8/26/2004 FCST: Central Plains

[guest]

Although a 4 to 5-day forecast for details is highly questionable, there exists some possibility of severe storms on Thursday. GFS is becoming more certain in the placement of the shortwave trough of interest. Thursday has a potential surface low setting up somewhere east of the Rockies-currently in the vicinity of S-central SD. Currently looks like a decent shear/moderate instability setup over eastern NE and E SD/W MN/NW IA. Probably too early to start a thread due to the high uncertainty, but for long-term planning, it is definitely worth looking at.

Chris

 

[Jeff Snyder]

Just quickly glancing at the 84h ETA output for Thursday evening, I'd think I'd put myself somewhere in IA... Now, CAPE will be explosive from southern MN southward, as low-mid 70 Tds invade the warm sector. Additionally, 850mb is pretty strong for this time of year, courtesy of decent lee-side troughing. There does appear to be perhaps two warm fronts stemming from the low in NE ... one that extends northeastward across southern/central MN, demarkated by the northern extend of the strong instability. The other boundary appears to stretch southeastward from the low, ahead of the dryline, into northwestern MO. This is reflected in both surface wind forecasts as well as theta-E forecasts. Oddly enough, south of the 'southern boundary', yet still ahead of the dryline, the surface/low-level moisture looks to mix out for the most part, as evidenced by only low-mid 60 Tds in OK (down from the upper-60 and low-70s). Between these two frontal zones, however, lies the best combo of high surface temperatures and high dewpoints, thus the nice theta-E and extreme CAPE. Additionally, backed surface flow, beneath a decent 25-30kt 850 SSW LLJ creates a mostly-favorable veering profile (with 0-1km helicity progged to be ~150 m2/s2 along the MN/IA border).

All that said, however, the main mid- and upper-level forcing remains locked in the longwave trough across the west. There does appear to be a pretty weak shortwave traversing KS and NE, and ready to advance into IA/MO by afternoon. This not only doesn't help with the forcing issue, but it also means the best mid-upper flow remains well west of the area. Indeed, mid level flow is progged to be pretty weak across the area, with 20-30kts at 500mb at best. Without the aid of DPVA, I don't think there'll be much upward motion to help weaken the cap, which means that the ETA-forecast 'no convection' may verify... With the 850mb low looking to remains across eastern CO on Thursday evening, assuming the surface system doesn't kick the moisture too far east, I think the main show may hold off until the trough advances more into the plains on Friday...

 

[Jeremy Lemanski]

the SPC seems to agree with your thinking, but slightly farther east:

THE POTENTIAL EXISTS FOR AN ACTIVE SEVERE
WEATHER EVENT INCLUDING SUPERCELL TORNADOES OVER PARTS OF IA/MN/WI
ON DAY3.

Just looking at the eta svr. page for this area, it has mid 70 points, possible its overdone a little as the eta is known for doing, A large area of 4000+ capes, -10+ lifted index across southern minn. and northern iowa, and between 100-250 of 3 KM Storm Relative Helicity. It also looks to bring in a strong jet into the region during the day with 250mb winds in the 70-90 knot range over minn by 18z. Things are starting to suggest a severe weather outbreak thursday.

 

[mikegeukes]

Latest Day 2 came out: Moderate Risk

THERE IS A MDT RISK OF SVR TSTMS THURSDAY AFTERNOON AND EVENING
OVER PARTS OF SOUTHEAST MN...NORTHEAST IA...AND MUCH OF WI...

Source: http://kamala.cod.edu/offs/KWNS/0408250648.acus02.html

Chasing can be somewhat difficult in the Driftless Region of
NW Illinois, NE Iowa, SE Minnesota and SW Wisconsin.
The Driftless Area is a area that was untouched by the glaciers.
The area is relatively hilly, and the roads to say are interesting.

Maps of the Driftless Region:
http://www.jdcf.org/glacial.htm
http://wisplants.uwsp.edu/DRIFTLESS1.gif

Relief Maps:
http://fermi.jhuapl.edu/states/maps1/wi.gif
http://www.lib.utexas.edu/maps/us_2001/wis...in_ref_2001.jpg

Mike

 

[Dan Cook]

DVN's HWO mentions "strong tornadoes" possible tomorrow.

 

[Glen Romine]

Chasing can be somewhat difficult in the Driftless Region of
NW Illinois, NE Iowa, SE Minnesota and SW Wisconsin.
The Driftless Area is a area that was untouched by the glaciers.
The area is relatively hilly, and the roads to say are interesting.

I'll second that Mike. It's not as bad as say se OK, but most roads are far from straight and lots of trees and rolling hills. But, I think this event could get an early afternoon start, as the forecast cap is not very stout, so probably back west of the I-35 corridor along I-90 where it is still plenty flat. Last night's WRF run suggests a target further north, say around MN hwy 12. 06Z MESOETA seems to agree with this northward shift - placing the boundary up near the UP at 00Z - even worse than the Driftless Area IMO. Forecast storm motions in the 35-40 knot range will make them tough to chase even in flat land. Maybe target of around Mankato at this early stage - but I won't be chasing this event.

Glen

 

[mikegeukes]

Day 1 just came out, the moderate risk has now shifted to Iowa, instead of Wisconsin.

THERE IS A MDT RISK OF SVR TSTMS MUCH OF CENTRAL/WRN IA.

Mike

 

[Glen Romine]

Another day, another bust opportunity...lol. Well, SPC is more excited about today than yesterday at this time - but a very different scenario and not much like what yesterday's model output suggested. Today's hope appear to hinge on the maintenance of a weak ridge axis roughly along I-80, which provides a focus for convergence along with enhanced backing of the surface winds. Both Edward's and Imy's forecast suggests enhanced tornado potential in IA today - with Imy giving a much larger region of 15% probs. Looking at the RUC forecast from this morning - the weak surface low is progged to remain more or less stationary through the day over nc KS, with enhanced moisture convergence along the quasi-stationary boundary to its east - particularly focused around the St. Joe area. No precip is eaked out of the model - not surprising given the 700 mb ridge axis overlies this region. The further south boundary appears to limit the flow further north, leading to only weak convergence at the boundary over nw IA. This scenario could be a real show stopper if it pans out. Guess my early thought would be to try for around the Nebraska City area - decent road choices if you need to go somewhere fast, just north of the 700 mb ridge, and near the forecast convergence max. This is well off of the front to the NW - so a riskier choice - but the 850 jet is very weak back along the front - as is the surface flow, and forecast instability also looks weak at the front. Prefer sw IA for all of the above. Looks like some potential further east as well, perhaps in my neighborhood - too bad I can't chase today.

Glen

 

[Mike Hollingshead]

I am getting so excited to bust 3 days in a row on this neato sfc convegence once again seeming to be stuck in ne KS. Move north damn it. Glen you are very correct in saying it could be a show stopper, as it truly is killing flow up here. I'm fairly tempted to drive south yet again to ne KS, basically out of the tor probs in my backyard. Yeah capping is stronger, but if flow and convergence is going to be crap again, but up here this time it might prove the better idea.

http://www.spc.ncep.noaa.gov/exper/mesoana...sis/s2/mcon.gif

I imagine that is just going to sit there all day and never move north. Look at the no flowness going on north of it towards the cf. It is wall to wall sunshine down there and no ofb's around, so maybe it will lift north...doubting it though.

Now I see some 10knt se winds from Lincoln ne through Omaha now, so who knows I guess. CU now starting to get going along the front in central NE. I suppose that will be todays target for me....beings it will be on me by 3pm or so. 28c at 850 at 0z in ne KS....I guess I'll stay north and see what unfolds.

 

[Jeff Snyder]

Difficult forecast here... High surface dewpoints and full radiational heating will provide another day of extreme instability, with RUC pusing CAPEs >6000 across southern IA and northern MO. Models show winds either remaining from the south or backing to the southeast slightly, depending upon exactly where you are. The RUC is depicting a nice low-level shear profile, particularly across southern IA. That said, however, upper-level jet dynamics aren't helping too much, since the area is largely in the right-front quad of the incoming jet streak, leading to some slight subsidence, which is indicated partially on the 500mb UVV plots, at least from the RUC. Additionally, dry air at 850mb isn't helping the matter much either, as that'll probably result in the surface dewpoints dropping a little this afternoon in the area.

I'd probably target southwestern IA today... Perhaps Creston or Atlantic... I don't think this area has the best chance for initiation, but I think this area is most favorable... Just'll have to watch vis sat later this afternoon I suppose...

 

[Glen Romine]

Back to take another look at things. I'm further discouraged with tornado potential along the "front" - here I'm referring to the wind shift from Hill City up to Souix City with NE winds on the NW side of the boundary. Dewpoints are dropping very fast on the backed side of the boundary - as you might expect given how shallow the moisture at OMA was at 12Z. Cells that form on this boundary could quickly become outflow dominant - though the forecast cell motion is slightly right of the boundary orientation, so cells might manage to get east of the boundary before the cold pools expand too much. The persistent low clouds over southern IA appears to have done a good job at slowing the northward progress of the boundary, but the western portion might start moving now that skies are sunny across sw IA. Cap is slowly weakening - but for the reasons Jeff mentioned, this may be an uphill battle. You currently can't buy a cloud in ne KS - despite the strong convergence - so that is a definite negative. 18Z OMA sounding shows a double near sfc inversion - and moisture that's unlikely to persist once the first inversion is overcome. The inversion near 500 mb is likely from the upper jet dynamics - a CAPE robber for sure. Moisture is only 90 mb deep at DVN as well - so I'd say head for an area with persistent moisture convergence and hope for some pooling. TOP 18Z not in yet, RUC analysis showing minimal cap in that region now though.

Did anyone see the 84 dewpoint at Belleville, IL? Wow. MLCAPES around the STL metro already > 4000 - too bad the shear wasn't a little better.

Heh - MCD just out. Focus is on the area in sw WI - not the KS area - so no help there. Sfc flow looks pretty disorganized in that region.

Glen

 

[Mike Hollingshead]

Latest sat shows some weak cu starting to bubble up in far ne KS...hmmmm. The cu around Hastings also interesting...as well as into IA I suppose. This sort of sucks with 3 possible areas at close to 20z. I bet new day 1 will be slight with 2% tors.

Area west of Lincoln now gaining my interest. Beatrice now se 90/70 at 10knts while Columbus and Norfolk both backed for some reason to due east at 10knts. Lincoln or Beatrice I am guessing are wrong as Lincoln reporting ssw at 10knts. Then again flow IS pathetic. Just checked outside here just north of Omaha and the flag looks like it is petrified. Now that I look out though the leaves are moving and the cu are getting fatter(but still very flat and hardly moving).

Guess I'm wrong with the day one..lol. It is interesting what the sfc winds are wanting to do now(or at least early on appearing to want to do).

 

[Amos Magliocco]

Latest sat shows some weak cu starting to bubble up in far ne KS...hmmmm. The cu around Hastings also interesting...as well as into IA I suppose. This sort of sucks with 3 possible areas at close to 20z. I bet new day 1 will be slight with 2% tors.

I would definitely be tempted that way if I were still in the area. Deep layer shear looks good and flux convergence bullseye looks co-located with new cu field you noted. RUC breaks out precip there again and LLJ is increasing. Plus northeast KS has produced tornadic (RITOR on Tuesday night late) supercells the last two nights, which suggests that it has the right combo of ingredients plus CIN to keep things isolated and spinning. Wish I was still there, frankly.

 

[Glen Romine]

Wish I was still there, frankly.

Wish I could be there too. I think this will be the place to be. latest vis showing improving cu field - maybe even a tcu peaking out there just across the NE border. 20Z outlook selling the same story as earlier - bit surprised. Supercells certainly a good bet in IA - and fastly developing now NNE of Ft. Dodge. But, cooler air is advecting toward this from the extensive cloudy area earlier - so maybe elevated or will become that way with time. RUC analysis also suggests this cell is elevated. It's an easy drive down I-29 - at least you'll see more clouds!

Glen

 

[Amos Magliocco]

Glen, I was just about to say I was surprised at the 20Z's omission of Kansas from the 15%, but I'm not following today's setup as closely as yesterday. Still, so many similarities in the three day set. Can you use persistence to forecast tornadoes? LOL