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June 28-30 Severe/Heavy Rain Threats


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1 hour ago, andyhb said:

Pretty violent looking tornado from near Bedford, IA.

 

Honestly, the motion starting at 0:13 is some of the most impressive I've ever seen. Looks like its being fast-forwarded. Certainly a violent tornado, but I haven't heard any reports of major damage in Taylor County. Must have avoided structured for the most part.

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3 hours ago, andyhb said:

Today another great example of how shear trumps instability. Relatively modest instability for the time of year, but with more southerly surface winds than projected, large low level hodographs and favorable storm relative wind profiles, it yields a regional tornado outbreak especially for Iowa. Favorable storm mode lasting for longer than expected as well, largely because of the aforementioned wind profiles.

A lot of things came together really, including low LCLs and rich boundary layer moisture (lower 70s dews pooled in southwestern Iowa) which helped allow the tornado threat to get going rather early in storm development. Whenever you have low LCLs and substantial low level shear, it won't take much for storms to rotate and become tornadic. Just noting climo, the mean MLCAPE for tornado events in southern Iowa is generally in the 1500-2000 J/kg range and tonight was at or above that range. 

Going back to LCLs, I didn't see any conclusive tornadoes, but saw plenty of very low wall clouds and funnels, even with cells that looked junky on radar. Rolling hills blocked my view of Arbor Hill, but the wall cloud could have easily been mistaken for a large wedge. The cloud base had to have been AOB 500m, but by the time I got there, the funnel was lifting. 

Another consideration is that storms fired along an apparent residual outflow boundary, ahead of the cold front. That alone was a key difference between a brief supercell tornado threat and a more widespread, longer-lived tornado event, like we saw. 

From a chasing standpoint, it was certainly not easy. A lone cell fired north of DSM by mid-afternoon and that decoyed some chasers north. Those who jumped on the early southern echo near Nebraska City probably had the catch of the day. I was slow to bite and if it wasn't for hills blocking view, I'd probably have had better footage of the tornadic storms up near I-80.

 

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3 hours ago, andyhb said:

Today another great example of how shear trumps instability. Relatively modest instability for the time of year, but with more southerly surface winds than projected, large low level hodographs and favorable storm relative wind profiles, it yields a regional tornado outbreak especially for Iowa. Favorable storm mode lasting for longer than expected as well, largely because of the aforementioned wind profiles.

Was CAPE values of 2000 J/KG and mid-level lapse rates of 7 C/KM really all that modest?

I mean, the numbers weren't earth-shattering, but definitely more than enough to trigger and sustain decent activity. In fact, when doing a comparison to the data from DVN's historical soundings, the level of instability today was slightly above average for this time of year.

EDIT: Never mind, I see Quincy beat me to it.

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1 hour ago, Powerball said:

Was CAPE values of 2000 J/KG and mid-level lapse rates of 7 C/KM really all that modest?

I mean, the numbers weren't earth-shattering, but definitely more than enough to trigger and sustain decent activity. In fact, when doing a comparison to the data from DVN's historical soundings, the level of instability today was slightly above average for this time of year.

EDIT: Never mind, I see Quincy beat me to it.

For the time of year.

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14 minutes ago, Stebo said:

2000 J/kg SBCAPE at this time of year isn't earth shattering, especially when we can get to 3000+ very easily on most hot and humid days in Iowa.

Yeah, and we are getting into the time of year where you can have those crazy 5000-7000+ days, although usually lacking good low level shear.  

The environment today was well suited for tornadoes.  But only focusing on CAPE, I agree that it was not that impressive by late June standards.

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Public Information Statement
National Weather Service Chicago IL
421 PM CDT Thu Jun 29 2017 /521 PM EDT Thu Jun 29 2017/

...NWS DAMAGE SURVEY FOR 06/28/2017 TORNADO EVENT...

.Cherry Valley to Belvidere Tornado...

Rating:                 EF-1
Estimated peak wind:    100 mph
Path length /Statute/:  6.0 miles
Path width /Maximum/:   200 yards
Fatalities:             0
Injuries:               0

Start date:             Jun 28 2017
Start time:             8:14 PM CDT
Start location:         2 miles SSE of Cherry Valley
Start Lat/Lon:          42.211/-88.948

End date:               Jun 28 2017
End time:               8:22 PM CDT
End location:           4 miles S of Belvidere
End_lat/lon:            42.197/-88.835

.Belvidere Tornado...

Rating:                 EF-1
Estimated peak wind:    100 mph
Path length /Statute/:  1.6 miles
Path width /Maximum/:   100 yards
Fatalities:             0
Injuries:               0

Start date:             Jun 28 2017
Start time:             8:23 PM CDT
Start location:         4 miles SSE of Belvidere
Start Lat/Lon:          42.208/-88.810

End date:               Jun 28 2017
End time:               8:25 PM CDT
End location:           5 miles SE of Belvidere
End_lat/lon:            42.208/-88.778

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.

$$

- Izzi
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