Supercell Type

[Donnie Beasley]

I have wondered about what all goes into forecasting what type of supercells may form on a particular day. I have heard on certain days that the storms are going to be a "HP Mess". I understand that this means that the storms are going to be high precipitation and tornadoes will be difficult to see, but what exactly goes into forecasting the supercell type? I understand that when the surface temperature and dewpoint are really close together that this indicates a really humid, moist atmosphere. Is this one of the main factors in Supercell type?

 

[Jeff Duda]

http://www.theweatherprediction.com/habyhints/42/

The above is a good intro page into the subject. The site in general is pretty good at explaining these types of things, so you can follow some links around to find more if you want.

As far as what atmospheric properties determine supercell type:
-HPs tend to form in atmospheres that have weak storm relative anvil level flow. This is because there isn't as much storm relative flow to separate the updraft and downdraft, and thus you tend to get lots of precipitation near the intense updraft of a supercell, making it HP. Also, the more total moisture in the column (shown by precipitable water), the more likely a supercell will be HP just because of the immense amount of water supply available.
-LPs tend to form in atmospheres that are dry. You generally only see LPs on the high plains or in mountainous regions. However, you can get LP supercells anywhere, as long as the column is generally dry. I don't necessarily think LPs require strong anvil level storm relative flow, but if it is stronger, it's probably more likely for you to get an LP or classic over an HP.

The supercell types are really on a continuum, with LPs at one end and HPs at the other. Thus the values of parameters that best result in classic supercells are generally between the extremes that result in HPs and LPs. Strong shear at all levels is always a good sign that classic supercells will occur.

 

[Donnie Beasley]

Very good info. Thank you Jeff. That part about Precipitable Water was interesting. I'm fairly new to forecasting and that was something I did not know about.

 

[Bob Hartig]

Another thing that can factor into rain-wrapped, messy storms is storm motion that parallels upper-level winds. The result is a line of training storms, each storm dumping its precipitation on the next one downstream from it.