Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:10 PM GMT on September 01, 2009
The most powerful hurricane on the planet so far this year, Hurricane Jimena, continues to maintain an intensity just below Category 5 strength. Jimena's 155 mph winds beat out the South Pacific's Tropical Cyclone Hamish (150 mph winds) as the most powerful tropical cyclone so far this year. Satellite estimates of Jimena's strength continue to show no weakening of the hurricane, and the Hurricane Hunters are currently enroute to Jimena to see exactly how strong the hurricane is.
Jimena is expected to make landfall Wednesday along the Mexico's Baja Peninsula. The hurricane is in an environment with low wind shear, 5 - 10 knots, and warm Sea Surface Temperatures (SSTs), 29°C. Shear is expected to remain low, and SSTs will decline to 28°C with a decrease in total oceanic heat content between now and landfall, and these conditions should allow Jimena to be a Category 3 or 4 hurricane at landfall. The computer models remain in good agreement that Jimena will miss the southern Baja resort town of Cabo San Lucas, and make landfall midway up the Baja Peninsula. However, just a small deviation in track would bring Jimena ashore in southern Baja. Cabo San Lucas has a 13% chance of receiving hurricane force winds, according to NHC's wind probability product. Serious flooding due to heavy rains will occur across all of the southern Baja today and Wednesday. Jimena is of similar intensity and is following a similar track to Hurricane Juliette of 2001, which brought 17.7" of rain to Cabo San Lucas. Juliette weakened to a 45 mph tropical storm before hitting Baja, but the storm killed 7 people and caused $20 million in damage to Mexico, mostly due to flash flooding and mudslides from the heavy rains. Jimena will probably be one of the strongest hurricanes ever to hit Baja.
After Jimena makes initial landfall on Baja, it will cross over the Gulf of California and make landfall on Mainland Mexico. Depending upon how up along the coast this second landfall occurs, Arizona may receive moisture from Jimena late this week that will be capable of causing flooding rains.
Figure 1. Rainfall totals from Hurricane Juliette of 2001, which had a similar track and intensity as Hurricane Jimena. Image credit: NOAA.
California fires creating major air pollution event
The wildfires burning east of Los Angeles, California have created a major air pollution event, and will continue to impact air quality for millions of people in Southern California today. The fires have burned over 90,000 acres and have led to Unhealthy to Hazardous Air Quality Index (AQI) levels (Code Red to Code Brown) for the past five days in nearby regions. Not only are fine smoke particles directly elevating the most deadly form of air pollution--particle pollution, the gases in the smoke are reacting with sunlight to form ground-level ozone, another dangerous pollutant. High pressure over the southwestern states today will keep temperatures hot and relative humidity low in the vicinity of the fires. Smoke has settled into the valleys of Los Angeles County overnight and in the eastern San Bernardino Valley. While onshore ocean breezes Tuesday afternoon are expected to move smoke into the mountains, winds will also increase fire growth potential. Air pollution levels will continue to reach the Unhealthy level (code red, see Figure 2) today over much of the Los Angeles area. By Wednesday, weather conditions will improve as the high pressure region over Southern California weakens and moves east, bringing cooler temperatures, higher humidity, and stronger airflow from the cool ocean, aiding firefighting efforts.
Health tip: People in areas directly impacted by smoke should avoid any vigorous exertion, indoors or outdoors. In addition, people with respiratory or heart disease, the elderly, and children should remain indoors. Individuals without air conditioning who stay inside with the windows closed may be in danger during extremely hot weather. In these cases, individuals should seek alternative shelter. For the latest information on this air pollution event, see the latest South Coast Air Quality Management District Air Quality Advisory.
Check out our wundermap for Los Angeles with the fire layer turned on to see where the fire and smoke are located, and track the temperatures and winds during today's air pollution event.
Figure 2. NASA MODIS satellite image of the California fires of August 31, 2009 (top) with air pollution levels at major cities superimposed. Bottom: air quality forecast for today, September 1, 2009. Image credit: EPA AIRNOW.
Invest 94L strong but disorganized
Tropical wave 94L, about 250 miles east of the Lesser Antilles Islands, continues to toy with the idea of becoming a tropical depression. Infrared satellite imagery shows the low has developed a region of heavy thunderstorms that extend high into the atmosphere. This morning's QuikSCAT pass showed surface winds of 45 mph beneath these thunderstorms. However, QuikSCAT showed no surface circulation, with a large and rather confused region of wind shifts and lines of convergence at the surface. Recent satellite wind shear analyses by NOAA/Colorado State University and the University of Wisconsin show that wind shear has oscillated between 10 - 20 knots over 94L this morning, and this shear is keeping the storm disorganized. Also, last night's upper-air balloon sounding from Guadeloupe is showing the presence of a narrow jet of 30 knot winds from the east at 700 mb (3000 meters). The relatively crude measures of shear we use, taking the difference between the wind at 200 mb and 850 mb, will miss seeing these sort of this layers of shearing winds that can have a strong negative impact on the organization of a developing tropical disturbance. It is apparent from visible satellite imagery that shear is causing some disruption to the southwest side of 94L, and that there is no surface circulation. The storm also has fewer low-level spiral bands than yesterday.
The forecast for 94L
The latest of 00Z and 06Z model runs continue to be split on whether 94L will develop or not, with the GFDL and HWRF models bringing 94L to hurricane strength in 4 days, and the NOGAPS and ECMWF predicting 94L will just be a disturbance 4 days from now. SSTs will be warm, in the 28 - 29°C range, and dry air should have only a minor inhibiting effect, but high shear may continue to delay 94L from developing into a tropical depression for several days. The storm is moving more slowly today, about 10 mph, and will be steered west-northwest to northwest over the next three days thanks to the presence to two upper-level lows to the northwest of the storm. The degree of northward motion of 94L will depend upon how soon the storm becomes a tropical depression and begins intensification. If 94L follows the GFDL and HWRF model's predictions, and intensifies into a hurricane four days from now, a more northwestward track taking the storm between the Lesser Antilles and Bermuda is likely. If 94L lingers a few more days as a tropical disturbance, a more southerly track over the northern Lesser Antilles Islands, Puerto Rico, and the Bahamas is more likely.
The Hurricane Hunters will investigate 94L at 2 pm this afternoon, and I'll update the blog by 3:30 pm.
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