Waterspout Along the St. Johns River
June 26th 2009
by Phil Peterson - Forecaster, WFO Jacksonville FL
1. Summary of Event
At around 5 PM EDT on June 26th, a waterspout touched down on the St. Johns river just north of the Buckman Bridge and travelled north along the river before moving inland across the riverside area of Downtown Jacksonville as a tornado at around 540 PM EDT and then dissipating. The waterspout travelled north over the river, staying northeast of the Ortega River Entrance and south of the Fuller Warren Bridge, before moving inland (Fig. 1). There were excellent eyewitness accounts and footage of the waterspout as it travelled north along the river and when it briefly moved inland as a tornado. Per TV web-cam footage, there may have been two other separate waterspouts near the Ortega River Entrance. The tornado damaged trees near
2. Radar and Meso-analysis for the Event
Meso-analysis of the event (mainly from WSR-88D) showed that the waterspout probably formed from the intersection of 3 boundaries. A fast moving outflow boundary (from earlier storms) southwest of the area raced northeast and collided with the east coast sea breeze. There was also an outflow boundary from a severe storm in southern St. Johns County. This boundary moved north and intersected the 2 other merged boundaries along the river (Fig. 7, Fig. 8). The interaction of these 3 boundaries was enough to produce a small area of strong rotation over the river. The 2 mile width of the river and the low surface friction made it an ideal location for the waterspout to develop. The orientation of the river allowed for an extended path length of around 5 miles before moving ashore Riverside. During the event there was a very small and weak storm relative velocity couplet (not shown). There was also a weak hook echo noted at about the time of the report of damage at 540 PM EDT (Fig. 10).
3. Synoptic and Sounding Analysis for the Event
The 12Z Surface Analysis showed that the forecast area was under a light southwest flow,between a stalled out trough to the northwest and a ridge to the south (Fig. 2). A look at the Jacksonville soundings (Fig. 3, Fig. 4) showed light winds between the surface and 500 mb. Such a setup was conducive for the formation of sea breezes and outflows which was the case that afternoon. A look at the soundings and also 300 MB analysis (Fig. 5) showed the passage of an upper level short wave trough at around the time of water spout. There was also cooling and moistening at the mid levels (500 MB temps fell to -10C) which steepened the lapse rates and made the atmosphere more unstable.
4. Actions Taken By WFO JAX
A busy afternoon at the weather office as there were a number of severe storms preceding this one. A severe thunderstorm warning was issued at 5 PM EDT to include southern Duval County for large hail and damaging winds. There was a report of quarter sized hail on the south side near Belfort and Butler Blvd at 515 PM EDT. At around the time the warning was issued, the reports of the waterspout started coming in and the severe thunderstorm warning was then upgraded to a tornado warning at 503 PM EDT and remained in effect until 615 PM EDT (Fig. 9). It should be noted that the initial decision to issue the tornado warning was based upon the reports of the waterspout and a severe storm in the area. While there was later a weak storm relative velocity couplet and small hook echo, the radar beam was too wide to adequately sample such a small feature and it therefore did not have the classic signatures associated with larger tornadoes.
A number of factors came together for the water spout:
1. A moist and unstable atmosphere.
2. Light winds.
3. The intersection of 3 boundaries over water.
4. A passage of an upper level short wave trough which enhanced lift.
5. A pocket of cold air aloft (-10C at 500 MB) enhanced the instability.
As a final note, the emphasis must be put on how important good storm spotting is since the initial tornado warning was based on reports.
Picture of water spout courtesy of Zisser, Robison, Brown, Nowlis, Maciejewski & Cabrey P.A.
Daily Weather Maps NOAA, June 26, 2009.
Storm Prediction Center (SPC)
Scott Carroll (Public Information Statement issued 6/26/09)
7. Other Credits