22 August 2015

Dry air and shear affecting Hurricane Danny

After yesterday's nearly flawless appearance on satellite images, Danny is now showing signs of a more hostile environment.  The micro-hurricane is surrounded by dry air, and is entering a belt of strong vertical wind shear.  The combination is expected to significantly weaken the storm over the next few days.  As of the 5am EDT NHC advisory, Danny's peak winds are down to 100mph from a peak of 115mph yesterday afternoon.

Enhanced water vapor image over the central Atlantic from 6:45am EDT today.  Warm colors indicate dry air in the mid-upper levels of the atmosphere.  The sloppy asymmetric cloud shield over Danny is evidence of wind shear. (NASA)
In a zoomed in visible satellite image, you can actually see Danny's cloud tops getting whisked away to the northeast, and the surface circulation is centered to the southwest of the thunderstorm activity. 

The track forecasts have not changed much at all, and a selection of dynamical model forecast tracks is shown below in addition to the NHC forecast (OFCL) and cone of uncertainty.  It is forecast to be near the Windward Islands and Puerto Rico on Monday --likely as a tropical storm-- and then most of the guidance is along or north of the Greater Antilles, which at first glance, looks quite threatening for the Bahamas and south Florida. 

However, if we look at the intensity associated with those tracks, we see that Danny is forecast to be much weaker in 5 days.  The GFDL suite of models (GFTI/GFDI/GHMI on here) offers an exception: the storm weakens over the next few days but then is able to regroup and reintensify over the Bahamas. 
Intensity guidance as of Saturday morning.  In 5 days, the range spans from a tropical depression to a Category 2 hurricane.
That scenario can't be ruled out, but the majority of models currently do not suggest that right now.  I've gotten a lot of questions from south Floridians about this storm, and as of now, there is no cause for concern... just cause for attention.  It's still about 2,000 miles away from south Florida, but *IF* it remains intact and *IF* it continues to track toward the area, it would arrive in the Friday timeframe.

Here is an interesting graphic that I produce... it's a week-long series of slices of infrared satellite images over the tropical Atlantic, with a slice every 12 hours.  The newest image is on top, and the oldest is on the bottom and is from August 14 when the wave that would become Danny left the African coast.  I tracked Danny with a red line.  These African easterly waves are a normal and regular feature of the tropical Atlantic from late July through October, and a small fraction of them end up becoming named storms and even major hurricanes.

Which leads me to the next potential system... a wave that exited the African coast yesterday.  More on that later if it shows signs of development.