Radar Tutorial
Case History / Elk River / Page 1
I have done a lot of thinking about this event. As a result, my understanding of this event has taken an about face.
My understanding of storms has taken what I believe is a big step forward. And the way I've taught my SkyWarn Class
over the past 4 years HAS to change, BIG time.
About 3:15 a small cell developed southwest of Alexandria. It developed quickly as it moved rapidly east. Velocity
radar suggested it was a supercell, but it only briefly developed an obvious "hook" still well west of the Twin Cities.
Now the single cell rapidly becomes several. The center of the line of cell surges ahead several times at different
points. At times, a visible bow appears. The most obvious bow swept across the Como Ave area.
During the net, there were several reports of wall clouds. As of this date, there was only one confirmed tornado touch down
in Greenfield, MN in extreme NW Hennepin County. Here is that part of the looped slowed to see what happened.
Note the bowed surge in the line that eventually tears up Como Ave with a downburst. Greenfield would be just about where
a comma head tornado might develop, right at the northern most point of the bow where counterclockwise rotation would be
facilitated. In fact, if you look closely, the northern most part of bow forms a noticeable hook, suggesting that
circulation was indeed there.
Todd Krause KBØSGH Warning Coordination Meteorologist at MPX had a few comments about the Greenfield tornado:
The Greenfield tornado was at the northern end of a surging bow, right where you'd expect a comma head. And there
was a brief velocity couplet. The tornado traveled northeast toward Maple Grove, but it was on the ground for only
1/2 mile or so.
People in Maple Grove claiming to see a tornado might be correct if they were in southwest portion of Maple Grove,
and looking southwest. But all the photos I've seen were looking at the feature in the area around I94/694/494 and
points northward, associated with a different storm coming from Big Lake.
I was saving radar loops throughout the storm. As it approached my area, my wife and I hopped in the car and headed
west intending to meet the advancing line and retreat home before the wind, rain and hail arrived. As we approached
Elk River, through the trees I noted on the leading edge a suspicious lowering of the cloud base that seemed to reach
almost to the ground. This is how it looked from Oak Grove.
Finally, when I found a clear view through the trees, I saw something close to this view (below) from Elk River.
I observed for quite awhile before heading back for cover. I knew I was on the leading edge. The rain in the background
of the image was part of another bow in the line in northern Anoka County, some 10 miles or more away. This feature was
relatively isolated, not spreading from horizons to horizon. Yet I have been thinking this was a small, but dramatically
emphasized shelf cloud. Since then I have been persuaded otherwise.
The next day, I exchanged emails with Larry Narikawa, another Skywarn Instructor. Like a seasoned spotter and instructor,
he was reluctant to make a definitive statement about what he saw. But he put together this image to illustrate the
view. You need to use your imagination a little. These two images comprise a panoramic view. Even though the cloud
base doesn't line up, imagine that it does. See if you can tell what's going on from this.
The view on the right shows a suspicious looking shape, at nearly the same moment on the left, we see unmistakably
evidence of outflow, dust kicked up in the air traveling in ONE direction. Is this a leading edge shelf cloud with a
small down burst or is this rear flank down draft from a HP supercell imbedding in the line? One very experienced
spotter says from looking at the pictures alone, he is convinced its the latter. Much thought about his comments have
led me to rethink my initial point of view.
Though one can never be sure without multiple confirmations, I'm pretty sure at least some of the wall cloud reports on
5/9/04 in the Metro Area were in fact shelf clouds. But I will never again simply assume a wall cloud can't be on the
leading edge. Checking velocity radar showed evidence of wind gusts, but no conclusive rotation.
However, given the strong bowed surge in the line, one has to wonder if that counterclockwise rotation at the northern
most point wasn't visible more than once in Maple Grove, Brooklyn Park and Shoreview as the bow surged into the central
cities. But there is no evidence of a tornado and no pictures of a funnel as was reported.
I have been laboring under an old concept: leading edge. The fact is storms are way more complex than the simple storm
structure we have been teaching. Six years ago, we gave up the concept of the "rain-free base". Now I have thrown out
the concept of the "advancing storm" and storm "types". Storms evolve continuously in a complex interaction between
layers of air of different temperatures, dew point, wind direction and speed in a free flowing dance akin to fluid
dynamics.
On May 9th, 2004, I think what I saw was a wall cloud. A supercell, caught up in the nearly continuous line of
thunderstorms, wrapped around itself far enough until the updraft was located in a pocket on the leading edge of the
storm. The rest of the cell was at least 10 miles away to the north.
Over the next few months, I will be thinking hard about how I should change my class for next year. Hmmmm.....
Email me if you have any comments at 
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