8/23/04 FCST: Midwest/Plains

Forecast thread started for Monday: August 23, 2004

Latest SPC Day 1 has the Northern and Central Plains
in a slight risk for severe weather.

Feel free to post your discussions below.

Mike
 
The 23rd and 24th look good for the Northern, Central and even the Southern Plains, but there are many factors that must fall in place for this to happen. I like WestCentral Oklahoma on the 24th, because the ETA Forecast puts the Left Exit Region of a relatively strong jet streak over the region and it also shows a very strong bulge in the dryline, so that strong forecasted CAP may break!
Storms that fire in Oklahoma may turn out to be very impressive supercells if the Shear and CAPE forecasted by the ETA verifies.
 
I don't know how many chasers are in spitting distance of South Dakota, but it's one of my big regrets of the summer that I didn't take tomorrow's setup more seriously and make plans for chasing tomorrow. Since nobody is talking about this setup anywhere, I suspect that the High Plains Mikes, Umscheid and Hollingshead, will have these long-lived cyclic supercells all to themselves. MikeH will have to issue a special "second" version of his new highlights tape and both chasers video will be peaceful and chaser hordes-free.

So the hordes should head north! I don't know what's not to like about this setup except it could use ten or twenty more knots in the midlevels, otherwise it's almost perfect. Supercells along the warm front sohuld produce tornadoes north and northeast of Pierre.
 
Well, I don't know when it will happen, but one of these days I expect to be able to agree with Amos' forecast. Unfortunately, today is not that day if the latest ETA forecast is to be believed. Appears the shortwave comes out too early, leading to convection before peak heating. Further, deep layer shear is initially marginal, and low-level shear is maximized associated with the post precip mesohigh. So, back west near the surface boundary, large scale subsidence behind the lead shortwave will not help, and although ETA builds in a narrow instability axis in the wake of the early convection, dewpoint depressions of 20F+ will make tornadoes rare, and cell motions are forecast to be E, quickly crossing the narrow instability axis if they do form. I suppose you could hope for a significant MCS to develop tonight and leave a strong cold pool to create an E-W boundary, and limit convective potential w/ lead short wave, and maybe something manages at the intersection point somewhere in northern SD to ND. Guess for those heading out - best of luck!

Glen
 
ETA builds in a narrow instability axis in the wake of the early convection, dewpoint depressions of 20F+ will make tornadoes rare, and cell motions are forecast to be E, quickly crossing the narrow instability axis if they do form.

Glen, not disagreeing with what you have mentioned, but just one thought. How high is the base in your avatar? More and more I want to blow off LCLs when all the other parameters look good.
 
Glen, not disagreeing with what you have mentioned, but just one thought. How high is the base in your avatar? More and more I want to blow off LCLs when all the other parameters look good.

Not as high as it appears - Tdd was around 12F this day, the shot is with a wide angle lens, so the view is somewhat misleading in my avatar. Yeah, I never said this type event couldn't sustain tornadoes - but this doesn't have the look I seek for such events. Extreme lapse rates can make up for high bases, but forecast LRs are only about 7K/km over the instability axis. I'd want better than that here, but of course forecast doesn't equal reality, though models lean toward the overly optimistic side. I generally won't chase > 30 Tdd days - but the OKC event from June of '98 is a prime example of what can happen with extremely steep lapse rates despite high bases. Good mid-level lapse rates are tough to come by in the fall season without strong upper level dynamics, and with the monsoon season underway, upper level clouds are often a good bet with emerging systems out of the Rockies. I think a good day will emerge from the upcoming pattern - but which day I think is still unclear at this point.

Obviously, you have the benefit of waiting until late tomorrow morning before having to worry about how this event is panning out - but I just wanted to express some words of caution to anyone considering a power drive to get in position for tomorrow.

Glen
 
Well, I don't know when it will happen, but one of these days I expect to be able to agree with Amos' forecast. Unfortunately, today is not that day if the latest ETA forecast is to be believed.

LOL! As a chaser it's probably a good sign to find youself disagreeing with me.

This might be the wrong approach, but I don't put much faith in standardized lists of indices and thresholds for those values. It seems to me that each setup makes its own rules. In this case, the key factors would appear to be low level instability and strong values of low level shear. I can't imagine LCL's will be a problem for storms along the boundary.

Enhanced streamwise horizontal vorticity along a retreating warmfront with excellent 0-1k SREH values and very good 0-3k SREH values are all you need for mesocyclones and tornadoes. Though deep layer shear likely won't suppprt long-lived cyclic supercells (and therefore tornadoes may be brief), storms that form on the front should easily root themselves in the boundary layer and rotate. I haven't looked the 0Z ETA tonight, so this is all based on the 12Z.

As of the 12Z ETA, I'd still say it's a go.
 
Yeah and I have to actually decide somewhat early. If I want the dl before 0z I'll need to be west of PIR, and PIR is a good 6 hour drive. So leaving at 8 could get me to PIR around 2...then drift w or n. I wouldn't want to leave much later then 8. But yeah, much better then anyone having to leave the day before. Then again I know this will be no more clear to me come morning, unless the southern half of SD is full of precip...which looks like could happen. WF is tempting but not if morning convection is screwing that up..and/or...if it actually blasts as far north as the eta wants it to. I am more undecided about this then I was earlier in the day. That dl is so damn far west still at 0z and we know how the models love to be too fast. I dislike driving more and more every year..hmmm.

http://weather.cod.edu/forecast/ETA/previo..._500_spd_36.gif
http://weather.cod.edu/forecast/ETA/etaUS_..._500_spd_24.gif

At least the 0z eta brought the 500mb winds about a good state south...I guess. Still thinking the only real play might be the sfc low and/or dl out west...WEST. I think it'd be wrong of me to sit this out given MikeU left DDC early today for it...lol. I might as well go to PIR at 8am.
 
This might be the wrong approach, but I don't put much faith in standardized lists of indices and thresholds for those values. It seems to me that each setup makes its own rules.

I agree you can't put too much faith in such things - but synoptic setups are about all you can have much faith in this far out. Mesoscale features forecast by the ETA are noise at best - so if the synoptics setup isn't in the ballpark - mesoscale enhancement often won't get it done. The 00Z ETA is breaking out considerable precip early with a lead shortwave - and forecast CINH is still > 50 J/Kg along the surface boundary in the wake of this convection - if this cap is in the ballbark - odds of new development in the wake of the early storms is low. That said - I think the current ETA forecast is verifying quite poorly. Precip is further north (near FSD) than forecast, and the ETA kills it off early, but would expect this region to maintain for some time - so a more substantial boundary may set up than forecast. The upper level wave forecast timing is probably pretty accurate by now - but I think the surface evolution for tomorrow is anybodies guess at this stage.

Glen
 
SPC Slight Risk

Looks like some of the forecasts are lining up. NWS has placed a SLIGHT risk for 08/23 over the Northern Plains and the Midwest. Its primarily in the ND-SD-NE regions with MN, IA and KS sitting on the fringe of the risk area.

Surface detail maps on the weather channel website is showing a stationary front right along the NE-SD border with a low pressure system centered in MT. I remember there was a SWW watch box for MT yesterday. I will not be surprised if NWS puts up another watch box for MT today. I looked at the dewpoint charts and NE, IA is pushing into the 60s already this early a.m.

I would put myself on standby and see where all of these weather elements are going to go. The closer that Low pressure system gets to the Midwest, the higher the chances of a severe weather outbreak.

Anybody know where I can find a website that shows the jet stream and its movements without having me muddle thru the windcharts?? Which wind charts best show the jet stream??
 
Not sure I'm liking where the ruc places the sfc trof by 21z...lol still in WY basically. SFC low in nc MT. Dews struggling to reach nw far enough. I could maybe see something in far nw NE, sw SD, but confidence is quite low, at least going by the 9z ruc.
 
Precip evolution forecast from this mornings RUC doesn't look to be realized based on the current precip trends, which throws out much of the sfc evolution forecast. As mentioned in the SPC outlook from this morning - ND might be ok for this afternoon as the warm front races northward - or you can take your chances with the ouflow boundary from the convective cluster developing over MO hooking up with the dryline over central KS. Both seem like considerable gambles at this stage - though I think I'd lean toward KS at this point - as supercells at least look to be a better chance there given any convective development. Definitely a frisbee in the trunk is a must if you do go out....

Glen
 
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