09 September 2011

Katia, Maria, and Nate still making waves

Katia remains a strong Category 1 hurricane with 75kt winds as it recurves into the north central Atlantic.  It's over 26C water, in about 20kts of vertical shear, and is about to begin its extratropical transition (ET).  This will be a major storm for the UK early next week.

 The baroclinic enhancement that commonly occurs with ET can keep the intensity high even amidst what we typically think of as hostile conditions.  In the next 24h, the SST will plummet to 18C and the shear will increase to about 45kts, but the storm could still pack hurricane-force winds.  ET involves losing the circular symmetric structure and becoming more frontal in nature (which implies the cyclone tilts with height rather than being vertically stacked), while at the same time transitioning from a warm-core system to a cold-core system (which implies the strongest winds move outward and upward).  Tropical cyclones have their strongest winds at the surface and close to the center, while extratropical cyclones have their strongest winds aloft and quite removed from the center.

Maria has regained a lot of deep convection, and the intensity as of 15Z today is 40kts, with a forecast for VERY slow intensification over the next 5 days as it heads WNW toward the Bahamas.  Tropical Storm warnings cover the Leeward Islands and Puerto Rico (http://www.nhc.noaa.gov/storm_graphics/AT14/refresh/AL1411W5+gif/145220W_sm.gif). The vast majority of forecast models now indicate that Maria will follow a track similar to Katia, so as of now, it appears that the US east coast could be spared another rainmaker.

Nate is organizing and wrapping up, but part of what it's wrapping up is the very dry air from Mexico and the northwestern Gulf.  The entrainment of dry air into the system has limited its intensification, but it's still a 55kt tropical storm.  It is forecast to reach hurricane status by tomorrow morning as it crawls slowly westward toward the northern Veracruz coastline in Mexico.  There are hurricane and tropical storm watches/warnings around the Bay of Campeche's perimeter: http://www.nhc.noaa.gov/storm_graphics/AT15/refresh/AL1511W5+gif/143800W_sm.gif

Finally, it's worth noting that we are in the peak of the season now... Sept 10 is climatologically when we have the most activity in the basin: http://www.nhc.noaa.gov/climo/images/peakofseason.gif

Please visit my tropical Atlantic headquarters.

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