Storm speed

[Elinor McLennon]

How can you forecast how fast storms will move once they become severe? I noticed the storms in arkansas on 4-3-08 were moving at approx. 50mph. Is there a way to know this info in advance?

[Bob Schafer]

Hi, Elinor,

There are several ways, but the two easiest, IMO, are to either find that in the models or on the text of a forecast sounding.

The forecast storm motion is often found on models forecasting helicity. Here's one example:

http://weather.cod.edu/forecast/ETA/SP/etaSP_1km_hel_0.gif

That's kind of hard to read, but if you squint you can kind of make out the wind barbs on there, except that they aren't "wind barbs" exactly, but storm motion. That's from the CoD site, of course:

http://weather.cod.edu/models/

Another way to forecast storm motion is to look at a forecast sounding.

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_overlays.cgi?fcsthr=036&STATIONID=KGBD

That's one example of a forecast sounding, and in the second column/second row it gives you the "Storm Dir/Spd". Other sites with fSoundings will display the forecast storm motions differently, but they can always be found in the text somewhere.

Unless that link has been updated since I posted this, it says "275.1/31.4" (from the WRF; I unchecked the GFS), so storm motion would be slightly FROM N of W at 31.4 knots.

Finally, it's important to note that storm motions can be way off from these forecasts for a number of reasons. Storms riding a boundary will not adhere to the forecast, nor will right and left-turners.

[Scott Weberpal]

I still use the old equation of taking 80% of the average of 850 and 500 mb winds...seems to be fairly accurate.

[Chris Nuttall]

Initially, we can assume that storms will follow the mean wind in the 0-6km AGL layer. Often, storms will deviate this motion, but that's a different topic. Several sources of soundings, such as CoD and Unisys have this plotted on the sounding itself. It's listed as "STM" and is shown as direction (deg)/speed (kts).

If you want to get hardcore, you plot the mean wind using a hodograph. There's a good hodograph module on the COMET website that demonstrates this. BTW, anyone can sign up and use the COMET website, and I highly recommend it.

So, in summary, to get an estimated speed, all you have to do is average the wind speeds from 0-6km. There are other things that come into play, but this is a good first estimate.

[Dave Gallaher]

I still use the old equation of taking 80% of the average of 850 and 500 mb winds...seems to be fairly accurate.

Thanks for that, Scott--that's the kind of thing that really helps those of us outside the field (and ignorant, in my case).

[Chris Nuttall]

See above post. Deleted this one since the threads were merged.

[Shane Adams]

I usually look at the 700mb and the 500mb winds. I take the average of those two speeds, and then adjust the forward track a few degrees right of that mean motion.

[Mikey Gribble]

I've heard 700mb winds are usually pretty close if you are just guesstimating. I usually take the Bunkers storm motion speed off of the WXCaster page and subtract 10%-20% or so. That works good enough. I really don't care too much about storm motion so long as it's below 35kts. Above that and it can be trouble IMO.

[Jason Harris]

And for the easy way out . . . I like wunderground.com with the Nexrad, which will tell you the storm speed for each storm when you use the storms label :) It's in knots by the way: 1 knot = 1.15 mph

[Elinor McLennon]

ha ha ha....i must admit i do like the easy way out post by jason. i've been chasing for several years and i am learning as i go. i appreciate the replies and this will give me something else to study. thanks so much for the help and for making a place to ask the "dumb" questions. keep up the great work and stay safe.