29 September 2015

Joaquin forms near Bahamas, could threaten northeast U.S.

On Sunday night, Tropical Depression 11 formed north of the Bahamas, and on Monday night it was upgraded to Tropical Storm Joaquin, the tenth named storm of the season.  It is currently centered about 400 miles north of the eastern Bahamas and is drifting west.

Joaquin actually was born of an upper-level low that sat nearly stationary for over a week and gradually built down to the surface and acquired tropical characteristics.  Evidence of the upper-level low extends back to beyond September 20!  In this figure, there are slices of infrared satellite images shown every 12 hours for a week, and you can spot the same feature at nearly the same location east of the Bahamas the entire time.

It is in an area with minimal steering flow, so it is not forecast to move much in the next couple of days.  By Thursday, it should begin to feel a trough approaching the area and start to turn to the north.  However, there is tremendous spread among the reliable models even in the near future (1-3 days), so the timing is quite uncertain.  But, aside from the timing, there is decent agreement a general track to the north and then toward the northeast U.S. coast.

Track forecasts from a variety of global and regional dynamical models and consensus from the 06Z guidance. (UWM)
The intensity forecasts, for the most part, indicate that Joaquin will become a Category 1-2 hurricane around Friday.  Some models are significantly stronger, and few are weaker (AVNI and NVGI are global models, and have a harder time with intensity because of their coarser resolution).

Intensity forecasts from a variety of global and regional dynamical models, statistical-dynamical models, and consensus from the 06Z guidance. (UWM)
The official forecast from the National Hurricane Center is shown here, which generally agrees with the model consensus for track, but is a bit lower than the model consensus for intensity.

Vertical wind shear will be fairly strong over Joaquin until Thursday or so, which will be a key day to see if it intensifies quickly or not... because once it starts interacting with the trough that is expected to come off the east coast, the shear will increase again. 

However, and this is really important, if it begins to lose tropical characteristics as it heads north, that does not make it less dangerous should it make landfall.  A Sandy-like scenario is not something I'd predict this far out, but it's also not completely impossible and is something to be aware of.  There are a lot of unknowns to be resolved before being too concerned about that possibility.

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