Hurricane Florence remains on track to severely impact Carolinas; Isaac becomes hurricane

Florence now Category 4 storm

By Bryan Norcross - Hurricane Specialist

PEMBROKE PARK, Fla. - Florence is now a Category 4 hurricane. In total, there are three systems of concern in the Atlantic:  

•    Hurricane Florence is now strengthening rapidly as forecast. It is still on track to severely impact the Carolinas and the southern Mid-Atlantic states later this week continuing into the weekend.
•    Hurricane Isaac is on track to move through the eastern Caribbean islands in a few days.  Questions remain about its intensity at that time.  
•    Disturbance No. 2 in the western Caribbean could develop in the western Gulf at the end of the week.

Hurricane Florence is an extremely powerful hurricane and expected to continue to strengthen.

Peak winds are now estimated at 130 mph -- Category 4 strength. The official National Hurricane Center forecast is for a high-end Category 4, nearly Category 5 storm approaching the Carolina coast on Thursday.  

Exactly what Florence's strength will be at landfall is uncertain. Super-strong storms always fluctuate in strength, but all of the evidence points to a destructive hurricane impacting coastline, and then slowing its forward motion to bring flooding rain to the coastal and inland sections of parts of North and South Carolina, Virginia, and the bordering states.

Florence's track and most likely landfall zone is unchanged – North or South Carolina.  And there is strong consensus that the hurricane will stall near or soon after landfall. The hurricane is expect to reach the coast on Thursday, with weather conditions deteriorating early in the morning.

Questions remain, however, exactly when and where the storm slow to a crawl? How far inland will it move before the meandering starts? How long will it dump torrential tropical rain over the already saturated ground in the Carolinas and the southern Mid-Atlantic states? When will an approaching cold front finally sweep it out? How will Florence interact with that front? And will the interaction spread the storm’s effects farther north at that time?

Slow-moving storms are notoriously difficult to forecast precisely, which leaves us with the bottom line: A highly destructive Hurricane Florence is expected to arrive at the North or South Carolina coastline on Thursday. Coastal effects will be felt in Virginia and points north. Then the entire region will be subject to flooding rain of a foot or two over a wide area.  

Hurricane Harvey comes to mind, obviously. But Houston is flat. The mountainous and hilly terrain in the eastern U.S. will make the flooding even more dangerous.

Key Messages from the National Hurricane Center concerning Florence:

1. A life-threatening storm surge is likely along portions of the coastlines of South Carolina, North Carolina, and Virginia, and a Storm Surge Watch will likely be issued for some of these areas by Tuesday morning. All interests from South Carolina into the mid-Atlantic region should ensure they have their hurricane plan in place and follow any advice given by local officials.

2. Life-threatening freshwater flooding is likely from a prolonged and exceptionally heavy rainfall event, which may extend inland over the Carolinas and Mid Atlantic for hundreds of miles as Florence is expected to slow down as it approaches the coast and moves inland.

3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina, and a Hurricane Watch will likely be issued by Tuesday morning. Damaging winds could also spread well inland into portions of the Carolinas and Virginia.

4. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week, resulting in life-threatening surf and rip currents.

HURRICANE ISAAC is still organizing. Top winds are now estimated at 75 mph.  Isaac is a very small-diameter storm, which contributes to the forecast challenges. Small storms can intensify very quickly, but they can fall apart quickly, as well.

As Isaac moves to the west toward the eastern Caribbean islands, the atmosphere appears somewhat favorable for the storm to strengthen, but not ideal as with Florence.  As it approaches the islands on Wednesday, however, it will encounter more hostile upper-level winds, mostly generated by powerful Hurricane Florence well to the north.  If that happens, Isaac would weaken, but the amount of weakening is uncertain.

There is also some question if Disturbance #1 in the middle of the Atlantic Ocean will be strong enough to pull Isaac north as it approaches the islands, as it is doing with Hurricane Helene, which is farther east.  The National Hurricane Center forecast is ignoring that possibility for now, going with the traditionally accurate European and U.S. GFS models.

The bottom line, the eastern Caribbean islands need to prepare for a hurricane arriving late Wednesday or Thursday.

Key Messages from the National Hurricane Center concerning Florence:

1. Isaac is a small hurricane and uncertainty in the forecast is greater than normal.  Although Isaac is forecast to begin weakening when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands.

2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days.

The system labeled DISTURBANCE #2 is a tropical wave that is moving slowly toward the western Gulf of Mexico.  It is looking somewhat more organize this morning.  The National Hurricane Center is giving this system a 40% chance of developing into at least a Tropical Depression this week.  Everybody on the Texas and northern Mexican Gulf coast needs to stay aware.  Systems like this can develop and intensify quickly.

The system labeled DISTURBANCE #1 has not yet developed, and will be of little consequence except that it might affect the track of ISAAC and is deflecting HELENE to the north.

HURRICANE HELENE is heading north in the Atlantic and will not affect land.

In addition, HURRICANE OLIVIA is heading toward Hawaii. Tropical Storm Watches have been issued for most of the state. The mountainous islands are especially susceptible to heavy tropical rainfall. The storm will begin its approach to the islands Tuesday. 
 

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