to the
center of an incoming region of surface high pressure. Thus,
confidence has increased enough in widespread frost developing
in Ford, Iroquois, Kankakee, and southern Will counties to
warrant the issuance of a Frost Advisory there.

Farther to the north, some uncertainty remains regarding whether
boundary layer decoupling will remain more limited, thus
permitting at least occasional mechanical mixing into 10-15 kt
flow atop the stabilizing boundary layer, which would tend to
inhibit frost development. While the stratocumulus should clear
out of there within the next 5-6 hours, the coverage and overall
influence of some incoming high-level cloud cover remains somewhat
uncertain, and it is possible that this also will end up being
a limiting factor for frost development closer to the Illinois-
Wisconsin state line. Our northwest Indiana counties will be
dealing with these same wind and cloud cover-related
uncertainties, plus the possibility that the lower stratocumulus
clouds don`t depart in time for efficient radiational cooling
to begin until just before dawn.

In spite of these lingering uncertainties, opted to err on the
side of caution and issue the Frost Advisory through pretty
much the remainder of our CWA, as at least patchy frost will
still probably develop in our wind- and cloud-beleaguered
counties with temperatures still likely to fall into at least
the mid 30s there overnight. The lone forecast zone that was
left out of the Frost Advisory was central Cook County, with the
expectation that the influence of the urban heat island effect
will keep temperatures there just "warm" enough to preclude the
development of widespread frost.

Ogorek

&&

.DISCUSSION...
Issued at 231 PM CDT Sat Apr 20 2024

Through Sunday night:

Northern IL/northwest IN remain within the southern periphery
of a deep upper trough over eastern Canada. Breezy west-
northwest flow was maintaining cold advection, with steep low-
level lapse rates helping to produce a dense, widespread
stratocu deck across the area this afternoon. Daytime
temperatures have struggled in the face of this cold advection
and opaque cloud cover, with readings only in the low-mid 40s
across most of the cwa as of 2 pm. Clouds are expected to
persist in this evening, though some thinning is likely with
time as we lose the diurnal component. Timing of trailing edge
over the upper Mississippi Valley per satellite imagery suggest
clearing from the northwest by mid-evening for our northwest
counties, and eventually around/a bit after midnight farther to
the east (perhaps a bit earlier with the loss of diurnal low-
level instability).

Clearing, continued low-level cold advection and a weaker
surface pressure gradient (and resulting winds) than last night
suggest we`ll likely have freezing temperatures across the
western half or so of the cwa by morning. In collaboration with
MKX/DVN and ILX have elected to issue a Freeze warning for areas
west and southwest of Chicago, where conditions appear most
favorable. Farther east, will likely see some areas of frost
toward morning, though air temperatures are expected to remain
above freezing. Had contemplated a frost advisory, though
lingering 10-15 kt flow above the boundary layer and slightly
warmer temperatures lend lower confidence to longer duration
widespread frost conditions. Better frost potential looks to be
across central IL, where winds will be lighter in closer
proximity to the surface ridge axis.

Sunny skies are expected for Sunday, with weak mid-level
ridging developing in the wake of an early morning positive-tilt
mid-level short wave. Guidance depicts 50-60 meter mid-level
height rises within deep subsidence behind this wave, with the
pocket of coolest mid-level air also moving off to the east of
the region. Under sunshine (perhaps just a shallow fair-weather
cu field), temps should "rebound" to the upper 50s/around 60
degrees for afternoon highs. We`ll still be a bit breezy, north
of the surface ridge, with west-northwest winds gusting around
20 mph during the day. Winds diminish and back west-southwest
Sunday night under clear skies. Could see some patchy areas of
frost away from the city again toward Monday morning, though
overnight lows are expected to be a bit warmer ranging from the
mid-30s to around 40 in Chicago as warm advection begins to
strengthen aloft.

Ratzer

Monday through Saturday:

Increasing low-level southwest flow behind a departing ridge
combined with strong diurnal mixing into a rather dry layer
above the PBL will support the potential of brush fire spread
Monday afternoon. Afternoon fine fuel moisture values are
already below 10% this afternoon, so with additional drying the
next couple days, current forecast RH values as low as 25-30%
and wind gusts possibly over 20 mph, we will continue to message
elevated fire wx concerns.

Strong upper-level diffluence ahead of a zonal jet streak over
the northern Great Plains will promote decent mid-level ascent
over an antecedent dry airmass. Modest mid-level lapse rates
around 7.5C/km should foster scattered high-based showers or
even a storm or two late Monday night. The surface low
associated with the upper jet streak and concomitant mid-level
trough/low will push a surface front across the CWA sometime
Tuesday afternoon. Deep mixing and marginally favorable low-
level moisture profiles should support diurnally driven showers
and some thunderstorms along/ahead of the front. More aggressive
guidance even suggests the potential for small hail given
freezing levels as low as 6kft and steep lapse rates into the
hail growth zone below EL values around 20kft.

In the wake of Tuesday`s system, a ~1030hPa high will settle
southeastward across the Great Lakes region Wednesday through
Thursday. With another dry and cool airmass in place during this
period and the southwest extent of the high edging into the
forecast area, widespread frost conditions and/or sub-freezing
temperatures are possible Wednesday night.

A highly amplified mid-level pattern Friday into next week will
bring another period of active weather to the central portion
of the CONUS. While consensus guidance favors keeping a
seasonably strong low well northwest of the forecast area,
several members (including the deterministic GFS) are
precariously close enough to warrant close monitoring of trends
for stronger convection in our area late Friday.


Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
-- Changed Discussion --
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
-- End Changed Discussion --

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

$$

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