Ernesto spares Florida, heads for the Carolinas

Tropical Storm Ernesto waited until the final hours before landfall to finally put its act together, much to the benefit of South Florida. The pressure dropped from 1005 to 1001 mb as the storm came ashore about midnight, but the winds did not have time to adjust to the lower pressure, and Ernesto still had just 45 mph winds at landfall. A tropical storm in the developing phase is a fussy thing, and a number of ingredients have to come together just right for rapid intensification. I believe that the presence of Cuba to the south and the Florida Peninsula to the north, along with the particular pattern of upper air flow that existed, combined to create a turbulent air pattern with multiple vortices that made consolidation of the storm around just one central vortex difficult. One could see these multiple vortices in long radar loops last night, and it was not until just before landfall that Ernesto managed to consolidate around a single center and start to intensify. Had the storm had another 24 hours over the warm waters, it would have been a hurricane.


Figure 1.Total precipitation from the Miami radar.

So today, residents of Florida should be feeling good. There will be some heavy rains moving through periodically, but flooding should be minor. So far, Ernesto has dumped rain amounts less than four inches. Winds are too low to do any damage, but the windsurfers in South Florida get an unexpected boon. Virginia Key, one mile from downtown Miami, had sustained winds of 33 mph, gusting to 41 mph this morning at 8am. The Rickenbacker Causeway connecting Virginia Key to downtown Miami is a popular windsurfing spot I've spent many days windsurfing at, and I'm sure the windsurfers out out in force today to take advantage of Ernestos's unexpectedly modest winds. Sustained winds at other offshore buoys were generally in the 20-25 mph, with a peak wind gust at 50 mph seen at Vaca Key in the past hour.

Ernesto's forecast track
There is no change to the forecast. Ernesto is maintaining a well-organized appearance on radar animations, and should only slowly weaken during its 1-day long plus passage up the length of Florida. The storm will re-emerge into the Atlantic early Thursday morning and re-intensify over water. If the storm stays close to shore and makes landfall in South Carolina, it will probably come ashore Thursday night as a tropical storm. If the storm moves more offshore and makes landfall near the Outer Banks of North Carolina, it has extra time over warmer water, and will have a chance to be a Category 1 hurricane. The chances of Ernesto becoming a hurricane are a little less than it appeared yesterday, since it made landfall in Florida as a weaker than expected storm.

Hurricane John
The most serious situation in the tropics today is off the west coast of Mexico, where Category 3 Hurricane John is. John has just completed an eyewall replacement cycle, and is expected to intensify into a Category 4 hurricane today. The Air Force Hurricane Hunters are in the storm now, since it presents a serious threat to the coast of Mexico from Acapulco to Manzanillo.

Super Typhoon Ioke
The incredible Category 5 Spertyphoon Ioke continues to trek over the Western Pacific, and is expected to submerge tiny Wake Island later today. The entire population of the island has been eveacuated to Hawaii.

African tropical waves
An tropical wave near 12N 34W, a few hundred miles southwest of the Cape Verde Islands, has a pronounced surface circulation one can see in QuikSCAT satellite data from 3:52am EDT this morning. The amount of heavy thunderstorm activity has decreased since yesterday, and any development should be slow to occur. The thunderstorm activity associated with this wave was being enhanced yesterday by a process known as upper-level divergence. When the the winds at high levels diverge (blow outward from a common center), then air from the surface must rise to fill the vacuum created. As this surface air rises, the moisture in it condenses, fueling thunderstorms. Thunderstorms created by this mechanism make a tropical disturbance look more impressive than it really is. The wave has moved away from this area of upper divergence, reducing the amount of thunderstorm activity.

The large spiral of low clouds near 18N 45W surrounded by a large cloud of dry air and African dust continues to spin, but the thunderstorm activity near the center has dropped to nearly nil this morning. The wave is under a very favorable upper-level environment for development, with low shear and an upper-level anticyclone on top. However, water vapor satellite imagery shows that the air on all sides is very dry. The GFS model is showing that this wave will not be able to find a moister environment until Sunday at the earliest, when it may be near Puerto Rico or the Bahamas.

I'll be back late this afternoon with an update on Ernesto, John, Ioke, and the rest of the tropics.

Jeff Masters