Tropical storm Dorian battling dry air, and losing

The increased vertical wind shear and persistent dry air have broken down tropical storm Dorian’s defenses, and its appearance on satellite indicates that it has lost a lot of organization in the past day.  And it’s probably not going to escape this hostile environment for a few more days.

As I mentioned on Tuesday (see “New disturbance exits Africa”), Dorian was expected to have a window of opportunity to strengthen on Tuesday and Wednesday, but then conditions would make a turn for the worse on Thursday, Friday, and possibly beyond.  Indeed, the storm intensified through Wednesday night, then plateaued, and then weakened again.

Today as of 5 a.m. EDT, Dorian is a mere 50 mph tropical storm (down from 60 mph Thursday), the same intensity as it was on Wednesday morning.

Satellite image of Tropical Storm Dorian at 7:45am EDT today.  The thunderstorm activity is disorganized, and generally displaced to the east of the surface circulation.

Satellite image of Tropical Storm Dorian at 7:45am EDT today. The thunderstorm activity is disorganized, and generally displaced to the east of the surface circulation.

Dry air is encroaching on the storm through a significant depth of the atmosphere.  Two types of satellite data we use to look at this are total precipitable water (TPW) and water vapor (WV).

TPW is the integrated amount of water in the atmosphere from the surface all the way up to the top of the atmosphere… but, the vast majority of the contribution comes from near the surface and low levels, so that’s essentially what you’re seeing.

WV is sensed over a rather deep layer too, but the instrument is most sensitive to water molecules in the upper atmosphere.  Sometimes features show up in both, while other times a feature exists only near the surface or only high up.  The two images below are shown at the same scale and same map area.

Top: total precipitable water around and ahead of Dorian.  Warm colors indicate higher values; cool colors are too dry for a tropical cyclone.  Bottom: water vapor around and ahead of Dorian.  Browns indicate dry air; grays and bright colors indicate more moist air.  (CIMSS and NOAA)

Top: total precipitable water around and ahead of Dorian. Warm colors indicate higher values; cool colors are too dry for a tropical cyclone. Bottom: water vapor around and ahead of Dorian. Browns indicate dry air; grays and bright colors indicate more moist air. (CIMSS and NOAA)

The forecast remains largely unchanged.  Dorian is still not expected to strengthen into a hurricane within the next five days, and it is still forecast to maintain a steady W-WNW track.  In fact, the longer it stays weak, the more westward it will move since the low-level easterly trade winds will govern its motion as opposed to a stronger storm being steered by winds higher in the atmosphere (for a more detailed explanation of this, see my blog post from July 10th).

It is forecast by the National Hurricane Center to pass close to Puerto Rico on Monday, Hispaniola on Tuesday, then the Bahamas or eastern Cuba on Wednesday.  IF it follows the northern envelope of this path, it would affect southern Florida and/or central-western Cuba on Thursday into early Friday next week.  However, long-range models are trending further and further south, so it’s possible that it will pass too close to the mountains of Hispaniola and eastern Cuba and get torn apart before ever reaching western Cuba/Florida.  It is still several days away from affecting any land, but it’s wise to at least be aware of possible scenarios.

Track forecasts from NHC (“OFCL”) as well as five other dynamical models.  (NCAR)

Track forecasts from NHC (“OFCL”) as well as five other dynamical models. (NCAR)

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science

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