Damaging Wind in Illinois, July 21, 2022

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
643 PM CDT Thu Jul 21 2022

.SHORT TERM...
Issued at 215 PM CDT Thu Jul 21 2022

Through Friday night...

Surface analysis reveals a cold front pressing into western
Wisconsin/NE Iowa early this afternoon. Ahead of it, moisture has
evidently pooled in sufficient depth to limit the degree of
downward fall with diurnal mixing with upper 60s to near 70 degree
dewpoints noted across the region. While recent AMDAR soundings
depict a slight region of mid-level warming, mesoanalysis
nevertheless indicates MLCAPE building towards 1500-2000 J/kg
across parts of north and northwest Illinois with rapidly-eroding
inhibition. Visible satellite loops depict increasingly agitated
cumulus entering far northwest Illinois, and this seems to be the
area from which we should eventually sprout developing convective
elements, with confidence in at least isolated-widely scattered
thunderstorm development increasing for locales north of I-80.
While deep layer shear isn't overly high, some modest local
augmentation to the mid-level flow is probable in the vicinity of
an incoming shortwave yielding 35 to locally 40 kts of effective
bulk shear which will likely translate into some degree of
storm/updraft organization. Deep mixing has promoted increases in
low-level lapse rates which will promote a strong to locally
damaging downburst wind threat. Some hail threat may also
materialize with sufficient storm organization/transient supercell
structures, but thinking the primary threat is wind. Given the
degree of dry air within the mixed PBL and roughly 20 kts of
shear, think some degree of dry air entrainment challenges may be
noted with incipient updrafts, but a few more robust/sustained
cores will be capable of delivering a localized severe threat. We
have sent out an updated "Graphical NOWCast" outlining these
thoughts into the late-afternoon. The main severe window should
close pretty abruptly after 7-8 PM.

Once convection dwindles this evening, the overnight looks pretty
uneventful. Warm advection will commence again across Iowa
overnight and into Friday morning on the nose of a modest low
level jet and this will promote additional scattered shower and
thunderstorm development off to our west. It's conceivable that a
few of these meander east of the river and towards our forecast
area during the late morning hours on Friday, although the
diurnally-weakening LLJ will eventually result in dwindling
isentropic upglide and shower/storm coverage. Forecast guidance
is looking pretty capped (to surface based convection at least) on
Friday afternoon, so any additional development would be tied to
localized increases in WAA which look to increase late in the
afternoon and early-evening, mainly across northwestern portions
of our forecast area and have attempted to limit our PoP forecast
to those locales.

For Friday night, am getting a little more concerned with a
potential for training elevated convection (which may also carry
with it at least a low-end threat for small hail and gusty winds).
Forecast guidance today has now come into better alignment,
depicting an increasing southwesterly low-level jet ramping up
through the evening and overnight hours. Attendant isentropic
ascent looks pretty impressive, especially along the 310-315 K
theta surfaces into early Saturday morning. This ascent will
intercept a steepened zone of lapse rates to promote elevated
convective development with at least some potential for re-
generative/upwind development which could promote a localized
heavy rainfall threat due to training cells. Main threat currently
looks to maximize near/north of I-80 into early Saturday morning.
Additionally, while an unfavorable time of night, veering through
a deep layer looks to promote sufficient deep layer shear to
suggest at least a low-end strong-severe threat with any taller
convective elements. Still too many uncertainties to commit to
likely precipitation chances, but we have nudged these up during
the Friday night period.

Carlaw

&&

.LONG TERM...
Issued at 321 PM CDT Thu Jul 21 2022

Saturday through Thursday...

Unsettled weather is expected across the Midwest this weekend and
into next workweek. Locally, this will entail almost daily chances
for showers and thunderstorms for at least part of our forecast area
from Saturday onwards through the middle of next week. This could
also come with the potential for strong to severe storms and at
least localized flooding late Saturday into Sunday.

Saturday morning could start off on a wet note for some as Friday's
overnight convection will still likely be ongoing somewhere in our
CWA after daybreak. The nature of this convection, as well as how
long it can persist into the day on Saturday will have consequences
on the rest of the forecast for Saturday afternoon and evening,
including on whether advisory-level heat will be observed later in
the day and potentially on how robust of a severe weather threat may
be realized during the evening.

Synoptically, a "double barrelled" region of surface low pressure
over the Dakotas will track northeastward into Minnesota by Saturday
afternoon and will eventually reach Lake Superior and/or Ontario as
a consolidated low by the late evening. A cold front will link the
two low pressure centers and extend farther out to the southwest
into the Great Plains while a warm front attached to the southern
low pressure center will attempt to progress northeastward into the
Great Lakes. Without much impediment from the early morning
convection, the warm front would likely clear the CWA by the late
morning. Recent trends in hi-res guidance though suggest that the
early morning convection could be more widespread than previously
thought and could cough out a decent cold pool that would combat
the northeastward progression of the warm front. If this were to
occur, this would at the very least delay the arrival of any 90+
degree temperatures, but if the ongoing showers and storms
continue to reinforce the cold pool, this could keep the warm
front at bay until very late in the day and outright keep
temperatures capped below 90 degrees throughout the course of the
day. Given that convective temperatures are progged to be in the
mid 90s through much of the forecast area, this would likely keep
a lid on any attempts at air mass convection producing at least
spotty showers and/or storms during the afternoon. However, there
is still a slight lean towards the morning convection subsiding by
the late morning hours and the warm front accordingly lifting
through most or all of the CWA by the mid afternoon. This would
allow for temperatures to climb to at least 90 degrees across much
of northern Illinois and northwest Indiana with heat indices
peaking near or above 100 degrees in most places and still a low
chance for spotty showers to develop during the mid/late
afternoon. In either scenario, it will still feel quite humid on
Saturday as between the morning rainfall, moist air advection, and
evapotranspiration, surface dew points should still climb into
the low-mid 70s across most or all of the CWA by the early
afternoon.

Attention then turns to our northwest where a shortwave trough
moving through the Upper Midwest will foster the development of
thunderstorms along the aforementioned cold front with somewhat
quick upscale growth expected into one or more MCSs somewhere in
Minnesota, Iowa, and/or Wisconsin by the mid-afternoon. While the
better overall overlap of instability, shear, and forcing for ascent
should remain to the northwest of our CWA (roughly where the SPC has
place their Enhanced Risk area in their Day 3 Convective Outlook),
Corfidi downshear propagation vectors pointing towards the east-
southeast suggests that any MCS that materializes to our west or
northwest could eventually track into at least part of our CWA,
where plenty of instability and shear will still be present to
support what would primarily be a damaging wind threat. However,
there are still questions over where exactly any potential MCS may
develop on Saturday, and it remains possible that our CWA will
remain devoid of any MCSs through Saturday night. Additionally, the
timing of the arrival of upstream convection could come late enough
where some degree nocturnal stabilization could mitigate the
strength of any potential strong/severe storms. Regardless, at least
scattered showers and storms look like a good bet for at least the
northern half of our CWA on Saturday night with the cold front
encroaching the area amidst the presence of a burgeoning low-level
jet.

Additionally, we're eyeing another potential for heavy rainfall
and an attendant flash flood threat late Saturday night and into
Sunday. PWATs are forecast to increase to around 2 inches through
Saturday night, indicative of an increasingly moist and unstable
airmass present in the free warm sector ahead of an incoming cold
front. One change we've noted from earlier model guidance is that
the orientation of the incoming front looks a bit more southwest
to northeast instead of west-east which may ultimately curtail the
potential for significant cell training to a degree with
individual storm motions that may be focused a bit more off the
cold front. That said, with the noted diurnal increases in the
southwesterly low-level jet, upwind Corfidi Vectors trend under
about 10-15 kts Saturday night which suggests a threat for renewed
updrafts to propagate very slowly. If these areas receive heavy
rainfall Friday night, this would further increase the flood
potential. This threat could then continue into Sunday,
though the main threat area then would be farther south as the
cold front continues to slowly sink southeastward, encouraged
southward by cool outflow.

The forecast from Monday onwards will be tied to how far south the
weekend front is able to get before it starts to stall out and
possibly lift northward by the middle of next week. The NBM
continues to advertise daily precipitation chances for at least part
of our CWA Monday through the end of the current extended forecast
period, though there will more than likely be some dry time for most
on Monday and/or Tuesday as the front/baroclinic zone ends up being
positioned south of our CWA during this time. The lone exception to
this could be our far southern CWA, where the NBM continues to have
at least slight chance PoPs prevailing during this period. By mid-
week, area-wide precipitation chances will return as another
shortwave enters the Midwest while the front/baroclinic zone begins
to lift northward towards our CWA. Cooler temperatures are expected
on the north side of aforementioned front/baroclinic zone with highs
expected to remain in the 80s for the most part through the
middle of next week.

Ogorek

&&

.AVIATION...
For the 00Z TAFs...

643 PM...Forecast concerns include...

Isolated thunderstorms early this evening.
Chance of thunderstorms Friday night.

A weak upper wave is moving across the area early this evening,
allowing isolated thunderstorms to develop ahead of a weak frontal
boundary. This boundary is now moving through northwest IL where
winds are turning to the northwest. Once this boundary moves
through the Chicago terminals, the chance of thunderstorms early
this evening will end. Opted to maintain thunder mention through
01z, but as trends emerge, may be able to amend prior to 01z to
remove thunder mention.

Southwest winds will gust into the mid 20kt range early this
evening until the boundary noted above shifts winds more west/
northwest. Some gusts are possible through sunset, but speeds and
gusts will diminish through the evening with light and variable
winds possible by sunrise. Winds will turn southwesterly later
Friday morning with speeds increasing into 10-12kt.

There remains quite bit of uncertainty for thunderstorm chances
Friday night. The consensus is trending toward late Friday evening
or possibly early Saturday morning for thunderstorm development
across parts of northern IL. This would be right at the end of the
new 30 hour ord/mdw tafs. Confidence is too low to include any
precip mention with this forecast, but thunderstorm chances for
early Saturday morning will likely need some mention in later
forecasts. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 8 PM
     Thursday.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
331 PM CDT Thu Jul 21 2022

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-222045-
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-
331 PM CDT Thu Jul 21 2022 /431 PM EDT Thu Jul 21 2022/

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

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk...with an associated:
    Limited damaging wind risk...up to 65 mph.
    Limited hail risk...up to quarter size.

DISCUSSION...

Scattered thunderstorms are expected to develop this afternoon.
The strongest storms may produce winds capable of downing tree
limbs, mainly near and north of Interstate 80. The severe weather
threat should ease after 7 to 8 PM.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Limited Thunderstorm Risk.
Friday Night...
  Limited Flooding Risk.
  Limited Severe Thunderstorm Risk.
Saturday...
  Limited Severe Thunderstorm Risk.
  Limited Flooding Risk.
  Limited Excessive Heat Risk.
Sunday...
  Limited Thunderstorm Risk.
  Limited Excessive Heat Risk.
Tuesday and Wednesday...
  Limited Thunderstorm Risk.

DISCUSSION:

An active weather pattern will set up across the region this
weekend, with periods of thunderstorms possible. Some of the
storms later Friday night into Saturday and Saturday night could
be strong to severe with damaging winds and hail the primary
threats. Locally heavy rainfall amounts will also be possible,
which could lead to a few instances of flash flooding, especially
Friday night and early Saturday morning, and again Saturday night
into Sunday.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed through early this evening, mainly north
  of I-80.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east-southeast at 40 mph.

LMZ740>745-222045-
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-
331 PM CDT Thu Jul 21 2022

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...This Afternoon and Tonight.

Weather hazards expected...

  Hazardous conditions for small craft.
  Limited Thunderstorm Risk...with an associated:
    Limited damaging wind risk...up to 65 mph.
    Limited hail risk...up to quarter size.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Limited Thunderstorm Risk.
Friday Night...
  Limited Severe Thunderstorm Risk.
Saturday...
  Hazardous conditions for small craft are likely.
  Limited Severe Thunderstorm Risk.
Sunday...
  Hazardous conditions for small craft are likely.
  Limited Thunderstorm Risk.
Tuesday and Wednesday...
  Limited Thunderstorm Risk.

Carlaw

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