The effective version of the Modified Severe Hazards in Environments with Reduced Buoyancy parameter (i.e., modified SHERBE or MOSHE), detailed in Sherburn et al. (2016), is now available in beta form on the SPC Mesoanalysis page.

In observing the parameter’s trends over the past couple of days, I’ve noticed that there tend to be some “hot spots” associated with enhanced values of effective shear and low-level lapse rates. This is not entirely surprising, as our NARR values of effective shear did tend to show a low bias compared to the mesoanalysis values (hence, our term is weighted based on these lower values), and our dataset consisted entirely of low-CAPE cases (which inherently have less steep lapse rates). The omega field is also dependent on model setup and could have a different distribution in the RAP than what we saw in the NARR.

At any rate, I have been in touch with Rich Thompson at SPC about the potential of plotting the individual terms of the MOSHE on an internal page for us to monitor. This will allow for some troubleshooting to account for differences between the NARR and other analysis/forecast platforms. I could envision some changes to the relative weighting of terms or perhaps instituting some “caps” on the contributions of certain terms (similar to what is done in the STP/SCP) to limit false alarms while still maintaining the discriminatory skill of the parameter.

I’ll keep everyone up to date on the progress of the project to plot individual MOSHE terms. Until then, please monitor the real-time plots when you can and share any feedback with us. Thanks!

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Thanks for the update, Keith! From the SPC mesoanalysis webpage, the modified SHERB or MOSHE is a product of the following:

MOSHE = ((0-3 km lapse rate – 4 K km-1)2 / 4 K2 km-2) * ((0-1.5 km bulk shear – 8.0 m s-1) / 10 m s-1) * ((effective bulk shear – 8 m s-1) / 10 m s-1) * (MAXTEVV + 10 K Pa km -1 s-1) / 9 K Pa km-1 s-1)

where MAXTEVV is the maximum product of theta-e decrease with height and upward motion over a 2 km deep layer from the surface to 6 km AGL .

1) So I’m thinking about this correctly… MAXTEW is the maximum value one would obtain by finding the greatest -d(Theta-E)/d(Z) over a 2km layer between 0-6 km multiplied by the greatest upward motion (presumably omega?) over a 2km layer between 0-6 km. Is this correct?

2) Is the max 2km upward motion confined to the 2km layer of maximum -d(ThetaE)/d(Z), or is the max 2km upward motion unbounded by the max -d(ThetaE)/d(Z) layer? In other words, can the max 2km upward motion come from, say, the 4-6 km layer, while the greatest -d(ThetaE)/d(Z) comes from, say, the 1-3 km layer.

IF this is the case, e.g. the max 2km layer of forcing is displaced from the max 2km layer of potential instability, (all else being equal) would the release of potential instability be reduced in magnitude? Would it be released at all? Is there a certain degree of “overlap” that must be present between the max 2km layer of omega and max 2km -d(ThetaE)/d(Z) layer for potential instability contained therein to be released?

Brandon Vincent

Meteorologist

NWS Raleigh, NC

Hi Brandon,

Thanks for the comment. I appreciate you bringing this to my attention, as I hadn’t read the “fine print” description of the parameter on the SPC Mesoanalysis page yet.

The intent of the MAXTEVV term is to combine the potential instability in a layer with the vertical velocity at the top of that layer in order to approximate the release of potential instability. We calculated this over layers from 0-2 km to 0-6 km at a 0.5-km interval. In other words, the MAXTEVV is the maximum product of the vertical theta-e difference over 0-2 km, 0-2.5 km, 0-3 km, 0-3.5 km, …, 0-6 km layers multiplied by the omega at 2 km, 2.5 km, 3 km, 3.5 km, …, 6 km, respectively.

The caption on the Mesoanalysis page implies that the MAXTEVV term may be calculated in a different way in the MOSHE there. As you noted, their formulation, as described, would allow for a vertical offset of potential instability and omega, which is not what we intended. I’ve contacted the SPC folks and will relay what I hear from them.

Thanks again!

-Keith

Great Keith. Thanks for getting this parameter added to the SPC mesoanalysis and passing the word. I’ve added the MOSHE to the pile of other SPC products we archive each hour for SPC sectors, 16, 17, 18 and 19.