Rare February tropical disturbance drenching the Florida Keys
Our calendars may say it's February, but Mother Nature's calendar says it's more like May in the waters of South Florida, where the year's first significant tropical disturbance is drenching the Keys. The disturbance, designated Invest 90L by NHC late Sunday morning, has dumped 1 - 3 inches of rain over much of the Florida Keys this morning, with Key West receiving 4.34" of rain on Sunday, a record for the date. The storm was close to developing a surface circulation last night, thanks to wind shear values to fell to 20 - 25 knots, and NHC gave 90L a 30% chance of developing into a subtropical depression in a special Tropical Weather Outlook issued last night. However, wind shear has increased to a prohibitive 30 - 40 knots this morning, and 90L is looking much less organized. In their 7 am EST outlook this morning, NHC gave 90L a 0% chance of developing. The system will continue to grow less organized today as it moves over Nassau in the Bahamas and heads out to sea.
Figure 1. Radar-estimated rainfall from Invest 90L.
What's going on?
Obviously, strong tropical disturbances capable of developing into named storms are very rare in February, and I've never seen one in my 30 years as a meteorologist. However, ocean temperatures are warm enough year-round to support a tropical storm in the waters of the Western Caribbean. Water temperatures today in the region were 26 - 26.5°C (79 - 80°F), which is near average for this time of year. If an unusual configuration of the jet stream allows wind shear to drop below about 25 knots in the Western Caribbean, there is the opportunity for a rare off-season tropical storm to form in February. I discussed in an appearance on NPR's All Things Considered on Friday just how unusual the atmospheric flow patterns have been this winter, and today's rare tropical disturbance over South Florida is symptomatic of how whacked-out our 2012 atmosphere has been. In isolation, the strange winter weather of 2011 - 2012 could be a natural rare occurrence, but there have been way too many strange atmospheric events in the past two years for them all to be simply an unusually long run of natural extremes. Something is definitely up with the weather, and it is clear to me that over the past two years, the climate has shifted to a new state capable of delivering rare and unprecedented weather events. Human emissions of heat-trapping gases like carbon dioxide are the most likely cause of such a shift in the climate, as I discussed in my post last week, Where is the climate headed?
A historical precedent: the 1952 Groundhog's Day tropical storm
There is a historical precedent for a tropical storm this time of year--the 1952 Groundhog's Day tropical storm that hit Southwest Florida. According to Wikipedia,
The 1952 Groundhog Day Storm was the only Atlantic tropical cyclone on record in the month of February. First observed in the western Caribbean Sea on February 2, it moved rapidly throughout its duration and struck southwestern Florida within 24 hours of forming. In the state, the winds damaged some crops and power lines, but no serious damage was reported.
Meteorologist Andrew Hagen performed a re-analysis of all the tropical storms between 1944 - 1953 for his Ph.D. thesis, and looked in detail at the 1952 Groundhog Day's storm. He noted that it didn't look like a classic tropical storm, but it didn't look like an extratropical storm, either, and should stay in the database as the first named storm of 1952. In the old teletype files for February 1952, he found a February 2 message from the Cuban Weather Service that expressed some concern about possible tropical development between Cuba and Florida. NHC responded: "TROPICAL STORMS DO NOT FORM IN FEBRUARY."
Figure 2. February 2, 1952 teletype message from the Hurricane Center to the Cuban Weather Service, explaining that there couldn't possibly be a tropical storm in February. Image credit: Andrew Hagen.