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Jimena nears Category 5 status; 94L getting sheared

By: Dr. Jeff Masters, 9:02 PM GMT on August 31, 2009

The most powerful hurricane anywhere on the planet so far this year is Hurricane Jimena, according to data from this afternoon's hurricane hunter mission. Jimena's 155 mph winds beat out the South Pacific's Tropical Cyclone Hamish (150 mph winds) as the most powerful tropical cyclones so far this year. The Hurricane Hunters have completed their mission into Jimena, so we will have to rely on satellite estimates of Jimena's strength until Tuesday afternoon's hurricane hunter flight to see if the storm intensifies to the 160 mph threshold needed for it to become a Category 5 hurricane.

Jimena is expected to make landfall Tuesday night or Wednesday morning 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), 30°C. Shear is expected to remain low, and SSTs will decline to 28°C with a corresponding decree in total oceanic heat content between now and landfall, and these conditions should mean that Jimena will be a Category 3 or 4 hurricane at landfall. The computer models have come into better agreement with their latest 12Z runs, giving confidence that a landfall north of Cabo San Lucas will occur, and that town is now outside of the NHC cone of uncertainty. 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 Tuesday 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 killed 7 people and caused $20 million in damage to Mexico, mostly due to flash flooding and mudslides from the heavy rains.

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. Visible satellite image of Hurricane Jimena at 4:35 pm EDT on Monday, 8/31/09.

Invest 94L getting sheared
Tropical wave (94L) about 450 miles east of the central Lesser Antilles Islands, continues to be a threat to develop into a tropical depression. However, recent satellite wind shear analyses by NOAA/Colorado State University show that wind shear has increased throughout the day over 94L, and is now near 20 knots, which is marginal for development. Satellite shear analyses from the University of Wisconsin show about 10 - 20 knots of shear, and it is apparent from satellite imagery that shear is causing some disruption to the west side of 94L. Visible satellite imagery show that 94L does not have a surface circulation center, and the storm has not significantly increased in organization this afternoon.

The forecast for 94L
Wind shear as diagnosed by the SHIPS model currently shows a low amount of shear, 5 - 10 knots, and this is not correct, so this model's forecast of low shear over 94L for the next five days cannot be trusted. The latest set of 12Z model runs are more restrained in developing 94L, and are depicting higher levels of shear will be affecting 94L over the next three days. SSTs will be warm, in the 28 - 29°C range, and dry air should have only a minor inhibiting effect, but the high shear may delay 94L from developing into a tropical depression for several days. The HWRF makes 94L into a strong tropical storm 5 days from now, but the GFDL and GFS models do not develop 94L at all. The ECMWF and NOGAPS models predict that development of 94L will not occur until five or so days from now, in a region between the Bahama Islands and Bermuda. NHC continues to give 94L a high (greater than 50% chance) of developing into a tropical depression by Wednesday. This probability may need to get scaled back to moderate (30 - 50% chance) if the high shear continues into tomorrow.

My next post will be Tuesday morning.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.