Pop quiz

[kg4ojj]

Not like I'm an expert....

What's the proper name (common, not Latin) of this cloud structure?

Would you report it, if you saw it?


Of course, this photograph is reduced in size and quality from the original. I'm looking off to the NW from east-central Jacksonville, FL. Taken tonight at 8pm ET, as a high-precipitation system slowly moved through NE Florida. There were horizontal striations that were clearly seen at the time of the photograph.

For those SKYWARN spotters, you should be able to identify this cloud type. Not a trick question, just something you don't see every time you chase or spot.

 

[radioscan]

That is a Roll Cloud. It signifies the approaching Gust Front of a thunderstorm.

In and of itself it does not necessarily mean severe weather. It could signify a microburst and
thus be worth reporting. It is a somewhat rare weather phenomenon. I would report it.

 

[Q101ATFD]

Actually, its a shelf cloud. The difference being that a shelf cloud is attached to the thunderstorm, while the roll cloud is independent. radioscan is right in that it signifies the gust front, or outflow boundary of a thunderstorm.

However, shelf clouds are not related to microbursts. The shelf cloud is just the boundary where the cool air outflow condenses as it mixes with the warm surface air ahead of the storm.

While shelf clouds are commonly found with larger, more defined, thunderstorms, it does not indicate the presence (or lack) of severe weather. I probably would not report it, and the leading edge is usually visible on WSR-88D anyways.

 

[k4swx]

Actually, The NWS would like to know of these features as per their low tech rule of if you have a shelf cloud you can add 30 mph more to the wind speed behind it like this:

Storms Fwd speed = 30mph
+ 30 mph under shelf cloud
equals roughly a 60 mph wind as the shelf cloud passes over a location. unless its a really slow moving storm ... The example storm would usually get a warning with it.

Altho they do not signify a severe thunderstorm you will usually see them present with them.

Of course the report is not as urgent as say a funnel or tornado but they would like to know about these so they can compare their radar data to what is seen on the ground.

 

[Raven95150]

According to the Skywarn class I attended last weekend, the presence of a shelf cloud is NOT a reportable condition, as it does not necessarily indicate any severe weather. Only the damaging winds (if present) that can come along with it are reportable.

 

[Q101ATFD]

Actually, The NWS would like to know of these features as per their low tech rule of if you have a shelf cloud you can add 30 mph more to the wind speed behind it like this:

Storms Fwd speed = 30mph
+ 30 mph under shelf cloud
equals roughly a 60 mph wind as the shelf cloud passes over a location. unless its a really slow moving storm ... The example storm would usually get a warning with it.

Altho they do not signify a severe thunderstorm you will usually see them present with them.

Of course the report is not as urgent as say a funnel or tornado but they would like to know about these so they can compare their radar data to what is seen on the ground.

Enter, WSR-88D's Storm Relative Velocity product. It adds/subtracts the velocity of the storm with that of the winds inside to get the surface winds (relative to the storm's motion).

The picture here shows a shelf cloud that rolled through a few weeks ago and didn't have a warning during the life of the storm:
http://www.centralmediaserver.com/WCPO/news/2007/06/21/noon/jun22_0002.jpg