Danny Forms In the Central Atlantic: Possible Threat to the Leeward Islands Next Week

The National Hurricane Center officially upgraded the central atlantic tropical cyclone to Tropical Storm Danny at 5pm ET with a forecast that could bring our boy to the Leewards by next Monday or Tuesday. Scatterometry wind passes overnight and during the afternoon confirmed that a closed center had formed, and winds have already begun to increase. At 22z/6pm ET, Buoy 41026 received sustained winds of 42kts/48 mph at a distance of 46nm from TS Danny. Winds out of the east were measured from the buoy, which at the time was located to the NW of the center of circulation.

081815 Danny 2345z IR

Current Information: 

  • Sustained winds: 40 mph (likely 50 mph according to the buoy)
  • Minimum pressure: 1008 mb (likely dropping)
  • Movement: W at 12 mph
  • Distance: 1595 miles from the Windward Islands

Early models even a couple of days ago alluded to the possibility that this fourth tropical storm of the season may not be a tropical storm for long. Even in the hour or two since the 5pm advisory, winds may have increased. A clear curved band has set up from the north clockwise to the southwest side. SHIPS guidance shows about a 50/50 shot at minimal rapid intensification.

Microwave imagery from 21z/NRL

Microwave imagery from 21z/NRL

Let’s talk about current conditions for the time being. Current water vapor imagery continues to show that the biggest decrement to this storm, dry air, remains to be pushed away by mid/upper level outflow. The above microwave image does show that there is room for dry air to be entrenched into Danny, however SHIPS guidance continues to show mid level humidity of above 60%. Slight increases in both sea surface and sea depth temperatures may actually increase humidity for a while. Shear continues to be amazingly low around this system, and is also decreasing ahead. Danny is well stacked, and an anticyclone is forming aloft. The in-up-out process is taking shape rather quickly. A glimpse at precipitable water, or in lay moisture that is throughout the entire column if air above the ground, actually shows a boost in moisture to the north and ahead of Danny trying to do its work to help Danny survive. A tropical wave closer to the islands is helping as well.

Total precipitable water over the last 72 hours/CIMSS

Total precipitable water over the last 72 hours/CIMSS

Aggregation of moisture is beginning as seen by the darker browns at the end of the above loop. I do have some worry that if intensification happens earlier rather than later, the turn to the NW that I talked about last night could break the umbilical cord that is providing some of the convergence from the south. As with a child, development could be stunted for a time if that occurs. That being said, that is a very minor concern since the envelope surrounding Danny is very moist and SST’s increase ahead.

Forecast Track and Intensity

It is still a tad bit early to talk about impacts since land interference is a week away. To be honest, models are still very well split at least in terms of intensity. At 00z, track models came together quite a bit. Short term mid-layer steering flow will bring Danny just north of due westward for the next day or so mainly due to a sliver ridge lying to the north. The Azores High, shunted east by disturbance #2 on the NHC maps, is forecast to protrude westward toward the Greater Antilles over the weekend, at a time when dry air may begin to approach Danny. The combination of these two factors should bend the track back toward just north of due west once again.

081915 Danny Track

I should note that the intensity forecast is rather clear cut: Up, up, and away…but where it stops…nobody knows! SHIPS continues to give Danny a decent chance of rapid intensification similar to what the HWRF has shown for a few days. The best chances of that happening are between tomorrow afternoon and Friday morning. Right now, it is hard to see anything more than an 25 kt increase in 24 hours, but I should caution that a few models are showing a small, tightly cored system. This could lead to more wild intensity swings. Where it stops? When dry air enters the system. That could happen as early as late Friday evening. It seems that at least the SHIPS model seems to be delaying the onset of the drop into humidity values below 40%…not a good sign for weakening. Maximum intensity from the NHC is in the category 2 range, and the middle of the intensity model range is in the high-end category 1, but a large spread exists. An extra glitch in the forecast may be if rapid intensification takes place, we may be able to expect an eyewall replacement cycle as that RI slows down and stops. That is not currently forecast.

The Bottom Line: 

Chances are at the very least decent that there will be a hurricane threat to the northern islands beginning at the end of the week. Stay vigilant to the forecast because this storm will be prone to intensity swings, mainly in the upward direction.

Hurricane Hunter deployment is likely by Friday or Saturday out of Barbados and/or St. Croix as Danny gets into range.

As always, you should use http://www.hurricanes.gov for all official information, especially if you are making decisions. Stay tuned to your local weather forecast office for more information.

–Jonathan, @JonathanBelles

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Posted on Tuesday, August 18th, 2015, in Tropical Weather and tagged , , , , , , , . Bookmark the permalink. Leave a comment.

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