We all know that the hurricane season starts in June, and after Hurricane Richard last year we're all on high alert. But in April? Really?
That's what we said after the National Hurricane Center in Miami issued a special Tropical Weather Outlook this afternoon. The Center says a low pressure area 460 miles northeast of San Juan, Puerto Rico has a 20% chance of forming into a tropical cyclone.

That is a very low chance, but it bears watching. The low pressure system has developed shower and thunderstorm activity near its center and satellite data and ship reports indicate gale-force winds north of the center.

Slow development of the system is possible during the next few days as it moves west northwest at about 10 mph. Going that way, it poses no threat to Belize, and may pose no threat to any land mass at all, but, as an outlier, it bears watching and could be an ominous precursor of what forecasters say will be an active hurricane season.

Channel 7

Right now i wouldn't care how the water was delivered!

we got about 10 minutes of rain last night.

Atlantic tropical disturbance 91L poses little threat

Posted by: JeffMasters, 2:15 PM GMT on April 21, 2011

A tropical disturbance (91L) near 24N, 63W, midway between the Virgin Islands and Bermuda, is moving north-northwest at about 8 mph. The system's heavy thunderstorm activity has increased since yesterday, but 91L has an elongated and poorly-organized circulation, thanks to a hefty 80 knots of wind shear. The storm is over waters of 25°C, and these waters will cool to 24°C by Friday as the storm continues to the north-northwest. Before 91L reaches Bermuda, steering currents will reverse and force 91L to the south-southwest on Saturday, into a region of higher wind shear. The latest SHIPS model forecast predicts wind shear will drop to 50 knots over 91L by Friday, then increase again to 70 knots by Sunday. The high shear and relatively cool water temperatures will make it difficult for 91L to organize into a subtropical depression. I give 91L a 10% of becoming a subtropical depression. Climatology argues against 91L becoming the first named storm of the year; there has only been once 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 necessarily a harbinger of a active season ahead.


Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L.

Special Tropical Weather Outlook

Statement as of 1:45 PM EDT on April 21, 2011

For the North Atlantic...Caribbean Sea and the Gulf of Mexico...

A low pressure area located about 450 miles north-northeast of San
Juan Puerto Rico is producing poorly organized shower and
thunderstorm activity. Upper-level winds are expected to become
less conducive for development on Friday...and development of the
system...if any...should be slow to occur as it moves northwestward
to northward at about 10 mph during the next day or two. There is
a low chance...10 percent...of this system developing into a
subtropical or tropical cyclone during the next 48 hours.
Additional information on this low can be found in high seas
forecasts issued by the NHC/Tropical Analysis and Forecast
Branch...under AWIPS header nfdhsfat2 and WMO header fznt02 knhc.
Another special outlook will be issued on Friday...or sooner if
necessary.

Elsewhere...tropical cyclone formation is not expected during the
next 48 hours.

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

Posted by: JeffMasters, 2:31 PM GMT on April 22, 2011

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.

The low pressure system that is Invest 91-L is quickly falling apart this morning due to wind shear. Visible satellite imagery showed that much of the deep convection has been pushed well to the east and northeast. Even higher wind shear values will impact Invest 91-L this weekend and no further development is expected from this system.