It’s only mid-June, but the level of tropical weather activity in the Atlantic is more typical of early August. There’s a tropical storm off the northeast U.S. coast, a tropical disturbance in the Gulf of Mexico expected to become a tropical depression late this week, and a tropical wave off the coast of Africa with a 10% chance of development.

Tropical Storm Bill formed along the remnants of an old cold front in waters 335 miles east-northeast of Cape Hatteras, North Carolina, at 3Z Tuesday (11 p.m. EDT Monday). Bill’s formation date of June 15 comes more than six weeks before the usual August 1 arrival of the Atlantic’s second named storm, according to the National Hurricane Center (NHC). Only 12 Atlantic hurricane seasons since 1851 have had two named storms form earlier in the year (thanks go to Sam Lillo for this fact).

However, this year is well behind the blistering pace of the record 2020 season, which had its second named storm on May 27 and third named storm on June 2.

Bill took advantage of the relatively warm Gulf Stream waters off the coast of North Carolina, where sea surface temperatures (SSTs) were near 26.5 degrees Celsius (80°F), about 1°F above average. Wind shear over Bill was a high 35 knots on Tuesday. These conditions are marginal for sustaining a tropical storm. Satellite loops on Tuesday showed that Bill had a respectable area of heavy thunderstorms that were changing little in organization and intensity.

NHC’s forecast for Bill

Bill, with sustained winds of 50 mph at 5 a.m. Tuesday, was speeding northeast toward Newfoundland, Canada, at 31 mph. In NHC’s wind probability forecast, only Canada was being given a chance of experiencing tropical storm-force winds from Bill of 39 mph or more.  The highest odds were for Cape Race, Newfoundland, with a 35% chance.

Bill has a very short window of time to exist as a named storm, as steering currents are carrying it over cold waters north of the Gulf Stream and into a region with higher wind shear. Bill is likely to degenerate into a remnant low by Tuesday night or Wednesday morning.

Bill is the type of weak and short-lived storm that might not have been named before satellite imagery was available in the Atlantic, as explained in this YCC mid-May post, “Why are there so many Atlantic named storms? Five possible explanations.” The large increase in recent decades in the number of so-called “shorties” – named storms that last two or fewer days at tropical storm strength – has thus far not been demonstrated to have a climate change connection.  Improvements in experts’ abilities to detect and diagnose these systems may be partially responsible for the apparent increase. Weak, short-lived early-season storms like Bill have grown more common, making it difficult for storms beginning with the letters A, B, and C to get their names retired: Only two have been retired in the past 28 years (Allison, 2001; Charley, 2004).  

Figure 1. 92L in the Gulf of Mexico as seen by the GOES-16 satellite at 9:20 a.m. EDT June 15, 2021. (Image credit: NOAA/RAMMB/Colorado State University)

92L in Gulf of Mexico poses heavy rain threat for central Gulf Coast

In a rare occurrence for mid-June, the National Hurricane Center was tracking two other invests* on Tuesday: 92L in the Gulf of Mexico and 94L off the coast of Africa.

The more concerning system is 92L, which has the potential to bring destructive flooding to the central Gulf Coast beginning on Friday. On Tuesday morning, 92L was meandering over the southern Gulf of Mexico’s Bay of Campeche, bringing heavy rains to southern Mexico and portions of Central America. The system had favorable conditions for development, with warm waters of 28-28.5 degrees Celsius (82-83°F), moderate wind shear of 10-15 knots, and a moist atmosphere with a relative humidity at mid-levels of 75%. Satellite loops showed that 92L had a broad circulation but was poorly organized, with limited heavy thunderstorms.

Figure 2. Total precipitable water for 8 a.m. EDT Tuesday, June 15, showed 92L embedded in a very moist tropical airmass, with more than 2.5 inches of precipitable water. (Image credit: University of Wisconsin CIMSS)

Steering currents are expected to push 92L slowly northwards at about 5 mph beginning on Thursday, which should result in a landfall over the central or northwest U.S. Gulf Coast by Saturday. Satellite measurements of total precipitable water (the amount of rain that would fall in a column of air if all the liquid, solid, and condensed water vapor fell as rain) showed 92L embedded in a very moist tropical airmass. This moisture will begin moving inland over the central U.S. Gulf Coast on Friday. Even if 92L does not develop into a tropical cyclone, its slow forward motion and high level of moisture likely will lead to rainfall amounts of 5-10 inches along the coast June 18 through June 20, causing damaging flash flooding. Already, soil moisture as of Sunday, June 13, ranked in the top 1% of climatology for the date across the lower Mississippi Valley.

Figure 3. Predicted seven-day rainfall amounts ending at 12Z Tuesday, June 22. The central Gulf Coast is predicted to receive 5-10 inches of rain from 92L. (Image credit: NOAA)

The 0Z Tuesday runs of two of the three reliable models for predicting tropical cyclone genesis, the GFS and European models, predicted development of 92L into a tropical depression by Friday, when it will be over the northern Gulf of Mexico. In an 8 a.m. EDT Tuesday tropical weather outlook, NHC gave 92L 2 two-day and five-day odds of development of 20% and 70%, respectively. The next name on the Atlantic list of storms is Claudette.

Figure 4. 94L off the coast of Africa as seen by the GOES-16 satellite at 7:55 a.m. EDT (11:55Z) June 15, 2021. (Image credit: Tropical Tidbits)

A way-too-early threat: 94L off the coast of Africa

The strongest tropical wave so far in 2021 emerged from the coast of Africa on Sunday, June 13, with an impressive appearance on satellite imagery for such an early-season system. Designated 94L by NHC on Monday, the system had a pronounced spin but little heavy thunderstorm activity on Tuesday morning. The system had favorable conditions for development, with warm waters of 28 degrees Celsius (82°F), moderate wind shear of 10-20 knots, and a moist atmosphere with a relative humidity at mid-levels of 70%. However, dry air to the north of 94L appeared to be infiltrating the system.

94L was headed west at about 15 mph on Tuesday morning, and a west to west-northwest motion at about 15 mph should bring the system close to the Lesser Antilles Islands by Sunday, June 20. None of the three reliable models for predicting tropical cyclone genesis predicted development of 94L into a tropical depression, and 94L will encounter drier air, cooler waters, and higher wind shear on its trek westwards this week, likely limiting any development.

Climatology is decidedly against development of 94L. Since accurate satellite data began in 1972, only two tropical depressions and no tropical storms have formed east of 40°W in June. In an 8 a.m. EDT Tuesday tropical weather outlook, NHC gave 94L two-day and five-day development odds of 10%.

* When NHC is monitoring a tropical disturbance that is a threat to develop into a tropical or subtropical cyclone, the disturbance can be designated as an “invest”. The disturbance is then given a tracking identification number of 90-99, followed by a single letter corresponding to the ocean basin – “L” for the Atlantic, or “E” for the Eastern Pacific. When the number of invests in a season reaches 99, the next disturbance gets the recycled number 90. There is no formal definition of what qualifies as an invest, and the purpose of doing so is primarily to allow generation of specialized computer model forecasts.

Bob Henson contributed to this post.

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Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...