16 October 2019

Odds increasing of Gulf storm this weekend

Surface wind streamlines as of Wednesday morning. Two circulation centers are apparent: one on each side of the Isthmus of Tehuantepec. (earth.nullschool.net)
Since yesterday's update, the disturbance over central America has drifted into the southern Bay of Campeche.  It's still disorganized, and is really close to another disturbance on the Pacific side of the low-lying 130-mile-wide Isthmus of Tehuantepec!  This has been a feature of interest going back to last *Thursday* when it was a disorganized blob off the east coast of Nicaragua.  These things can take a long time to cook (remember watching Michael for about a week last year before it became a Depression?).  There are zero indications that this will have much else in common with Michael though.

The most recent European model ensemble indicates a pretty decent chance of this developing in the coming days as it heads north to northeast. Some of the ensemble members have a trackable low pressure center near the northern Gulf coast on Saturday, but none are strong (shown here).  If this becomes a tropical storm, the next name on the list is Nestor.  Nestor was introduced to this list in 2013 after Noel '07 was retired.

As I shared yesterday, there's a short list of seven named storms that developed in the western Gulf of Mexico after October 1 since 1960.  The one that took off toward the northeast into Florida's Big Bend region was Josephine in 1996.

Tropical Depression 15, the one that was way out by Cabo Verde, has dissipated and never reached tropical storm status and so was never named.

As of today, the Atlantic has had 13 named storms, 5 of which became hurricanes, and 3 of those became major hurricanes (Category 3+).  I whipped up a seasonal tracking map through today to refresh your memories on when and where things happened.

15 October 2019

TD 15 forms by African coast, also closely watching Gulf of Mexico

Enhanced satellite image of Tropical Depression 15. (EUMETSAT)
On Monday afternoon, a very strong and late African wave developed into Tropical Depression 15, way out at 20.2°W -- just 200 miles offshore! The Cabo Verde season, in which tropical waves exit the African coast near the Cabo Verde islands, typically spans mid-August through mid-October... but by mid-October it's pretty meager. As Phil Klotzbach at CSU pointed out, this is the easternmost tropical cyclogenesis this late in the year on record.

The forecast is not very threatening to anyone though. It may never reach tropical storm status as it passes by the Cabo Verde islands then off toward the northwest where it is forecast to dissipate.

Elsewhere, a disturbance over central America has some potential for development in the Gulf of Mexico later this week.  The National Hurricane Center is giving it a 40% chance of development into at least a depression by the end of the weekend.

Global model ensembles are divided on the system developing on the west side of Mexico or the east, but the trend has been shifting eastward, favoring the Gulf.  At this point, model guidance does not suggest that it becomes anything too strong, but that can change at this long lead time. So we will just keep a very close eye on it and model trends... especially interests in the western Gulf this weekend.  The next name on the list is Nestor.

Looking back to the start of the satellite era in 1960, I could find just seven named storms that developed in the western Gulf after October 1.  The two most recent were Marco (2008) and Matthew (2004). Jerry (1989) is the one that made landfall near Galveston as a Category 1 hurricane on October 16.

Tracks of seven named storms that formed in the western Gulf after October 1 (1960-2018).
Today is also the 14th anniversary of Hurricane Wilma's formation (Tropical Depression 24 at the time).  If you missed it on Twitter, there's a thread with new and interesting graphics at https://twitter.com/BMcNoldy/status/1184106020299104261

12 October 2019

Melissa forms off northeast U.S. coast

Tropical Storm Melissa, the season's 13th named storm, began its journey over North Carolina on Monday, then left the coast on Tuesday and continued to slowly organize... it was finally upgraded to a subtropical storm on Friday morning.  However, it may only be around for another day or so. Melissa will be yet another short-lived and messy storm this year, similar to Andrea, Chantal, Erin, Fernand, and Imelda -- those five storms combined were named for less than four days!

Peak sustained winds are at 50 mph and further weakening is expected as it accelerates toward the east into higher wind shear and colder water.  There are no associated tropical storm or storm surge watches/warnings. The storm is very tiny; the tropical storm force winds extend an average of just 45 miles from the center!

The recent transition from a subtropical cyclone to a tropical cyclone is nicely illustrated in this cyclone phase space diagram. It's a bit technical, but the low starts at location A on Wednesday, it's presently at location C on Saturday, and the forecast is at location Z on Friday. In this model-based analysis of the storm's structure, we see that it began as an asymmetric warm-core system (subtropical), it migrated into the symmetric warm core realm (tropical), is currently in a near-neutral zone, and is headed for the asymmetric cold core realm (extratropical).  So, while these classifications have a bit of subjectivity to them, there is also an objective foundation on which the transition is based.

Cyclone phase space diagram for Tropical Storm Melissa. (FSU)

Elsewhere, the Atlantic basin should be quiet for the foreseeable future. But don't tune out just yet... even late October has a history of producing some infamous storms!