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Tim from Springfield (IL)

Sept. 2-4 Severe Threat

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ENH risk for severe on this Labor Day from eastern ND to W WI, with slight including most of the northern half of WI:

https://www.spc.noaa.gov/products/outlook/day1otlk.html

...Northern Plains through upper Mississippi Valley...

   A shortwave trough will amplify as it moves east southeast through
   southern Alberta and Saskatchewan, reaching the northern Plains by
   early this evening and the upper MS Valley late tonight. Downstream
   from this feature, a warm front will advance northeast, and will be
   situated from a weak surface low in northwest ND through northeast
   SD and central IA by early evening. A cold front will extend
   southward from the low through the western Dakotas and advance east
   during the evening. 

   A moist warm sector will reside south of the warm front. Surface
   dewpoints will range from the mid 60s to upper 70s F beneath a plume
   of steep mid-level lapse rates associated with the eastward
   expansion of an elevated mixed layer (EML). Warm air at the base of
   the EML will result in a substantial cap in warm sector during the
   day. Theta-e advection along a southerly low-level jet will
   contribute to northeastward destabilization through the Dakotas and
   into the upper MS Valley this afternoon and evening with MUCAPE
   increasing from 2000-3000 J/kg north of the warm front. Isolated
   storms with a marginal hail threat may be ongoing from SD into NE
   within zone of warm advection. However, primary severe threat is
   expected during the afternoon into the evening when forcing for
   ascent associated with approaching shortwave trough and attendant
   strengthening low-level jet will result in development of storms
   north of the warm front over northern  ND. These storms will be
   elevated, but effective bulk shear from 40-50 kt and a favorable
   thermodynamic environment will support a few supercells with large
   hail the initial primary threat. Storms may eventually evolve into
   an MCS with primary threat transitioning to damaging wind during the
   evening and overnight as activity continues southeast through MN
   into WI along instability gradient. Given favorable low-level
   hodographs, a conditional threat will exist for a couple of
   tornadoes with any surface-based storms that can develop on the warm
   front. However, current indications are that the warm sector will
   probably remain capped to parcels originating in the boundary layer.

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Day 2 includes slight for SE WI, and most of the N half of IL and Southern Lower MI, as well as N IN and NW OH:

https://www.spc.noaa.gov/products/outlook/day2otlk.html

 ...Great Lakes...
   Models are in good agreement in a scenario with morning storms over
   the western Great Lakes in association with a strong southwesterly
   LLJ.  A lingering severe risk may accompany this activity mainly in
   the form of isolated damaging gusts before dissipating by the late
   morning.  A plume of steep 700-500mb lapse rates will advect
   northeast into the southern Great Lakes during the day.  Despite a
   capping inversion, convergence near the boundary and strong heating
   will likely lead to local erosion of the cap and isolated to
   scattered storm development by late afternoon from northern IL east
   into southern Lower MI.  A moderate to very unstable airmass is
   progged by guidance with 3000+ J/kg MLCAPE possible across IL and
   decreasing to 1500 J/kg into Lower MI.  Hodographs are forecast to
   enlarge during the late afternoon/evening across the southern Great
   Lakes concurrent with diurnal storm development.  A mix mode of
   supercells and multicells capable of all severe hazards are possible
   primarily during the 21z-03z period.  

   Farther southwest over MO into western IL, lower storm coverage is
   forecast but large CAPE and a wind profile supportive of organized
   storms would lend a conditional risk for severe with the stronger
   storms.  

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The ol 15% hatched day 2.  Certainly a bit unusual for these parts (actually can't remember the last time that happened) as hatching is usually accompanied by higher probabilities.  

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LOT

.LONG TERM...
311 PM CDT

Tuesday through Monday...

The main item of interest for the rest of the short workweek will
be tied to a cold front, both gusty winds and thunderstorm
chances ahead of it on Tuesday, some possibly strong to severe,
as well as a heightened rip current risk for the Lake Michigan
beaches behind this front on Wednesday.

Tuesday`s thunderstorm chances continue to look as if they fall in
two primary time windows: 1.) the morning north of I-80 with
activity shifting out of Wisconsin and 2.) potential for
redeveloping scattered activity ahead of a cold front in the mid-
afternoon, most favored south and east of a Chicago O`Hare to
LaSalle-Peru line. While there is fairly good confidence in those
time windows, sizable uncertainty exists in convective morphology
and coverage within these. In addition, any morning activity will
likely play into the afternoon location of redevelopment and
potentially its magnitude. The main forecast changes were
primarily to refine timing some. Also collaborated with
neighboring offices and SPC to continue to reflect a Slight Risk
type verbiage, though the instability and shear spectrum that is
tapped includes isolated higher end potential conditional on
supercell mode being able to develop in the afternoon. That`s on
the lower end of confidence spectrum right now.

Upstream observational trends this afternoon across the Dakotas
and Minnesota play somewhat into what are expectations are for
early Tuesday morning. The more organized MCS behavior there is
across central Minnesota this evening and into
western/southwestern Wisconsin early overnight, the better
likelihood some activity makes it into far northern Illinois
Tuesday morning. The effective warm front has been struggling to
reach north downstream of the current North Dakota MCS, and as
such this may not yet be the primary convective show, with
potentially more developing further south through the night in a
semi-organized state, especially given the strength of the low-
level jet. This jet will orient across northern Illinois early
Tuesday morning with likely a convectively-enhanced short wave
traversing central Wisconsin. Whether some remnant MCS or elevated
scattered activity, it is likely some convection will be flirting
with far northern Illinois and if organized could make it down to
I-80 by later morning. The severe threat with this looks isolated
at this time, and tied to steepening mid-level lapse rates and
effective shear in the 1-6 km layer increasing to 45 kt.

The footprint of convection and any debris will be key for
afternoon pre-frontal recovery, but given the strength of the
surface pressure gradient and wind field, warm and moist advection
in the low levels should not be completely hampered in the case
of organized morning convection. The surface low, likely at or
just below 1000 mb, will be crossing the U.P. of Michigan through
Tuesday. This is still a ways from the CWA but is closer with a
stronger gradient than last Thursday`s setup, and thus this late
summer cold front will have greater moisture to work with to spark
renewed convection in the region. Southwest wind gusts by early
afternoon are forecast to be 30-35 mph with potential for 40 mph
gusts again dependent on cloud cover. The confluence along and
ahead of the boundary for maximized moisture/instability and
lifting of parcels improves the further northeast along the
boundary, especially into lower Michigan and northern/northwestern
Indiana by later afternoon. If little activity in the morning
though, this may be slightly further west including over Chicago
during peak heating. Dew points should be in the lower 70s at
least with possible mid 70s given upstream conditions.
Temperatures into the 80s, possibly upper, support mixed layer
CAPE in excess of 3000 J/kg in any pre-frontal locations that
receive even just filtered sun into the afternoon. The regime
favors plenty of deep layer shear again with 45-55 kt, and some
turning in the lowest 3 km.

If afternoon convection can redevelop in the CWA...again most
possible favored south and east of Chicago to LaSalle-Peru line...
the mode is likely to be semi-discrete given mainly perpendicular
flow to the boundary, with a trend into early evening to have
some more convective segments as it evolves southeast. Hazards
would include hail especially initially, and wind in segments.
The high instability and wide nature of the CAPE in the vertical
predicted on NAM and RAP soundings would support some significant
hail threat especially in any temporary supercell structures that
can initially develop. Any bowing segments too could be a little
more robust, but that`s even lower confidence in our area right
now. The tornado threat is low in our area compared to lower
Michigan where there is more of a warm advection, slightly backed
low-level flow regime, but any initial supercells would have a
non-zero threat. Overall if having to highlight an area that is
the better looking within the broader Slight Risk for our area,
that would be far northwest Indiana right now.

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I am really interested in this one as long as the morning activity doesn't ruin the potential. 

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5/15/15 probs (5% TOR) on the 1AM Day 1.  Still SLIGHT.  Higher probs possible later in lower MI:

https://www.spc.noaa.gov/products/outlook/day1otlk.html

...Midwest into the Great Lakes and Mid MS Valley...
   A long-lived MCS over far southeastern MN and western/central WI
   early this morning will likely continue southeastward across WI for
   at least the next several hours along an instability gradient. This
   line of storms should continue to gradually weaken with
   southeastward extent this morning as it encounters a less unstable
   airmass. However, strong low/mid-level flow associated with a
   shortwave trough over the northern Plains may allow these storms to
   persist for longer than most model guidance suggests. An isolated
   hail/wind threat may be ongoing at the start of the period across
   parts of northern IL and vicinity if these storms can maintain their
   intensity, especially on the southwestern flank of the line where
   instability is relatively greater.

   The convective scenario for today remains rather uncertain given the
   potential for this morning MCS to delay/disrupt diurnal
   destabilization, particularly with northward extent into Lower MI. A
   relatively greater chance for moderate to strong instability to
   develop may occur along a trailing outflow boundary from these
   morning storms across parts of northern IL/IN into far southern
   Lower MI. Strong large-scale forcing for ascent attendant to a
   southeastward-moving shortwave trough will overspread these areas by
   this afternoon. Additional convective development appears likely
   along or just ahead of a surface cold front by peak afternoon
   heating.

   The presence of a 50-70+ kt mid-level westerly jet will support
   strong effective bulk shear values over this region. Any storms that
   can form in this environment will likely become organized and pose
   an isolated large hail and damaging wind threat. Initially
   semi-discrete storms will probably grow upscale fairly quickly along
   the cold front or remnant outflow boundary, with a damaging wind
   threat potentially continuing into the evening across parts of
   central IL/IN and northwestern OH. The degree of the tornado threat
   will be dependent on storm mode, but a southwesterly low-level jet
   will overspread much of the warm sector through the afternoon.
   Isolated tornadoes will be possible given the strong low-level shear
   that will be present. Greater severe probabilities may be needed
   across some part of the Midwest into southern Lower MI, but too much
   uncertainty exists regarding morning storms to introduce them at
   them at this time.

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43 minutes ago, Hoosier said:

Had a period of very gusty winds earlier.  Was that a wake low or something?

Yep.  There were several reports of 40+mph winds in the Chicago area around mid-morning, with 50mph estimated winds in Aurora.  

Festering zombievection and cloud shield over Iowa this morning won't help things for the DVN cwa.  This one looks DOA for this area.  

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22 minutes ago, cyclone77 said:

Yep.  There were several reports of 40+mph winds in the Chicago area around mid-morning, with 50mph estimated winds in Aurora.  

Festering zombievection and cloud shield over Iowa this morning won't help things for the DVN cwa.  This one looks DOA for this area.  

Yeah, I had a chance to check LSRs and there are some damage reports in there from the wake low.

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New day 1 outlook maintains a slight risk.  It is loaded with uncertainty but the duscussion even mentioned a strong tornado being possible if things come together. 

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21 minutes ago, Hoosier said:

New day 1 outlook maintains a slight risk.  It is loaded with uncertainty but the duscussion even mentioned a strong tornado being possible if things come together. 

Slight risk has been expanded further into northern and western Ohio.

1230 PM Day 1 SPC Outlook.JPG

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Sigtor numbers are progged to pop later.  Low level flow does veer for the most part but mid level WNW flow does mean some directional shear.

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1 hour ago, nwohweather said:

Rain stayed away in Toledo and even sunshine is out right now. Liking the chances a little more

Been pretty socked in here, though there are breaks looking upstream in IL/WI.

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Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
502 PM CDT Tue Sep 3 2019

.MESOSCALE DISCUSSION...
500 PM CDT

Convective evolution continues to be monitored, with capping
from the morning convection and lingering clouds severely
impeding any mature development thus far. This will likely prevent
any rapid development and may very well keep any development that
can occur isolated, such as has been the case. A conditional
severe risk does exist though if any sustained mode can develop.

Convection in the southeast CWA over the past hour to hour and a
half will have moved mainly east of the area by 530 pm. This
activity is along an old outflow boundary and pseudo warm
front advancing higher theta-e air into the area. While this
convection has fair potential to be supercellular, it has
struggled to be mature and sustaining thus far, likely owing to
the cap.

Further northwest along the cold front -- our now best chance for
storms -- convection has been isolated in southern Wisconsin with
agitated cumulus struggling along the boundary. Aircraft
soundings from across northern Illinois and even into Wisconsin
show capping around 850 mb, that had weakened little since ILX`s
18Z sounding. In Wisconsin there are more noteworthy height falls
and associated cooling with the ascent that looks to only just
graze far northern Illinois. This is why we think development is
likely only to be isolated at least for the next few hours.
Observational trends indicate the front is slower than forecast
trends, and so did slow the forecast timing of chances slightly,
along with lower them some. The nature of development should
continue to be discrete given the orthogonal flow and shear with
respect to the front. Given the values of deep layer shear (50+
kt) and CAPE, including in the low-levels thanks to the high
moisture ahead of the front, supercellular mode is still favored
and a conditional severe threat if sustained enough. If convection
can grow more scattered into early evening as the front works
into eastern Illinois and northwest Indiana, it`s possible some
segments/clustering may develop and have a little more potential
for wind.

MTF

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Strange but intersting mode around here today.  Been on the road most of the day.  I've experienced elevated thunderstorms, strong ones, but not as cellular as these.  There was one from my location I literally could watch it work it's way down to the surface.  I've never visibly seen that, was pretty cool.

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6 hours ago, Hoosier said:

Yeah, I had a chance to check LSRs and there are some damage reports in there from the wake low.

It must have maintained across Northern IN and Northwest Ohio, because IWX also reported scattered non-T-storm wind damage this afternoon

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Chad from LAF sent IND a pic from their tower cam of a wall cloud with the cell that moved through earlier. Several reports of it rotating and looked pretty good on radar for a couple of scans.

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Helicities are fairly high this evening. Looks like the cap is in place but could be eroding and instability is fairly high. Honestly wouldn’t be shocked to see a potential tornado or two tonight, this isn’t a terrible setup

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3 hours ago, nwohweather said:

Helicities are fairly high this evening. Looks like the cap is in place but could be eroding and instability is fairly high. Honestly wouldn’t be shocked to see a potential tornado or two tonight, this isn’t a terrible setup

Wasn't enough in the end

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Fun fact... the Waukegan tornado is going to be only the 3rd tornado in the LOT cwa in the first half of September since 1978.  The other 2 were EF1 in Jasper county, IN on 9/4/2008 and EF0 in LaSalle county, IL on 9/1/2013.  Looking at all of IL/IN in general, the first half of September is a bit of a downtime.

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Public Information Statement
National Weather Service Chicago IL
412 PM CDT Wed Sep 4 2019 /512 PM EDT Wed Sep 4 2019/

...NWS DAMAGE SURVEY FOR 9/3/2019 BEACH PARK/WAUKEGAN AREA TORNADO...

.National Weather Service staff on Wednesday surveyed the storm
damage from Tuesday evening in far north Waukegan. With the
assistance of Lake County Emergency Management Agency (EMA) and
Waukegan EMA, along with trained spotter reports, we were able to
conclude a brief tornado occurred before moving over the lake.

.WAUKEGAN TORNADO...

Rating:                 EF-1
Estimated peak wind:    90 mph
Path length /Statute/:  3.1 miles
Path width /Maximum/:   200 yards
Fatalities:             0
Injuries:               1

Start date:             9/3/2019
Start time:             6:39 PM CDT
Start location:         3.5 miles north-northwest of Waukegan
Start Lat/Lon:          42.4113 / -87.8624

End date:               9/3/2019
End time:               6:43 PM CDT
End location:           3.0 miles north-northeast of Waukegan
End_lat/lon:            42.4030 / -87.8024

Damage from the tornado was minor to commercial buildings and
residences, including windows blown out, minor roof damage, fence
destruction, and damage to a gas station canopy. There was a
vehicle flipped just east of the intersection of York House Road
and Lewis Avenue where the one minor injury occurred. Numerous
trees felled or experienced heavy branch damage. The tornado was
at its widest and strongest in the latter half of its path.

The one mile stretch between the last damage point and the lake
shore was unable to be surveyed due to a nature area. A
waterspout was reported just off shore and so by interpolation we
took the tornado path to the lakefront.

EF Scale: The Enhanced Fujita Scale classifies tornadoes into the
following categories.

EF0...Weak......65 to 85 mph
EF1...Weak......86 to 110 mph
EF2...Strong....111 to 135 mph
EF3...Strong....136 to 165 mph
EF4...Violent...166 To 200 mph
EF5...Violent...>200 mph*

NOTE:
The information in this statement is PRELIMINARY and subject to
change pending final review of the event and publication in
NWS Storm Data.

$$

LENNING/FRIEDLEIN/CARLAW

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