XM Shear Markers

[Dustin Wilcox]

Just curious if anyone knows what must occurr for the shear markers to show up on the XM radar screen. Will they become visible when there is rotation at any level of the storm or does it have to be all levels and how strong does the shear have to be? I know it tells the amount of shear but it does not tell the speed for each direction is there a standard requirment for that?

 

[Ben McMillan]

I believe its what they call "gate to gate" shear of more than 50 kts in each direction towards, and away from, the radar. Jeff Piotrowski, can you confirm?

 

[Dan Robinson]

My guess (and it's just that) is that the SCIT marker is an indicator of a low-level velocity couplet. From what I've seen first-hand, a supercell that is tornadic (or about to become tornadic) will almost always have multiple shear SCITs in the same area (usually 3 or 4) with values over 92mph. Typical tornado machine on WxWorx will have three or four SCITs with values peaking at 115 to 138mph. The 52mph and 69mph shear scits are usually not reliable indicators of a promising circulation, unless one shows up in the right spot on a large, isolated cell. These small-value SCITs seem to show up all the time, such as on multicell clusters and the leading edge of gust fronts.

If you see an 80mph or a 92mph SCIT, that's one to watch. 115mph and above with multiple markers on the south side of a big, isolated blob means get there ASAP if you're not already.

 

[Greg Stumpf]

My guess (and it's just that) is that the SCIT marker is an indicator of a low-level velocity couplet.[/b]

I've always been confused about why folks, both in ST and in the media, constantly refer to the shear markers in XM and VIPIR as shear "SCITs". The term "SCIT" is actually the WSR-88D Storm Cell Identification and Tracking algorithm (reference). Therefore, the proper terminolgy would be to call the storm centroids "SCIT markers", not the shear centroids.

 

[rdale]

As I recall that's what Barons referred to them as when they first came out - Shear SCIT's was clearly used in their training. So once it starts, it's hard to stop...