Ernesto closing in on the Yucatan

Tropical Storm Ernesto is closing in on the coast of the Yucatan Peninsula as a strong tropical storm with 65 mph winds. Latest data from the Air Force hurricane hunter aircraft in the storm found the pressure had dropped to its lowest value yet--988 mb at 9:12 am EDT. Top surface winds as seen by their SFMR instrument were in the 60 - 65 mph range, and the plane found 72 mph winds at their flight level of 5,000 feet, on the northeast side of the eye. Ernesto does not have an eyewall, but the Hurricane Hunters noted an eyewall may be the process of forming, from the north to the south-southeast side of the center. Visible and infrared satellite loops show that Ernesto's heavy thunderstorms have expanded in areal extent and intensity to form a Central Dense Overcast (CDO), a feature of intensifying tropical storms. Ernesto is encountering light wind shear of 5 - 10 knots. The dry air that mixed into Ernesto's core and disrupted it on Monday is no longer apparent on water vapor satellite loops.

Winds at the Yucatan Basin buoy, about 140 miles north of the 10 am EDT position of Ernesto, were sustained at 34 mph, gusting to 40 mph, at 10 am EDT. Winds along the north coast of Honduras have been light the past day, and a personal weather station on Roatan Island off the north coast of Honduras picked up 1.51" of rain from Ernesto as of 10 am EDT. Sporadic heavy rains from Ernesto's outer spiral bands have affected Belize City, Belize most of the morning; these bands can be seen on Belize radar.


Figure 1. True-color MODIS image from NASA's Terra satellite of Tropical Sotrm Ernesto, taken at 11:35 am EDT August 6, 2012. At the time, Ernesto had top winds of 65 mph. Image credit: NASA.

Forecast for Ernesto
Ernesto does not have an eyewall, and this will severely limit the storm's chances of rapid intensification until the storm can build one. However, Ernesto is under light wind shear and over warm ocean waters of 29°C with very high heat content, so some modest intensification to a Category 1 hurricane is possible before landfall occurs near midnight tonight near the Belize/Mexico border. Heavy rains will be the main threat from Ernesto. The storm will take about a day to cross the Yucatan, and its winds will probably diminish by 15 - 25 mph. Once Ernesto re-emerges over water into the Bay of Campeche in the southern Gulf of Mexico, wind shear will be light and ocean waters warm with high heat content. I expect Ernesto will increase its winds by 15 - 25 mph while over the Bay of Campeche, and the storm could be near Category 1 hurricane strength when it makes a second landfall near Veracruz, Mexico.

Crossing the Yucatan: a history
Hurricanes and tropical storms regularly cross Mexico's Yucatan Peninsula and enter the Bay of Campeche, the Gulf of Mexico's southernmost region. Since the crossing usually takes less than a day and the peninsula is surrounded by warm ocean water that can help feed the storm during the crossing, the great majority of storms survive the trek. Once in the Bay of Campeche, most storms regenerate, even though there is not much room for the storm to go before a second landfall in Mexico occurs. This is because the curved shape of the mountain-lined coast helps boost counter-clockwise spin of the air, and the waters in the bay are among the warmest in the North Atlantic. Typically, a storm that crosses the Yucatan with a mostly westward track and enters the Bay of Campeche will intensify by 15 - 25 mph before making a second landfall in Mexico. Let's consider two historical analogue case for what might happen to Ernesto.


Figure 2. Track of Hurricane Karl of 2010.


Figure 3. Tracks of all major hurricanes since 1851 near Mexico's Bay of Campeche. Karl was the most southerly major hurricane on record in the Gulf of Mexico. Image credit: NOAA Coastal Services Center.

Hurricane Karl of 2010
Hurricane Karl of 2010 hit the Yucatan near the Belize/Mexican border as a tropical storm with 65 mph winds. Karl took 18 hours to cross the Yucatan, and weakened to a 45 mph tropical storm during the crossing. Remarkably, though, conventional and microwave satellite imagery indicated that the storm’s organization and vertical structure improved during the crossing, with the appearance of an eye-like feature and an increase in low-level spiral bands. This probably occurred as a result of frictional convergence--when air flowing over the smooth ocean surface moves over land, the increased friction causes the air to slow down and flow at a sharper angle towards a center of low pressure. Once the storm reached the Bay of Campeche, Karl took advantage of low wind shear, ocean temperatures of 29 - 30°C, and a moist atmosphere, to put on an impressive show of rapid intensification. Karl took only 12 hours to regain its strength, and within 36 hours of exiting the Yucatan, had intensified to a Category 3 hurricane with 125 mph winds. This sort of intensification so far south in the Bay of Campeche was unprecedented, and Karl was the strongest major hurricane ever observed so far south in the Gulf of Mexico. Karl dumped 10 -15 inches of rain over most of the northwestern half of the state of Veracruz, triggering floods that killed 22 and did $400 million in damage.


Figure 4. Track of Tropical Storm Hermine of 1980.

Tropical Storm Hermine of 1980
A more likely historical analogue storm for Ernesto may be Tropical Storm Hermine of 1980. Hermine hit the Yucatan near the border between Belize and Mexico with 70 mph winds. The 24-hour crossing of the Yucatan weakened Hermine's winds to 50 mph. After emerging into the Bay of Campeche, Hermine turned to the west-southwest and made landfall southeast of Veracruz 30 hours later, with top winds of 70 mph.

Other storms of the past 30 years with a similar landfall location to Ernesto's
Hurricane Dean of 2007 hit the Yucatan near the Belize/Mexican border as a large Category 5 hurricane with 175 mph winds. Dean took 10 hours to cross the Yucatan, and weakened to a 75 mph Category 1 hurricane during the crossing. Upon reaching the Bay of Campeche, Dean strengthened by 25 mph to a Category 2 hurricane with 100 mph winds, before its second landfall occurred.

Hurricane Keith (2000)
Hurricane Dolly (1996)
Hurricane Diana (1990)

92L
A tropical wave in the far Eastern Atlantic (Invest 92L) is disorganized, with limited heavy thunderstorm activity and a modest amount of spin at mid-levels of the atmosphere. Of the six main models used operationally by NHC, only one--the HWRF--develops 92L. The storm is at least 6 days from the Lesser Antilles Islands, if it maintains a westward motion.


Figure 5. NOAA-19 AVHRR image of the big low pressure system in the Arctic, taken at 9 am EDT August 7, 2012. At the time, the GFS analysis gave a central pressure of 970 mb for the low. Image credit: NOAA and Environment Canada. Thanks go to wunderblogger Grothar for pointing out this image to me.

Big storm in the Arctic
A remarkably intense low pressure system formed in the Arctic north of Alaska Monday, bottoming out with a central pressure of 963 mb at 2 pm EDT. A pressure this low is rare any time of the year in the Arctic, and is exceptionally so in summer. The storm is stacked vertically with the upper-level low, and will spin in place and slowly weaken over the next few days, but remain unusually strong. Strong winds behind the low's cold front caused a 1.3' storm surge Monday in Prudhoe Bay, on Alaska's north shore. As noted in Neven Acropolis' sea ice blog, the strong winds around this low have the potential to cause a large loss of Arctic sea ice, due to churning, increased wave action, pushing of ice into warmer waters, and the mixing up of warmer waters from beneath the ice. According to the latest analysis from the National Snow and Ice Data Center, Arctic sea ice extent was at a record low extent as of August 1. This week's big storm will likely keep Arctic sea ice at record low levels for the next week or two.

Angela Fritz will have an update on Ernesto late this afternoon or early this evening.

Jeff Masters