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Area Forecast Discussion
Issued by NWS Chicago, IL
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FXUS63 KLOT 231853
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Lincoln IL
153 PM CDT Tue Apr 23 2024

.KEY MESSAGES...

- There is a level 1 of 5 (marginal) risk that scattered
  thunderstorms through around 8 pm this evening turn severe
  roughly along and north of I-80. Strong wind gusts and hail
  are the main concerns.

- 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.

&&

.DISCUSSION...
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

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1228 PM CDT Tue Apr 23 2024

* Concern through 06Z: Timing of convection with the front, wind
  shift from southwest to northwest/north
* Concern after 06Z: MVFR ceilings overspreading KORD/KMDW
  overnight through about mid morning Thursday

Pronounced dry layer below 3 km remains in place per most
recent ACARS soundings from KORD, which has kept the rain and
lower ceilings south of the terminals thus far. High-res models
have been trending a bit later with convection developing along
the incoming cold front, focused on the period from 21Z-01Z,
with a few of these storms potentially on the stronger side.
Have maintained the TEMPO periods at KORD/KMDW during this time
frame. Wind shift with the front will become more sharp as the
front gets further into the Chicago metro and the marine flow
begins to kick in. More of a sharp switch from about 330 degrees
to 010-020 degrees looks to take place sometime around 03-04Z.

As the lake flow kicks in, concern shifts toward lower ceilings
at KORD/KMDW. Latest HREF guidance suggests this could happen as
early as 05Z, but more focus is on 06-07Z for MVFR conditions.
By 09Z, probabilities of ceilings below 3,000 feet are up to
80-90% at both sites, with improvement not until 15-16Z.

Outside of these sites, a more transient period of MVFR ceilings
is hinted at for KRFD around 05-09Z. With KGYY being on the
south end of the lake, once MVFR ceilings take hold, they are
more likely to remain in place until midday.

Geelhart

&&

.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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