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March 17-18 Severe Weather Event


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2 minutes ago, jpeters3 said:

I think that was the original poster's point.  A single violent tornado doesn't verify a High risk - many tornadoes do.  We've had many tornadoes today, hence high risk verified.

I think what a lot of us (myself included) forget is that tornado probabilities in SPC outlooks reflect confidence in a tornado outbreak.  They are not direct forecasts of violent tornadoes.  A smaller outbreak that verifies a high risk, and that will confidently occur, warrants a 45% tornado threat.  Another larger outbreak, but with greater uncertainty from the forecasting perspective, might warrant 15% or 30%.  Just because the second outbreak was bigger than the first doesn't mean the forecast was wrong - it just means that there was more uncertainty leading up to the big outbreak, compared to the smaller outbreak.   We've likely had a several long tracked strong tornadoes today which fits well well with the forecast.


In other words, tornado % (i.e, 15%, 30%, 45%, 60%) in the forecast doesn't necessarily correlate with outbreak size.  It correlates with confidence leading up to the outbreak.

Indeed.  There are so many variables in forecasting something like this, and forecasters like yourself always have to be wary that the public is constantly finding reasons to doubt forecasts if they don't line up exactly with expectations.  It's a serious catch 22, and why I generally chafe when I hear "bust" talk thrown around on days like this -- I remember reading from no fewer than 2 posters that 4/27/11 was already busting around noon, and that sort of dialogue is problematic in general.

I think they were pretty close to bang-on with this.  We're fortunate that we (likely) didn't see violent tornadoes today (and hopefully won't through the night hours).  But it might anger people who believe that a high risk day always indicates a massive outbreak with dozens of fatalities.

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8 minutes ago, nwohweather said:

The bust talk is a bit extreme, although I’m shocked Mississippi didn’t get a little more as the main show today has definitely been Alabama. Atmosphere is too worked over at the moment across the Mid South for this to really take off tonight in my opinion. 

Still it’s been one hell of an afternoon. And truthfully I think tomorrow in the Carolina’s offers a potentially bigger threat than today to be honest 

 

I hope not. I think it all depends on how long it takes to clear things out overnight and when the warm air gets here. A lot of times we get saved by the clouds hanging around longer than expected. 

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7 minutes ago, nwohweather said:

Still it’s been one hell of an afternoon. And truthfully I think tomorrow in the Carolina’s offers a potentially bigger threat than today to be honest 

Ironically, 5/20/19 played out similarly. Everyone got so burned by that day they ignored the massive outbreak that ensued over the next ten days 

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I think that was the original poster's point.  A single violent tornado doesn't verify a High risk - many tornadoes do.  We've had many tornadoes today, hence high risk verified.

I think what a lot of us (myself included) forget is that tornado probabilities in SPC outlooks reflect confidence in a tornado outbreak.  They are not direct forecasts of violent tornadoes.  A smaller outbreak that verifies a high risk, and that will confidently occur, warrants a 45% tornado threat.  Another larger outbreak, but with greater uncertainty from the forecasting perspective, might warrant 15% or 30%.  Just because the second outbreak was bigger than the first doesn't mean the forecast was wrong - it just means that there was more uncertainty leading up to the big outbreak, compared to the smaller outbreak.   We've likely had a several long tracked strong tornadoes today which fits well well with the forecast.

In other words, tornado % (i.e, 15%, 30%, 45%, 60%) in the forecast doesn't necessarily correlate with outbreak size.  It correlates with confidence leading up to the outbreak.

This is exactly what I was getting at.

Let’s say you have a High risk but only 7 tornadoes. 1 EF5 and everything else is EF0/1. High risk does NOT verify by given standards from the SPC.


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A large concern with this setup is the potential for wedge tornadoes overnight in a region notorious for after-dark tornado outbreaks. As the evening wanes and the nocturnal LLJ ramps up, this setup is screaming danger in the 0-1 km layer over Alabama. Textbook long/looping hodographs, shear vectors with a large magnitude component normal to the front, and extremely low LCL heights. Not what you want to see at any time, let alone 12-4 am. These rain wrapped wedges may be hard to make out during the day, let alone at night. Nobody should attempt to confirm these.

Was going to post this last night, I'll post it now that some skeptics have showed up before the nocturnal LLJ has fully ramped up.

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I mean come on is this the third time today Selma has at least had a decent shot at a tornado? Just ridiculous in terms of the probabilities of that happening... There was definitely that one confirmed earlier. 

 

DC652DBB-6170-4860-A0DA-9347D7BBC361.jpeg

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Cullman and Selma cells are really intense - some of the strongest rotation we've seen today. Could be the distance from the RADAR, but they're intensifying.

 

Edit: Cullman had a good 10 minute window but weakened - no longer warned.

 

Sent from my Pixel 3 XL using Tapatalk

 

 

 

 

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1 hour ago, Jim Marusak said:

yea, it's starting to look like the initial warm sector wave is diminishing. But the 2nd, frontal wave of strong storms will be developing within the next few hours. we're not done yet.

Ding ding ding. 

 

For ref, until this AM everyone was talking the evening threat. True instability is lower. But more important (frankly) params are higher. The day ain't over yet.

 

As for critiera: the probability refers to the severe weather type, period. To verify you calculate the density of tornadoes (or wind or hail) in given area and correlate against the percentages delineated. However, hatching verifies separately. That signifies sig severe..sig tor, sig hail, sig wind. To verify that you need requisite # events at the specified threshold per unit area. So far today, neither high risk nor hatching seems to have verified--and particularly that 45% prob did not. Cannot say for certain of course. What is obvious though is, not every tor warning verified (entirely normal), so # tor warnings =/= # tornadoes. 

Generally, for sig severe vs threat level: USUALLY, the conditions that produce ef4/ef5 and/or long-track tornadoes tend to also be the conditions that produce a lot of tornadoes. It's a venn diagram with significant overlap-- but it is not 100%. In theory you could get 5 EF5s and nothing else and your high risk would bust. You could, conversely, get 100 EF0 and EF1 spin ups, which would verify...given the right delineation of area of couse. It's just that typically (nearly always) we see EF3-5 stuff on days when conditions are outbreak-y. 

 

To that end, today has, thus far, had a high end enhanced to mid grade moderate feel to it. If we stopped the clock now, yeah, this isn't the national news event that has been advertised. We've all seen moderate days, and enhanced, that have exceeded what we have seen so far today. To be clear, on a true outbreak day, you (typically) have at least one major tornado hit a populated area and be in the news, with yet more tornadoes ongoing at the time. That's not a rule that pertains to outbreaks and metros--you can of course have 100 tornadoes and have all hit unpopulated forests...it's a probability game. With a true outbreak you almost always get enough tornadoes, several strong, that at least one hits a population center (think Bassfield sized town) and causes major damage. We haven't gotten reports today of mass fatalities. You don't have governors pre declare state wide SOEs unless, being blunt, they expect a body count and a need for S&R and state level disaster recovery resources. That is GOOD, but is an indicator that we probably have not seen a density of tornadoes at any intensity that would verify a HIGH. This is fairly subjective reasoning so take that fwiw, but just my observational experience. 

But, fun fact, that was round 1. Thermals are more marginal in the evening but the dynamics very much are not. Helicity and such are much higher tonight. That 45% zone...I don't expect it was made for the afternoon stuff. It was made for round 2. I'd bet money on that. Now that may or may not verify vis a vis how discrete the evening cells remain to take advantage of said dynamics but the threat isn't over yet. 

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HIGH risk still 

DAY 1 CONVECTIVE OUTLOOK    
NWS STORM PREDICTION CENTER NORMAN OK  
0744 PM CDT WED MAR 17 2021  
  
VALID 180100Z - 181200Z  
  
...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF  
ALABAMA...  
   
..SUMMARY  
  
THE POTENTIAL FOR SIGNIFICANT TORNADOES CONTINUES TONIGHT. MUCH OF  
THE GREATEST RISK IS OVER ALABAMA.  
   
..01Z UPDATE  
  
LLJ IS STRENGTHENING ACROSS THE CENTRAL GULF STATES THIS EVENING  
AHEAD OF A STRONG MID-LEVEL SHORT-WAVE TROUGH THAT IS PROGRESSING  
INTO THE LOWER MS VALLEY. LARGE-SCALE FORCING APPEARS TO BE  
CONTRIBUTING TO AN ELONGATED MCS THAT IS GROWING UPSCALE FROM  
EASTERN MS INTO SOUTHEASTERN LA. WHILE THIS MCS IS GRADUALLY  
MATURING, NUMEROUS EMBEDDED MESOS ARE NOTED ALONG THIS CORRIDOR AND  
THE STRENGTH OF THE WIND FIELDS CONTINUE TO FAVOR THE POTENTIAL FOR  
SUPERCELLS. OF MORE CONCERN ARE THE DISCRETE STORMS THAT CONTINUE  
AHEAD OF THE MCS, AND THE POTENTIAL FOR NEW SUPERCELLS.  
STRENGTHENING WIND FIELDS FAVOR TORNADOES WITH THIS ACTIVITY. THE  
PRIMARY CHANGES AT 01Z ARE TO LOWER SEVERE PROBABILITIES IN THE WAKE  
OF THE EVOLVING MCS.  
  
..DARROW.. 03/18/2021  

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Seen this act before. This afternoon was definitely very active, but the large amount of rain in the risk area combined with a large maturing MCS moving through is going to mitigate the risk this evening and overnight.

NOT that there won't be a few more tornadoes, and possibly significant at that, but the risk level has definitely decreased.

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Current trends don’t support long track violent tornados overnight in my opinion. Sure as hell hope I’m right. Nocturnal violent tornados are straight up nightmare stuff in the southeast. Either way hope all the folks down there stay vigilant and stay tuned into the warnings through the night. Worse atmospheres have produced way more serious TORS then what we have seen so far. 

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