Damaging Wind in Illinois, June 19, 2021

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 AM CDT Sat Jun 19 2021

.MESOSCALE DISCUSSION...
849 PM CDT

Evening surface analyses places a surface cold front stretching from
near Lansing, Michigan southwestward to Valparaiso, Indiana to
Moline, Illinois and Omaha, Nebraska. South of the front, a
secondary boundary (outflow from convection this morning?) is noted
extending from explosive severe convection near Indianapolis,
Indiana westward to Springfield, Illinois to just north of Kansas
City, Missouri. Between both boundaries, a moist and unstable
airmass resides with surface dew points in the lower 70s and RAP-
analyze MUCAPE of 3000+ J/kg. Satellite imagery this evening shows
extensive accas and elevated showers from central Iowa to central
Illinois, with occasional lightning strikes from time to time.

The 00Z RAOBs from ILX (located between both aforementioned
boundaries) exhibited substantial inhibition, with some 100-120 J/kg
of MUCIN and 140 J/kg of MLCIN. Yet, as the nose of a developing 850
mb low-level jet stalls in the vicinity of the IA/MO border, low- to
mid-level moistening and low-level confluence (and hence lift)
should erode any remaining convective inhibition affording explosive
convective development, likely sometime in the 9-11 pm timeframe.
However, we have noted a few storms developing already in central
Illinois. With steep mid-level lapse rates and appreciable deep-
layer shear (~8 K/km and 40-45 kt of 0-6 km flow from the 00Z ILX
RAOB, respectively), the thunderstorms are likely to become
organized with a mix of both splitting supercells and multicell
clusters as they move eastward into central Illinois after midnight.
The main threats with the storms will be large, damaging hail and
perhaps strong surface wind gusts--the latter dependent on whether
the storms can grow "upscale" into an organized bowing cluster or
two. Torrential downpours and frequent lightning would be likely,
too.

The northern edge of the severe weather threat will be close to the
southern edge of our forecast area, perhaps along and south of a
line from Pontiac, IL to Fowler, IN, in agreement with SPC Level
3 out of 5 threat area. However, a few storms cannot be ruled out
as far north as I-88. Anyone living south of I-88 should
accordingly have multiple ways to receive warnings this evening
should a severe storm impact their area. The thunderstorms will
gradually sink south with time, with the threat ending across our
southern zones likely by daybreak.

Borchardt

&&

.SHORT TERM...
331 PM CDT

Through Saturday evening...

Concerns for tonight center around another round of strong to
possibly severe convection along with an axis of heavy rainfall
somewhere in the region.

The mid level wave that drove some scattered thunderstorms along the
US Highway 24 corridor earlier today has shifted eastward this
afternoon. At the surface, the cold front have moved to a Chatsworth
to Rensselaer corridor. The atmosphere is capped behind the front
and we are void of any strong upper forcing, thus the lake breeze
and the front will likely not support much development this
afternoon.

Looking upstream there are some subtle shortwaves in the westerly
flow aloft which may drive some showers or a storm or two well south
into early evening, but stronger forcing mechanisms are still back
across the Dakotas and then back toward Nebraska. Both of these
upper waves will act to drive another round of thunderstorms late
this evening, and expand overnight. Ample CAPE in the hail growth
region and steep lapse rates aloft coupled with the approaching
speed max will support a concern for hail initially. Things get a
bit messier overnight as the flow becomes parallel to the boundary
with the upper jet, increasing low level jet aid in storm
development. PWATs in excess of 2" will support heavy rainfall,
although ensemble guidance does suggest the heaviest rain may fall
just to our southeast. Still a swath of 1-2" or locally more, may
still be plausible along the US Highway 24 corridor. Also, a wind
threat could materialize as well if storms congeal.

The boundary may still be hung up over our far south and east into
Saturday. This coupled with a lake breeze and there are some forcing
mechanisms. It will also be warm and humid along the boundary, thus
a marginal risk for severe weather exists, mostly southeast of
I-57. Upper forcing seems to have cleared the area, thus there are
some concerns for what type of coverage we would have.

KMD

&&

.LONG TERM...
351 PM CDT

Late Saturday night through Friday...

Late Saturday night into Sunday Morning: After the more unstable
airmass is suppressed south on Saturday, steep mid-level lapse
rates quickly advect back into the area late Saturday night into
Sunday morning ahead of a convectively-enhanced wave moving ENE
through the central Great Plains. Meanwhile, a low-level boundary
left behind from the ongoing stretch of convection will remain
begin to lift back north as a warm front. Enough low-level
(925-850hPa) moisture may exist that any axis of isentropic ascent
within the airmass gradient could isolated to scattered strong to
locally severe storms with large hail. The period of most concern
is in the 4-9am window.

sunday afternoon and evening: The forecast on Sunday then depends
heavily on the northward progress of the warm front. There are
increasing signals that the front may not clear the northern CWA,
leaving the boundary somewhere between the WI state line and I-80.
This boundary may also be reinforced by a potential cluster of
elevated convection across Wisconsin in the morning. By mid to
late afternoon, a deep trough axis with an associated cold front
will swing through the mid-Mississippi River Valley. While low-
level capping will again be an initial limiting factor, increasing
deep layer forcing should erode much of the cap by late
afternoon. Sufficient deep-layer shear and instability along and
south of the warm front exists to support a mix of supercell and
multi-cell bowing clusters conditional upon convection initiating.
While the fine details will need to be refined as we are still
nearly 48hrs out, the presence of the warm front across the area,
low-level turning of the winds, and potential storm mode does
support at least a low-end tornado risk late in the afternoon, and
possibly well into the evening on any QLCS structures. Continue
to monitor for forecast updates through the weekend as more
details emerge.

Monday and Tuesday: Much cooler air will arrive behind the cold
front Sunday night. Lingering low-level moisture in the southeast
half of the area may support isolated showers through early
afternoon before drier air advects in. Gusty northwest winds will
also create dangerous swimming conditions for the northwest
Indiana beaches, primarily late Monday afternoon into Tuesday
morning.

Tuesday night through Friday: The cooler weather will be short-
lived, with more potential convection late Tuesday night into
Wednesday morning as WAA and steep lapse rates again advect into
the region from the west. With the area in WNW flow through late-
week, seasonable temps with the potential for a round or two of
convection is expected during this time.

Kluber

&&

.AVIATION...
For the 06Z TAFs...

A challenging aviation forecast through today at area airports,
with the following concerns:

* A chance of scattered showers...and an outside chance of some
  convection...near the airports between 09Z-13Z

* A low chance winds briefly swing northeast sometime mid morning
  (~13Z-15Z)

* Probable that winds do turn northeast during the late
  afternoon/early evening; low confidence in timing of this

Strong convection is found far southwest of the terminals as of
0530Z, as well as scattered convection well to the northwest near
La Crosse, WI. ORD and MDW should find themselves in-between this
convection for a fair amount of the time through daybreak, with
the convection to the south becoming even further widespread and
prevalent with deep cloud tops. Isolated to scattered convection,
or more likely showers, may develop to the north of this area near
the airports in the pre-dawn hours through the dawn time.
Confidence in this occurring is low though, as some of the
ingredients are in place but typically with such robust convection
to the south, it can rob some moisture northward and that trend
has been seen in observations thus far. As for the convection in
Wisconsin, this should remain safely north, with an outside
possibility of creating an outflow boundary that briefly turns
the winds northeasterly after daybreak.

Low pressure will move over northeast Illinois during midday
today, and while this could trigger some showers or even
convection, that remains an outlier possibility for over/near ORD
and MDW. This does however lower certainty with the wind
forecast. Initially, west-southwesterly winds will be gusty and
gradually veer to west northwesterly into the afternoon. But
under the low pressure itself coupled with the incoming cold
front, the winds are likely to bend to northeasterly at some point
late in the day into early evening. There is a great degree of
spread in the model solutions, further supporting the low
confidence in specific timing of this, and further TAF refinements
are expected for this.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

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Hazardous Weather Outlook...CORRECTED
National Weather Service Chicago/Romeoville IL
448 PM CDT Fri Jun 18 2021

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-192100-
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-
448 PM CDT Fri Jun 18 2021 /548 PM EDT Fri Jun 18 2021/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
     Elevated hail risk...up to ping pong ball size.
     Elevated damaging wind risk...strong enough to down tree limbs.
  Limited Excessive Heat Risk.

DISCUSSION...

Expect another round of thunderstorms tonight, mainly after 10 pm.
There is a threat for hail and damaging winds, mainly south of
Interstate 80. Rainfall amounts may also approach 2 inches in this
corridor as the storms will be efficient rain producers.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Saturday and Sunday...
  Limited Severe Thunderstorm Risk.
Monday...
  Limited Thunderstorm Risk.
Wednesday and Thursday...
  Limited Thunderstorm Risk.

DISCUSSION...
Strong to severe thunderstorms are possible this weekend, with
a slightly higher chance Sunday afternoon and evening.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed tonight, especially after 10 pm CDT.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east southeast around 40 mph.

LMZ740>745-192100-
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-
448 PM CDT Fri Jun 18 2021

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...

  Limited Thunderstorm Risk...with an associated:
     Limited hail risk...up to quarter size.
     Limited thunderstorm wind gust risk...up to 40 knots.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Sunday...
  Hazardous conditions for small craft are likely.
  Limited Severe Thunderstorm Risk.
Monday...
  Limited Thunderstorm Risk.
Thursday...
  Limited Thunderstorm Risk.

KMD/Castro

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