Lowtop, rotating thunderstorms in the Pacific Northwest!!


As we continue to dig into the SWAT-V case database, we discover a spectrum of supercell and tornado events. Presented here are some intruiging storms that occurred near Portland, Oregon, on October 3, 1998. Two tornadoes were reported from two individual storms. One produced some minor damage. WSR-88D data depicts lowtopped storms with a significant lean in the vertical, and modest rotation.  These low-topped, mini-supercells can occur anywhere under the right conditions (e.g. low equilibrium level, adequate shear, and instability).  The SWAT-V case study page conatains examples of low-topped, mini-supercells from West Virginia, California, Arizona, Oklahoma, and now Oregon and Washington.

The NSSL Mesocyclone Detection Algorithm (MDA) and Tornado Detection Algorithm (TDA) have been run on this case in a research mode. This page presents some algorithm output as well as a radar overview of the storms.  A cyan circle represents a 2D vortex feature (from a constant elevation angle) as detected by the MDA.  Yellow, and red-in-yellow circles represent 3D vortex detections.

The Tornadic Storms:

Description:  The first storm was very small and shallow, but resulted in the report of one weak tornado.  WSR-88D velocity data indicates weak cyclonic shear.  Interestingly, the shear is displaced from the core of highest reflectivities.  This is indicative of the kind of strong shear that existed at low levels.

The second storm occurred south of Portland, Oregon, producing a brief tornado around 23:45 UTC.  The MDA identified a Rank 3 circulation within this storm at 23:31 UTC.  The storm appeared somewhat less impressive on radar when it produced the tornado.  However, base scan reflectivity revealed a miniature weak echo region, and the base scan velocity image showed a 15 m/s gate to gate velocity difference.  The storm was low-topped, miniature, and is a borderline supercell.  It showed sustained organization for a little over one hour.

A Marine Storm:

Description:  This storm may have produced severe weather, but it weakened just as it came onshore.  The storm existed on radar for at least one hour.

An Impressive Storm in Washington

Description:  The most impressive storm of the day, judging by radar, took place in the state of Washington.  This storm lasted longer, and exhibited a more well defined relectivity structure than its counterpart storm in Oregon.  The MDA identified some type of 3D circulation within the Washington storm for 5 consecutive volumes (approx. 1/2 hour).  Curiously, no severe weather was reported with this storm.  Severe weather may have occurred, but then slipped through the spotter network as the storm moved away from the coast, and into a lightly populated area.

The radar presentation of the storm is quite impressive.  The Washington storm can certainly be called a low-topped, mini-supercell.  Some perspective may be gained by comparing it to other supercells.  The Washington storm is not as small as one that took place near Phoenix in 1995.  It is many times smaller, however, than some of the large supercells that occur more frequently east of the Rockies.  The end result is the same, to a degree, a thunderstorm capable of producing damage regardless of location.