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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 240000
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
700 PM CDT Tue Apr 23 2024

.KEY MESSAGES...

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- There is a 50-70% chance for a freeze Wednesday night into
  Thursday morning outside the Metro, with the highest chances
  generally west of I-57. Tender vegetation may be damaged if
  protective actions are not taken.

- Multiple rounds of thunderstorms late Friday through the
  weekend will pose a risk for severe weather and heavy rain.
  Damaging wind gusts, large hail, localized flooding, and a
  tornado are all concerns, though confidence is low in when and
  where chances will be highest at this time. Have multiple ways
  to receive warnings, and a plan for where you`ll seek shelter.
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&&

.DISCUSSION...
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Issued at 149 PM CDT Tue Apr 23 2024

Through Wednesday Night:

At 145pm, current radar imagery shows showers with a few
embedded thunderstorms roughly south of an El Paso to Valparaiso
line. Over the next couple hours, the pockets of sunshine
behind this wave of precip will result in steepening low level
lapse rates for some weak instability favoring isolated showers
and thunderstorms ahead of the cold front slated to reach I-55
around 6-7pm. The stamps from the 12z HREF are looking a little
less aggressive than they did in the 00z iteration, though still
around 50-60% of HREF membership generates 500 J/kg or more of
SBCAPE at some point between 3 and 8 pm along a narrow corridor
extending from roughly Chicago- Waukegan southwest to Galesburg.
Given low LCLs and 30-40 kt of 0-3 km wind shear along the
front, there`s a nonzero (though very small) chance for a
landspout tornado with this activity. However, the steep lapse
rates and dry mid level air would seem to favor more of a
momentum transfer wind concern with the stronger cells; the HRRR
shows this in its wind gust field, with pockets of 40+mph gusts
under showers this evening. In other news, it`s turning quite
warm behind the front given the drier airmass fostering
efficient radiational warming with the anticipated sunshine; the
last few iterations of the HRRR suggest a few locations will
top out around 70 degF for highs before the evening`s over.
Short term guidance suggests northerly winds will stay 5-10 mph
tonight, except quite a bit higher near the Lakeshore, which
should in theory prevent frost from developing and
simultaneously offset radiational cooling to diminish chances of
a freeze (NBM gives 10-20% probabilities north of a Georgetown
to Waukegan line) as well.

The cool advection in the wake of the front will suppress
tomorrow`s highs, most notably near Lake Michigan where the
Lake-chilled boundary layer airmass will be advected inland
throughout the afternoon to keep temps in the 40s in many
locations, including the Chicago Metro. Tomorrow night, there is
a higher chance of widespread frost as high pressure building
into the region leads to light/calm winds allowing for more
optimal radiational cooling. The lowest temps will occur away
from the Lake and west of I-57; further east, the weak
northerly/northeasterly flow will advect southward a shallow
marine layer and attendant stratus which should offset nocturnal
cooling. The NBM suggests west of the I-57 corridor there will
be a 50-75% chance for a freeze, with a 15-30% chance for a hard
freeze (temps < 28 degF) north of Rockford, so frost and/or
freeze headlines will likely become necessary.

Bumgardner


Thursday through Tuesday:

As the high pressure ridge axis reaches northern IL, winds will
veer to southeast on Thursday, advecting into the region a
warmer airmass that will bring afternoon highs into the low 60s
inland, mid 50s Lakeshore. All signs indicate Friday will be
even warmer, though it`ll come with a price: The deterministic
GFS and ECMWF each show a pronounced theta-E gradient lifting
north into the area at some point Friday, and while they
disagree on timing each model suggests precip riding along that
front. Fortunately for us, the associated sfc low will be
displaced well to our west - across the central Plains - though
locally backed winds enhancing low level shear and low LCLs
along and south of that front could prove sufficient for a
couple severe storms, especially across our south and west CWA
where the NAM suggests around 1000 J/kg MUCAPE materializes by
00z Sat (7pm Friday). We`ve still got several days before that
system arrives, so a lot is subject to change, though the CSU
MLP paints a 15%+ probability area across virtually the entire
CWA (save northwest IN).

Convection will certainly have an effect on the warm front`s
movement, but at this point there`s fairly good agreement in the
extended range models and their respective ensembles that
Saturday will be much warmer; the NBM, in fact, gives a 50-65%
chance for highs over 80 degF. This, of course, could be a bust
if there`s more cloud cover than currently forecast or
thunderstorms develop during the day. Instability will certainly
be higher on Saturday compared to Friday, with 50-60+% chances
for more than 1500 J/kg SBCAPE during the evening west of I-55.
However, it`s uncertain whether there will be any trigger for
storm initiation with the warm front expected to be so far
north; it might come down to outflow boundaries from nocturnal
convection Friday night into Saturday morning. CSU again depicts
a 15% contour for severe weather on Saturday, so we`ll be
watching carefully.

Another sfc low will spin up in the southern Plains Saturday
night and lift into IA Sunday afternoon/evening, increasing wind
shear across our area and potentially favoring another round of
thunderstorms which, once again, could turn severe. The SPC has
highlighted our area with a level 2 of 5 (slight) risk for
severe weather on Sunday, with all hazards (damaging wind gusts,
large hail, and tornadoes) seemingly in play; of course, this
will be conditional on Saturday night-Sunday morning convection,
afternoon cloud cover, the precise track of the sfc low, and
the strength of the low - all of which are difficult, if not
impossible, to pin down at this time step. We`ll continue to
monitor.

Bumgardner
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&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
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Issued at 700 PM CDT Tue Apr 23 2024

Aviation concerns:

- A few showers linger through mid-evening (~1-2Z)
- Sharp northeast wind shift with backdoor front late evening
- Period of MVFR cigs at ORD/MDW/GYY overnight into Wednesday AM

The earlier thunderstorm potential has mostly waned. Spotty
showers (and perhaps a lightning strike or two out over Lake
Michigan) will remain possible for the next couple of hours as
they continue to redevelop just east of a surface boundary
extending from Peru, IL to near ORD.

Winds to the south of this boundary (ORD/MDW/GYY) are SSW to SW
and are W to NW to the north of this boundary (DPA/RFD) with
gusts in the mid 20kt range. As this boundary gradually sinks
southward winds may turn northwest at ORD/MDW over the next
couple of hours. A sharp wind shift to northeast is anticipated
late in the evening as a backdoor front dives down the lake
during the 4-6Z timeframe, earliest at the Chicago area
terminals. Winds then ease with time during the daytime hours on
Wednesday

There remains the potential for a period of MVFR stratus to
push inland in the wake of the backdoor front overnight near the
lake (ORD/MDW/GYY) and is expected to scatter out during the
morning on Wednesday.

Petr
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&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT Wednesday for the IL and
     IN nearshore waters.

&&

$$

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