Invest 91L more organized, but has little time to develop

By: Dr. Jeff Masters , 2:31 PM GMT on April 22, 2011

Share this Blog
2
+

Hurricane season is more than a month away, but we have a tropical disturbance (91L) typical of what one might see in June or November. 91L is spinning over the waters a few hundred miles south of Bermuda, and has improved considerably in organization since yesterday, thanks to a drop in wind shear. The latest SHIPS model output is showing shear of 40 - 55 knots over 91L, but shear analyses from the University of Wisconsin CIMSS group is showing lower shear values of 20 - 30 knots over the main circulation center and to 91L's north, where the heaviest thunderstorms are. The system has a warm core at low levels, but a trough of low pressure lies over the storm at upper levels, and this trough is pumping cold, dry air into 91L, making it not completely tropical. One characteristic of subtropical systems like 91L is the presence of the main band of heavy thunderstorms removed several hundred miles from the circulation center, and 91L fits that description. 91L has two centers of circulation competing to be dominant, and this competition is slowing the storm's development. The storm was headed north at 5 - 10 mph early this morning, but that motion has halted, and 91L appears to be moving more south-southwesterly now, away from Bermuda. Sea surface temperatures are 23°C, which are very cold for a tropical storm to form in, but could support development of a subtropical storm.

As 91L moves south today, shear will steadily rise, and the storm likely has only until Friday night before shear grows too high to permit development. NHC is giving 91L a 20% chance of developing into a subtropical or tropical depression, which is a reasonable forecast. There has been only been one named April storm in the Atlantic since 1851, Tropical Storm Ana of 2003. The formation of a tropical disturbance at this location this time of year is unusual, but is not a harbinger of a active season ahead. Had this been going on in the Caribbean, that would be a different story.


Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L. Note the two centers of circulation competing to be dominant. I expect the northern center will become dominant.

Jeff Masters

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 5 - 1

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 — Blog Index

Thanks for the update Dr Masters
Member Since: Posts: Comments:
From previous blog


Quoting PolishHurrMaster:

So let'em choose a dominant one!They are professionalists,should be able to do this.



They won't name it with 2 vortice. Should 91L acquire enough organization to be named 91L will eat that second center for lunch.
Member Since: Posts: Comments:
Thanks Dr. Masters.
Member Since: Posts: Comments:

Viewing: 5 - 1

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 — Blog Index

Top of Page

About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.