Particularly Dangerous Situation

[Karen St John]

I get that it means a whole mess of nasty tornadoes are likely in the watch area. Nevermind one on the ground coming for a populated area.

What I really want to know is:

What criteria do forecasters use to establish a watch area as being a Particularly Dangerous Situation? Is there a specific set of numeric conditions used?

 

[rdale]

FAQ 2.7 - http://www.spc.noaa.gov/faq/

 

[StephenHenry]

Interesting, I didn't know about their goal of 3/4 of PDS watches verifying with multiple intense tornadoes. It might take some work, but I'd be curious to know how close SPC is to that level of verification.

 

[Tim Paitz]

I get that it means a whole mess of nasty tornadoes are likely in the watch area. Nevermind one on the ground coming for a populated area.

What I really want to know is:

What criteria do forecasters use to establish a watch area as being a Particularly Dangerous Situation? Is there a specific set of numeric conditions used?

Most of the time a PDS watch goes with a mod or high risk and are usually associated with major tornado outbreaks or derechoes. Not always though.

 

[Ethan Schisler]

I believe the criteria is based upon the issuing forecaster's discretion.

 

[Mike Marz]

What criteria do forecasters use to establish a watch area as being a Particularly Dangerous Situation? Is there a specific set of numeric conditions used?

This is quoted from the link that rdale posted above, it gives a pretty good answer to your question.

"The Particularly Dangerous Situation wording is used in Tornado Watches for rare situations when long-lived intense tornadoes are likely. This enhanced wording may also accompany Severe Thunderstorm Watches for widespread significant severe events, usually produced by exceptionally intense derechos. PDS watches are issued, when in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions. Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective with no hard criteria. However, the SPC goal is to have 3 out of every 4 PDS Tornado Watches verifying with multiple intense tornadoes. PDS watches are most often issued with a High risk in Day 1 Convective Outlooks."

 

[Daniel van Os]

I looked at the watches issued during the first 5 months of 2011, for 396 of those I did get the probabilities from SPC.

17 out of 396 were marked as PDS (4.3%)

Looking at the probabilities for 2 or more tornadoes:

5-70%: 0 PDS
80% : 8 No PDS, 4 PDS
90% : 1 No PDS, 8 PDS
>95% : 5 PDS

Similarly, looking at the chances for 1 or more F2-F5 tornadoes:

2-50%: 0 PDS
60% : 2 No PDS, 6 PDS
>70% : 11 PDS

Two of the PDS watches were for cases with 80% prob for 2 or more tornadoes, combined with 60% prob for strong tornadoes. Hail/Wind probs for these cases also were not extremely high.

So indeed it seems there is no 100% strict mapping from these probabilities to a Yes/No decision on a PDS Watch.