6/26/10 DISC: SD/IA/MN

[Andy Jackson]

I was kind of curious about this event. After seeing the first storm south of the i-90 and the subsequent wall cloud my first thoughts were I was going to see a big tornado. This didn't occur, yet with that storm and subsequent storms that fired to the south it seemed like if just one ingredient was different this day could have had quite a different outcome. Maybe I was hyped by the 15% hatched spc tornado probabilities. Does anyone have insight on what was missing. I personally think 0-1km helicity values needed to be higher, I would like to hear what others think about this.

 

[Dean Baron]

I was at work until 3:00 on Sat. so I only had a couple brief moments to look at data, but from what I could tell the warm front was expected to move north and light up which from what I could tell was what caused a 15% hatch tor to be issued. A cold front extended across the eastern Dakotas and was also expected to fire off storms, but as is usual with cold fronts these storms were expected to go linear quickly and pose only brief tornado potential. Storms didn't fire along the warm front, and the storms that did fire went up along the cold front. Also, instead of the warm front pushing north, it actually appeared to retread south further in to IA. The models showed the warm front retreating south in to IA between 18z and 0z, which was why I was surprised to see such a high tornado probability, but figured I was missing something (and probably still am). So, the only storms that fired (besides the one that went up southwest of FSD, which I think got over run by the cold front rather quick too) went up along the cold front and the linear forcing of a cold front caused things to line out quickly, and nothing went up along the warm front. Please let me know if I'm wrong, because I'd really like to learn from this too. Looking forward to what others have to say.

 

[Andy Jackson]

Here's a surface analysis taken after the before mentioned storm had fired. It seems like the surface winds were weaker and less uniform in the area of the above mentioned storm.

 

[Matthew Piechota]

I was semi-nowcasting this one and it seemed to me that the shear was partly the problem with the wind fields not all that great. If they were better I think it could of been alot bigger day. Also, there were alot of storm mergers with more linear forcing then what people thought that was playing havoc on certain storms that would of had the chance to do something. the environment was pretty set for big storms with dew points pooling into the middle 70's right on the warm front in the afternoon. just my 2 cents.

 

[Derek Weston]

Was never impressed with the low level shear. 850 on down... just not too much to work with. OK directionally...

Based on model runs I was seeing... was surprised SPC went 15% hatched and mod...

 

[Jeff Duda]

Was never impressed with the low level shear. 850 on down... just not too much to work with. OK directionally...

Based on model runs I was seeing... was surprised SPC went 15% hatched and mod...

Agreed on both accounts. I also agree with others that deep layer shear struggled to be impressive/sufficient over the boundary and the 15% hatched region. Plus, 700 mb temps really surged over the area during the day, so capping was probably an issue. Also there was a band of low-mid level clouds over NC/NW IA and into far SE SD and extreme S MN that probably reduced instability near the boundary. Surface winds were okay until later in the day.

 

[MatthewCarman]

Was never impressed with the low level shear. 850 on down... just not too much to work with. OK directionally...

Based on model runs I was seeing... was surprised SPC went 15% hatched and mod...

I thought the MOD risk was for large hail and destructive winds? If so it verified as there was reports of winds over 60 MPH and large hail.

Edit: The main risks for that day were damaging winds and large hail. There was a 45% chance of large hail and a 30% chance of damaging winds with only a 15% chance of a tornado. These risks were highest in the Moderate risk outlook. In and near the Moderate risk area hail as large as 2.75 inches fell and winds were reported as high as 74 MPH from the reports page. Numerous trees and power lines were knocked down from winds over 70 MPH. What made you think the Moderate risk was for tornadoes? There was 12 tornadoes reported in and around the Moderate risk area so it could have been for tornadoes but I thought someone else said in the forecast thread the risk was for damaging winds?

 

[Danny Neal]

15% hatched for tornadoes is at the least a moderate risk for tornadoes among other wind/hail.

 

[MatthewCarman]

I did not know that. Thanks for letting me know. I think with multiple EF2 tornadoes, hail to 2.75 inches and winds over 70 MPH knocking numerous trees and power lines down in and around the moderate risk area verified it but I could be wrong. I think the moderate risk was for a combination of the three weather types. Anyways I think I made my point and will leave it at that.

 

[Jayson Prentice]

I thought the MOD risk was for large hail and destructive winds? If so it verified as there was reports of winds over 60 MPH and large hail.

Both the 45% and hatched area for damaging winds and the 15% and hatched area for tornadoes fall within the moderate categories. And remember that verification of the risks means a certain number of reports per square mile covered by the risk. For verification purposes, a moderate risk should result in at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (at least 30 reports that likely would be associated with a squall line, bow echo or derecho, all occurring within a 50,000 sq mile area. The area in question was ~35,000 square miles with the 1630z outlook and therefore could withstand slightly lower numbers and still verify. There were 8 tornadoes that occurred within the moderate risk, and ~50 large hail reports within the moderate risk area. Therefore, the moderate risk for this day did verify with SPC standards...

So despite the lack of photogenic and potentially strong tornadoes that could have occurred, the day was still an impressive severe weather day that warranted the moderate risk.

 

[MatthewCarman]

Both the 45% and hatched area for damaging winds and the 15% and hatched area for tornadoes fall within the moderate categories. And remember that verification of the risks means a certain number of reports per square mile covered by the risk. For verification purposes, a moderate risk should result in at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (at least 30 reports that likely would be associated with a squall line, bow echo or derecho, all occurring within a 50,000 sq mile area. The area in question was ~35,000 square miles with the 1630z outlook and therefore could withstand slightly lower numbers and still verify. There were 8 tornadoes that occurred within the moderate risk, and ~50 large hail reports within the moderate risk area. Therefore, the moderate risk for this day did verify with SPC standards...

So despite the lack of photogenic and potentially strong tornadoes that could have occurred, the day was still an impressive severe weather day that warranted the moderate risk.

Thanks Jason. That is what I was trying to figure out.

 

[Jeff Duda]

What really got under my skin was the issuance of a tornado watch with probs of 80% and 50% for at least 2 tornadoes and at least one EF2 or higher rated tornado. If you check the SPC database and add in Andy G.'s report, there were only 2 total reports in the watch area. I haven't seen any damage surveys or PNSs from KFSD, but I would be surprised if either was EF2 or higher rated. To me, an 80-50 watch implies a stronger confidence in a higher end event than what occurred. Hell, on May 22nd (the Bowdle, SD EF4 day), the tornado watch was a 70-40.

 

[Jayson Prentice]

There were a total of 8 preliminary reports of tornadoes, none of which appear to be duplicates, thus potentially 8 different tornadoes that occurred within the initial watch (tornado watch #415). I cannot find information regarding the ratings of these tornadoes, but I think it is safe to say that the 80% was warranted given that storms were likely to develop and with the overall wind shear they would produce tornadoes.

The 50% probability of significant tornadoes is in question, however to compare the probabilities to the Bowdle day: If I remember correctly the Bowdle day wasn't a guaranteed day of convection, which when taking into account the probabilities you have to believe that it lowered them substantially. I think if you were to 'guarantee' convection on the Bowdle day you would have seen those numbers much higher than they were with the initial tornado watch issuance.

In summary, yes the day was not as good as it could have been or would have been given storms fire along the warm front ahead of the cold front/outflow. But, to say that the SPC over-played the event is highly speculative... More likely is that some chasers got hyped over the idea of the 15% and hatched and ended up with nothing but outflow given the circumstances.

 

[Jeff Duda]

There were a total of 8 preliminary reports of tornadoes, none of which appear to be duplicates, thus potentially 8 different tornadoes that occurred within the initial watch (tornado watch #415). I cannot find information regarding the ratings of these tornadoes, but I think it is safe to say that the 80% was warranted given that storms were likely to develop and with the overall wind shear they would produce tornadoes.

The initial tornado watch (WW #0415) expired at 8:00 PM CDT (01Z). There was only one tornado report in the database (not counting Andy G.'s tornado) within that watch box while it was valid. The remainder of the tornadoes occurred after the watch expired and/or outside the watch box, and really, they were front-of-the-line spinups, not supercellular tornadoes.

 

[Jayson Prentice]

Valid point on the timing of the watch, didn't look close enough at the report times.

My summary point still remains valid, the watch was definitely warranted and the moderate risk which was issued did indeed verify. Although it wasn't the ideal verification for chasers, it still was a valid moderate risk.