Help me with today's (2012/3/29) models for SW Iowa

[BBauer]

I am a student and do not have a lot of extra time to study forecasting but I do try and take a few minutes on severe weather days to learn something. Ok, so the SPC has a slight risk coverage up into SW Iowa for today. This interests me because I'm live in the Des Moines area. So I am running through the parameters of the RUC (CAPE, shear, etc) and I am not understanding something I am seeing in the models. While the shear looks decent across Iowa this afternoon and into the evening I am not seeing much CAPE at all. As I scan through the hourly models from mid day through later this evening it shows the CAPE holding to the southwest of IA, mainly in Kansas, NW MO, and SE NE. I have noticed similar phenomena on other severe weather days as well, where in the models it looks like things are not lining up properly. Now I should have been paying more attention to what actually happened as those days unfolded. I'm fairly sure I have seen a disconnect between what actually happens and what the models show (e.g. no CAPE). Now, I understand that the models are just models and things change on a real-time basis. I understand you can get bust days when the parameters are off the hook and seemingly marginal days can produce tornadoes like up in MI recently. But, I am just curious as to why a slight risk would extend up to Des Moines with the models showing practically zero CAPE in SW Iowa.

 

[Tim Supinie]

There are a couple things going on here, I think.

Firstly, what you're probably looking at on the RUC is surface-based CAPE, which is a measure of the available energy a surface parcel. Surface parcels aren't the only parcels that can convect; there can be parcels well above the surface feed the updraft for thunderstorms (these are known as "elevated" parcels), even when the parcels right at the surface are too stable. Since there is a warm front currently across northern MO, we would expect a warming with height across southwest IA. This means that the surface parcels are probably stable, as you noted in the RUC forecasts. However, given the large amount of moisture streaming over the warm front, I would expect the parcels just above the warm front to be unstable. Thunderstorms that form from elevated parcels usually aren't much of a wind or tornado threat; mostly hail. And accordingly, the SPC 15% hail outlook extends farthest to the northeast across IA, implying they expect some of the threat to be from elevated convection.

Secondly, the outlook mentions that they expect the clouds to break in southwest IA quicker than forecast. This would imply that the cloud cover is inhibiting the northward progress of the warm front, which in turn inhibits the northward progress of the surface-based CAPE. The models don't handle clouds very well, and they *really* don't handle the inhibited surface heating from the cloud cover very well. So my guess is that the SPC is calling the RUC out on the forecast of no surface-based CAPE in southwest IA this evening.

 

[BBauer]

Thank you for that explanation, severe t-storm watch in south central Iowa now so it looks like you are right as some CAPE is being fed up into central Iowa per the SPC mesoanalysis in the area that was issued recently. Thanks again!