The area of disturbed weather (90L) off the Southeast U.S. coast finally gained a well-defined circulation center and enough heavy thunderstorms near its core to be designated Subtropical Storm Ana
at 11 pm EDT Thursday. Ana's formation date of May 7 is the earliest appearance of a named storm in the Atlantic since a previous incarnation of Subtropical Storm Ana was recognized on April 20, 2003.Long-range radar out of Charleston
Thursday evening showed only a modest amount of heavy rains associated with the storm, and satellite loops
showed a large circulation with most of the heavy thunderstorms more than 100 miles away from the center. This is a typical appearance for a subtropical cyclone. As explained in wunderground's subtropical storm tutorial
, a subtropical cyclone has characteristics of both tropical and extratropical cyclones. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate, but tropical storms generate more rain, and tropical storms have the potential to rapidly intensify into hurricanes, while subtropical storms do not. Wind shear
over Ana was a moderate 20 knots late Thursday night, and water vapor satellite loops
showed a large area of dry air to the west over the Southeast United States. This dry air is retarding development, thanks to strong upper-level winds out of the west driving the dry air into the storm's core. Ocean temperatures were near 25 - 26°C (77 - 79°F), which is just at the limit of where a tropical storm can form. Given these relatively cool ocean temperatures and the fact Ana will need at least a day to transition to a fully tropical storm, I expect that the worst Ana will be able to do is intensify to a 65-mph tropical storm that brings 4 - 6" of rain to the coasts of South Carolina and North Carolina on Saturday and Sunday. The 11 pm EDT Thursday forecast from NHC, which calls for a 50-mph storm bringing 2 - 4" of rain to the coast, is more likely.Figure 1.
Latest satellite image of Ana.Figure 2.
Sea Surface Temperatures (SSTs) from the AVHRR satellite instrument clearly show the warm Gulf Stream current off the Southeast U.S. coast for the 6-day period ending on May 2, 2015, before the arrival of 90L (AKA Ana). The storm took advantage of the warm waters of the Gulf Stream to aid its intensification into Subtropical Storm Ana. Image credit: Johns Hopkins University Applied Physics Laboratory.How unusual would a landfall be?
It’s not so rare to get a tropical or subtropical cyclone developing somewhere in the Atlantic basin before the official June 1 start of the hurricane season. Since 1851, a total of 39 “pre-season” systems have developed across 32 separate years. We’ve averaged about one pre-season tropical or subtropical system in the Atlantic every two to three years since the modern satellite era began in 1960. Interestingly, among these early-starting seasons, 38% ended up producing more storms than a typical season, while 62% produced near- or below-average totals. Pre-season named storms may be getting more common. In 2008, Dr. Jim Kossin of the University of Wisconsin published the paper, "Is the North Atlantic hurricane season getting longer?"
in the journal Geophysical Research Letters.
He concluded that there is an "apparent tendency toward more common early- and late-season storms that correlates with warming Sea Surface Temperature but the uncertainty in these relationships is high".
Very few of the tropical/subtropical systems that develop prior to June 1 make landfall. The earliest U.S. landfall on record occurred on February 3, 1952, when the Groundhog Day tropical storm struck near Cape Sable, Florida. Several U.S. landfalls have occurred in May, with Beryl the strongest of the group in recent years, packing 65-mph winds during its landfall near Jacksonville Beach, Florida, on May 28, 2012. However, a recent paper in the Bulletin of the American Meteorological Society
documented a hurricane landfall on the Florida Panhandle on May 28, 1863, with estimated peak winds of 105 mph (see Figure 3).Figure 3.
Track of the May 28, 1863 hurricane, which struck northwest Florida as a Category 2 storm with 105 mph winds, killing at least 72 people. This was the only hurricane on record to hit the U.S. in May, and stuck nearly two weeks earlier than the next earliest U.S. landfalling hurricane, Hurricane Alma of June 9, 1966. (Tropical Storm Beryl of May 28, 2012 came close to being a May hurricane, bringing 65 mph winds to the coast near Jacksonville Beach, Florida.) According to a 2013 paper by hurricane historians Mike Chenoweth and C. J. Mock in the Bulletin of the American Meteorological Society, "Among the most unusual and unexpected hurricanes in United States history is the only hurricane to make landfall in the month of May. This recently re-discovered storm that struck northwest Florida on 28 May 1863 created a natural disaster in the area that became lost to history because it was embedded in a much larger and important manmade event, in this case the U.S. Civil War. We document the arrival of this storm both historically and meteorologically and anachronistically name it Hurricane “Amanda” in honor of the Union ship driven ashore by the hurricane. The hurricane revealed deficiencies and strengths in combat readiness by both sides. Meteorologically, the storm nearly achieved major hurricane status at landfall and its absence from modern data bases of tropical cyclone activity is a useful reminder to users of important gaps in our knowledge of tropical cyclones even in the best-sampled storm basins."
Image credit: Mike Chenoweth and the Bulletin of the American Meteorological Society.
Here is the full list of peak winds observed on U.S. shores from named systems in May since 1900, including both landfalling and offshore systems, as compiled by tropical meteorologist Andrew Hagen (StormGeo) using the HURDAT database
. These are ranked in descending order of observed winds and listed with the state(s) where the winds were recorded. If the current low were to develop into Ana and bring winds of at least 45 mph (40 knots) onshore, it will be the strongest U.S. impact by a tropical/subtropical storm on record for so early in the season.
5/28/2012 – 55 kt – FL (Beryl)
5/29/1908 – 55 kt – NC
5/27/1972 – 50 kt – GA (Alpha)
5/23/1976 – 40 kt – FL/GA (unnamed)
5/30/1959 – 40 kt – LA (Arlene)
5/10/2007 – 35 kt – GA (Andrea)
5/14/1916 – 35 kt – FL
5/17/1951 – 35 kt – FL (Able)
Only five tropical cyclones since 1851 have reached hurricane strength prior to June 1. The only one that exceeded Category 1 on the Saffir-Simpson scale was Hurricane Able of 1951, which carried out a counter-clockwise loop off the Florida coast before swinging northeast just off the North Carolina coast. Able attained peak winds of 115 mph about 70 miles east of Cape Hatteras, NC, on May 22. As noted above, Able also produced 40-mph (35 knot) winds in Florida, though it didn’t make landfall there. Many thanks to Andrew Hagen (StormGeo), Michael Lowry (Weather Channel), and wunderground member TropicalAnalystwx13 for information used above.
Jeff Masters and Bob Henson