The cause: a waterspout that developed near Anderson Island in the southern Sound and which remained intact for about a half-hour. Here are some pics I found on the KOMO and KING-5 web sites. An extremely well-formed funnel and you can see from the first that the winds reached the surface, kicking up lots of spray. Beautiful pictures.
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Nearly all of the waterspouts and tornadoes in our area are from the non-superstorm type. That usually involves a fairly strong thunderstorm passing/developing over a region where the wind changes rapidly with horizontal distance. We call that horizontal wind shear. Such wind shear is associated with some inherent rotation around a vertical axis that get spun up by the vertical motion of the thunderstorm. A schematic of this effect is shown below and I have a whole section on this process in my book on Northwest weather (which is a good gift for the upcoming holidays by the way).
Most Puget Sound tornadoes seems to develop on the shear associated with a Puget Sound convergence zone or the strong wind shear near terrain. This one appears to be an exception.
The last time a Tornado Warning was issued for the greater Seattle Metro was on Dec. 12, 1969, so the younger folks reading this blog will never have experienced such excitement.
Waterspouts are the weaker cousins of the strong tornadoes one finds over the Midwest U.S.
According to the official Storm Prediction Center definition:
A waterspout is a tornado over water--usually meaning non-supercell tornadoes over water. Waterspouts are common along the southeast U. S. coast--especially off southern Florida and the Keys--and can happen over seas, bays and lakes worldwide. Although waterspouts are always tornadoes by definition; they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn boats, damage larger ships, do significant damage when hitting land, and kill people.
This waterspot, and virtually all of our waterspouts/tornadoes around here, are associated with non-supercell thunderstorms. Supercells are the big Kahunas of the thunderstorm world with very high tops (reaching 40-60K ft), intense rain, hail, and most importantly rotation.
This waterspout came out of a relatively wimply NW thunderstorm. Let me show you. We start with the composite reflectivity--a measure of the highest precipitation rate in the storm. Got some red...that is pouring rain, probably with some small hail in the south Sound. Impressive for around here. But equally strong thunderstorms were hitting in the north Sound with no waterspouts. No sign of any hooked echoes...which indicate supercell storms.
How high were the thunderstorm tops?....we have a radar-based diagnosis of this. Only 20,000 ft! Folks in the Midwest would laugh at such tops.
And since the National Weather Service radar is Dopplerized, it can show the radial winds toward or away from a radar. Neither this shot from the Camano Island radar nor one from the Langley radar showed any hit of thunderstorm rotation as we would see in a supercell storm. Rotation would be indicated by a pairing of warm (red,orange) and cool (green, blue) colors. Nothing there.
Nearly all of the waterspouts and tornadoes in our area are from the non-superstorm type. That usually involves a fairly strong thunderstorm passing/developing over a region where the wind changes rapidly with horizontal distance. We call that horizontal wind shear. Such wind shear is associated with some inherent rotation around a vertical axis that get spun up by the vertical motion of the thunderstorm. A schematic of this effect is shown below and I have a whole section on this process in my book on Northwest weather (which is a good gift for the upcoming holidays by the way).
Looking at the surface map at noon on Saturday (see below), I wasn't impressed by the amount of shear, but clearly there was enough. Perhaps we had 15 knots over southern Puget Sound and much weaker wind over land (Kitsap, Anderson Is.).
Most Puget Sound tornadoes seems to develop on the shear associated with a Puget Sound convergence zone or the strong wind shear near terrain. This one appears to be an exception.
The last time a Tornado Warning was issued for the greater Seattle Metro was on Dec. 12, 1969, so the younger folks reading this blog will never have experienced such excitement.
Dawg Dash: Weather Looking Decent!
One of my favorite events will occur Sunday morning at 9AM: the UW Dawg Dash fun run. For more information, check out their site: http://depts.washington.edu/alumni/blogs/dawgdash/ The latest forecast suggest no major storm at that time. Little or no rain.
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