2012-02-10 FCST: IA

[Skip Talbot]

I've seen many setups like this fail to materialize when presented at this time range, but thought it might be interesting to open a discussion for a possible severe weather event in western Iowa on Sunday. 0z and 12z NAM are both showing a cut off 500 mb and surface low stacked over southeastern South Dakota with a fairly narrow warm sector extending down through western Iowa bounded by warm and cold fronts. Dews at the surface are well below par for a typical severe weather event at about 45F into central Iowa, but for a cold core like event may be sufficient for low topped supercells or even tornadoes given the extremely cold air aloft: -30C pooling around the cut off low at 500 mb. It may seem way too early in the season to be chasing tornadoes in Iowa and a modest play at best, but there are several plots that are really jumping out at me:

Very impressive low level instability by mid afternoon in southwest and central Iowa. Some of the fattest low level cape you can expect to see in fact, and a crucial although fairly unpredictable parameter for these types of setups.

A well defined dry slot wrapping into the warm sector by noon, which should pave the way for strong solar heating and destabilization.

And finally, the cap opening up at noon and a 100(!) knot midlevel jet nosing into the warm sector.

If this verifies, I'd expect southwest Iowa to be destabilized by noon and storms exploding on the nose of that excessively strong mid level trough. Forecast soundings show very impressive lapse rates, "fat" low level cape for robust albeit low topped updrafts, and very favorable shear parameters. Storms may produce tornadoes as they approach the warm front draped across central Iowa where 1km SRH exceeds 200 m2/s2 and while low level instability is maximized from daytime heating. If this setup materializes I'll be out on my first '13 chase targeting an area between Atlantic and Creston, IA (adjust as the models converge a solution for the surface features). On the other hand, these cold core setups love to fall through and this one already stands on the edge of a knife. If we drop a few more degrees on those dews, or the low ejects or occludes early, this is going to be a non event in Iowa and a Louisiana squall line.

 

[Skip Talbot]

Couple of concerns I forgot to mention. The first is snow cover:

Snow on the ground would kill your low level lapse rates, but forecasts for Saturday in southwest show temps in the upper 40's. We should see the line marking the start of the snow pack in central Iowa receding some by Sunday.

The second is storm motion, which is going to be crazy with low topped storms and a 100 knot mid level jet. Expect a very fast north, northeast storm motion. You'll want to be well downstream and ready to catch storms, which will look fairly unimpressive on the radar.

 

[Jeff Duda]

The 00Z NAM appears to have reduced the potential for the setup in Iowa. It seems to keep the snow pack line a bit further south, thus making surface temperatures struggle to get into even the 50s. However, the surface and lower atmosphere is nearly saturated with steep lapse rates, so some (albeit very minimal) SBCAPE remains. Skip, that 0-3 km CAPE you saw is probably due to the very low LFC-EL layer, where the EL is at about 600 mb. Still a ton of shear, however, so should a buoyant plume happen to be particularly vigorous, there may still be some rotating columns of air in that area.

Recent SREF runs are in agreement with the 00Z NAM on the reduced instability.

SE IA remains the best bet.

 

[TJKLECKNER]

I'm pretty much right under this event if such an event were to happen. Right now the local forecasts call for a very light rain and snow mix event. Not a single bit of confidence for anything more than a few pesky showers.

I hope you are right though! I would love some spring type storms to wash away this drought.

 

[Chris Cooper]

I have been watching for SE Missouri. The news has said everything from moderate rain to possible severe tstorms.

 

[Bob Hartig]

This is a reminder that more stringent rules apply to the forecast discussions and storm reports than to other parts of the forum. In particular, the rules state, "Your posts must have substantial discussion of the event." This means providing your own analysis of the situation, referring to various forecasting tools (i.e. surface and upper air maps, model soundings, instability and shear indices, and so on) in a way that lets other members see how you personally are piecing together a developing situation. Because the forecast discussions are the crown jewel of this forum, they are monitored more closely to ensure quality of content. Long-time members will appreciate how important this is and how difficult it can be to maintain consistent, high-quality Target Area interactions. We moderators sometimes tread a fine line between exercising too much control and too little.

I am leaving the last couple of posts in place because I don't expect much more in-depth commentary on this event, and another part of the forecast discussion rules says, "Extraneous chatter is allowed if the post already contains substantial weather discussion and the remarks don't disrupt or derail the thread." However, if this were a different, more promising setup, comments that merely cite local weather forecasts or quote SPC text would get deleted offhand.

This is no offense, T.J. and C.T.--I know you're not well-acquainted with the expectations of the Target Area discussions. But this post provides a good opportunity, here in the off-season, to encourage you and all new (and even not-so-new) members to familiarize yourselves with the rules that govern Target Area discussions--forecast discussions in particular. These discussions are upper-level. That doesn't mean you should be afraid to try your hand at them. Just please consider whether you're ready to do so. If not, feel free to lurk here and learn, but use the Beginner's section as the place to first try your wings. The expectations there are more relaxed. The two comments following Jeff Duda's would fit perfectly there.

Again, this post is intended as purely instructive, with a goal of enhancing Target Area discussions once storm season 2013 arrives.

Thank you,
Bob

 

[Skip Talbot]

I hope you are right though! I would love some spring type storms

I don't think there are going to be any large thunderstorms like you'd expect with a typical warm season severe weather/tornado setup. If there is any sort of tornado threat, it's going to be from low topped showers, and there may very well be no thunder st all. Even though these aren't huge storms, the updrafts in them can still be extremely strong, and given the cloud bases only hundreds of feet off the ground, it's not too difficult for them to spin up tornadoes given favorable low level shear and enough moisture. On the radar these look like 30-40 dbz blips with no hook, but they still have compact, rotating updrafts.

Maybe I'm out to lunch on this. We're getting no love from SPC at all. However, I think the 12z NAM looks great. The surface low dropped to the south a bit, which I was hoping to see. This allows the moisture to advect and wrap around that low a bit more where low level convergence and shear would be best, and the cold air aloft is pooling. The 12z run even has a narrow ribbon of 50's dews streaming into southeast Iowa. That's more than enough for a cold core like setup. Surface based and mixed layer cape are not as pronounced as they were in earlier runs, dropping from 750-1000 j/kg to about 500 at best. Still, I've been a little more interested in the low level CAPE since these updrafts are going to be shallow, and the 3km cape remains as impressive as ever if it's to be believed. 3km Cape near Ottumwa, IA is forecast at 280, 1km SRH over 300 just to the north, and bulk shear over 80 knots just to the east. If we can get an updraft to tap this favorable mix of ingredients, I don't see why there wouldn't be the possibility of a few tornadoes in the area. With the southern shift in surface features, I'd probably target the Ottumwa, IA to Chariton, IA area between noon and 4pm when low level instability is maximized.

 

[Jeff Duda]

Maybe I'm out to lunch on this. We're getting no love from SPC at all. However, I think the 12z NAM looks great. The surface low dropped to the south a bit, which I was hoping to see. This allows the moisture to advect and wrap around that low a bit more where low level convergence and shear would be best, and the cold air aloft is pooling. The 12z run even has a narrow ribbon of 50's dews streaming into southeast Iowa. That's more than enough for a cold core like setup. Surface based and mixed layer cape are not as pronounced as they were in earlier runs, dropping from 750-1000 j/kg to about 500 at best. Still, I've been a little more interested in the low level CAPE since these updrafts are going to be shallow, and the 3km cape remains as impressive as ever if it's to be believed. 3km Cape near Ottumwa, IA is forecast at 280, 1km SRH over 300 just to the north, and bulk shear over 80 knots just to the east. If we can get an updraft to tap this favorable mix of ingredients, I don't see why there wouldn't be the possibility of a few tornadoes in the area. With the southern shift in surface features, I'd probably target the Ottumwa, IA to Chariton, IA area between noon and 4pm when low level instability is maximized.

Skip, consider the different scenario for this event. This is not a typical warm-season, nuclear type updraft event. Therefore, the amount of 0-3 km CAPE probably does not add much value to this setup. 0-3 km CAPE generally gives you an idea of how low the LFC is. During the cold season, with lapse rates as they are, it's not surprising that the LFC is so low. However, at the same time, the level of neutral buoyancy for a surface parcel is also very low - around 600 mb. Therefore, the 0-3 km CAPE simply contains the majority of the entire CAPE of a surface parcel in the sounding. If this were April-June and the EL was at 40000 ft with a total of 3000 CAPE, then 250 0-3 km CAPE would be impressive because it would indicate that parcels don't have to be lifted very far to access that 3000 CAPE. That of course allows for greater potential of vorticity in parcels to be tilted and stretched closer to the ground, presumably increasing the potential for tornadoes. With this setup, it's good that the LFC is so low, because it means the same thing as for the 3000 CAPE setup. However, now a parcel only has to go up to the low LFC to access a few hundred J/kg of CAPE, so it's not like a parcel reaching its LFC is going to result in nuclear type deep convection.

That said, the degree of shear in the layer where the parcel is buoyant is quite substantial, so if an updraft can avoid being sheared apart, there does still appear to be a good chance it could produce a tornado. I would guess if a tornado did form it would be weak and brief due to the overall lack of instability, but if a tornado is all one wants to see, there probably is decent potential.

Personally, I don't care for setups like this. I want to see nuclear bombs of convection go off with tornadoes below. To me it's not worth a 500 mile drive just to add to my tornado count. But, everyone is different. To each his/her own.

 

[Alex Heimberger]

Couple of concerns I forgot to mention. The first is snow cover:

Temps haven't raised like forecast here. With cloud cover and 15-25mph winds the snow covers seems to be sticking around. I've been watching this system run for a week or so. The dew points is what really concerns me. I believe the best if any play would be LA out in front of the dry line.

 

[Skip Talbot]

Thanks for the explanation, Jeff. I've read in a couple spots, including papers by Davies that 3km cape was one of the strongest indicators for cold core tornado events which are also characterized by LFC's and EL's much lower than a typical warm season setup. An EL at 600 mb seems quite low even for a cold core setup, I agree, but I'm not sure I follow on why the EL would disqualify very large amounts of low level buoyancy. If a 14,000 foot updraft can spin up funnels owing to the updraft acceleration and stretching for the low level buoyancy, steep low level lapse rates, and favorable shear profiles, is it a problem that there is really no instability in the mid levels or that the equilibrium tops out at such a low level? I guess what I'm getting at with my wishcasting is that that the 3km cape would be compensating for the low updrafts heights, and not that the low updraft heights would disqualify the 3km cape. Dynamic forcing from that jet may take over in the midlevels where buoyancy is lacking as well.

Temps haven't raised like forecast here. With cloud cover and 15-25mph winds the snow covers seems to be sticking around. I've been watching this system run for a week or so. The dew points is what really concerns me. I believe the best if any play would be LA out in front of the dry line.

Cedar Rapids is quite a bit further north than where I've been looking. I agree that the snow cover would be a deal breaker on this event, but given that the models have shifted the target a bit further south (well south of 80), I think snow is going to be less of an issue.

 

[Jeff Duda]

I'm not sure I follow on why the EL would disqualify very large amounts of low level buoyancy

It doesn't. Either I misunderstood your enthusiasm when discussing the amount of 0-3 km CAPE or you misunderstood my reply. What I meant to say was there is a difference between having 250 J/kg of 0-3 km CAPE in May when there is 3000 total CAPE versus having 250 J/kg of 0-3 km CAPE when there is only 500 total CAPE. With an appropriate wind profile, tornadoes are certainly possible with both profiles. However, the overall storm updraft in an environment with 500 CAPE is going to be much weaker. But if the degree of acceleration is still enough to generate sufficient vertical vorticity, then there may still be tornadoes.