9/30/07 FCST: IA/NE/KS/MO/WI

[Brian Stertz]

Very nice wave on the 12z GFS...shear, strong divergence, and very strong ascent looks adequate for sustaining a few tornadic supercells by late Sunday afternoon and evening. Based on the position of the wave, extreme SE Nebraska, WC/SW Iowa, and extreme NW Missouri appear to be the favored spots. If deeper moisture was in place, this could be a significant and very focused severe wx event. Right now, things could be a bit moisture starved for things to get too out of control. Stiill should be some pretty "robust" supercells to check out. This may coincide nicely with my trip this weekend to my sister's in West Des Moines. Better bring the chase gear...I am ready for some chasing !!

 

[Craig Maire II]

This may be one of the few times where being a chaser who lives in Iowa could actually be a good thing! LOL The amount of shear and dynamics in place with this system may definitely produce the storms I desperately need to get rid of this SDS ailment that currently afflicts me! (I will be targeting southwest Iowa as it stands for now).

 

[Jeff Snyder]

Pretty significant model differences for this next trough... I won't say much given the differences, but a quick look at the forecast Supercell Composite Parameter shows this pretty well:

12z NAM 84hr (valid Sunday evening)
12z GFS 84hr

Obviously, the difference is evidence of the speed at which the two models move the trough... The 12z NAM is quite a ways faster east / faster than the 12z GFS, with the cold frontal passage, correspondingly, considerably faster as well. Given a general bias for NCEP models to move troughs out of the western US too quickly, I'm thinking that a slower solution may verify (supported by other models, as well as by dprog/dt). Moisture appears as though it'll be quite a bit more rich for any possible event Sunday (compared to Sat, at least), particularly S of I70 (>65F Td). The kinematic structure of the trough portends the possibility of a significant convective event, but timing / speed may be key for (a) chase terrain concerns, (b) those who reside in the southern Plains, and/or (c) those looking for anything beyond marginally-severe storms. I get quite giddy when I see a jet streak rounding the base of a compact upper-level trough, so I hope timing works out nicely. For that matter, we may also end up with squall line action given the intensity of the powerhouse vort max seen in the 78hr 500mb GFS plot (HERE).

I'd be happy if the GFS verifies, save for the actual outcome to be ~6 hrs slower than even the GFS fcst. If the NAM verifies -- yuck. In fact, last night's ECMWF run shows this slower-than-GFS solution very nicely (see http://wxcaster4.com/ecmwf/NORTH-AMERICA_ECMWF_500_GPH_96HR.gif ). That is almost perfect timing and location of a good short-wave trough for me to get quite excited about a central Plains chase. The GEM (500mb plot at http://grib2.com/gem/CONUS_GEM_500_GPHTMPVORT_96HR.gif ) seems to support the GFS. At least they all seem to be quite a bit slower / farther west than the NAM.

 

[Brian Stertz]

Yep agreed Jeff...timing is a bit willy nilly on the models. The Euro and GFS look quite interesting, especially with a small pocket of "deeper" LL moisture shown over SW Iowa on the GFS. ETA looks rather mundane and fast. Depth of this system will probably mean slower movement. Yes that nose of the upper jet is very impressive. May not have a lot of supercells, but the ones that do get their act together could be pretty tough. If things are slower, this could put SE Nebraska, NE Kansas, NW Missouri, and SW Iowa in "fat city" for the best action.

 

[Jeff Snyder]

Geez, I had to do a double-take to make sure I was looking at forecasts for the same day... Tonight's GFS and NAM runs are quite a bit different from previous runs, with the trough quite a bit faster / farther east by 00z Mon (Sunday evening), and with much flatter / considerably less amplified flow S of I70. I'm not sure what to make of the latest model forecasts, but the reversal of the previous trend of slowing down this trough is troubling. It still looks like it MAY work out in IA, MN, WI, and IL, but it looks like a non-event farther south. I haven't had enough time to interrogate GFS and NAM output in much depth, but it's quite a shift. The latest ECMWF, GEM, and UKMET that I've seen (which would be last night's runs) still showed what the previous GFS runs indicated, but I assume the new runs will probably also show this faster and considerably weaker solution. I hope I'm wrong, however. The trough axis coming through the Plains Saturday night and Sunday morning doesn't make me happy.

 

[Brandon Lawson]

I didn't want to believe the models, but they all have the trough moving from central Wyoming to Wisconsin in 24 hours. Talk about screaming its way through the country. Is there any chance the models might be off a bit? Good gracious.

 

[Mike Hollingshead]

0z run of ECMWF

It leaves hope for eastern IA. Might even be some cold core ops in nc or nw IA if it is right. The others seem awfully fast.

 

[Brian Stertz]

There is a nice kicker system behind this first wave so it could move pretty quickly...especially as strong as the one behind it is. If this one had a bit more "dig-iness" to it, then the moisture may have had a bit more time to stream farther northward. I can still see there being some supercells IA/MN/WI but this one is kind of disjointed with the dynamics and deeper moisture. I am not totally ruling out a few tornadoes, but just not as excited as I was yesterday about the Sunday event. I am fairly excited though about Tuesday's forecasted setup which I will comment on a separate thread.

 

[Jim Bishop]

The models have trended slower with the speed of the shortwave into Iowa and this setup is looking much better. It reminds me of the November 12, 2005 setup in Iowa but a little further east.

The north-south orientation of the front shown to blast east from Eastern Nebraska into Central Iowa will promote discrete storm development. You may even have a dryline setting up over Eastern Kansas into Southeast Nebraska, which would further promote discrete storms.

Once storms develop they will rapidly move north-northeast to northeast into a 40-50knot south-southwesterly 850mb jet. That jet will allow for great low level shear as well as bring rich 850mb moisture to the area, helping to enrich the surface as well, allowing for low 60s dewpoints. The very cold temperatures aloft associated with the very strong upper trough will provide for ~1000 J/kg CAPE. That's plenty of CAPE considering the amount of upper forcing that will be present.

This is a delicate setup because of the speed of the upper system as well as the amount of forcing aloft. But from the models this morning it looks like things will come together in such a fasion that low-topped suppercells with tornadoes will develop over Central Iowa. Again the similarities to Noveber 12, 2005 are quite interesting.

 

[Scott Weberpal]

0z WRF looks better than any previous runs. Though it appears moisture is overdone, thus yielding slightly higher CAPE values than what may be realized (forecast 2000 bullseye in C. IA at 0z tomorrow night); as Jim mentioned upper forcing should augment a lower-CAPE environment. I'll be starting tomorrow somewhere west of ALO along Hwy. 20. I just keeps looking better with every run...

 

[Joel Wright]

The system is forecasted to be about six hours slower than what was suggested last night. However, everything still looks to be coming together for a late season event, focusing on Iowa.

Looks like we'll be heading towards the Des Moines/Ames area by noon tomorrow, and then go from there. If the system continues to look slower, we may end up chasing in western Iowa...

 

[Scott Lincoln]

0z WRF looks better than any previous runs. Though it appears moisture is overdone, thus yielding slightly higher CAPE values than what may be realized (forecast 2000 bullseye in C. IA at 0z tomorrow night); as Jim mentioned upper forcing should augment a lower-CAPE environment. I'll be starting tomorrow somewhere west of ALO along Hwy. 20. I just keeps looking better with every run...

I'd be careful of starting off that far east... the latest model runs seem to have the triple point near Denison around 3-4pm, moving into central Iowa through the evening as the cold front overtakes the dryline (if it holds together this far east).
As of right now the preliminary target for the ISU chasers is somewhere near Denison, probably heading out around noon.

 

[Jeff Snyder]

It's interesting that the NAM has slowed down this trough/low and FROPA now, after being quite fast with their (trough/low + cold front) progression the past few days. Unlike the past few runs, strong westerly mid-level flow extends all the way into northern OK, which yields sufficient deep-layer shear for supercells across all of eastern KS, MO, and northeastern OK (>35 kts 0-6km shear). NAM forecast soundings and forecast hodographs indicate that, if Tds can stay/get in the mid-60s, shear + instability should be sufficient for a supercell threat. Though the strongest LLJ will remain closer to the low, low-level shear may also be strong enough to support a tornado threat as far south as northeastern OK and southwestern KS. As is/was the case today, instability and moisture will probably be a limiting reagant,

Note: I'm biased towards these areas since I'm in OKC, so a lack of discussion of areas farther north should not imply my feeling that those areas are at a lower risk.

 

[Brian Stertz]

I am going to target the Fort Dodge-Carroll IA area as my initial chase target. I am inclined to think there will be the best of everything (as good as it gets today) stacked up there by mid afternoon. Feel this may end up being decent low topped supercell day after all, as this system seems well timed with the sfc low and strong vort, and strong shear just glancing over the 00z ETA and GFS. I am in good intercept position today here in DSM. 5% TOR probabilities appear to be justified.

 

[Scott Weberpal]

I'd be careful of starting off that far east... the latest model runs seem to have the triple point near Denison around 3-4pm, moving into central Iowa through the evening as the cold front overtakes the dryline (if it holds together this far east).
As of right now the preliminary target for the ISU chasers is somewhere near Denison, probably heading out around noon.

West of ALO...was thinking Fort Dodge area. Don't want to be too far south as the storms will be screaming northeast. May sit in an area with a good road network and pick off the storms as they come past hoping at least one is producing a nice tornado as it whizzes past