4/07/06 NOW: Midwest and South

[Dave Gallaher]

Back home from chase (thank you, Bob Shafer, for excellent guidance) but no time to report---line of storms continues to impact Madison county, including TOR warned cell currently affecting Decatur, AL, that sprang up suddenly between two other warned cells. Line on convection still trains across Mississippi into the area.

 

[Dave_Hilbert]

Huntsville, AL is about to get hit hard w/ tornado on ground. GR3 shows about 10 miles out.

Decent radio live feed in huntsville now telling everyone in city to take cover: http://mmslb.eonstreams.com/wvnn.asf

 

[Jeff Snyder]

You don't read this too often:
<div class='quotetop'>QUOTE("NWSFO HUN")</div>

TORNADO WARNINGS AND FLASH FLOOD WARNINGS ARE IN EFFECT FOR SEVERAL
OF OUR COUNTIES. OUR RADAR OPERATOR SAYS WE MAY NEED TO LEAVE THE
OFFICE IN A FEW MINUTES...DUE TO A TORNADIC CELL APPROACHING FROM
THE SOUTHWEST. [/b]

The circulation associated with this storm looks to be south of the main part of Huntsville, but I don't know where the NWSFO is located. Softball-sized hail is also being reported near that area. Warning text from KHUN indicate tornado touchdown(s) near Huntville Int'l Airport. It's been a little difficult to see a lot on HTX, since the radar is looking down the line of supercells / most intense convection, resulting in attenuation issues, along with some dubious velocity 'measurements' as one moves farther away from the radar. The supercells appear to be moving E or ENE, so this will become less of an issue with time.

These storms have been interesting to watch the past hour or two, with an OFB clearly evident on KHTX. Widespread convection (likely elevated) is continuing to develop as th strong low-level flow impinges upon and over the OFB and associated cold pool / outflow. The supercells near and southwest of Huntsville appear to be riding along this OFB. Dewpoints in the area remain in the 59-61F range, though dewpoints rise into the 63-64F range to the south. Temperatures in the inflow are of these supercells have dropped about 10F in the past 5 hours, likely resulting in rather significant CINH. With established mesos (and vert. pert. PGF), along with convergence along that mesoscale OFB ahead of the convection and ENE and NE of these supercells in northern AL, these supercells are apparently still ingesting surface-based parcel (i.e. their updraft source level is near the surface). CAPE profiles can't be very good given the T/Td obs in the area... Regardless, it seems as though mesos are developing right where the various OFBs intesect the storms, with a new TVS indicating 150kt max differential velocity associated with some rotation along the southern portion of the supercell moving into Marshall county (where the softball hail is being reported). I'm not sure if this is legit, or whether its a radar dealising issue. At any rate, given the look of the convection, I think the tornado risk remains with the supercells as long as they are able to stay ahead of the OFBs. I think that the storms may slip behind the OFB(s) and become more elevated in time. It looks like the supercells would need to move due east in order to continue to feast on uncontaminated (though only slightly potentially unstable) near-surface inflow. Otherwise, I think we'll see a transition to a more quasi-linear, elevated mode.

 

[Jason A.C. Brock]

Look at the cool front firing off in LA and TX. I bet thats a site to see along the leading edge with strong updrafts and lots of lightning. Looks like dry air is slamming into the front that was over North Texas earlier today

 

[Joe Nield]

Now getting some really good low level rotation and a spectrum width maximum at the center in the storm to the east of Woodbury. Looks like the rotation may be tightening and the chance for a tornado, perhaps a strong tornado, is increasing.

Also looks like the tornado might be in the center of the intense hail core. That would be one heck of an HP.....

At 5:00pm the spectrum width maximum has increased dramatically and a tornado may be forming or on the ground now 3.5 miles south of Ecre, MS....
[/b]

Just for future info, low spectrum widths are generally associated with intense updrafts and deep, coherent, and persistent mesocyclones. A spectrum width maximum in this situation, although I say this without having seen the data in question, is more likely to be related to the presence of very large hail, or Mid-Altitude Radial Convergence (MARC).

 

[Scott Lincoln]

Just for future info, low spectrum widths are generally associated with intense updrafts and deep, coherent, and persistent mesocyclones. A spectrum width maximum in this situation, although I say this without having seen the data in question, is more likely to be related to the presence of very large hail, or Mid-Altitude Radial Convergence (MARC).
[/b]

True for large mesocyclones and areas with generally low velocity noise/variability, however research suggests a small area of spectrum width maximum can be used to aid in the detection of developing tornadoes. This is research done here at ISU and i have seen the results of this. Many cases when the rotation of the mesocyclone is tightening up at the center and increasing in intensity, it will cause a spectrum width maximum if the intense area of rotation ends up being around the size or smaller than the size of a radar bin.

 

[Matt Haworth]

Tornado warning in effect of Jefferson county and Birmingham Al now with what looks like a developing couplet on SRM and possible hook on the ref across the western and northwestern parts of the county. Sirens being sounded now in Birmingham.

 

[Jeff Snyder]

Scott,

The only paper I could find about this made special mention that spectrum width was "surprisingly" low for isolated severe storms (supercells), especially when compared to other modes such as squall lines. See:
Ming Fang, Richard J. Doviak and Valery Melnikov. 2004: Spectrum Width Measured by WSR-88D: Error Sources and Statistics of Various Weather Phenomena. Journal of Atmospheric and Oceanic Technology: Vol. 21, No. 6, pp. 888–904.

To bring to on-topic... A tornado warning has been issue for metro Birmingham! A nice mesocyclone has developed to the WNW of the city... From velocity data, it looked more like a developing bow echo 30 minutes ago, but a pretty respectable mesocyclone / couplet now resides about 14mi WNW-W of downtown Birmingham, moving east. The cell in northern AL appear to have weakened a bit (as expected), though svrs and one or two occassional tor warnings will go up for those storms. Low-level shear is currently ridiculous per the Birmingham VWP, with gobs of 0-1km SRH (note: VWP tends not to perform too well near intense convection). Of course, a storm would need to ingest air in the 0-1km layer to work with any of that, but it appears that some storms will occassionally do so.

 

[Joe Nield]

True for large mesocyclones and areas with generally low velocity noise/variability, however research suggests a small area of spectrum width maximum can be used to aid in the detection of developing tornadoes. This is research done here at ISU and i have seen the results of this. Many cases when the rotation of the mesocyclone is tightening up at the center and increasing in intensity, it will cause a spectrum width maximum if the intense area of rotation ends up being around the size or smaller than the size of a radar bin.
[/b]

Very interesting. Thanks for passing that along. I'll have to keep an eye out for that in the future.

I think Les Lemon wrote a paper at one point calling spectrum width the forgotten stepchild of the 88D base products. Has anything been published on the results you cite, and if so, where can I find it?

Thanks!