21 August 2019

Chantal forms as active part of hurricane season begins

Chantal, the season's third named storm, formed on Tuesday night in the north-central Atlantic.  As of Wednesday morning, it's a tropical storm with 40 mph sustained winds centered 450 miles south of Newfoundland.


Similar to Barry back in July, Chantal's origins were over land, and it eventually acquired tropical cyclone characteristics once over water long enough.  Last Thursday (15th), the disturbance that would become Chantal was over the Florida panhandle, and it subsequently meandered its way across Georgia, South Carolina, North Carolina, then out over the ocean where it began to take shape.


It is forecast to marginally maintain tropical storm status for a couple days before weakening in the face of dry air, strong vertical wind shear, and cooler water.

So far this season, the southernmost named storm was Barry at about 28°N... nothing has formed or existed in the tropics yet. But we're just now entering the peak of the "Cabo Verde" season, where we closely watch disturbances coming from Africa for development (Cabo Verde is the archipelago off the west coast of Africa). The vast majority of major hurricanes have an African pedigree.

Over the past fifty years, the average date of third named storm formation is August 13, so this is about a week late. Using the same climatology, 84% of the season's ACE (Accumulated Cyclone Energy) occurs after August 20. As of today, 2019 is at just 22% of average for the date. But things should start waking up soon.


Closer to home, there's a disturbance near the Bahamas that has a slim chance of becoming a tropical cyclone and will approach Florida this weekend. If nothing else, it will bring elevated chances of heavy rain to the Florida peninsula starting on Friday.  (Miami has already had its 9th wettest August on record, with 11 days to go!)

The yellow X marks the approximate center of the disturbance today, while the yellow filled "blob" is the area where tropical cyclone formation might occur in the coming five days.


13 July 2019

Barry becomes season's first hurricane as it makes landfall in central Louisiana

On Saturday morning, Barry was upgraded to a Category 1 hurricane (with peak sustained winds of 75mph) just before it made landfall south of Lafayette. Dangerous storm surge of 6-8 feet is occurring east of the center (Morgan City area), and heavy rain is just making its way onshore... fortunately, the storm remained lopsided and the majority of the rain is still south of the center. But it is not over; now that it's onshore, the rain will be too!


Again, the classification of the storm (tropical storm, Category 1 hurricane, etc) *only* pertains to the peak sustained wind speed found somewhere in the storm -- it does not tell you anything about the size of the wind field, the amount of rain it will produce, or the depth and extent of the storm surge.  You can find a few long, updating radar loops covering Hurricane Barry's landfall at http://bmcnoldy.rsmas.miami.edu/tropics/radar/


As you can see, rainfall remains a major concern today and in the coming couple of days as Barry moves inland. The graphic below shows the flask flood risk from Saturday through Tuesday morning:


This event fits within what we expect from climatology: the Gulf of Mexico is a favored region for tropical cyclone formation during July.

Barry is the 8th hurricane to make landfall in Louisiana in the past two decades... the other recent hits include Lili (2002, Cat1), Cindy (2005, Cat1), Katrina (2005, Cat3), Rita (2005, Cat3), Gustav (2008, Cat2), Isaac (2012, Cat1), and Nate (2017, Cat1).

Tracks of the seven hurricanes to make landfall in Louisiana from 1999-2018.
Elsewhere in the Atlantic, an easterly wave left the African coast back on July 8th and continues to maintain a minimal amount of organization. The National Hurricane Center is giving it just a 10% chance of becoming a tropical depression by Thursday. It, in whatever condition, would reach the Lesser Antilles around Monday-Tuesday. At this point, it's very likely to be nothing more than a tropical breeze.