Today modern radars paint out a wonderfully nuanced image of such echos, most of which are associated with huge supercell convective storms:
So why I am bringing this up now? On Thursday, I received an email from a Canadian forecaster who was excited to see a well-defined hook echo along the Washington coast (viewed by my favorite new radar, Langley Hill). Here is an example (the times was between 10 and 11 AM on Thursday, 1800-1900 UTC)
You can see the well-defined hook just offshore south of Quillayute (UIL). So was there a supercell storm and tornadoes/hail off the WA coast, or was there some other explanation of this amazing image? We are fortunate that this feature was extremely well positioned to be seen by the Langley radar. Ok, lets play detective!
First, one would expect very high reflectivities (intense radar echoes) and some rotation (called a mesocyclone) as viewed by the doppler capability of the radar. Here is what the radar showed. First, the reflectivity:
The radial velocities (towards or away from the radar) are next.
Mesocyclones (regions of rotation with supercell storms) generally show couplets of velocities towards or away from radar--that shows rotation. Looks like this:
No hint of that pattern here. Strike 2. Furthermore, supercell storms generally have high tops. Here is the top of the radar echos for this feature:
5-10 thousand feet. VERY unimpressive. Big storms in the Midwest can get to 50-60K feet and supercells around here up to 25-30K ft. Strike 3.
So when we look under the hood, it does not appear that this is a supercell storm. So why the hook echo?
We had a nice line of heavy showers approaching the coast relatively slowly. If there was enough horizontal wind shear, that could roll up the convective shower line into such hooked features. In fact, if you look at the first image above, there are hints of other rolled-up echoes. The Doppler velocity figure suggests that there was some shear at low levels offshore. There are some very limited surface wind observations that indicate the same thing:
|Observation at 10 AM|
Moderate southeasterlies at Destruction Is and southerlies offshore. Put a pinwheel in there at the right spot and it would tend to rotate. I suspect that is what happened... the tendency for rotation in the winds (we call it vorticity in the biz)..caused a roll-up of the convective shower line. A fun oddity, but no big storm along the coast (if any of you were out there then, let us know what it was like!)
PS: the western weather satellite is repaired and back online! And mild weather is ahead.