Tuesday, September 1, 2009
Sunday evening a rapidly developing thunderstorm developed off the Galveston Coast. The storm developed over two intersecting outflow boundaries from earlier storms. The result was a waterspout which moved on shore to become an EF-1 tornado. Here is the official report from the National Weather Service in Houston:
...NWS STORM SURVEY RESULTS FOR GALVESTON TORNADO...
AT AROUND 948 PM ON SUNDAY...AUGUST 30TH...A WATERSPOUT MOVED
ONSHORE ON GALVESTON ISLAND NEAR THE INTERSECTION OF 29TH STREET AND
SEAWALL BLVD. THE TORNADO THEN PRODUCED AN INTERMITTENT DAMAGE PATH
FOR ABOUT ONE THIRD OF A MILE TO THE NORTHWEST...ENDING NEAR THE
INTERSECTION OF 31ST STREET AND AVENUE R. SEVERAL STRUCTURES WERE
DAMAGED INCLUDING THE DOLPHIN WORLD SOUVENIR SHOP...WHERE WINDOWS
WERE SHATTERED AND MOST OF THE METAL ROOF WAS PEELED OFF. A
GALVESTON POLICE DEPARTMENT PATROL CAR HAD ITS WINDOWS BLOWN OUT IN
THE SAME AREA. HEADING FURTHER INLAND...THE TORNADO PUSHED OVER A
HOME ON STILTS AND REMOVED MOST OF THE ROOF OF A MOBILE HOME.
ANOTHER HOME AT THE INTERSECTION OF 31ST STREET AND AVENUE Q 1/2 HAD
ITS ROOF COMPLETELY REMOVED. BEYOND THIS LOCATION DAMAGE WAS LIMITED
TO SMALL TREE LIMBS AND SOME SHINGLES REMOVED FROM ROOFS.
THE OBSERVED DAMAGE SUGGESTS WIND SPEEDS IN THE 80 TO 90 MPH
RANGE...WITH MAXIMUM WINDS LEADING TO AN EF-1 RATING ON THE ENHANCED
FUJITA SCALE. DAMAGE PATH LENGTH WAS AROUND ONE THIRD OF A
MILE...WITH A WIDTH OF AROUND 100 FEET. THERE WERE THREE INJURIES
REPORTED. TWO POLICE OFFICERS...WHO WERE IN THE PATROL CAR WHEN ITS
WINDOWS SHATTERED...WERE TREATED AND RELEASED. ANOTHER MAN...WHO WAS
THROWN AGAINST A WALL...REMAINS HOSPITALIZED AT UTMB.
Most waterspouts form under developing cumulus towers along sea breeze and land breeze fronts where the surface vorticity can be stretched in the vertical under the developing cloud updrafts. These so called “fair weather spouts” are usually short lived and remain off shore. Every once in a while one of these spouts will make land fall becoming officially a tornado but rarely producing more than EF-0 damage. The water spout that formed Sunday night appears to have formed in the same way the Beaumont Tornado formed earlier in August. By looking at the radar data from Sunday night we can see that two outflow boundaries were interesting each other just off the coast of Galveston.
At this intersection a new storm cell rapidly developed indicating an area of strong vertical growth. This rapid upward movement was able to stretch the surface vorticity located over the intersection allowing for the waterspout to form.
Thanks to the amount of low level energy available(08/31/09 OZ RUC 0-3km CAPE over 200 J/kg off shore) for storm development, a strong updraft was able to form translating to a stronger wind speed in the tornado as it moved on shore, producing EF-1 damage. Below are a couple of radar loops of the develop storm which spawned the tornado. In the first you can see the two boundaries with the cell rapidly developing over the intersection. In the second loop you can see the rapid vertical development showing signs of a strong updraft over the intersecting boundaries.