9/20/07 NOW: MN/SD/ND

[Chad Cowan]

Potentially significant tornado threat increasing this afternoon and evening in eastern ND and west-central MN... 60+ kt ssw LLJ beneath 90 kt westerly h25 jet streak nosing in, and consistent moderate wind speeds in the mid-levels are resulting in incredibly strong wind shear (see hodograph below). Moderate instability is on the rise resulting from daytime heating into the mid-80s and dewpoints AOA 70. OFB's from ongoing convection and WF should provide enough forcing for surface based discrete cells to form. Cu field over SE SD should be focal point for surface based convection in the near future as per RUC guidance.


MPX 17z sounding

 

[Michael O'Keeffe]

I am in Sioux Falls, SD we have strong south winds and a great CU field as Chad mentioned. There is good reason to believe tornadoes are possible later today. We are going to wait it out and hope the RUC is right.

 

[Danny Neal]

Very nice TVS showing up southwest of Litchfield, MN. Slightly elevated convection is potentially going to start riding the warm front, if this happens I expect to see this supercell to head east toward northern MSP areas.

 

[Jeff Snyder]

If I were chasing right now, I think I'd be very near Marshall, MN. There may be a small mesolow near there giving rise to the E and ENE sfc winds at Madison and Montevideo MN (La Qui Parle and Chippewa counties). Certainly, the wind and shear profiles (from the Wood Lake profiler and MPX VWP) are quite impressive throughout the risk area, particularly east of the SD/MN border, with 0-6km shear in the 45-60kt range and 0-1km SRH 150-350 m2/s2 in the warm sector (increasing to ~800 m2/s2 north of the warm front, but that's moot giving the high stability in the cool sector).. I'm a little less concerned nearer FSD, where sfc winds are more out of the southwest. Cap is still an issue, but 88/66 at Marshall, with a strong southerly wind just south of the warm front (near Granite City), is telling me "go here, go here". That warm front is slicing through the middle of the MSP area, so there could be trouble if a sustained supercell can develop out of the somewhat messy convective area near Litchfield (W/WNW of MSP). Latest BR data from MPX indicates that the warm front, as seen by the fineline that stretches WSW/ENE on the western side of the MSP metro, is moving little, or maybe even moving slowly southward currently. The cell farther north near Mora is likely quite elevated, with downstream sfc ob (Rush City) indicating 57/52!

18z ABR sounding did show a rather stout / sharp cap nosed near 750mb, but the combination of near 90F temp, 67-68F Td, and very strong deep moisture convergence noted on latest Mesonanalysis may be enough to allow for initiation in western MN. HCRs / cloud streets in much of the warm sector south of the warm front still suggest that convective inhibition remains, and any mid-level forcing associated with DPVA ahead of any shortwave trough / vort max is not going to be present much south of the warm front. There are a couple Td=70F readings coming in Windom and Jackson (to the immediate east of some sort of confluence boundary that stretches N/S across southwestern MN and delineates SSE winds from SSW winds). A definate high reward, high bust potential, that's for sure. Kory Hartman looks to be in a good place, though I'd rather be a tad farther east / downstream, particuarly since reflectivities are increasing on a possible developing cell east of Brookings. I think there's a pretty good shot at a significant tornado if a sustained, discrete supercell can develop along or near that warm front from the look of the wind/shear profiles and instability (which is not particularly impressive, but may be "sufficient" for a siggy tor).

Edit: Sure, as soon as I brush off the Rush City storm, I see that there was just a law enforcement report of a tornado with that storm... I certainly wouldn't have thought that the storm would have presented much of a tornado threat given the degree of stability for any sfc or near-sfc parcel that far north of the warm front.

 

[Kory Hartman]

Have chased a tiny cell that initiated near Lake Benton... chased it toward the NE for a while, but it died quickly.

Started heading back southwest toward Pipestone as a couple more towers went up... but no dice again.

Listening to wall-to-wall coverage of storms moving through the Twin Cities on WCCO 830 AM. Tornado Warnings in the metro with confirmed wall/funnel clouds and flooding.

 

[Danny Neal]

Looks like a nocturnal event shaping up for the rest of TOR watch. TC may be hit twice tonight, my guess would be the initial supercells that MAY develop near Morris/Benson MN area may congeal into an MCS. Although this area is north of the warm front as Benson is sitting in at 64/63 while 22 miles to the south in Montevideo it is 78/63 while 14 miles to the south of there in Granite Falls its 86/64. Pretty nice boundary there if I say so myself! It will be interesting to see those of you that travelled from KC or SD if you will stick it out?

 

[Michael O'Keeffe]

Us KC guys are on our way home. Have to be back tomorrow so we just had to bust. Really dissipointing day. Though some interesting pics and a beautiful sunset. Will post pics on a report thread later.

 

[Danny Neal]

Storm initiation has begun just northwest of Morris, MN, it was interesting to watch this pop out of nowhere. GR3 mentioning a possibility of 2' hail but the storm seems to be elevated as it is about 30-40 miles north of the boundary lying across south central and southwest MN.

Edit: It was only a matter of time before a tornado watch went out again.....looks like supercell is splitting, but I am still not totally convinced it will root on the better moisture. However the wave out of WY and LLJ may play a big role in tonights activity.

 

[Jeff Snyder]

There is ample instability for parcels just off the surface, per MUCAPE analysis off SPC/RUC mesoanalysis. In addition, effective SRH is still relatively impressive. I agree that there's a very good probability that any supercell will be elevated, implying a significantly reduced chance of tornadoes. That said, I found the Watch Probabilities very interesting for the new tornado watch:

Tornadoes
Probability of 2 or more tornadoes

Mod (60%)
​

Probability of 1 or more strong (F2-F5) tornadoes

Mod (50%)​

I don't think I've ever seen such a high probability for "1 or more strong tornadoes" without the watch being classified as a PDS tornado watch... I though T watches were typically PDS for >=30% prob of 1 or more strong tornadoes, particularly if the prob of 2 or more tornadoes is >50%. It's not a big deal -- just an observation.

Deep moisture convergence remains strong throughout portions of MN and the eastern Dakotas, and significantly cooler mid-level air is currently advecting into the northern 1/2 of MN and eastern ND, which will help steepen lapse rates. This is associated with a weak vort max that appears to be traversing the watch area attm.

 

[Danny Neal]

There is ample instability for parcels just off the surface, per MUCAPE analysis off SPC/RUC mesoanalysis. In addition, effective SRH is still relatively impressive. I agree that there's a very good probability that any supercell will be elevated, implying a significantly reduced chance of tornadoes. That said, I found the Watch Probabilities very interesting for the new tornado watch:

Tornadoes
Probability of 2 or more tornadoes

Mod (60%)
​

Probability of 1 or more strong (F2-F5) tornadoes

Mod (50%)​

I don't think I've ever seen such a high probability for "1 or more strong tornadoes" without the watch being classified as a PDS tornado watch... I though T watches were typically PDS for >=30% prob of 1 or more strong tornadoes, particularly if the prob of 2 or more tornadoes is >50%. It's not a big deal -- just an observation.

Deep moisture convergence remains strong throughout portions of MN and the eastern Dakotas, and significantly cooler mid-level air is currently advecting into the northern 1/2 of MN and eastern ND, which will help steepen lapse rates. This is associated with a weak vort max that appears to be traversing the watch area attm.

Hey Jeff, I was just going to comment on that, it is pretty odd to see a 60% chance of 2 or more tornadoes and 50% that they may be strong to violent. Just odd thats all, maybe a typo? But with all the shear and relatively low LCL's with a 61/60 in Alexandria anything is possible.

Storm still showing 2' hail marker in over the Nashua area and I am noticing some weaker convection to the south of that trying to fire up. I can see a tornado or two out of the storms further south near the warm front.

 

[Danny Neal]

At 11:55 PM CDT law enforcement reported golfball to baseball hail 8 NE of Wheaton, MN.

 

[Chad Cowan]

Better late than never I guess These cells are blowing up in an extremely sheared environment with low LCL's and moderate instability on the order of 3000 j/kg mlcape... as was the same this afternoon, any tornado that does form has the distinct possibility of being quite strong. The deepening low in central SD and weak vort max that Jeff mentioned appear to have sparked this new convection, which may be slightly 'elevated', but with such low dp depressions and resulting low LCL's, IMO, this term is entirely relative.

The storm in Otter Tail county has really been trying to produce a left split. Hopefully this will be a precursor to the main cell deviating to the east and taking it in a more tornado conducive motion relative to the LL inflow winds. Too bad it is 100+ miles from the nearest radar site, so we're only sampling the mid-levels.

As for the Tor watch probabilities, obviously the size of the red box contributes to the first percentage being so high. I'm not sure about this but isn't a PDS watch more of a judgment call by the SPC?

 

[Danny Neal]

Storm in northern Douglas county, has a bit of a turn toward the east as Chad mentioned, looks like tomorrow will be another very active day into the N. WI. area all depends if an MCS really does form out of this cluster of severe storms.