probabilities

[Alex Goldstein]

Here in NJ today, we got our first severe thunderstorm watch of the year. Like many of you I'm sure, I went and looked at the probs for severe events, and I became confused. Here were the probs:

Tornadoes
Probability of 2 or more tornadoes


Low (20%)
Probability of 1 or more strong (F2-F5) tornadoes


Low (5%)
Wind
Probability of 10 or more severe wind events


Mod (50%)
Probability of 1 or more wind events > 65 knots


Mod (30%)
Hail
Probability of 10 or more severe hail events


Mod (30%)
Probability of 1 or more hailstones > 2 inches


Low (20%)
Combined Severe Hail/Wind
Probability of 6 or more combined severe hail/wind events


High (70%)

I was hoping some of you more familiar with the SPC than I could clear this up. If the individual wind/hail threats are moderate, then how does the combined threat become high? Why is the combined not moderate as well, especially considering how low the other probs are.

Thanks.

 

[Jeff Snyder]

I'm sure one could do a detailed statistical explanation, but I'll just say that the def'n of the "combined" category only requires 6 severe reports, while the individual wind and hail categories have probabilities for 10 severe reports. Ideally, if the met thinks that 4 hail and 4 wind reports are possible, they verify well with 0% 10+ hail report, 0% 10+ wind reports, and 100% 6+ total severe reports. It looks like it was a rather low-end watch, with a mix of several severe hail and severe wind reports expected. Thus, there was an expectation of>6 combined severe reports, but severe hail and severe wind reports weren't expected to be particularly common (so the individual probs for 10+ reports were relatively low).

 

[Darrin Rasberry]

Perhaps they count the severe hail/wind events separately? It seems odd that such a high probability for 6 or more combined events is 70% while 10 or more hail events is 30%. That's a confident dropoff if the two categories overlap IMO.

 

[Alex Goldstein]

I'm sure one could do a detailed statistical explanation, but I'll just say that the def'n of the "combined" category only requires 6 severe reports, while the individual wind and hail categories have probabilities for 10 severe reports. Ideally, if the met thinks that 4 hail and 4 wind reports are possible, they verify well with 0% 10+ hail report, 0% 10+ wind reports, and 100% 6+ total severe reports. It looks like it was a rather low-end watch, with a mix of several severe hail and severe wind reports expected. Thus, there was an expectation of>6 combined severe reports, but severe hail and severe wind reports weren't expected to be particularly common (so the individual probs for 10+ reports were relatively low).

Ahh. That makes sense now. thanks.