07/08/04 FCST: Northern Plains

[Jonathan Merage]

Mike,
I agree. There's a well-defined instability axis (LIs -12 to -14) that's forecast to extend over n.e. Kanasas leaning back to just west of the GRI area. Dews supposed to be in the 70s across the entire area, and CAPEs stay nice & high straight into SE Nebraska as well. As the warm front advects into northern KS/ s. central NE, hopefully synoptic lift from the front combined with the shortwave moving through will spark off a few sustained supercells before dark tomorrow. SPC highlights the strong cap as their biggest concern, but I'm sure all that instability will break through before late. Hopefully that GRI Skew-T will actually verify & yield some explosive supercells around that area. Man, I'm printing out that sounding & FRAMING it! :shock:
Jon

 

[Glen Romine]

Guess I'll be the only one to add some negativity to this forecast - but I think R. Edwards discussion is spot on. Very potent cap - note the 700 mb thermal ridge axis lies right over the region of interest at 00Z:

http://www.wxcaster2.com/CENTRAL_ETA_700_G...WINDRH_36HR.gif

I'd ignore the trough / moist anomaly over western IA into eastern NE - this feature is pretty suspect, as it is a continuation of convection from western Dakotas tonight - the hard right turn 12Z Thursday is a new feature in the morning ETA that is not supported in previous runs or in other models. Expect the complex to skirt northern SD. SPC also discounts this convective feedback feature in IA/NE. Expect instead for there to be a lack of sharp boundaries present across NE during the day, and w/o any upper level support (instead shortwave ridging - adding to the cap strength), I'd be surprised to see convection much before dusk along the WF. Agree also w/ SPC that convection should begin over the Black Hills region, where better moisture should be present than forecast owing to overmixing by the ETA. If I could be anywhere, I'd probably play the storms rolling off the Hills as they approach the better moisture axis - perhaps somewhere from VTN to PHP. Back further east, storms probably have the best chance in the 01-03Z time frame w/ the decoupling of the boundary layer, w/ consistent model guidance suggesting the SUX area. Otherwise, looks like convection to the NW should roll out into the LLJ axis over E NE overnight.

Others are welcome to hack/comment on my forecast - which is of course subject to change as the real atmosphere starts to reveal its cards.

Glen

 

[Bill Tabor]

You folks are all hyping the positives, granted there is extreme cape and instability. However there are negatives and concerns to consider. Note that the 700mb temps will be high and may prevent convection. In the areas that have the best sfc to 850mb flow there is no decent 700mb and especially no 500mb flow. The upper jets are weak and mostly absent which could eliminate storm venting. IMO there will certainly be some hail monsters and some high winds from storms with no vent collapsing into microbursts. Tornadoes could occur (big maybe) near magical boundary intersections if there are any or more likely IMO behind the boundary in cooler air - these being very diffuse and spread out tornadic events.

If the 500mb and up winds can come further south and intersect the boundary area and convection can initiate in that area then certainly there is a chance. Right now I don't see a strong showing of this on the models though.

Will SPC go Moderate? I think they will base that on their belief for where convection will actually occur and what the risk is in that area. My guess is they will go Moderate initially probably in a fairly small area to cover their butts, and if convection and ingredients don't seem to be coming together they will downgrade to Slight.

I think this could potentially be a significant severe event based primarily on instability if a large area becomes uncapped and convective. Once again primary risks will be large hail, damaging downbursts, and a few scattered tornadoes possible.

 

[Jeff Snyder]

Originally posted by Bill Tabor

If the 500mb and up winds can come further south and intersect the boundary area and convection can initiate in that area then certainly there is a chance. Right now I don't see a strong showing of this on the models though.

I would have been saying the same thing as you last year, but not after experiencing some of the events this year... Last year I would have said that we needed >55kts at 500mb for a good event. However, this year certaintly didn't have too many of these setups. For example, check out the conditions near the 6/12 Mulvane supercell... Winds were, by more typical cyclic tornado events, relatively weak, with general 25-40kts flow above ~800.. For most decent chase days this year, mid-level flow has been 35-50kts...

More importantly, despite relatively weak mid-upper flow, 0-6km shear is still progged by the ETA to be quite favorable for signficant supercells: http://www.wxcaster2.com/CENTRAL_ETA_250M-...VECTOR_36HR.gif ... General 40-55kt 0-6km shear is very favorable for supercells, especially given the strong/extreme instability. I longer put much into absolute wind flow. Rather, I put more into 0-6km bulk shear, which has worked well so far this year...

 

[Kyle Masters]

Hey if anyone is coming up for this event, drop me a line...i live up here in SE SD and know my way around quite well. Maybe I'll see some of you out there! Keep you're eye out for the black monte carlo driving in circles trying to find the best observation point.

Anways, the numbers seem to be adding up, I'm expecting a signifigant event. The models are reminding me of a souped up picture of June 24, 2003, but hey, I doubt that'll happen again. Time to go dust off the camera!

 

[Richard Halter]

Now I asked 3 days ago if the cap was going to be a big issue and a chaser to be named later said "no" as he droolled all over the floorboards of his truck while stuffing it with supplies for a 2000 mile road trip. I just hate to think that he may end up driving all that way for nothing, but boy if that cap should break - KABOOM!!!!!!!! :blob6:

 

[mrobinson]

Originally posted by Anthony Petito
Perhaps we might see some local Omaha area action. I was out of town during the big Hallam storm that came through the area.. but perhaps this could be a good show

Omaha got some wind and small hail but no real action that day. The city of Omaha has been REALLY lucky this year and with as many high risks as there have been in this area I always worried its luck would run out.

As far as this system goes, I just charged the camera, got tapes and got the laptop ready to go.

 

[James McCormick]

Hm, enticing day, but a big risk for a bust. 7000+ J/kg suggested by the ETA near HDE by 00Z ? Mind boggling! Not worth much though with 14+ C 700 hpa air. I'm more inclined to play as far east as I absolutely can. May take til 8 PM or so for shortwave energy/warm front to break cap near H14 in C Nebraska.

I like southern Nebraska, but just too many questions until we see the sun rise and get some real data. But boy, if it's sunny north of the warm front tomorrow, watch out. Right along I80 and points a bit north are where I'm interested in. As long as the 700 hpa winds are 15 kts + and the 500 hpa winds are 30 kts +, any storm that develops will have a very good chance for deep rotation. (I might be a bit different in that I love events between 30-50 kts at 500 hpa, and not a fan of anything stronger, because it makes storm motion too fast.) The low level tornado parameters are fantastic. UL winds typical for July in Nebraska, more typical forecast soundings for supercells that sit and drop 5 inches of rain in one area.

Finally, I am really intrigued by subsiding air ETA projects ahead of the shortwave. Might keep us mixed a bit but will also keep us mostly sunny. I am thinking as of tonight of starting near Superior, NE, playing the southerly warm front, and seeing what happens from there. Will stay home as absolutely long as I can. There may also be a high plains play near the surface low out to the west. 14-15 C 700 hpa air isn't as big of a problem out there it seems. I'm not that gutsy though.

 

[Mike Hollingshead]

Yeah when you say the mid-levels are too weak one should look at some historic events and see what CAN happen. Due south sfc winds at 10knts and 30-35knts at 500mb might not be that great. Back those sfc winds and 850 flow and that 30-35knts become much more favorable. As for all the cap talk. Screw the cap. It's nearly pointless to even look at the thing, I mean seriously. If you are a chaser, you CHASE the cap. I'd worry more if it was weak then if it were insanely stout. I've seen too many days where the cap was progged to hold all day into the night and crap went up early, while I sat at home feeling sorry for myself. No thanks. I'll drive and kick myself for 5 minutes if it holds(10hours driving today doing just that)...beats the hell out of kicking myself for days, months, years? For forecasting for others, ok, fine lets forecast what we think will happen with the cap. If you are a chaser, you'll learn to not waste too much time fussing over the cap. Fuss all ya want but you're best off going and finding out 'the hard way'. And ya know, same almost goes with weak mid-level flow. Amazing what a storm propegating and back building down a boundary does not need.

Mike....here's to the positives.....(1 being it's not winter)

 

[Bill Tabor]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

I would have been saying the same thing as you last year, but not after experiencing some of the events this year... Last year I would have said that we needed >55kts at 500mb for a good event. However, this year certaintly didn't have too many of these setups. For example, check out the conditions near the 6/12 Mulvane supercell... Winds were, by more typical cyclic tornado events, relatively weak, with general 25-40kts flow above ~800.. For most decent chase days this year, mid-level flow has been 35-50kts...[/b]

I don't have quite as high of a threshold as >55kts at 500mb. I've always put the minimum closer to 20kts to 25kts or so. True Mulvane had about 35kts 0-6km shear.

<!--QuoteBegin-Jeff Snyder
More importantly, despite relatively weak mid-upper flow, 0-6km shear is still progged by the ETA to be quite favorable for signficant supercells: http://www.wxcaster2.com/CENTRAL_ETA_250M-...VECTOR_36HR.gif ... General 40-55kt 0-6km shear is very favorable for supercells, especially given the strong/extreme instability. I longer put much into absolute wind flow. Rather, I put more into 0-6km bulk shear, which has worked well so far this year...

As I understand it 0-6km shear is calculated by summing and then averaging the incremental layers of air in 50mb increments from near srf to 500mb. This results in an overall average value for calculating the shear across this area. However the problem with this as I see it is significant layers could be missing and the value would still look reasonable. However it would still be possible to kill tornado potential. I certainly don't rely just on the bulk shear values, and I think that's one of SPC current problems with their forecasting tools. I had a conversation with Rich Thompson and he mentioned that parameters such as the STP can be misleading because they don't always look at individual layers of air. He said he plans to revamp it so that if values fall below a certain threshold then the parameter would go to zero. Anyway 0-6km is still something to consider in the mix.

For Thursday I was down on it because I saw 500mb being near 20kts - possibly lower on the 12z ETA for the 7th. The upper jets were almost absent as well. Looking now at the latest 0Z run I see it is showing slightly more favorable 500mb, 700mb, 850mb and surface IMO, the upper winds are still a bit lacking though and CINH is still questionable. I notice however that the ETA does show precip and so does the WRF for portions of NE that I would be interested in.

I'm thinking there could perhaps be a few torns in NE if convection breaks out, but the very slow upper winds may mostly prevent the supercells from lasting long enough to do it well. I think upper winds and cap is the greatest threat right now - all else is basically a pseudo reasonable go.

 

[Bill Tabor]

Mike makes some points about the cap, and midlevel flow - basically that we live and die with it and that is true. However from a long distance planning perspective I am in Austin, Tx and most of you are already in NE - LOL! I'd absolutely love to chase another good one before this season dies down, but the odds and distance (especially since it's already nearly 4am) are weighing me down heavily. Looking to SPC to break the tie, I see they have a large 2% torn area. I think that will focus more later in the day when they see the real threats emerge, but for now their lack of faith, gives me a bit more lack of faith. Looks like I'll be sitting this one out. Good luck to you locals - if it's good it could be real good!

 

[mikegeukes]

Area Forecast Discussions from this morning, common theme for
Nebraska and South Dakota is concerning the low clouds & cap
and for Kansas the cap is the issue.

Area Forecast Discussions:
Aberdeen Des Moines Hastings Kansas City North Platte Omaha Sioux Falls Topeka Wichita

Mike

 

[JHadorn]

I'm hoping the cap will erode somewhat by this afternoon and something will happen in SE NE. I like the forecasts I've seen, but I'm a little wary of the "all dressed up with nowhere to go" aspect of it.

Last night I wound up eating my words (somewhat) with all the action in Kansas (co-workers asked, "what's gonna happen today" and I said "nothing'). Hopefully today I'll eat my words again when that cap pops like cherry on prom night.

But I'm not going to get my hopes up.

 

[JHadorn]

SPC just went friggin' psychotic in their latest update.

If all goes as they currently think Nebraska is going to get rocked.

On a day I have to work. Go figure.

 

[Robin Tanamachi]

I hope SPC isn't going ballistic just because they got caught with their pants down yesterday... :wink:

Good luck to those of you who do go out today!