*******Note that forecast and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********
...TUESDAY DECEMBER 6 2022 5:40 PM EDT...
Potential remains high for the formation of a subtropical or tropical storm in the open central Atlantic within the next couple of days... see area of interest #48 section below for more information.
New to this site this year... I will be sequentially numbering up areas of interest for possible Atlantic tropical development. In this scheme... will reset back to #1 at the start of next year (January 2023). The current area of interest in this blog post is designated #48 as the other numbers were used in previous birdseye view posts. This scheme is to reduce confusion as Atlantic tropical activity increases during the peak of the hurricane season... when multiple simultaneous areas of interest begin and end which previously required shuffling around the area of interest numbers from update to update.
AREA OF INTEREST #48... The cut-off upper trough in the western Atlantic over the last 24 hours has become larger and re-enforced while absorbing the south fracture of another upper trough fragment that was pushed southeastward from eastern Canada by the current Gulf of Mexico upper ridge. The eastern divergence zone of the expansive upper trough has evolved the broad area of central Atlantic low surface pressures into a better-defined subtropical surface low pressure system... featuring a broad area of rotation in the vicinity of 26N-55W and a fanned out area of showers and thunderstorms on the north side of the broad rotation. Over the next 24 hours the warm core upper ridge in the Gulf of Mexico is expected to be amplified by southerly surface flow ahead of the current central Canada frontal cyclone... resulting in the parent western Atlantic upper trough of the subtropical low amplifying into an upper vortex with a better-defined upper divergence maximum on its east side. This should help the subtropical surface low strengthen and establish a well-defined center... and given the amount of thunderstorm activity the system already has I anticipate subtropical cyclone formation. Therefore I have begun a subtropical cyclone formation forecast with specific track and intensity points as outlined below. As far as the NHC tropical weather outlook... odds of subtropical cyclone formation have been increased to 50% as of this writing... however given the above-listed current observations and expected evolution it would not surprise me if these odds are notably increased in the next 24 hours.
The track forecast is generally straightforward and to the northeast. The main steering feature will be the east side of a large scale mid-ocean upper trough that forms when the north Atlantic and Gulf of Mexico upper ridge cells push the current northeast Atlantic upper vorticity southwestward into the forecast parent upper vortex of the subtropical storm... with the merger between the two resulting in the upper trough. The northwest quadrant of the large outer circulation of the subtropical storm will also pull the cold air associated with the approaching northeastern Atlantic upper vorticity southward... accelerating the merger of its parent upper vortex and northeast Atlantic upper vorticity into the mid-ocean upper trough. A recent ASCAT-C descending pass (https://manati.star.nesdis.noaa.gov/datasets/ASCATData.php) suggests the max strength of this system... as of this writing... is 45 knots (40 mph) and occurring in a broad area on this system's north side. The structure/intensity forecast that I project for this storm is as follows:
(1) Over the next 24 hours I forecast the surface center of the subtropical system to whirl beneath the center of its forming parent upper vortex as is typically seen with a mid-latitude non-tropical or subtropical system... but not before gaining some strength while initially underneath the east side of the upper vortex where there is upper divergence.
(2) Between 24 and 48 hours... despite being over water temps below 26 deg C I currently forecast this system to esbalish a core of thunderstorms with latent heat release driven outflow due to the cool upper air temperatures in the envrionment (200 mb heights just below 1200 dekameters). The establishment of the warm core upper outflow will be easier as this system ends up on the east side of the aforementioned mid-ocean upper trough instead of underneath its generally suppressive axis. Furthermore the upper divergence on the east side of the upper trough may further aid the thunderstorm core of this system... therefore I forecast brisk intensification to a top-end and fully tropical storm by 48 hours.
(3) Between 48 and 96 hours... despite tracking northeast into water temps in low-20s of deg C... the upper air temps are forecast to cool as the northwest quadrant of this system's surface circulation advects in colder northeast Atlantic upper vorticity as discussed above (200 mb heights forecast to fall to 1168 dekameters). Therefore I forecast lingering intsability/thunderstorms to bring this system into category 1 hurricane strength... with the intensification also aided by the divergence on the east side of the mid-ocean upper trough. I forecast intensification to cease after 72 hours as the system reaches even cooler 19 deg C waters by 96 hours... and as the mid-ocean upper trough amplifies into an overhead upper vortex from ongoing cold air advection... with the overhead center of the upper vortex reducing upper divergence over the surface circualation. I forecast a rare 19 deg C water tropical system at 96 hours due to the potential instability brought by the rather cold 1168 dekameter 200 mb height.
(4) I forecast a loss of core thunderstorms and tropical characteristics... and hence a downgrade to a remnant north Atlantic frontal cyclone... by 120 hours as this system moves into even cooler 15 deg C waters. This system should also begin a classical post-mature decay beneath the core of the regional upper vortex where upper divergence is lacking... however hurricane-force strength is anticipated through 120 hours as this system will take time to spin down. Hurricane-force strength of the remnant cyclone could very well linger well past 120 hours as the regional upper vortex merges with the current North American upper vortex and current upper trough pivoting into the northwestern US to make a regional upper trough west of the cyclone which in turn maintains the cyclone for a bit longer with its eastern divergence zone... before the regional upper trough once again becomes an overhead and suppressing upper vortex once the cyclone wraps in the cold air associated with the trough.
Regarding impacts... this system through 72 hours... and again sometime after 120 hours... is forecast to retain a large outer circulation due to a sprawling area of upper divergence as discussed above. This will result in a larger/stronger area of coastal sea swells than typically seen with a smaller storm of equivalent intensity... at first developing across much of the open central Atlantic through 72 hours and then reaching the Azores and north Atlantic by 96+ hours.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Dec 6)... Subtropical low pressure centered at 26N-55W
IOH 24 Hr Forecast (1800Z Dec 7)... 50 mph maximum sustained wind subtropical storm centered at 29N-53W
IOH 48 Hr Forecast (1800Z Dec 8)... 70 mph maximum sustained wind tropical storm centered at 30N-51W
IOH 72 Hr Forecast (1800Z Dec 9)... 85 mph maximum sustained wind hurricane centered at 35N-45W
IOH 96 Hr Forecast (1800Z Dec 10)... 85 mph maximum sustained wind hurricane centered at 39N-41W
IOH 120 Hr Forecast (1800Z Dec 11)... Hurricane-force remnant frontal cyclone centered at 43N-37.5W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z (Dec 6) CMC Model Run...
**For area of interest #48... potential subtropical cyclone suggested near 28.5N-53.5W at 36 hours... accelerates northeast to 35.5N-41.5W through 90 hours where it strengthens into a potential hurricane... as a remnant frontal cyclone over cooler waters reaches 42.5N-35.5W by 120 hours.
0000Z (Dec 6) ECMWF Model Run...
**For area of interest #48... potential subtropical cyclone suggested near 26.5N-55W at 42 hours... accelerates northeast to 32.5N-41.5W through 78 hours where it becomes more elongated/less tropical... remnant frontal cyclone reaches 43N-34.5W by 120 hours.
1200Z (Dec 6) GFS Model Run...
**For area of interest #48... potential subtropical cyclone suggested near 28N-55W at 24 hours... accelerates northeast to 35N-41.8W through 84 hours where it strengthens into a potential hurricane... as a remnant frontal cyclone over cooler waters reaches 41.8N-36.5W by 120 hours.
1200Z (Dec 6) NAVGEM Model Run...
**For area of interest #48... potential subtropical cyclone suggested near 28.5N-54.5W at 12 hours... potential hurricane suggested near 29N-48.5W at 54 hours... as a remnant frontal cyclone over cooler water reaches 45N-30.5W by 120 hours.