Tropical Strorm Isaac – What’s Next
A 10am update from Dr. Jennifer Collins (8/27/2012)
Here in Tampa, we are quite a distance away from center of Isaac but already we have seen some flooding in parts of Tampa for example at Dale Mabry and Neptune, but that water has receded. The tornado watch in Tampa has expired at the time of writing (10am). We are no longer in the watch because the outer rain bands passed us, but that does not exclude a new tornado watch from happening later as these rain bands MAY get pulled back into Tampa as the storm moves away from us further. Climatologically, tornadoes spawned from hurricane landfalls typically occur in the afternoon or evening hours so another tornado watch cannot be ruled out later in the day. Overall later today we may see isolated tornadoes as well as thunderstorms, lightning and heavy rain, from intermittent outer banding which was on the east side of the state this morning, with more localized flooding possible. We have not yet had sustained TS force winds (39 mph or greater). So far we have had a northeast wind which has shifted to east in the last couple of hours. These winds have been offshore, and I expect only minimal coastal surge in the Tampa Bay area – 2 ft, and even then, there is a low probability even of this. I don’t expect any major surge as Isaac is tracking further offshore than previously forecast. Surge in Tampa is no longer a story and will not affect the Convention Center, but risk of significant surge along the Northern Gulf Coast is high. Total rainfall in the last 6 hours at Tampa International Airport was 1.74”.
We should be clear in Tampa by tomorrow morning. The NHC has the center being latitudinally adjacent to us in the middle of the night tonight (2am) based on the 8am forecast. Remember, even though it may be at the same latitude, it will still be far out in the Gulf. Isaac is nowhere near as close to Tampa as was forecast 2 days ago. I believe it was absolutely the right call the RNC made to delay by one day (as well as USF to close USF today to non-essential personnel and students) due to possible tornadic activity this afternoon. With landfall expected along the Northern Gulf Coast, here in Tampa we expect rainfall and winds here for the next day. By tomorrow, we may return to our typical summertime pattern, afternoon thunderstorms particularly, as Isaac moves away.
The track from NHC shows it coming right in at New Orleans. Remember though the cone shows other track possibilities and that everyone within the cone is at risk. Katrina was not an absolute worst case scenario for New Orleans due to its trajectory from south to north. The maximum surge from Katrina was not in New Orleans, but along the Mississippi coastline. Isaac is coming from southeast to northwest which could drive the surge right up Lake Pontchartrain, which is the worst case scenario “IF” it intensifies further. New Orleans could get inundated if the storm intensifies beyond the current 8am forecast.
Landfall is expected along the Northern Gulf Coast, with the models consolidating on hitting southeast Louisiana (though everyone should consider the cone of possibilities). The question really is the INTENSITY. The model which has been consistently showing the strongest hit is the HWRF with a Category 3 at landfall (yesterday it forecast even stronger) with the right front quadrant coming in at Lake Pontchartrain. Atmospheric and ocean conditions are conducive for intensification – warm sea surface temperature, lower wind shear, strong upper level outflow. However, most models have it as a strong Category 1, and the NHC forecast has it making landfall at 90 mph – a high end Category 1 hurricane (based on the 8am forecast). It could be stronger with further development of the inner core. Recent GFDL model runs have Isaac tracking further west towards the Louisiana/Texas border, and doesn’t have it strengthen into a hurricane at all. This is unreasonable in my opinion. Since yesterday evening the models have mostly come into closer agreement, where before the models showed more spread for possible landfall locations. While we have a WNW track at 14 mph, NHC says that “a turn toward the northwest is expected by Tuesday. On the forecast track… the center of Isaac will move over the eastern Gulf of Mexico today and approach the northern gulf coast in the hurricane warning area on Tuesday”.
Isaac could make landfall in Louisiana, seven years to the day from Katrina, but it is expected the storm could be quite different. The southeast to northwest trajectory is the text book case to bring maximum surge into New Orleans with the direct impact of the right front quadrant of the storm. So IF it were to intensify to a major hurricane, it could be more dangerous than Katrina. If it was to stay weak or weaken, it may not be too bad. We expect landfall Wednesday morning. There could be a sizeable amount of moisture and energy associated with it, so we could remain tracking it for one to three days. It is worth noting that the remnants of Ivan in 2004, for example, continued to put tornadoes on ground until three days after landfall.
My advice: If evacuation order occurs for the Northern Gulf States, heed it and get out of the way.
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