Saturday, June 5, 2010

Yes, Tornadoes Can Develop In New England, Too...

I can't remember the last time the Boston area was under a Tornado Watch (I'm not saying it's been years or anything like that, and I know I could find the date if I really wanted to dig for it, but I'm just not in the mood to dust-off the archives right now).  Anyway, wouldn't you know it, the Boston (KBOX) doppler radar is down this afternoon (I'm sure George W. Bush is somehow at fault here, but that's really not important right now).  Parts are "on order" and the last status message said that the NWS hopes to have it online by 7 o'clock this evening (central time).

(I digress slightly, but if you ever notice that your local radar seems to be showing old imagery - or no imagery at all - and want to see what the heck is going on, visit the NEXRAD Status Page for more info. than you probably want to know on the subject).

So, what is a guy (or gal) to do when an interesting storm is developing in the KBOX warning area but the radar isn't available?  Well...go South to Long Island, of course.  A cell that just moved through Windsor-Locks, CT looks pretty nasty and is trying to get better organized.  Here are a few images from the KOKX radar in Upton, NY (on Long Island):

Reflectivity (precipitation echoes)

Vertically Integrated Liquid (VIL) - Hail Anyone?

VIL image with Hail Algorithm & Data Table

The first image is pretty obvious, the reflectivity data (rain, hail, etc.) from the Upton radar site.  The last 2 are Vertically Integrated Liquid (VIL) displays of the same storm.  As I've blogged about in the past, VIL can (among other things) give you a rough idea as to hail potential, partcularly in large/well organized and/or severe storms.  The white areas on this particular VIL image correspond with the highest values on the scale (77 kilograms per square meter to be exact - in this case).  

NEXRAD's (the doppler radars that are used to obtain the above images) don't just produce pretty images of various kinds of weather information.  The computer programs that drive the units also derive other kinds of data, via algorithms that are programmed into the system.   For example, on the last image, I have overlaid the hail algorithm output (note the green triangle East/Northeast of Rockville).  That particular triangle denotes what the radar believes is the potential for hail up to 1" in diameter falling at or near that location.

If you click on the image to enlarge it, then look at the data table (a.k.a. "Storm Attributes Table") that is overlaid in the upper right hand portion of the screen, you'll find other information as well.  The particular storm we've been looking at has been labelled "Q8" by the computer program.  Upon further examination of the data table, you'll see that the computer is forecasting a 50% chance of severe hail (POSH) and a 90% chance of hail (POH) with that storm.  The maximum expected hailstone diameter is noted under the "Size" column, in this case 1.00 inch.

GRLevel3 users know to select "Show Storm Attributes Table" from the "Windows" menu of the program in order to display the above table.

I'll provide additional information on what some of the other data included in the table means in another post...

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