Santa Ana winds ease; tropical wave nearing Puerto Rico could develop

A surface low pressure area has developed near 17.5N, 60.5W, about 150 miles east-northeast of the northernmost Lesser Antilles Islands. This morning's QuickSCAT pass showed an elongated circulation that is not well organized. Top winds were about 25 mph to the northeast of the circulation center. Winds in the northern Lesser Antilles Islands were less than 10 mph this morning, but pressures have been falling for the past two days. Satellite loops show little heavy thunderstorm activity--less than yesterday. However, this activity appears to be increasing just north of the circulation center this morning.

Wind shear has dropped to 10-20 knots in the region, and some slow development of this system is possible as it moves to the west at about 5 mph. This westward motion is forecast to bring the low over Puerto Rico Thursday night and Friday morning, then to a point between Haiti and Jamaica on Saturday. All of these islands can expect heavy rains during passage of this system. Both the ECMWF and NOGAPS models develop the disturbance into a tropical depression on Saturday near the southern coast of Haiti. The GFS models does not, because of high wind shear. All of the models forecast falling pressures and low wind shear over the Western Caribbean late this week and early next week, and it would not be a surprise to see a tropical depression form in the region. Today is the 2nd anniversary of Wilma's strike on South Florida, so powerful hurricanes are still a concern at this point in the season. Wunderblogger Mike Theiss has posted a blog today recounting his experiences in the eye of Wilma.

Heavy thunderstorm activity has increased in the central Caribbean south of Haiti this morning. There is some rotation evident at mid levels of the atmosphere in satellite loops, but QuikSCAT showed no evidence of a surface circulation in this morning's pass. This disturbance will need to be watched for development. Activity in the western Caribbean between the Yucatan Peninsula and Cuba is associated with a cold front.

California's fires
Surface maps show a high pressure system centered over Nevada and Utah. This high is weakening and moving eastward away from California. The clockwise flow of air around this high is still driving northeasterly Santa Ana winds over Southern California, but these winds are much weaker than yesterday. By Thursday, the Santa Ana winds will be gone, to be replaced by a weak flow of moist air off the ocean. The new weather pattern will bring increased humidity, cooler temperatures, and lighter winds, which should allow firefighters to gain the upper hand. The long range forecast shows light winds for Southern California for most of the coming week, but no rain, and not as much onshore ocean winds as firefighters would like to see.


Figure 1. Visible satellite images from Monday and Tuesday with satellite-derived particulate air pollution levels overlaid. Image credit: NASA and EPA.

Air quality
Air quality due to particulate matter continues to be awful in Southern California (Figure 1). Exceedances of the Federal air quality standards by more than a factor of two have occurred the past four days in both Los Angeles and San Diego. The air pollution problem is expected to linger for several days after the fires are out. The onshore winds that are expected to form will recirculate smoke that is over the ocean back over land. As seen in a vertical cross section of the smoke taken by NASA's Calipso satellite Monday (Figure 2), smoke from the fires has stayed confined to the lowest 1 km (1,000 meters) of the atmosphere over the ocean regions southwest of San Diego. The cold waters of the California Current creates stable air above it that resists moving upwards, keeping the pollution trapped near the surface. However, some of the smoke is expected to rotate clockwise along the California coast, moving back over the U.S. over the northern half of California Thursday and Friday. Trajectory forecasts indicate that this smoke will be lifted as it circulates back over the U.S, thanks to the lifting motion associated with the low pressure system approaching the Pacific Northwest. The smoke should color sunsets over much of Northern California and northern Nevada over the next few days, but should not cause significant pollution problems at the surface, since most of the smoke will be aloft.


Figure 2. Altitude of smoke as measured by NASA's Calipso satellite at 3:07 am PDT Monday Oct 22. Calipso uses a LIDAR (LIght Detection And Ranging) to detect particles suspended in the atmosphere. Lidar is like radar, except that it uses a visible light beam instead of a radar-wavelength beam. Image credit: NASA.

The University of Wisconsin's CIMSS group has another excellent blog on yesterday's fires, complete with satellite animations and detailed analysis.

San Francisco Climate Challenge
Today is the last day for San Francisco residents to sign up for the San Francisco Climate Challenge, an innovative contest designed to encourage residents to reduce energy usage. The content offers prizes up to $5000 for those residents able to reduce their energy consumption the most over the coming month. For more information, see the new Weather Underground climate page at http://www.wunderground.com/climate/.

Ricky Rood has posted a blog this morning on the ongoing Georgia/Southeast U.S. drought.

Jeff Masters