A frontal boundary stretching across the subtropical Atlantic during the first week of November decayed, leaving a trough of low pressure and unsettled weather in its wake. Around November 7, a non-tropical low formed in association with the system well southeast of Bermuda. The low was generating significant convection in the form of a curved frontal band extending north and east of the center of circulation. Eventually, the low became separated from the band, but was still located within a broad upper-level low. The system also was generated gale force winds, so it was designated Subtropical Storm Theta. This designation of a 29th named storm officially broke 2005's record for the most ever recorded in an Atlantic hurricane season. It was the first ever use of "Theta" as well.
The storm was moving just north of east at a moderate pace. Sea surface temperatures were not especially warm, but a great deal of instability was present in the atmosphere to fuel Theta. Satellite estimates indicated that it strengthened over the following day to near hurricane strength. During the afternoon on November 10, Theta transitioned into fully tropical storm. The cyclone was dealing with wind shear near 50 knots, which would ordinarily overwhelm a tropical cyclone, but things are often different for late-season storms in the subtropics: one factor keeping Theta going was very cold air in the upper atmosphere. This meant that, despite cooler ocean temperatures, the altitude/temperature gradient was quite pronounced and supported deep convection.
After a little weakening overnight, Theta regained a bit of strength on November 11 when shear abated a tad. It was still riding the north edge of a mid-level ridge eastward with a very consistent forward speed. By November 13, ocean temperatures had dropped even further, and wind shear was bringing stable air out of the north. Theta started to weaken, slow down, and turn south of east. Enough convection persisted on the southeastern edge of the cyclone for it to stay tropical through the next day. It also was the first Atlantic tropical storm on record to travel so far east in November, past 20° W. Unfavorable conditions eventually overcame Theta though. It weakened to a tropical depression overnight and to a remnant low the next morning just norhwest of the Canary Islands.
The above image shows Theta as a subtropical storm on November 10.
Theta did not have any land impacts along its journey across the eastern Atlantic.
No comments:
Post a Comment