Tornadoes and Flood in Illinois, February 27, 2024

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
251 PM CST Tue Feb 27 2024

.KEY MESSAGES...

- Thunderstorms, some strong to severe, expected this evening

- Power-house cold front will rip across the area late this
  evening with a remarkable temperature drop

- A quick hit of accumulating snow late tonight into tomorrow
  morning with a threat for slick morning travel

&&

.DISCUSSION...
Issued at 251 PM CST Tue Feb 27 2024

Through This Evening:

We find an adolescent low pressure system that's taking shape
over far northern Missouri. This system is forming along the
cold front of a parent low that's currently traversing central
Wisconsin. SPC mesoanalysis resolves strong upper divergence
overlaid with some less impressive, albeit present, low level
convergence over northern MO. The low has been deepening at a
good pace dropping its central pressure from 992 to 990 between
11AM and 1PM. Nearby obs and RAP mesoanalysis data suggest that
this low has begun to reel in moisture and instability from the
east along a corridor that will shape into the storm's warm
front over the next several hours before moving into northern
Illinois.

In our south, efficient low level moisture advection has been
evident on satellite for some time. Now we find low 60s
dewpoints reaching as far north as the Kankakee River valley.
RAP mesoanalysis places 2,000-2,500 J/kg of MUCAPE across areas
south of I-80 and around 1,000 J/kg in far north central
Illinois. A capping inversion rooted around 850 mb remains in
place over most of our area being the strongest in south of the
Kankakee River and east of I-55.

Guidance remains in pleasantly good agreement on this low
tracking right through the heart of the CWA, specifically along
and north of the I-80 corridor. Forcing will be maximized near
the low pressure center and in the system's narrow warm sector.
A lot of guidance has been consistently convecting on the
northern and northwestern periphery of the low pressure center
which equates to areas northwest of I-55. Here we'll find the
cold front of the northern parent low catching up to the
mesolow's warm front and result in strong low level convergence.
Here is where we will also see the best potential for surface-
based convection with the weakest capping in place (in the
vicinity of the I-80 and possibly I-88 corridors). It's looking
rather likely that we'll go uncapped for a period late this
afternoon and early this evening. We're looking at a rather
narrow window of opportunity to see surface based storms before
the cap regenerates early-mid evening. Surface based convection
would mean a notably higher tornado threat with impressive 0-1
km shear. Large hail and damaging winds will be possible with
any storms near the low, but the likelihood will be maximized
with surface-based convection. The other item of note is that
the low-level jet may not arrive until after 6-7 PM, which may
modulate the tornado potential prior to its nocturnal
intensification.

Farther south of I-80 and into northwest Indiana, the capping
inversion will be harder to break later today and could possibly
inhibit CI altogether. However, model soundings have been
hanging on to less and less CIN this evening than with previous
runs. The HRRR has been trending toward a more aggressive
solution in this area since later this morning. Another driver
to consider will be a dryline-like feature that will develop in
this area this evening. This could help surface based parcels to
break through the cap and convect. The density discontinuity
doesn't look quite as impressive as previously forecast, but,
again, we'll be working with a weaker cap. With the much greater
instability overhead, robust convection with strong, broad
updrafts look very attainable with any surface based storms that
do develop in these parts. Should the cap hold, elevated
convection isn't out of the question. However, the environment
isn't nearly as progged for elevated severe storms as areas
farther north.

The severe potential looks to wrap up late this evening, though
the  potential for surface based storms should drop off by 8 or
9 PM at the latest when the low levels become too stable. A
stout low level jet will approach our south late this evening
which would almost certainly aid in severe convection south of
I-80 if it wasn't expected to arrive just a little too late.

Doom

Tonight and Wednesday...

A truly powerhouse cold front will blast through the region this
evening, ushering the severe weather threat east of the area.
Temperatures will fall at an exceptional rate tonight as winds
flip northwesterly--air temperatures are already in the teens
and single digits across the Dakotas and Minnesota, and this is
the airmass headed our direction (with just a bit of
modification along the way). We're explicitly indicating
temperature changes on the order of 45 to 60 degrees into
tomorrow morning which is obviously pretty remarkable.

With the strong baroclinicity involved, am a bit concerned that
winds will "pop" during the frontal passage as a zone of 6+ mb/3
hour pressure rises blasts across central Illinois. Wind gusts
have been overperforming model guidance immediately upstream,
and suspect that will continue given the notable pressure
rise/fall couplet and density discontinuity associated with the
front. Have elected to hoist a Wind Advisory to cover this
potential late this evening/early overnight for much of the
forecast area.

In addition to the winds, guidance continues to suggest that a
narrow but potentially robust/sharp frontogenetic circulation
will mature in the post-frontal airmass overnight tonight as
the left exit region of a 150+ upper jet blasts across downstate
Illinois and central Indiana. Unfortunately, the placement of
this f-gen circulation remains hopelessly unclear, with some
model output indicating this sets up across far northwest
Illinois into SE Wisconsin, while other guidance is farther
south. The multi-model consensus favors this somewhat farther
south solution at this time. Regardless, if this f-gen band
develops, there's plenty of instability aloft (near-upright
instability with theta-e lapse rates around or less than 0) to
support a brief burst of heavier rates as the columns rapidly
cools, allowing for a full changeover to snow. Main window for
this occurring is about 3 to 8 am, with the best guess that
we'll see a coating to perhaps an inch across the central half
of the CWA, although will note there's a big degree of
uncertainty in placement of any snow band tonight. On the high
end, a 2 inch amount isn't out of the question.

Finally, given such a robust temperature fall, there's some
concern for a flash freeze with any lingering wet pavement
overnight, particularly across northwest Illinois where there
will be less time between precipitation ending and sub-
freezing temperatures arriving. Something we'll also be keeping
a close eye on.

Windy/blustery/cold conditions will occur on Wednesday, with
morning wind chills either side of 0 degrees in most spots.
Temperatures should claw their way towards the mid and upper
20s, but the strong winds will hold wind chills in the single
digits and teens all day. Wind gusts could get close to advisory
criteria (45+ mph) through the day, although current
indications point to winds--on the whole--coming up just shy at
this time.

Carlaw

Wednesday Night through Monday:

No changes to the period beyond Wednesday night given the short
term focus. The previous discussion is included below.

While the below normal temps Wednesday and Wednesday night will
feel harsh coming off the exceptional, record breaking warmth of
Monday and Tuesday, the pendulum will swing right back to spring
through the weekend.

A brief reinforcing surge of cold air advection Wednesday evening
will keep breezy/blustery conditions going for a few hours, with
west-northwest winds gusting up to 25-30 mph. Wind chills will
bottom out in the single digits to lower teens. Ridging from
1030-1035 mb surface high pressure passing to our south will
cause winds to quickly diminish and turn light southerly
overnight. Good radiational cooling will support lows in the mid
teens to around 20F.

The southerly pressure gradient will quickly tighten as the
surface high slides east Thursday afternoon. Southerly winds
gusting to 25-30 mph, warm advection, and full sunshine will
allow afternoon high temperatures to recover to the lower to
mid 40s, or a bit above normal for leap day of what will go down
as one of, if not the warmest Februarys on record. A stout low-
level jet overhead Thursday night should result in breezy
conditions at times, limiting the temperature drop.

A minority of model guidance brings a short-wave far enough
north on Friday to spread clouds and showers into the area, so
our official forecast has a sliver of slight (15-20%) chance
PoPs over the far southern CWA. If the minority solution does
end up coming to fruition, temps may under-perform. On the other
hand, if the short-wave remains to our south, the mid 40s to
lower 50s highs in the forecast may be a bit conservative.

The weekend will offer another taste of early spring fever
thanks to well above normal mid-level heights downstream of
deep western troughing. Persistent warm advection, breezy south-
southeast winds, and plenty of daytime sun suggests highs well
into the 60s if not warmer on Saturday and upper 60s to mid 70s
on Sunday, if not warmer in spots. The official forecast is
above most of the raw guidance, though below the 90th percentile
of the NBM distribution, which has performed better (even
slightly underdone at times) on our recent mild days. With the
ground drying out again and breezy/gusty southerly surface flow,
we'll have to watch for elevated fire weather concerns if dew
points and RHs end up lower and temps a bit warmer than
forecast, as appears possible. The only minor exception to the
weekend warmth is along the Illinois shore where any east of
south component to the wind will keep daytime temps several
degrees cooler.

Western troughing should eject eastward, at least piece-meal,
early next work week. This will result in another fairly sharp
cold front passage Monday PM/night or Tuesday, though unlikely
to  the magnitude of tonight's per medium range guidance.
Showers and even thunderstorms are plausible out ahead of the
cold frontal to start the work week. The likely (55-70%) PoPs on
Monday and Monday evening in the official blended forecast
appear reasonable enough at this range given good ensemble
support.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1130 AM CST Tue Feb 27 2024

Key Aviation Impacts for the current TAF period:

- Scattered thunderstorms, potentially severe, expected this evening

- A robust cold front moves in with strong northwesterly wind gusts
overnight and into Wednesday morning

- A period of accumulating snow showers expected early Wednesday
morning. If accumulations do develop, prevailing gusts over 30 kts
could create blowing snow conditions.

VFR through the early afternoon with southerly wind gusts around 20
kts before thunderstorms are expected to develop ahead of a cold
front. Maintained the previous forecast's thinking, just adjusted
the timing a little. Storms will likely start developing
farther inland sometime between 4 or 5 PM and moving toward the
Chicago terminals around 00Z. As the storms develop, there
remains a risk for storms to become severe. Hail remains the
main threat. However, the slight chance for a tornado or two
that could potentially develop, but the better chances are away
from Chicago terminals but potentially near KGYY. As storms
approach terminals, cloud bases are expected to create MVFR
cigs.

Behind the storms, a robust cold front moves in changing the wind
direction to the northeast and increasing wind strength again. The
expectation is for prevailing gusts around 30 kts, though there is
the chance for a brief period of max gusts over 40 kts as the
initial front itself moves through.

The cold front will usher in a much, much colder air mass that will
sharply plummet temperatures overnight. With the colder air, there
is the potential for a period of accumulating snow at terminals
Wednesday morning with cigs returning to MVFR levels. Winds will
remain strong through the end of the TAF period, and keeps the risk
for BLSN in the TAF.

DK

&&

.MARINE...
Issued at 251 PM CST Tue Feb 27 2024

A sub-990 mb low will track across Lake Michigan tonight,
dragging a powerful cold front across the region. As the front
passes, winds will shift sharply out of the northwest and gust
to 40 to 45 knots, with this wind shift expected to occur around
(or just prior to) midnight tonight. Thereafter, northwesterly
winds will gust frequently to 35 to 40 knots through Wednesday.
A Gale Warning remains in effect for the Illinois and Indiana
nearshore waters from midnight tonight through 6 PM Wednesday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning from midnight tonight to 6 PM CST Wednesday for
     the IL and IN nearshore waters.

&&

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Hazardous Weather Outlook...CORRECTED
National Weather Service Chicago/Romeoville IL
404 PM CST Tue Feb 27 2024

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-282115-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
404 PM CST Tue Feb 27 2024 /504 PM EST Tue Feb 27 2024/

This Hazardous Weather Outlook is for portions of north central
Illinois...northeast Illinois and northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Limited Tornado Risk.
     Elevated Hail Risk...up to 3 inches in diameter.
     Limited Damaging Wind Risk...up to 60 mph.
  Elevated Non Thunderstorm Wind Risk.
  Limited Snow Risk.

DISCUSSION...

Scattered thunderstorms are likely this evening, some could
become severe. The primary severe risk will be large hail,
possibly up to 3 inches in diameter. In addition, locally
damaging winds over 60 mph and a few tornadoes are possible. The
tornado threat looks to maximize near and south of I-80.

Strong northwest winds are likely late this evening and overnight
behind a strong cold front. Frequent gusts of 30 to 40 mph are
expected with brief gusts over 50 mph possible.

Snow showers are likely after midnight. Some areas could see a
coating to one inch or so of accumulation and slick travel
conditions into Wednesday morning.

Wind chills of 5 below zero to 10 above zero are likely late
tonight.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Wednesday...
  Limited Excessive Cold Risk.
  Elevated Non Thunderstorm Wind Risk.
  Elevated Fire Danger.

DISCUSSION...

Northwest winds will gust as high as 35 to 45 mph Wednesday
morning into early Wednesday afternoon. This will produce wind
chills as low as 5 below zero to 5 above zero Wednesday morning.

In addition, afternoon relative humidity values are expected to
fall to around 30 percent and may fall a bit lower than that. This
may result in an elevated fire danger.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east northeast at 35 to 40 mph.

LMZ740>745-282115-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
404 PM CST Tue Feb 27 2024

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Gale force winds and or very hazardous waves.
  Elevated Thunderstorm Risk...with an associated:
     Elevated Hail Risk...up to 2 inches in diameter.
     Limited Thunderstorm Wind Gust Risk...up to 50 knots.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Wednesday...
  Gale force winds and or very hazardous waves are likely.
  Limited Freezing Spray Risk.
Thursday and Friday...
  Hazardous conditions for small craft are likely.
Sunday and Monday...
  Hazardous conditions for small craft are likely.

Doom

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