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Author Topic: Memorial Day Weekend Storms  (Read 20793 times)

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Offline JayCee

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Re: Memorial Day Weekend Storms
« Reply #105 on: May 29, 2017, 12:29:51 PM »
Quote
Models differ on the location and timing of convective activity
across the area today. The NAM keeps storms across southern
sections, while the HRRR spreads storms north all the way to VA.
The RAP, GFS, and SREF support the general idea of the southern
sections having most of the precip today, and given the W-NW flow
at 850 mb and lack of instability that is expected in the northern
half, this solution seems realistic. So the forecast will have
likely PoPs in southern sections but take chance PoPs into NE TN.
A weak cold front and a midlevel shortwave trough will move
through in the evening and take precip out of the area around
midnight.


Looks like the HRRR wins the battle of the models for at least today.  Most of the models showed the rain staying to my south today, except for the HRRR.  It is now raining hard enough to force me and my hot grill under the porch, and from the look of the radar, it may rain for several more hours.  There is a whole lot of green spreading northeast from Chattanooga toward my area.  I see no evidence of storms, though.  Just a lot of light rain.  Bleh.   ::coffee::
"For many years I was self-appointed inspector of snowstorms and rainstorms, and did my duty faithfully, though I never received one cent for it.." 
Henry David Thoreau

Offline bugalou

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Re: Memorial Day Weekend Storms
« Reply #106 on: May 29, 2017, 02:26:16 PM »
Hey guys, hate I missed the event on here, was tied up.  We did take a pretty good beating down my way too.  At my house we easily got 75-80 MPH winds and trees were down all over the place.  My parents lost power, but I lucked out and my leg of the grid is fed from the east which fared better off.  I only lost 1 tree in my yard. 

It looks like there was a swath of wind enhancement running nw to se right across the heart of Memphis.  This was caused by intersecting outflow boundaries.  As that vector propagated to the south it enhanced the winds at the surface to well above severe levels.  As you see on the radar below, I got most of the wind before the heavy rain and the actual storm cells got to me.  This is probably the case for most of us in Memphis and the metro.   Looking forward to MEG's analysis.

A couple of  pictures:







EDIT

Found a TMEM loop that MEG posted on twitter that shows the OFB collision I mentioned:




« Last Edit: May 29, 2017, 03:59:40 PM by bugalou »

Offline mempho

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Re: Memorial Day Weekend Storms
« Reply #107 on: May 29, 2017, 09:07:36 PM »
950,000 people live in Shelby Co in an incredibly dense urban core surrounded by trees. The city is carpeted with dense forest like not many I've been in before which is a recipe for power outages.

The effects of straight line winds in a derecho are much more spread out vs. tornado thus the damage you see here. I might even say that power restoration might be harder with these kinds of winds than a tornado- but that's just opinion.

I'm going to guess the worst hit areas of the city itself had wind gusts nearing 90mph, potentially more.
Keep in mind that the insurance risk rating for wind in Memphis is extremely high at 75/100.  Insurance company risk ratings are based not just on the historical record (which can often be skewed) but on meteorological expertise.  I found it surprising, but Memphis is apparently one of the highest risk cities for derechos.  I'm not sure that the extreme power outages are all that avoidable. 

I have long speculated that the slight orographic lift of the bluffs may help to bring the more extreme winds down to the surface. 

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Put on my big snow boots and
I boarded the plane
Touched down in the land of the Delta Blues
In the middle of the freezing rain

Snow up high
Won't you pour down over me
Yeah, I got some accretion
But I'm as blue as a boy can be

Offline mempho

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Re: Memorial Day Weekend Storms
« Reply #108 on: May 29, 2017, 09:17:31 PM »
Still 113,000 out, myself included.

Here's a pic of blowdown in North Memphis at Hwy 51 and Watkins. This is probably 3-5 miles NW of Midtown in Frayser. Populated areas begin near here.
That is near the pocket of 120+ that was showing up on WREG radar -  it was so high that they looked for rotation and found none.  They makes it look like it actually could have been a pocket of Cat 3 winds. 

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Put on my big snow boots and
I boarded the plane
Touched down in the land of the Delta Blues
In the middle of the freezing rain

Snow up high
Won't you pour down over me
Yeah, I got some accretion
But I'm as blue as a boy can be

Offline Skillsweather

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Re: Memorial Day Weekend Storms
« Reply #109 on: May 29, 2017, 10:01:44 PM »
Is that a down burst probably? Because thats very very isolated yet very intense.. Its like a down burst look of damage in a small area.. Almost looks non nature related because of the isolation of it.
Snow, flood and storm lover.

Offline mempho

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Re: Memorial Day Weekend Storms
« Reply #110 on: May 29, 2017, 10:05:39 PM »
Is that a down burst probably? Because thats very very isolated yet very intense.. Its like a down burst look of damage in a small area.. Almost looks non nature related because of the isolation of it.
It was a small pocket on radar-  the TV guys at first suspected the high velocities over a small area was due to rotation but they found none-  obviously, it was just straight line and probably an early microburst

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Put on my big snow boots and
I boarded the plane
Touched down in the land of the Delta Blues
In the middle of the freezing rain

Snow up high
Won't you pour down over me
Yeah, I got some accretion
But I'm as blue as a boy can be

Offline bugalou

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Re: Memorial Day Weekend Storms
« Reply #111 on: May 30, 2017, 05:04:49 PM »
It was a small pocket on radar-  the TV guys at first suspected the high velocities over a small area was due to rotation but they found none-  obviously, it was just straight line and probably an early microburst

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Look at my TMEM loop from above.  The way those OFBs are perpendicular and intersect would almost certainly create some rotation, I am almost certain there were high rotational velocities.  I didn't get a capture of it, but as I watched the storm on radar scope I got several TVS indications, but a quick flip between RV and BV on the radars and you could see it was not a tornado. 

I think when all it said in done we will see this was not a pure derecho straight line wind event driven by a rear inflow jet from a well established MSC, but this was moderately strong outflow boundaries from several different cells that interacted in a way that sheared on the the boundaries and greatly increased it's speed though conservation of angular momentum.  The path with the most damage aligns quite nicely with that vector.

Here is a terrible drawing to try and explain better than above.



Also note I am not a professional.  I am sure NWS/MEG will have a lot more data than me.  This is just are my thoughts as I watched the event unfold and looked at radar data.
« Last Edit: May 30, 2017, 05:07:43 PM by bugalou »

Offline harlequin

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Re: Memorial Day Weekend Storms
« Reply #112 on: May 30, 2017, 09:45:38 PM »
About to pass the three day mark. Hopeful about tomorrow.

Also, great posts above.

Offline mempho

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Re: Memorial Day Weekend Storms
« Reply #113 on: May 30, 2017, 10:04:27 PM »


Look at my TMEM loop from above.  The way those OFBs are perpendicular and intersect would almost certainly create some rotation, I am almost certain there were high rotational velocities.  I didn't get a capture of it, but as I watched the storm on radar scope I got several TVS indications, but a quick flip between RV and BV on the radars and you could see it was not a tornado. 

I think when all it said in done we will see this was not a pure derecho straight line wind event driven by a rear inflow jet from a well established MSC, but this was moderately strong outflow boundaries from several different cells that interacted in a way that sheared on the the boundaries and greatly increased it's speed though conservation of angular momentum.  The path with the most damage aligns quite nicely with that vector.

Here is a terrible drawing to try and explain better than above.



Also note I am not a professional.  I am sure NWS/MEG will have a lot more data than me.  This is just are my thoughts as I watched the event unfold and looked at radar data.

Wow, awesome post.  When they said rotation, the TV guys did was look at doppler velocities with the red/green directional overlay.  While there may have been some rotation and there may have been a TVS signature, they couldn't find winds moving in opposite directions in that radar scan-  I believe they were focusing on near-surface winds (which makes sense since there was no rotation).

Sent from my SM-G900R7 using Tapatalk



Put on my big snow boots and
I boarded the plane
Touched down in the land of the Delta Blues
In the middle of the freezing rain

Snow up high
Won't you pour down over me
Yeah, I got some accretion
But I'm as blue as a boy can be

Offline bugalou

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Re: Memorial Day Weekend Storms
« Reply #114 on: May 30, 2017, 10:18:29 PM »

Wow, awesome post.  When they said rotation, the TV guys did was look at doppler velocities with the red/green directional overlay.  While there may have been some rotation and there may have been a TVS signature, they couldn't find winds moving in opposite directions in that radar scan-  I believe they were focusing on near-surface winds (which makes sense since there was no rotation).

Sent from my SM-G900R7 using Tapatalk
I think it was the close proximity to the radar combined with the hybrid nature of the thunderstorms (being something like a bow echo/derecho with near by cellular storms) caused some measureble Eddies.  Def nothing mesocyclonic or even derecho head tornadic caused any of this.  The partially veering from the opposing winds definitely played into the strength though.

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Offline bugalou

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Re: Memorial Day Weekend Storms
« Reply #115 on: May 30, 2017, 10:24:33 PM »
About to pass the three day mark. Hopeful about tomorrow.

Also, great posts above.
I got a generator you are welcome to borrow if things look like they will drag out over the weekend.

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Offline Curt

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Re: Memorial Day Weekend Storms
« Reply #116 on: May 30, 2017, 10:31:49 PM »

Offline mempho

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Re: Memorial Day Weekend Storms
« Reply #117 on: May 31, 2017, 12:07:26 AM »
https://youtu.be/A9hv1_r-QiM

Nice vid W Frogge
City, what kind of wind wind speed are we looking at there?

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Put on my big snow boots and
I boarded the plane
Touched down in the land of the Delta Blues
In the middle of the freezing rain

Snow up high
Won't you pour down over me
Yeah, I got some accretion
But I'm as blue as a boy can be

Offline NashRugger

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Re: Memorial Day Weekend Storms
« Reply #118 on: May 31, 2017, 02:07:04 PM »
City, what kind of wind wind speed are we looking at there?

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That easily is a 90-100mph microburst because you have nearly total failure of the entire tree canopy in such a localized area.

Offline @NashSevereWx

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Re: Memorial Day Weekend Storms
« Reply #119 on: May 31, 2017, 02:17:35 PM »
I cosign the OFB explanation in Memphis. The highest BVs were colocated with the OFB moving SW thru Memphis Metro.
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