Isaac leaves Florida; threatens New Orleans

Chart by the National Hurricane Center/NOAA


Outer storm bands of Isaac continue to impact Florida, as shown in this 4:45 p.m. EDT satellite photo from NOAA.

FINAL UPDATE 5:00 p.m. Aug. 27: The effects of Isaac still are being felt in the Sarasota area, despite the storm now moving toward the northwest and an expected landfall near New Orleans in about 36 hours. Occasional torrential rains and winds gusting up to tropical storm force will continue for several more hours.

According to the National Hurricane Center, Isaac appears to have begun the long-expected intensification process and is forecast to make landfall with sustained winds of 100mph (161kph). The storm also is predicted to slow down as it approaches the northern gulf coast, which might cause its passage across New Orleans to take as much as 36 hours. Flooding from heavy rains and a possible 12-foot storm surge at high tide will create the greatest weather hardships for that area.

This doppler radar image from noon Monday shows the extensive effects of Isaac on Florida, even as it moves away from the state. Image by NOAA.

UPDATE 11:00 a.m. Aug. 27: Isaac is pulling away from Florida as it continues northwest toward an expected landfall along the north central gulf coast. However, its effects still are being felt across large sections of the state, as shown in the radar image to the left. Heavy rains and strong winds are expected to persist in the Sarasota area through much of the day.

The hurricane forecasting models are in significant disagreement as to the actual landfall, with some calling for landfall near the Louisiana-Texas border, and others projecting a strike as far east as Gulfport, Miss. The official NHC forecast is calling for Isaac to come ashore near New Orleans, although the NHC cautions that no one should rely too heavily on the predicted forecast track, given the wide disparity in the computer models.

Isaac still is a tropical storm, inhibited mostly by environmental conditions and its large size. The forecast is for it to become a Category 1 hurricane before reaching the north gulf coast. However, a contraction in the wind field and other factors could impact the intensification process, making Isaac a stronger hurricane upon landfall. Like the forecast track, residents should not rely too heavily on the projected strength of Isaac either. Given that Isaac is expected to reach New Orleans on the seventh anniversary of Hurricane Katrina, preparing for a stronger storm might be the more prudent course.

The size – and impact – of Isaac can be seen from this satellite photo of the storm. Outer bands already are affecting the Sarasota area. Photo from the National Hurricane Center/NOAA

UPDATE 11:00 a.m. Aug. 26: Sarasota County already is beginning to feel the effects of Tropical Storm Isaac as the outer feeder bands cross our area. Heavy rains and strong, gusty winds will continue for the next day or so, regardless of the final path of Isaac. A tropical storm warning is in effect for our area.

The exact path of Isaac over the next several days is rather uncertain. The computer models are divided as to whether the storm will be steered by a short-wave trough creating a break in the sub-tropical ridge and proceed northwesterly before recurving back to a more northerly course to make landfall in the Florida panhandle, or whether it will be bypassed by the trough and continue northwesterly across the Gulf of Mexico to make landfall along the Louisiana coast. It might be a day or more before the models coalesce around some sort of consensus as to the eventual path of the storm.

However, Isaac will affect our area regardless of its direction later in the forecast period. If it intensifies significantly in the next day or so — as is anticipated — the effects might be even more significant. Isaac is forecast to strengthen to sustained winds of 105mph (170kph). Given the very warm water temperatures in the gulf, the possibility exists for Isaac to become a major hurricane before its eventual landfall.

UPDATE 11:00 p.m. Aug. 25: The National Hurricane Center is reporting that Isaac is moving mostly parallel to the northern coast of Cuba. The timing of the storm’s expected turn to the northwest will affect the impact on the Florida Keys and other points along the gulf coast. The models have again diverged on the extended forecast scenarios, now calling for a five-day spread in landfall between Ft. Walton, Fla. and the Louisiana delta.

Hurricane hunter aircraft will survey the storm around 2:00 a.m. EDT. This data will have considerable bearing on later runs of the forecasting models. The 11:00 a.m. NHC update on Sunday, Aug. 26 should provide a clearer picture of Isaac’s eventual long-term path.

Regardless of that eventual path, the possibility continues to exist for tropical storm force winds to impact the Sarasota area. In addition, high surf created by the storm can impact beaches hundreds of miles away. After the considerable erosion of Debby, any storm damage could have significant consequences for area beaches … and the structures that are built upon them.

UPDATE 11:00 a.m. Aug. 25: According to the National Hurricane Center, Isaac has been disrupted somewhat by its passage over western Hispaniola. Any further weakening will be determined by the extent of its interaction with eastern Cuba. However, the NHC expects fairly rapid intensification of the storm once it emerges into the Florida Straits, and calls for Isaac to become a hurricane within 36 hours.

A tropical storm watch has been issued for our area. The NHC urges residents not to focus on the precise forecast track of the storm, as errors of hundreds of miles in the fourth and fifth days of those forecasts are common. The Sarasota area can expect, at the least, tropical storm force winds and high surf as Isaac passes to the west. The storm is expected to have maximum sustained winds at that time of 85-100mph (137-161kph), which will create large waves well out from the center.

Local residents should remember that Tropical Storm Debby originated in the central Gulf of Mexico and came ashore well to our north in the Big Bend area, at a right angle. It also was a minimal tropical storm. Yet significant beach erosion and damage in the millions of dollars occurred in Sarasota County. Isaac, on the other hand, will be a category 1 or 2 hurricane, with winds more than twice the strength of Debby. In addition, the storm will be parallel to our coast, maximizing the impact on our shores.

UPDATE 11:00 p.m. Aug. 24: Other models followed the earlier model that was predicting a more eastward break in the subtropical ridge during the 8:00 p.m. computer runs, so the forecast track has been shifted to the right over the next 48 hours. Despite this, the ridge still is expected to build back to the west and keep Isaac on a northwesterly heading during the remainder of the forecast period. The main effect of the shift is to bring Isaac higher across the Florida Keys by Sunday night. The forecast still calls for the storm to come ashore near Pensacola sometime Tuesday night, passing just to the west of the Sarasota coastline Monday afternoon and evening. Conditions locally should begin to deteriorate overnight Sunday and worsen throughout the day on Monday.

UPDATE 5:00 p.m. Aug. 24: Isaac has begun its turn to the northwest. However, several of the models have shifted the track eastward over the next 48-60 hours. In fact, one model now calls for a landfall in southeastern Florida, traversing the length of the state. The official forecast continues to show Isaac remaining off of the west coast of Florida, but with slightly more curvature. Depending on the steering currents, the storm could pass close enough to the Southwest coast to bring strong tropical force winds of about 60mph (97kph) to Sarasota County.

Because of the shift in the models since the last run, it is even more important to see how the models perform with the overnight runs, when sampling data from hurricane hunter aircraft are factored in. Continued bias toward a more eastward path overnight could change the forecast reasoning — and change the threat level to the southwest coast.

Isaac is forecast to be a minimal Category 1 hurricane as it passes Sarasota County, with sustained winds of about 75mph (120kph) and gusts to 90mph (145kph). Since the storm is expected to pass only about 100 miles (162km) to our west, it is almost inevitable that our area will experience tropical storm force winds, which will extend outward about 175 miles (275km) from the center. The closer the storm’s approach, the higher the winds our area will experience.

UPDATE 11:00 a.m. Aug. 24: Isaac has managed to gain strength and the National Hurricane Center now places the sustained maximum winds at 60mph (97kph). The storm has the potential to attain hurricane strength prior to its landfall on Hispaniola, but is expected to lose some strength as it crosses that island and Cuba. Once it reemerges over open water in the Florida Straits, it has the potential for additional strengthening as it proceeds northward. While the official forecast track has the storm passing well offshore, with an expected landfall in the Pensacola-Ft. Walton area, it should be noted that tropical storm force winds will extend up to 185 miles (297km) from the center on the eastern side of the storm. In addition, there remains quite a bit of disagreement among the forecast models as to the exact track of the storm between days 3 and 5.  Isaac likely will be at hurricane strength as it passes along the west coast of Florida, and any deviation eastward could bring hurricane force winds ashore, possibly with little time to prepare. Therefore, residents of Southwest Florida should continue to monitor the progress of Isaac closely.

UPDATE 11:00 p.m. Aug. 23: Over the last six hours Isaac has not intensified very much. As it will begin to interact with Hispaniola in the next 12 hours or so, not much strengthening is expected. As for the projected path, many of the models are expecting the storm to move farther to the west late in the forecast period, as the break in the sub tropical ridge appears to be occurring farther west than anticipated only a day ago. The data from hurricane hunter aircraft tonight will make the models’ computations overnight more precise, so a clearer picture of Isaac’s eventual path may emerge then.

(Original story: Posted at 5:00 p.m., Aug. 23) Tropical Storm Isaac has continued its march across the central Caribbean Sea, with forecasters at the National Hurricane Center predicting the storm could intensify to hurricane strength within 36 hours.

At that point, the storm is predicted to be between Hispaniola and Cuba. It then is forecast to traverse the mountainous terrain of eastern Cuba. Storm intensity forecasts are taking into account the impact of that terrain on the core of the storm, which is expected to lose some strength before emerging into the Florida straits around mid-afternoon on Sunday, Aug. 26.

Once back over water, the storm is expected to regain that lost strength and continue to intensify as it moves northwestward. Isaac is expected to be a minimal hurricane as it passes near Key West Monday morning.

This diagram, updated as of 11 a.m. on Aug. 25, shows the projected path of several computer forecasting models, with widely divergent outcomes late in the forecast period. Chart by Weather Underground

The forecasting computer models that have been doing the best job of tracking the storm’s short-term gyrations are regarded by forecasters as the most reliable predictors of Isaac’s behavior in the next 48 to 72 hours. However, each of those models provides widely divergent scenarios further into the forecast period. At five days, they range from Isaac coming ashore along the central coast of the Gulf of Mexico — bypassing Florida completely — to making landfall in Southeast Florida.

For that reason, NHC forecasters have low confidence in the forecast path for Isaac after the next three days.

Another important consideration is intensity. While Isaac has been struggling in its current environment, and will be further challenged by the mountainous terrain of Hispaniola and Cuba, conditions in both the Caribbean and the southern Gulf of Mexico are conducive for potential rapid intensification. For that reason, it is best not to rely too heavily on the long-term intensity forecasts.

Southwest Florida residents should continue to monitor the progress of Isaac. Emergency managers typically recommend that preparations begin now, however, since conditions locally could begin to deteriorate as early as Monday morning if the storm track moves closer to the Sarasota area.