Wednesday, September 6, 2017

Irma Track Troubles

Hurricane Irma remains an extremely powerful and dangerous category 5 hurricane, after passing over St. Martin this morning.

Source: NOAA/NWS/National Hurricane Center
Here's a shot as the system passed over Barbuda overnight.

Source: NOAA/NWS/National Hurricane Center
Wind observations from Barbuda show the remarkable increase in wind speed with the approach of the eye, reaching a maximum sustained velocity of nearly 120 mph and a peak gust of 155 mph before the sensor gave out.

Source: MesoWest
These are simply incredible numbers, especially that the winds at the time were out of the north and the site is on the south side of Barbuda.

From now through Saturday, the center of Irma is expected to track just to the north or along Puerto Rico, the Dominican Republic, Haiti, and eastern Cuba.

Source: NOAA/NWS/National Hurricane Center
Forecasts for Florida and the southeast US are remarkably tricky and depend strongly on when Irma takes the anticipated northward shift in track.  To highlight the abrupt nature of the anticipated track shift, I've put together a loop of the GFS forecast below (based on lower resolution grids).  Note the very steady WNW progression of Irma until it approaches the Florida Peninsula, when it decides to alter course and move northward.

The loop above is just one model solution.  Below shows the tracks produced by the GEFS ensemble, which fan out from the eastern Gulf of Mexico to tracks that don't even reach the US mainland.
Source: Tropical Tidbits
Forecasters will have these and additional tracks from the ECMWF and Hurricane models to ponder.  Today's forecasts are especially critical given the long time scales needed for hurricane preparation and evacuation from the Keys and South Florida. Keep in mind that we are still talking about a 4-5 day forecast from now until Irma is flirting with Florida.  The issues at play at these lead times are well summarized by the 5 AM AST forecast discussion from the National Hurricane Center.

Source: Source: NOAA/NWS/National Hurricane Center
The bottom line is that there is a wide range of possible tracks, and as a result, details of the timing and magnitude of Irma's impacts on Florida and other southeast US states remain uncertain.

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