Infrared satellite loop of Tropical Storm Erika (NOAA)

After struggling to puff out a thunderstorm yesterday, Tropical Storm Erika is becoming better organized and stronger as it zips (west at 17 mph) towards the northern Caribbean.

The storm could approach Florida by late this weekend and there is chance tropical storm conditions begin as early as Sunday afternoon.

A tropical storm warning is in effect for Puerto Rico and the northern Leeward Islands, including the U.S. and British Virgin Islands, with the Bahamas squarely in Erika’s path this weekend.

Maximum sustained winds are near 45 mph and the National Hurricane Center (NHC) forecasts slow but steady intensification to hurricane strength by Sunday, when it forecasts the storm to be approaching Florida.  But there is significant uncertainty in both the storm’s track and intensity forecast and a Florida landfall is by no means an inevitability.

Track forecast

The National Hurricane Center forecast  track, shown below, presents a “cone of uncertainty” based on the range of most likely storm tracks.  Note that the cone only conveys the geographic area in which the storm center has a 2 out of 3 chance to pass.  It is not an “impacts” cone; rain, storm surge, and strong winds could certainly occur outside of this cone.

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While some models forecast the storm to either dissipate or hook out to sea east of the Bahamas, a large number of their simulations forecast the storm center to be very close to South Florida Sunday night.


Track guidance from the 06Z model runs. (UWM)

Clearly, this storm poses a potential threat to both the Bahamas and Florida where residents and tourists should be starting to think about hurricane preparedness and a plan of action.

[Why Florida’s record-setting hurricane drought portends danger]

Coastal areas further north, from Georgia to the North Carolina Outer Banks should also pay attention to this storm.

Intensity  forecast

Several ingredients support a gradual strengthening of this system:

  • The vertical wind shear, which can tear storms apart, is not very strong
  • There is less environmental dry air getting wrapped into the circulation, which can disrupt thunderstorm development
  • The ocean temperatures below the storm are becoming increasingly warm
  • The ocean heat content is increasing along the storm’s forecast track

The biggest brake to intensification is an expected increase in wind shear on Thursday and Friday before relaxing some Saturday when Erika would be near the Bahamas.  (Note that in the plot below, the GFS model actually dissipates the storm due to the destructive wind shear, while it survives in the other models.)

Model-diagnosed wind shear over the next five days for Erika. (CIRA/RAMMB)
Model-diagnosed wind shear over the next five days for Erika. (CIRA/RAMMB)

Forecast model intensity forecasts vary widely, from low-end tropical storm to major hurricane strength by Sunday.  The GFS model (designated as AVNI below), which dissipates the storm, is an outlier.


Intensity guidance from the 06Z model runs. (UWM)

Storm scenarios

There are three general scenarios presented by forecast models:

1) The storm stays further south and tracks over Puerto Rico, Hispaniola, and Cuba and dissipates completely;

2) The storm maintains a west-northwest heading, avoiding the Greater Antilles, and is able to gradually intensify and head for south Florida;

3) It strengthens sooner and is steered to the northwest/north around the subtropical ridge and out to sea.

Scenario 2 currently has the greatest probability, while Scenario 1 is the least likely in my opinion.  If the south Florida scenario verifies, tropical storm force winds could arrive as early as Sunday afternoon.

Simulations from last night’s four leading computer models for Sunday evening, including two global ones (ECMWF and GFS) and two regional ones (HWRF and GFDL) present vastly different scenarios.  Again, never interpret a single model run as a reliable forecast, they are used for guidance purposes only.


850mb wind and surface pressures from four dynamical models, all valid on Sunday evening. (tropicaltidbits.com)

One thing a tropical system does well is rain.  The latest 7-day rainfall forecast from the Weather Prediction Center includes a 1-foot bullseye right off the southeast Florida coast, with significant totals all along the entire peninsula.  This is actually much-needed rain, since south Florida has been in an extreme drought all summer.


Seven-day accumulated rainfall outlook, valid through next Wednesday morning. (NOAA/WPC)

Depending on the storm’s intensity (and track), high winds and coastal flooding from storm surge could also impact the Florida coast.

Stay tuned for additional updates on this very uncertain and dynamic storm.

(Jason Samenow contributed to this post.)