5/21/08 FCST: NE/KS/CO/SD/IA

[Chad Cowan]

Confidence in a widespread severe outbreak next week has grown significantly over the past 36 hours with the GFS finally trending toward the ECMWF's consistent doomsday(s) solution. Although Wednesday will likely be the smallest day of the bunch in terms of areal coverage and probabilities, there is a slight prospect of late evening tornadic activity near the I-80 and I-70 corridors. Very warm mid-level temperatures will be in place due to the amplified trough just entering the picture from the West. This pattern will effectively cap most of the warm sector but will also result in a considerable amount of divergence aloft. Cyclogenesis will begin early Wednesday morning over the high plains and the sfc low will rapidly deepen throughout the day. Before nightfall, we could be looking at a sub-980mb low. When is the last time that happened in May? The LLJ will rapidly strengthen to over 60 knots after dark which will contribute to very favorable shear profiles for any updrafts that can break through the cap.

17/00z GFS forecast for 0z Thursday:

 

[Simon Brewer]

If mid/long-range forecast models verify then it appears the peak of the 2008 Great Plains tornado season is about to begin next week. The western trough and eastern ridge is typical of an active period for tornadic thunderstorm activity across the plains and upper midwest. I've been waiting, impatiently, all year for this patturn to develop.

Simon

 

[Brandon Sullivan]

Woot!

Yes, it appears things are coming together quite nicely. GFS and ECMWF continue to be in more agreement.. Looked like the 0z GFS was slightly faster with the scenario though. Regardless, as long as the classic omega block holds on, things should be very interesting!

I will agree with Chad that while there is potential wed. night, the main focus will be later time periods (Thurs/Fri)

However, I do see tornado potential near the I80-I70 Corridor on this one.

And yes, it's been a while since we have seen a low like that in May! I won't complain though!

 

[Bob Hartig]

My forecasting skills are still formative, but I'm developing more confidence in them, particularly in reading this thread AFTER picking my present area of interest. I've been watching the GFS solidify big CAPE and nice moisture return on into the north/central plains. While a few days ago, the surface winds looked abysmal, they began falling in line yesterday, and this morning they're backing nicely, with 850mb showing a nice southerly flow. Everything else seems to be falling into place. If this setup were for tomorrow, I'd be narrowing things down to the area along SE South Dakota/NE Nebraska/far western Iowa, where 500mb and 300mb jets look to be nosing in Thursday evening. CINH doesn't seem to be an issue. Curious why precip doesn't appear to break out farther south.

 

[Paul Knightley]

Hey Bob - I would suggest that the cap will be very stout at first, especially further south - the ridge over the south-west over the next few days is going to allow a hot airmass to form, and as the trough first begins to dig, this hot dry air will be advected as a very stout EML out across the Plains. Until deep tropospheric cooling starts via lifting ahead of the system, this cap will be very tough to break.

ECMWF dew points are only up to around 60F at most by Wednesday afternoon across much of the central plains, east of the dryline. Yesterday's 12Z run had better moisture by that stage, and somewhat better shear. I think the moisture issue on the 00Z run is down to the model digging the trough more strongly than the 12Z run.

 

[Brian Stertz]

Yep true on that Paul. The cap and weakish flow is going to be a big player Wed. and possibly even Thur. I think the models are truly struggling with the massive size of the upper system and how it will progress and morph as things shift out into the High Plains and then points east into the Plains after Sat. 5/24. Its not an easy forecast to really bite on any one solution yet. Not a big fan of omega blocks and Euro was really on board with that on this latest pkg. Kind of a radical change from the progressive pin-wheeling neg. tilted action that was suggested on prev. model runs.

 

[Shane Adams]

The best thing I see so far for the entire Thu-Sun window is the slowing of this trough. Yesterday we were going to E OK/AR again. Today looks like more of a traditional KS/OK/TX play....possibly further north into NE/SD. Either way (north/south) it all looks to be setting up further west with each successive run. Cap is advertised to be stupid strong on Thursday, but we'll see how this works out as models begin to fuse with reality in 3-4 days.

 

[Mikey Gribble]

I think this converstaion will be shifting to Thursday as the GFS is a little too quick IMO on bringing strong mid level flow over the plains. Even if the GFS was right on bringing decent mid level flow over the plains on Wednesday, there just isn't going to be enough time to get quality moisture north that quickly and I don't think there is any way we'll break the cap. Thursday is more likley the day before the day IMO. Moisture return should be better by this point and the cap is going to be easier to break, but still formidable (as it usually is on the day before the day). That being said, I think Thursday will be the day before the day and then Friday will be the main show with a widespread severe weather outbreak. Maybe if the trough can stall we will get more than that out of it, but I'm only banking on those two days for the plains ATTM.

 

[Mike Kovalchick]

I am still holding out for Wednesday evening to be the first of a series of chase days. Cap shouldn't be any problem near the Black Hills with a nice upslope flow there. Enough shear/moisture should be in place for Sups..certainly it will happen there before anywhere else.

 

[Mike Hollingshead]

I am still holding out for Wednesday evening to be the first of a series of chase days. Cap shouldn't be any problem near the Black Hills with a nice upslope flow there. Enough shear/moisture should be in place for Sups..certainly it will happen there before anywhere else.

I agree with that whole idea. Looks like that area should be up on the north edge of the cap, with good upslope against the black hills. 40 knots of 0-6km shear even with such horribly weak mid-levels, thanks to the strongly backed low levels. Hell it has storm motions a little nw up there. Given mositure return looks at all decent by then(mid-50s into the target) I plan on that being my first day. Then it's going to likely be several days without having to go terribly far from there.

 

[Mike Hollingshead]

Yeah, if the cap can erode along the dryline that'd be nice. I wish I could see some forecasts soundings out that far in that location. I don't know what is a breakable 700mb temp out there. Sure it'd be higher since elevation is higher, but would 15c be a bit much? Must not be. 850s are skyrocketing, but they aren't that terribly far above the sfc.

Guess this sort of answers my questions, at least in model land.
http://beta.wxcaster.com/models/eta/central/CENTRAL_ETA212_ATMOS_LSI_84HR.gif

http://beta.wxcaster.com/models/eta/central/CENTRAL_ETA212_ML_CINH_84HR.gif

 

[Mike Hollingshead]

I'm starting to think more and more the upslope north of Cheyenne may be the only option on this one. Can't get any mid-upper flow to push much east of there. Upslope should be rather intense. Just need *some* dews. Thursday almost looks like a copy of it too. I don't think upper 40 dews will get me there though. I'm growing to hate this system.

 

[JR Hehnly]

I'm keeping an eye out for something farther south, SE Colorado to Southern Kansas and maybe even Northern Oklahoma. The latest NAM has the very strong cap disintegrating between 21z and 0z, though it may not erode enough for convective initiation before we lose daytime heating. The upper support is weak, so if storms do form the lack of venting will likely make them the slow moving, HP variety.

Check out the 0z forecast sounding for Ponca City, OK

 

[Mike Hollingshead]

Where did you get that JR? I found it out to 72 on earl's but only looping. Here's one for Douglas WY.

 

[JR Hehnly]

Mike,

Came from here: http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=NAM&STATIONID=KPNC

I'm not going to get excited this far out since I'm sure the models will change half a dozen times before Wednesday, but it's nice to see there may be a southern option coming into play.