Subtropical Storm Theta Forms In The Northeastern Atlantic

2020 has broken the record for the most number of named tropical cyclones in Atlantic history with the formation of Subtropical Storm Theta. This is the first time Theta has been operationally used in Atlantic history.

Theta has pushed the number of storm formations in the Atlantic to 29, breaking the 2005 record of 28 tropical storms. 2005 still holds the record for the most number of systems at tropical depression strength at 31. 2020 is now at 30.

This tropical cyclone is not forecast to directly impact Trinidad, Tobago, or the Lesser Antilles as it moves generally eastward.

List of names for the 2020 Atlantic Hurricane Season. The next named tropical cyclone will be called Iota.
List of Greek names for the Atlantic Hurricane Season. The next named tropical cyclone will be called Iota.

What is a subtropical storm?

A subtropical cyclone is a non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Subtropical cyclones originate over tropical or subtropical waters and have a closed circulation about a well-defined center. Across the North Atlantic, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere.

Those with sustained winds below 62 KM/H (33 knots or 38 MPH) are called subtropical depressions, while those at or above this speed are referred to as subtropical storms.

At 11:00 PM AST, the center of Subtropical Storm Theta was located near latitude 28.8 North, longitude 40.3 West. The storm is moving toward the east near 15 mph (24 km/h), and a general east-northeast motion is expected to continue during the next two to three days.

Maximum sustained winds are near 50 mph (85 km/h) with higher gusts. Some slight strengthening is possible during the next 12 to 24 hours followed by little change in strength through Thursday.

Winds of 40 mph extend outward up to 115 miles (185 km) from the center.

The estimated minimum central pressure is 1000 millibars.

Subtropical Storm Theta Forecast Track as of 11:00 PM AST Monday 9th November 2020. (National Hurricane Center)

Watches & Warnings

Subtropical Storm Theta Watches and Warnings as of 11:00 PM AST Monday 9th November 2020. (National Hurricane Center)

There are no coastal watches or warnings in effect.

Hazards Affecting Land

Subtropical Storm Theta probabilities for tropical-storm-force winds as of 11:00 PM AST Monday 9th November 2020, as well as the most likely time of arrival of tropical-storm-force winds. (National Hurricane Center)

There are no hazards affecting land at this time.

This system is of no direct threat to the Windward Islands, including Trinidad and Tobago.

Subtropical Storm Theta Forecast Discussion

Subtropical Storm Theta (

Over the past 12 h convection has slowly consolidated near the center of a non-tropical area of low pressure over the far northeast Atlantic. Earlier, there was a 2325 UTC ASCAT-B pass that showed that the low-level circulation had become more detached from the warm frontal boundary located to its northeast. In addition, the ASCAT pass showed that winds have increased to near 45 kt on the southeast portion of the circulation near where the coldest convective cloud tops were located. Both a 2137 UTC SSMIS and 2324 UTC AMSU microwave pass showed convective banding under the cirrus canopy near where the highest winds were observed. However, water vapor satellite imagery still shows the presence of a broad upper-level low tangled up with the low-level circulation embedded in the convection. Therefore, Theta has been designated as a subtropical storm with a maximum wind of 45 kt.

Theta has been on a nearly due east heading over the last 12 h at 90/13 kt. Currently, the cyclone is located south of a broad deep-layer longwave trough. This trough is expected to bypass the cyclone to the east and Theta will primarily be steered by southwesterly flow on the northern periphery of a mid-level ridge centered over the Cape Verde islands. This flow will steer Theta slowly to the east-northeast over the next 3-5 days. The track guidance is in good agreement with this solution, with only minor differences in the across-track spread. There are somewhat larger differences in the along-track spread related to different forward motion, but the latest NHC track forecast elected to stay close to the track consensus at this time.

Thetas structure has evolved from a frontal cyclone to a subtropical cyclone, though a frontal boundary remains nearby on the northeast side of the circulation. This boundary is expected to gradually dissipate and Theta is expected to fully transition to a tropical storm in 24 h as convection erodes the upper-level cyclonic flow overhead. The maximum sustained winds were increased slightly in the first 12 h of the forecast but are kept at 50 kt through 120 h which is on the high end of the intensity guidance.

Theta is the 29th named storm in the record-breaking 2020 Atlantic Hurricane Season, breaking the previous record of 28th named storms in 2005.

Forecast discussion by NHC Forecaster Papin/Brown.

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