*******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.**********
...MONDAY DECEMBER 5 2022 2:46 PM EDT...
Potential for a central Atlantic subtropical to tropical storm is increasing and the National Hurricane Center has also begun issuing tropical weather outlooks for this system as of 9 AM EDT today. 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 that was previously in the central Atlantic near 25N-50W has lost its identity to the additional cut-off upper trough approaching from the western Atlantic. The sprawling eastern divergence zone of the overtaking cut-off upper trough has expanded upon the field of low surface pressures in the central Atlantic... with the shower and thunderstorm activity that was southeast of Bermuda also expanding in size and now located well northeast of the Lesser Antilles. Off to the northwest... the south part of the upper trough from eastern Canada has broken through the north Atlantic-to-Gulf of Mexico upper ridge... however the Gulf of Mexico upper ridge cell will still be strong enough to cause it to merge with and hence re-enforce the cut-off western Atlantic upper trough. The warm sector of additional North American frontal systems (to be supported by the current North American upper vortex and upper trough currently pivoting into the northwestern US) is expected to re-amplify the Gulf of Mexico upper ridge cell and allow it to re-link with the north Atlantic upper ridge... with this re-amplification expected to cause the western Atlantic cut-off upper trough to amplify into an upper vortex. The tremendous eastern divergence zone of the upper vortex in turn is expected to evolve the field of central Atlantic low surface pressure into a bonafide surface cyclone over the next 48 hours.
Because the surface cyclone has potential to form near 25N latitude where water temps are currently running at 26 deg C... thermodynamic conditions are conducive for continued thunderstorm activity and acquisition of tropical characteristics. Once the cyclone drifts northeastward toward water temps below 26 deg C... the parent upper vortex mentioned in the previous paragraph is forecast to be cold enough to maintain the potential for thunderstorm activity (200 mb heights forecast to be below 1200 dekameters). As is typically seen with a surface cyclone supported by an upper vortex... the cyclone is forecast to whirl beneath the vortex which will result in deep-layer cyclonic flow keeping wind shear levels also low enough for potential tropical development. Should the thunderstorms of the tropical core release enough latent heat to generate warm core outflow... the outflow could result in the intensification of this system's core into a fully tropical storm or hurricane (similar to Hurricanes Martin and Nicole from this past November). Because the major global models (GFS... CMC... NAVGEM... and ECMWF) remain in agreement on developing a bonafide surface cyclone... and due to the already-occurring widespread showers and thunderstorms associated with this area of interest... I have increased my peak odds of subtropical cyclone formation to 70% (as of this writing the NHC has set peak development odds to 40%). Also noting that as of late the models have trended with a slightly slower re-amplification of the Gulf of Mexico upper ridge... which in turn delays the amplification of the western Atlantic cut-off upper trough into a vortex. This means the formation of a focused eastern divergence zone on the east side of the vortex could be delayed... potentially resutling in an initially broad surface cyclone with multiple centers. Therefore my peak 70% odds are reserved for the 72 hour window... when the models agree that the upper divergence zone/surface cyclone consolidates.
In the long range (96+ hours)... the central Atlantic surface cyclone/upper vortex is expected to interact with the upper vorticity currently in the northeast Atlantic. The interaction begins when the north Atlantic and Gulf of Mexico upper ridge cells push the northeast Atlantic upper vorticity southwestward toward the cyclone... and is exacerbated as the northwest side of the large outer circulation of the surface cyclone pulls the cold air associated with the approaching northeastern Atlantic upper vorticity southward. Essentially the parent upper vortex of the surface cyclone and northeast Atlantic upper vortiicty merge into a strong upper trough that sends the cyclone northeastward toward the northeast Atlantic while the eastern divergence zone of the upper trough gives extended intensity and life to the surface cyclone. Even though the surface cyclone moves northeast into low-20 deg C waters by 120 hours on the current forecast... upper air temperatures in its environment are also expected to drop (200 mb heights dropping to 1170 dekameters) as the northwest quadrant of the cyclone pulls in the rather cold air associated with the northeast Atlantic upper vorticity which could prolong the instability and thunderstorm activity. Therefore I maintain subtropical development odds above 0% through 120 hours. However I do taper the odds down to 40% as it is not clear yet if the upper trough (that forms from the merger of the cyclone's parent upper vortex and northeast Atlantic cold core upper vorticity) will have a focused or elongated upper divergence maximum... with an elongated maximum resulting in an elongated circulation to the cyclone that would no longer allow it to be classified as a subtropical or tropical system.
Regardless of whether or not this system ultimately becomes classified as non-tropical... subtropical... or tropical... a strong and long duration cyclonic storm is anticipated for the central to northeast Atlantic in 2+ days which will be a concern for marine interests.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Dec 6)... 20% chance of subtropical cyclone formation (central Atlantic near 26N-54W)
IOH 48 Hr Outlook (1200Z Dec 7)... 50% chance of subtropical cyclone formation (central Atlantic near 29N-53W)
IOH 72 Hr Outlook (1200Z Dec 8)... 70% chance of subtropical cyclone formation (central Atlantic near 30N-51W)
IOH 96 Hr Outlook (1200Z Dec 9)... 50% chance of subtropical cyclone formation (central Atlantic near 32N-49W)
IOH 120 Hr Outlook (1200Z Dec 10)... 40% chance of subtropical cyclone formation (central Atlantic near 35.5N-43W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z (Dec 5) CMC Model Run...
**For area of interest #48... consolidated surface low develops near 26.2N-56.5W at 30 hours... potential subtropical cyclone suggested near 27.8N-55W at 66 hours... accelerates northeast to 35N-41W through 108 hours where it becomes more elongated/less tropical... remnant frontal cyclone reaches 40N-36.5W by 126 hours.
0000Z (Dec 5) ECMWF Model Run...
**For area of interest #48... consolidated surface low develops near 25.5N-56.5W at 24 hours... potential subtropical cyclone suggested near 28.2N-52.5W at 72 hours... accelerates northeast to 34N-41.2W through 108 hours where it becomes more elongated/less tropical... remnant frontal cyclone reaches 40N-38.5W by 126 hours.
1200Z (Dec 5) GFS Model Run...
**For area of interest #48... develops into a strengthening subtropical low with multiple centers in the vicnity of 29N-53W through 39 hours... potential subtropical cyclone suggested near 28N-53W at 60 hours... while strengthening further into a potential hurricane continues northeast to 35.5N-42W through 120 hours.
0600Z (Dec 5) NAVGEM Model Run...
**For area of interest #48... consolidated surface low develops near 26.2N-55.5W at 12 hours... potential subtropical cyclone suggested near 26.2N-53W at 36 hours... potential hurricane suggested near 28N-55W at 66 hours... potential hurricane accelerates northeast to 37N-42W by 120 hours.