Subtropical Storm Andrea Weakens to a Depression; Jaguar Roams the South Atlantic

May 21, 2019, 11:09 AM EDT

Above: Subtropical Storm Andrea as seen by the GOES-16 satellite at 9:10 am EDT May 21, 2019. Image credit: NOAA/RAMMB.

Subtropical Storm Andrea formed in the waters a few hundred miles southwest of Bermuda on Monday evening, but weakened to a subtropical depression at 11 am EDT Tuesday, and is expected to dissipate by Tuesday night without bringing tropical storm winds to any land areas. Andrea’s formation date of May 20 came twelve days before the official June 1 start of the Atlantic hurricane season. The Atlantic was flanked on Monday by not only Andrea but another subtropical storm, this one in the South Atlantic (see below).

Satellite loops on Tuesday morning showed that Andrea had degraded in appearance since Monday, with a small area of heavy thunderstorms to the east of its center. These thunderstorms were well removed from the center of circulation, something that is characteristic of a subtropical cyclone.

Wind shear over Andrea was a low 5 knots on Tuesday morning, but sea surface temperatures (SSTs) were near 24°C (75°F)—a temperature marginal for development. The atmosphere in which Andrea was embedded was dry, with a relative humidity at mid-levels of the atmosphere of 50%. This dry air has helped lead to the disheveled-looking appearance of the storm on Tuesday.

Rain for Bermuda

Andrea was headed north at 8 mph on Tuesday morning, and is expected to turn to the northeast by Tuesday evening, then eastward by Wednesday morning. This track will keep the storm to the west-southwest of Bermuda until dissipation occurs. Andrea (or its remnants) will be passing close to Bermuda on Wednesday, but in their 11 am EDT Tuesday wind probability forecast, NHC was giving only a 4% chance that Bermuda would see sustained tropical storm-force winds of 39 mph or greater from the storm. However, satellite measurements of atmospheric moisture (see tweet above) show that Andrea is bringing a substantial plume of moisture with it, and Bermuda can expect rains of 1 – 2” from the system.

According to CSU hurricane scientist Phil Klotzbach, the Atlantic has now had named storms form prior to June 1 in five consecutive years: 2015-2019. This breaks the old record of named storm formations prior to June 1 in four consecutive years, set in 1951-1954. Last year, Subtropical Storm Alberto formed on May 25. Andrea is the type of weak and short-lived storm that could well have been missed getting named in the days before satellite imagery was available in the Atlantic.

A simultaneous subtropical storm in the South Atlantic

Monday, May 20 marked the first time in recorded history that simultaneous named storms existed in both the North Atlantic and South Atlantic. In the South Atlantic, where named storms are uncommon, Subtropical Storm Jaguar formed on Monday, according to the Brazilian Navy Hydrographical Center (BNHC). On Tuesday morning, BNHC rated Jaguar as a minimal subtropical storm with 40 mph winds and a central pressure of 1010 mb. Steering currents are taking Jaguar to the south and southeast, out to sea, and the system is not a threat to any land areas.

Subtropical storms form about once every year in the South Atlantic, but this year saw the first fully tropical cyclone there since 2010, when Tropical Storm Iba formed back on March 23. There is a single list of names maintained by the BNHC, and Iba and Jaguar are just the ninth and tenth South Atlantic storms ever to get a name from the list.

Severe weather outlook
Figure 1. Severe weather outlook for Tuesday, May 21, 2019, issued by the Storm Prediction Center at 7:48 am CDT

A renewed threat of severe weather on Tuesday

It was a wild day of severe weather on Monday over the Midwest, with over 20 reports of tornadoes in addition to over 100 reports of large hail and damaging thunderstorm winds. Full details are at weather.com. Fortunately, no one was killed in Monday’s action, and damage was less than expected from a weather situation that was rated as one of the highest-risk scenarios in the history of NOAA’s Storm Prediction Center (SPC).

On Tuesday afternoon, SPC is advising of a large “enhanced risk” area for severe weather over much of Missouri and portions of Arkansas and Illinois. This risk level is two notches down from the “high risk” scenario they advised of for Monday’s action. Damaging thunderstorm winds will be the main threat on Tuesday, though tornadoes and large hail will also be a threat.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995, and flew with the NOAA Hurricane Hunters from 1986-1990.

jeff.masters@weather.com

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