Blog post submitted by Mark Luther
Hurricane Irma caused only a minimal storm surge in Tampa Bay. Storm surge is the difference between predicted (or astronomical) tidal water level and the observed or actual water level (called storm tide) and is what causes the majority of damage in a land falling hurricane. In relatively shallow coastal waters like those of the Tampa Bay region, wind blowing across the water surface drags water in the direction of the wind. If it piles up against the coastline, it causes a storm surge. We operate a network of water level gauges and wind measurement sites around the bay in cooperation with the NOAA National Ocean Service called the Tampa Bay Physical Oceanographic Real Time System (TBPORTS; see http://tbports.org/). Here is what our observations from TBPORTS tell us about the surge from Irma.
As Irma approached the southwest coast of Florida, forecasters were unsure if the center of the storm would stay just offshore or would move inland. The predictions of storm surge at that time assumed a worst case scenario where the eye of the storm stayed just offshore of Tampa Bay, with possible storm surge of 8 to 12 feet above predicted tide or 5 to 9 feet above ground level. Had that happened, Tampa Bay would have been devastated. Fortunately for us, the storm moved inland and weakened quickly once it went over land.
The eye of the storm tracked well to the east of the bay. By the time that the eye of the storm passed to our north and winds turned to the west (about 5 am EDT), we were only seeing 30 to 34 knot sustained wind speeds from the WNW with gusts to just under 40 knots. By 10 am the winds were 25 to 28 knots from WNW with occasional gusts to 32 knots. By 5 pm wind speeds were less than 20 knots, still from W to WNW. The wind never went south of west. If the wind had come from the southwest, the surge would have been greater, as wind would be pushing water straight up the bay. The storm also moved quickly through the area so that the winds didn’t have time to push as much water toward the coast and up the bay. For comparison, in Hurricane Francis in 2004, the eye of the storm stalled just to the northwest of the bay region and we saw sustained winds of 50+ knots from the southwest for approximately 12 to 18 hours. That drove a surge of about 6 ft. into the St. Petersburg area (flooding my garage) and an even larger surge into downtown Tampa.
The most striking aspect of the storm surge from Irma was the large negative surge seen as the storm approached. Strong winds from the east to northeast pushed water out of the bay, leading to water levels in the northern parts of the bay that were more than 6 feet lower than the predicted tide level. In Irma, the strongest wind speed from the north at our mid-bay site was 56 knots from the NNE gusting to 65 knots. Those NE to N winds drove the water out of the bay as expected. That factor may have delayed the return of water into the bay but not by much. Water flows downhill pretty fast. In the plots below, you see the water level rises to predicted tide level very quickly once the winds turned west of north, and then first goes above predicted water level at 4 am at the Port Manatee gauge. Maximum storm surge occurred at Port Manatee of 2.17 ft at 11:18 am, at Old Port Tampa (south of Gandy Bridge) of 2.35 ft at 11:54 am, at St. Petersburg of 2.17 ft at 12:36 pm, and at McKay Bay (Port of Tampa) of 3.08 ft at 1:24 pm, all very nearly coincident with low tide. If the maximum surge had occurred at or near high tide, coastal flooding would have been 2 to 2.5 feet greater than what we observed.
We were very lucky to have escaped major damage.