Reviewing Storm Standards

[Eric Friedebach]

Elise Comtois, Columbia Missourian, May 13, 2005

Experiment tests threshold for severe thunderstorm warnings

Fewer severe thunderstorm warnings and a new definition of severe weather could be the result of a National Weather Service experiment in western Missouri and Kansas.

Currently, the threshold for a severe thunderstorm warning is at least three-quarter-inch hail or winds in excess of 58 mph.

In the experimental area, the weather service is using a standard of 1-inch-diameter hail.

James Kramper of the National Weather Service in St. Louis said the regular warning guidelines were established years ago by the National Weather Service and the U.S. Air Force. This experiment marks the first time the weather service has changed the criteria for severe thunderstorms.

One of the reasons for the experiment is the frequency of severe storm warnings. Some people have come to give them short shrift, said Steve Runnels of the National Weather Service in Springfield.

http://columbiamissourian.com/news/story.php?ID=13869

 

[StephenLevine]

Severe Storm Warning Criteria

I have the thought that perhaps we ought to warn storms that are producing 2 or more inches of rain per hour as severe. Such storms cause intensive flash flooding pretty much anywhere, and I believe would warrent severe criteria.
I acknowledge that this would overlap flash flood warning criteria at times, though non-convective systems such as tropical depressions and heavy rains falling on snowpacks also cause flash flooding.
Such severe thunderstorms could then be added as a 4th record on storm reports.
This has already been alluded to with forecasts here in the DFW area that state "some storms may be severe with flash flooding".

 

[John Diel]

I tend to agree with NWS on this matter. Too many warnings cause the public to ignore them. Especially if the storm doesn't actually produce the hail. The wind criteria isn't being evaluated and remains the same.

The information still goes out though. In the form of the LSR or Significant Weather Statements. The only real complaints that I've heard so far are from the farming communities. Their concerns are valid as crops are damaged with hail far less the current 3/4 inch standard. As most farmers don't generally access the NWS products, but get much of their weather over radio and TV, I think it's up to the media to point out if there is a significant chance of crop damage. This is especially true of rural areas where Metro TV doesn't normally cover well. It's already being addressed to a small degree with Media talking about freezes and the like as well as High Wind issues. It would be a small matter to add in some hail advice into a broadcast.

 

[Ryan McGinnis]

Hey, wow -- I was wondering why I was hearing "Significant Weather Statements" in Kansas back in April, and what the heck the product was for. Pretty cool, if you ask me; raising the severe threshold isn't a bad idea. Actually, I wouldn't mind having an even higher product for unusually severe storms. If a storm is cranking 90mph straightline winds or baseball hail, that deserves a whole different class of warning. Back in '03, when a particularly strong squall pushed through Kearney, NE, the local WO actually reccomended that towns in the path of the storm crank up the tornado sirens, as the winds were exceeding hurricane force.

 

[Todd Beal]

The town I lived in on July 22nd, 2003 Mid-South derecho actually did sound the sirens. I woke up around 645am that morning to the tornado sirens going off. Winds that morning were around 110mph. I would agree in sounding the sirens in situations like that.

 

[Richard Halter]

Makes sense in that situation

110mph winds (or even close to that) can cause significant structural damage and people would need to take shelter in interior rooms or go to a stronger or underground shelter.

or get the video camera out and head out :lol:

 

[Robert Dewey]

I think the wind criteria should be upped to 70MPH. I rarely see damage at 58MPH, but it seems like once things hit 70MPH or higher, some pretty extensive tree damage results. Once you hit the 90MPH mark or higher, structural damage to buildings results...

 

[John Diel]

60 mph winds can cause plenty of damage no their when they knock down larger branches. This is the reason why NWS isn't looking to change the wind criteria. I think one of the major reasons for the Severe threshold changes was to reduce the "False Alarm" ratio. This was discussed at length at the Air Mass symposium in Wichita this year and though there was some dissension in the ranks of Met's there, most agreed with the new thresholds.

I like the new thresholds.

 

[Chris Allington]

I think it would be good to change the hail standard to 1-inch... many severe t storm warnings are issued for 3/4 inch hail... and often are cancelled because no reports of severe weather are received...I agree that the wind standard should stay around 60 mph... we had a 62 mph gust from a thunderstorm on one of the nights this week here in Omaha and it caused significant amounts of tree damage and downed powerlines... it also destroyed a couple of sheds... I dont know what too think about the 2 in/hr rain thing... while that does tend to cause flooding in some form...it is usually covered just fine by a flash flood warning...

 

[rdale]

"I have the thought that perhaps we ought to warn storms that are producing 2 or more inches of rain per hour as severe."

Not sure I understand... The rain rate per hour would not determine whether or not flooding occurs. It's the amount of water already in the ground, the rain rate AND how long the rain will last. If it is going to cause flooding, issue a Flood Warning. If not, it doesn't make sense to issue a SVR Warning just because it's raining heavily?!?

"I think it's up to the media to point out if there is a significant chance of crop damage."

1) How would the media (or anyone) determine if hail will damage crops?

2) What would the farmer do if he was notified of crop-damaging hail?

 

[Walker Ashley]

Originally posted by rdewey
I think the wind criteria should be upped to 70MPH. I rarely see damage at 58MPH, but it seems like once things hit 70MPH or higher, some pretty extensive tree damage results. Once you hit the 90MPH mark or higher, structural damage to buildings results...

A 50 kt wind here in the South will do quite a bit of damage. A couple hurricanes this past fall only had measured gusts of about 35-40 kt here in Athens, yet produced extensive tree damage. Power was out and major roads were impassable for a day or two.

Yes, a 50 kt wind in western Kansas won't do anything. Hell, a surfacing LLJ at night will gust that high no problem. My point ... it depends on your landcover/forest types. Perhaps region-specific criteria would be better?

 

[Chris Foltz]

Originally posted by rdale
\"I have the thought that perhaps we ought to warn storms that are producing 2 or more inches of rain per hour as severe.\"

Not sure I understand... The rain rate per hour would not determine whether or not flooding occurs. It's the amount of water already in the ground, the rain rate AND how long the rain will last. If it is going to cause flooding, issue a Flood Warning. If not, it doesn't make sense to issue a SVR Warning just because it's raining heavily?!?

\"I think it's up to the media to point out if there is a significant chance of crop damage.\"

1) How would the media (or anyone) determine if hail will damage crops?

2) What would the farmer do if he was notified of crop-damaging hail?

I was wondering the same thing with regard to what a farmer would do (other than pray he/she had purchased crop hail insurance beforehand... :wink: ) I do like the upgrade to 1" requirement. Feel it will lessen the false alarm rate which is always a concern for the NWS.

 

[B Ozanne]

A lot of these criteria depend on the location. A 60mph wind where I live in Connecticut would do significant damage. Especially on fully leaved trees over wet soil.

2 inches or rain in an hour here would be impressive, but it would not cause flash flooding. The terrain where I live is so well drained flooding is almost never a problem here.

The point is that a national standard might not be appropriate. There certainly is not a national standard for winter storms. A 6-12 inch snow event in Alta, UT would only require a snow adivsory, while many other locations would need a winter storm warning or even a heavy snow warning.

 

[Nick Smith]

The 3/4 inch hail criteria was introduced in 1954 by the Weather Bureau based on a study that showed that was the "smallest size of hailstones that cause significant damage at airplane speeds between 200 and 300 mph." The wind criterion was changed several times over the years... Aviation severe thunderstorm watches used 50 knots as the threshold, while public watches used 65 knots (1967). In 1970, the theshold was changed and has remained 50 knots for public and aviation watches.

Reference:
Galway, J.G., 1989: The evolution of severe thunderstorm criteria with the weather service. Wea. Forecasting, 4, 585-592.