Caribbean disurbance 96L nearly a depression; hottest day ever in Los Angeles

Pressures continue to fall over the Western Caribbean this morning as a strong tropical disturbance (96L) organizes. Surface observations suggest that 96L has a large circulation covering most of the Western Caribbean, as evidenced by winds out of the southwest sustained at 29 - 34 mph observed at Buoy 42057 to the southeast of 96L's center this morning. Rotation of 96L can be seen on radar loops out of Pico San Juan, Cuba, and well as satellite imagery. The heavy thunderstorms are currently quite disorganized, but are bringing torrential rains to the Cayman Islands, Jamaica, Cuba, and Honduras. A Personal Weather Station in George Town on Grand Cayman Island has picked up 3.64" of rain in the twelve hours ending at 8am this morning. 96L is not the typical sort of disturbance one sees in the Atlantic, since it is much larger than normal. What has happened is that the atmospheric flow pattern of the Eastern Pacific has shifted eastwards into the Western Caribbean, bringing in the Eastern Pacific ITCZ (Intertropical Convergence Zone, a region of converging surface winds that creates a band of strong thunderstorms). 96L resembles the "monsoon depressions" common in India's Bay of Bengal or the Western Pacific. A monsoon depression is similar to a regular tropical depression in the winds that it generates--about 30 - 35 mph near the outer edges (and usually stronger on the eastern side of the circulation.) Monsoon depressions have large, calm centers, and can evolve into a regular tropical storms, if given enough time over water to develop a tight, closed circulation. The Hurricane Hunters are scheduled to fly into 96L this afternoon near 2pm EDT to see if it has become a tropical depression.


Figure 1. Morning radar image of 96L. Image credit: Cuban Institute of Meteorology.

Forecast for 96L
Because 96L is so large and lacks a well-defined surface circulation, it will take more time than a typical disturbance for it to spin up into a strong tropical storm. Given that the steering currents are expected to pull 96L north-northeastwards over Cuba and into South Florida and the western Bahamas on Wednesday, the storm lacks sufficient time over water to be any stronger than a 55 mph tropical storm for Florida. I think the top winds in Southeast Florida are likely to be in the 30 - 45 mph range on Wednesday. By the time 96L makes landfall in North Carolina or South Carolina on Thursday morning or afternoon, it could be as strong as a 55 - 65 mph tropical storm, but I think it is only 20% likely that 96L will make it to hurricane strength on Thursday. The primary danger from the storm is heavy rainfall. A potent upper-level low and stationary front over the U.S. East Coast have been bringing moist, tropical air from the Caribbean northwards over the past few days, bringing heavy rains that have saturated the soils. Wilmington, NC received 10.33 inches of rain yesterday, its second greatest one-day rainfall since record keeping began in 1871. Only the 13.38" that fell during Hurricane Floyd on September 15, 1999 beat yesterday's rainfall total. With 96L expected to bring another 6 - 8 inches of rain to the region later this week, serious flooding is likely, and flash flood watches are posted for the North Carolina/ South Carolina border region. South Florida is also under a flood watch, for 3 - 5 inches of rain. Flooding rains of similar magnitude can also be expected in Cuba, the Cayman Islands, and the Western Bahamas through Wednesday night.


Figure 2. Radar-estimated precipitation since Saturday for the North Carolina/South Carolina border region. Widespread rain amounts of 5 - 10 inches have occurred.


Figure 3. Forecast precipitation for the 5-day period from 8am today through 8am EDT Sunday, October 3, 2010. Image credit: NOAA Hydrometeorological Prediction Center.

Elsewhere in the tropics
Once 96L moves out of the Caribbean, the GFS model predicts that the Western Caribbean will "reload" and produce another tropical disturbance capable of developing into a tropical depression late this week or early next week. The other models are not showing this, but do predict a continuation of the disturbed weather pattern over the Western Caribbean. A second disturbance, if it develops, would be subject to similar steering currents, and may also move northwards across Cuba, Florida, and the Bahamas, then up the U.S. East Coast. This second disturbance might be more dangerous, since it would be dumping heavy rains on regions already drenched by 96L.

Hottest day in Los Angeles history
The mercury hit a blistering 113°F (45.0°C) at 12:15 pm PDT yesterday in downtown Los Angeles, making it the hottest day in Los Angeles history. It may have gotten hotter, but the thermometer broke shortly after the record high was set. The previous record in Los Angeles was 112°F set on June 26, 1990; records go back to 1877. Nearby Long Beach tied its hottest all-time temperature yesterday, with a scorching 111°F. And Christopher C. Burt, our new featured blogger on weather records, pointed out to me that Beverly Hills hit 119°F yesterday--the hottest temperature ever measured in the Los Angeles area, tying the 119°F reading from Woodland Hills on July 22, 2006. Yesterday's record heat was caused by an unusually large and intense upper-level high pressure system centered over Nevada that generated winds blowing from the land to the ocean, keeping the ocean from exerting its usual cooling influence. Remarkably, Los Angeles had its second coldest summer on record this year, and temperatures just five days ago were some the coldest September temperatures in the region for the past 50 years.

The remarkable summer of 2010
Wunderground is pleased to welcome a new featured blogger--weather historian Christopher C. Burt. Chris is a leading expert in the U.S. on weather records, and is author of the world's most popular weather records book published to date, Extreme Weather: A Guide and Record Book. He's spent a lifetime collaborating with like-minded individuals from around the world, and no one--including official sources such as the National Climatic Data Center and the National Extremes Committee--has done as thorough a job correlating the various weather records available and determining the most accurate extreme values of such. Each month he'll be reporting on the notable records for heat, cold, and precipitation set world-wide, and his first post takes a look at the remarkable summer of 2010. It's great to have someone like Chris who stays on top of weather extremes, and I hope you'll pay a visit to his blog and welcome him to the wunderground site!

"Hurricane Haven" airing this afternoon
My live Internet radio show, "Hurricane Haven", will be airing again today at 4pm EDT. The call in number is 415-983-2634, or you can post a question to broadcast@wunderground.com. Be sure to include "Hurricane Haven question" in the subject line.

Today's show will be about 30 minutes, and you can tune in at http://www.wunderground.com/wxradio/wubroadcast.h tml. The show will be recorded and stored as a podcast.

I'll have updates as the situation with 96L requires.

Jeff Masters