5/7/14 Questions

[kmreid]

I have been watching the discussion on the possibility of severe weather this Wednesday. I have a few questions. I see that Iowa is an area of interest, but I am confused where the moisture is and why this area looks favorable when there has been a cap? From what I can gather, there is a definite dewpoint depression for areas of Iowa. I see the triple point and I know that a triple point can be a very playable area, but will there be enough moisture and enough lift to make this work? Also, will directional shear be enough to compensate for speed shear in some instances? I was using Iowa as an example, but if someone can show me how/why this may be or not be a good area, I would love to hear it! Other options are always welcome, because this time has me scratching my head a bit.

I really need someone that can take me on as an apprentice of sorts, because as soon as I think I have something figured out, I get thrown for a loop.

 

[chrisbray]

Moisture should arrive over the next few days as southerly flow at the surface brings up moisture from the gulf. It's already started with upper 50s/60f dewpoints in texas. I personally don't think the area looks that favorable because of the strong cap, but let's see what happens over the next few days and if the hi rez precip models agree

 

[Tim Supinie]

The moisture question is a big one, and I'm not sure how well the NAM and GFS have a handle on it. As I've mentioned elsewhere, they seem to be in their summertime "crops transpiring like crazy" mode, which isn't quite realistic yet. Example: the GFS has been blowing 12-hour dewpoint forecasts pretty badly for the last couple days, and I'm 99% sure this is the reason. Another issue is that we currently have high pressure sitting in the Gulf of Mexico, which means weak winds. Weaker winds generally means the air mass takes longer to moisten, and the models won't handle that well, either. Granted, there don't look to be any big interruptions in the flow for the next little while (until GFS fantasyland), so once we can get the Gulf to moisten up, it should stay that way for a while.

I don't usually like "directional shear" when looking at hodographs. The reason is that what matters is not directional shear, but the curvature of the hodograph. So if you take all the points on a hodograph with good "directional shear" and shift them to the right by 10 or 20 m/s, you've got a wind profile that looks "straight", but actually has a lot of curvature to it. Granted, southwesterly surface winds in the plains carry other problems (dry air transport, lack of surface convergence, and others), but none of those have to do with the magnitude of the shear being felt by any storms that form.

One problem with capping in IA on Wednesday is that the best mid-level forcing for ascent is still to the southwest in KS and OK. Also, IA is in a region of height rises on both the GFS and NAM Wednesday afternoon. Height rises usually mean sinking air, which means more capping. Both models do remove the cap in the region of the triple point in northwest IA, but they also mix out the moisture, which means LCL heights will be pretty high, and that could get worse, depending on how the moisture forecasts verify. Lots of people like to target the triple point because the surface winds are usually backed there, but as you note, this assumes the moisture can get there!

Many of these issues will be at least somewhat alleviated on Thursday, so if I lived in IA, I'd keep an eye on Wednesday, but not plan to chase until Thursday.

 

[kmreid]

I agree with you on directional shear in terms of hodos. Maybe I am not looking at the proper time (between 18Z and 00Z), but some of the hodographs for that area are just a mess. I haven't had much time to really delve into the timing of everything, which is likely why I am not seeing what the fuss is about currently. I am also glad that you touched on the LCL, because I kept thinking that the cloud bases that were being projected were less than desirable at 1,250-1,750m (although GFS looks more favorable at the moment). I agree that Thursday is looking better than Wednesday. There are some nice sweeping hodographs and hopefully that moisture will move in in time to get things going.

Speaking of moisture, at what levels is moisture the most imperative to storm initiation? I would think that low and mid-level moisture would be the most important in terms of lift?

 

[kmreid]

Moisture should arrive over the next few days as southerly flow at the surface brings up moisture from the gulf. It's already started with upper 50s/60f dewpoints in texas. I personally don't think the area looks that favorable because of the strong cap, but let's see what happens over the next few days and if the hi rez precip models agree

I saw where you had posted about your concerns on the cap, which is part of the reason that I posted over here. I have to agree that the cap is looking like an issue.

 

[Tim Supinie]

I agree with you on directional shear in terms of hodos. Maybe I am not looking at the proper time (between 18Z and 00Z), but some of the hodographs for that area are just a mess.

Actually, most of them from IA aren't too shabby. This one from the triple point area is pretty good. Again, the question there is capping and/or moisture.

I am also glad that you touched on the LCL, because I kept thinking that the cloud bases that were being projected were less than desirable at 1,250-1,750m (although GFS looks more favorable at the moment).

The GFS couldn't mix boundary layer moisture to save its life. It's likely over-doing the moisture.

Speaking of moisture, at what levels is moisture the most imperative to storm initiation? I would think that low and mid-level moisture would be the most important in terms of lift?

Mostly just boundary-layer moisture. However when you have weak forcing, moisture just above the boundary layer can help, too (not something IA has a lot of in the latest forecasts).

 

[kmreid]

Actually, most of them from IA aren't too shabby. This one from the triple point area is pretty good. Again, the question there is capping and/or moisture.

Ahhhh....I see. Every one that I glanced at earlier wasn't anything to write home about but that one is pretty nice. Not sure how I missed that.

Thank you for the insight! I still have a lot to learn, as you may well be able to tell, so I appreciate the input that I get from everyone on here.

 

[Greg Flint]

Have there been ANY improvements to weds/thurs forecast that you've noticed in the last 48hrs - Or does this kinda look as expected?

 

[chrisbray]

I'll admit I have never in my limited experience seen a setup like today in Colorado, with Northerly winds behind a front, 500mb winds in the opposite direction, and surface dew only in the 50s, and the result is tornados. I wonder how that works? Total opposite of what I normally look for.

 

[Matt Ashworth]

I'm personally not complaining. I got my first tornado off yesterday's setup just outside of Akron.

Akron, CO rope. This, along with the few landspouts this storm spawned, all appeared to be anti-cyclonic as well.

-EDIT-

Actually, now that I think about it, every rotation I spotted seemed to be anti-cyclonic. Perhaps this is a result of the unusual conditions?

 

[kmreid]

Have there been ANY improvements to weds/thurs forecast that you've noticed in the last 48hrs - Or does this kinda look as expected?

I am not sure who you were asking, but if it was me, I didn't really look. I was pretty busy yesterday so I didn't spend any time looking at the models.

 

[kmreid]

It looks like you had a pretty productive day!

 

[mkavulich]

Dew points in the 50s are often sufficient for good convection in the high plains. There is technically no limit to how low the dew points can be for a tornado event (reference this event; dew points were in the 20s and 30s); it's the lapse rates that matter. It's just harder to get a good moisture lapse rate with lower surface dew points.

In Wednesday's particular case, colder than normal mid-level temperatures associated with a strong upper-level low enhanced the CAPE to 1500-2000 J/Kg, making for the strong supercells seen that day.

 

[Rob H]

Because the cap is a mysterious thing that we don't fully understand/model and is sometimes worth gambling on. People were whining about the cap on the morning of Bowdle. It's really easy to be a negative forecaster from the comfort of your basement and say "told ya so" in hindsight, but that first time you sit out a "cap bust" that actually breaks you vow to never do it again.

 

[Marcus Diaz]

If I'm not mistaken, Bowdle had cap bust written all over it. In fact, the NAM didn't break any precip on any run up until the 18z run. So yes, the models never get a good handle on the cap.