|Above: Visible GOES-16 satellite image of 91L at 10:20 am EDT Saturday, June 1, 2019. Image credit: RAMMB/CSU/CIRA.|
The official June 1 start of the Atlantic hurricane season is upon us, and right on cue, an area of disturbed weather with the potential to develop into a tropical storm has formed in the Gulf of Mexico. This disturbance, dubbed 91L by NHC on Friday night, is part of a broad area of low pressure called a Central American Gyre (CAG) located over Central America (see Philippe Papin’s excellent series of tweets discussing Central American Gyres). 91L was centered off the west coast of Mexico’s Yucatan Peninsula, in the southern Bay of Campeche, and was bringing heavy rains to the Gulf coast of Mexico.
Satellite images on Saturday showed that 91L was headed west to west-northwest at about 10 mph and had a moderate-sized area of heavy thunderstorms that were steadily growing more organized. The system was under low wind shear of 5 - 10 knots, was over warm waters of 29°C (84°F) and was embedded in a moist atmosphere with a mid-level relative humidity of 65 – 70%. These conditions favor development. An Air Force hurricane hunter aircraft is scheduled to investigate 91L on Sunday afternoon.
|Figure 1. Predicted rainfall for the period Saturday, June 1 - Thursday, June 6, 2019, from weather.com.|
Forecast for 91L
The 12Z Saturday run of the SHIPS model for 91L predicted that wind shear would remain low, 5 - 10 knots, over the next five days, the atmosphere would remain moist, and water temperatures would remain warm, at 28 - 29°C (82 - 84°F). These conditions favor development, and the only impediment to development is likely to be the amount of time 91L has over water, since its west to west-northwest motion could carry it inland over Mexico between Tampico and Veracruz as early as Sunday evening. Steering currents in the Gulf are expected to weaken by Monday and force 91L on a more northwesterly to northerly track at about 5 mph. It is possible the center could remain offshore until Tuesday, allowing the system more time to develop. Regardless of development, 91L is likley to spread heavy rains of 3 - 6” along a stretch of coast from Veracruz to just south of Brownsville, Texas by Wednesday. Late in the week, moisture from 91L may reach Texas, and could potentially stream into Louisiana and Arkansas by next weekend.
91L has good model support for development, with 40 – 70% of the ensemble members of the 0Z Saturday runs of the GFS and European models calling for development. The 8 pm EDT Saturday 5-Day Tropical Weather Outlook from NHC gave 91L 2-day and 5-day odds of development of 60%. The next name on the Atlantic list of storms for 2019 is Barry.
We’ve already had one named storm that jumped the gun this season—Subtropical Storm Andrea, which formed on May 20 a few hundred miles southwest of Bermuda. Andrea was a minimal subtropical storm with 40 mph winds that lasted less than a day, and is the type of weak short-lived storm that would likely not have been named in the pre-satellite era.
Climatology of June tropical cyclone activity in the Atlantic
June is typically inactive in the Atlantic, with about one named storm forming every two years, on average. The usual location for June storms to form is in the Gulf of Mexico, Western Caribbean, or the waters between The Bahamas and Bermuda, several hundred miles east of the Southeast U.S. coast, where sea surface temperatures (SSTs) are warmest. SSTs are nearly always too cold in June to allow storms to develop between the coast of Africa and the Lesser Antilles Islands. There have only been three such developments in the historical record—Tropical Storm Ana of 1979, Tropical Storm Bret of 2017, and an unnamed hurricane in 1933.
June storms often form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. Another way to get Atlantic June storms is for a disturbed weather area in the Eastern Pacific Intertropical Convergence Zone (ITCZ) to push north into the Western Caribbean and spawn a storm there. This was the case for Tropical Storm Alberto of 2006 (which may have also had help from an African tropical wave). June storms often form from Central American Gyres, like 91L is poised to do.
African tropical waves, which serve as the instigators of about 85% of all major hurricanes, are usually too infrequent and too far south in June to trigger tropical storm formation. Occasionally, a tropical wave tracks far enough to the north to act as a seed for a June storm in the Western Caribbean or southern Gulf of Mexico. This was the case for the strongest and deadliest June hurricane on record, Hurricane Audrey of 1957. Audrey formed from an African tropical wave on June 24, 1957 in the southern Gulf of Mexico’s Bay of Campeche. The rapidly intensifying storm struck the Louisiana/Texas border region as a Category 3 storm with 125 mph winds, tying it with Hurricane Alex of 2010 as the strongest June hurricane on record. Audrey killed over 400 people in Louisiana and Texas.
|Figure 2. Radar image of Hurricane Audrey on June 27, 1957, a few hours before landfall. Image credit: US Air Force/NOAA.|
Majority of forecasters predicting a near-average Atlantic hurricane season
As detailed in our May 23 post, the majority of the best-regarded pre-season Atlantic forecasts are predicting a near-average hurricane season. Since then, one major player, TSR, has put out an update to their forecast, calling for slightly more activity than their April 5 forecast (see below). The next major forecast update to watch occurs on Tuesday, June 5, when Colorado State University (CSU) issues their widely-watched forecast.
Comparing 2019 to 2004
It’s well-known that El Niño events tend to cause quiet Atlantic hurricane seasons due to an increase in wind shear over the tropical Atlantic. We currently have a weak El Niño event in progress in the Pacific, but most of the warming of the waters there is occurring in the Central Pacific, rather than the Eastern Pacific. Central Pacific-focused El Niño events tend to have less of a dampening effect on Atlantic hurricane activity. It is good to remember that 2004, which featured a weak Central Pacific-based El Niño event similar in some respects to the 2019 El Niño event, was one of the most active on record for U.S. landfalling hurricanes: six hurricanes, including three major hurricanes (Charley, Ivan, and Jeanne), made U.S. landfalls that year. Taking a look at the departure of SST from average at the end of May for both 2004 and 2019 (Figure 3), we see some striking similarities. Thus, it would not be a surprise if the 2019 Atlantic hurricane season ended up more active than average, given the late-May similarities in SST and El Niño between 2004 and 2019.
|Figure 3. Departure of sea surface temperature (SST) from the 1985 – 1993 average for 2019 (top) and 2004 (bottom). Both years featured weak central Pacific-based El Niño events, and above-average SSTs in the Main Development Region (MDR) for hurricanes from the Caribbean to the coast of Africa. Image credit: NOAA.|
TSR predicts a slightly below-average Atlantic hurricane season: 12 named storms
The May 30 forecast for the 2019 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a slightly below-average Atlantic hurricane season--about 10% below the long-term (1950-2018) norm and 20% below the recent 2009-2018 ten-year norm. This is more activity than their April 5 forecast, which called for a season 20% below the long-term (1950-2018) norm and 30% below the recent 2009-2018 norm.
TSR is predicting 12 named storms, 6 hurricanes, 2 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 88 for the period June through December. The long-term averages for the past 69 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 104. TSR rates their skill level as low-to-moderate for these May forecasts: 17 - 26% higher than a "no-skill" forecast made using climatology. TSR predicts a 32% chance that U.S. landfalling ACE index will be above average, a 24% chance it will be near average, and a 44% chance it will be below average. They project that two named storms and one hurricane will hit the U.S. The averages from the 1950-2018 climatology are three named storms and one hurricane. They rate their skill at making these May forecasts for U.S. landfalls just 0% - 2% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects two tropical storms and no hurricanes. Climatology is one tropical storm and less than 0.5 hurricanes. The next TSR forecast will be issued on July 4.