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000
FXUS63 KLOT 201948
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 PM CDT Sat Apr 20 2024

.KEY MESSAGES...

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

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

&&

.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 /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1227 PM CDT Sat Apr 20 2024

No major forecast concerns at the terminals for this TAF period.
Breezy west-northwest winds will prevail through the rest of the
afternoon with gusts around 20 kts. Gusts will subside late this
evening as a surface high drifts closer and weakens the pressure
gradient. Though, winds will pick up again Sunday afternoon
with gusts in the 15 to 20 kt range possible. Otherwise, expect
BKN to OVC VFR clouds to persist through this evening before
scattering out overnight into Sunday morning.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Freeze Warning from 3 AM to 7 AM CDT Sunday for ILZ003-ILZ004-
     ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-ILZ021-
     ILZ032.

IN...None.
LM...None.

&&

$$

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