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Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at 

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*******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 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 dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********


Although the large subtropical low pressure system currently in the central Atlantic has not so far seen an increase in core thunderstorm activity... it continues to be monitored for subtropical/tropical cyclone formation. See area of interest #48 section below for an update on this system.

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... As expected the cut-off upper trough in the western Atlantic has amplified into a central Atlantic upper vortex as the northwest quadrant of the sprawling subtropical surface low pressure in the region has pulled cold air associated with the trough southward... and also as the adjacent Gulf of Mexico upper ridge has also re-amplified in warm southerly flow out ahead of the frontal low that has recently moved into the eastern Great Lakes from the central US. The increased divergence on the east side of the upper vortex helped to strengthen the subtropical low further... however intensification of the subtropical low is taking a pause as the core of the upper vortex has become stacked over the subtropical low where upper divergence is lacking. The thunderstorm activity around the core of the subtropical low was becoming better organized through 1200Z today while wrapping around in all quadrants... however over the last 6 hours this outer activity has weakened especially on the west side of the system and no core activity has developed. Therefore the subtropical low has not been upgraded to a subtropical storm by the National Hurricane Center.

Over the next 24 hours... the northwest quadrant of the subtropical low will be wrapping in the cold air associated with the adjacent north Atlantic upper vorticity... causing the upper vorticity to shift south and merge with the overhead upper vortex. The merger will create a mid-ocean upper trough whose east side will both strengthen the subtropical surface low further with upper divergence and send the subtropical surface low northeastward. Moreover the incoming colder north Atlantic upper vorticity will drop upper air temperatures perhaps enough to generate instability and core thunderstorms for the subtropical surface low. Given the outlook of potential increased instability and strenghtening... I still forecast the subtropical surface low to become a subtropical storm in my updated forecast below. However given that the subtropical surface low has not yet generated core thunderstorm activity... the window for this system to become fully tropical and strengthen with thunderstorm latent heat release driven outflow is reducing and so my updated structure/intensity forecast no longer calls for a fully tropical system and a slightly lower intensity. As a result in the updated forecast I also transition this system to a remnant frontal cyclone sooner (the timeframe that is now 72+ hours out).

A subtle change has also occurred with the longer-range computer model solutions... specifically the frontal low currently over the eastern Great Lakes and its eventual small upper trough it will carve out from the current north American upper vortex are now forecast to be stronger as they move in tandem into/across the western/central Atlantic. This in turn knocks down the longer-range amplitude of the Gulf of Mexico upper ridge and hence also knocks down the mid-ocean upper trough's amplitude. The slighlty lower amplitude mid-ocean upper trough in the latest solutions results in a more east forecast track and the updated track forecast below is a reflection of this. The lower-amplitude of the mid-ocean upper trough also results in a less focused and more elongated divergence zone on the trough's east side... resulting in a slightly weaker and more elongated surface subtropical low. The elongating structure would make it less likely to have a focal point of surface convergence for core thunderstorms and tropical characteristics... hence increasing the chances of an even earlier transition to a remnant non-tropical frontal system as the cold air associated with the mid-ocean upper trough wins over the lack of core thunderstorms/latent heat release. The current lack of core thunderstors and prospect of an earlier transition to an elongated less tropical low could be why the NHC has not raised subtropical cyclone formation odds above 50% in regards to the subtropical surface low. The slightly lower amplitude Gulf of Mexico upper ridge may also play a role in shorterning the long-range life (beyond 72 hours) of the remnant frontal cyclone of this system as it potentially fails to send the current North American upper vortex close enough to the remnant cyclone (if the energy from this upper vortex stays too far west... it will fail to extend the life of the remnant cyclone with its eastern divergence zone).

Regarding impacts... the already-large size of the subtropical low pressure system's circulation is translating into a large/strong sea swells event across much of the open central Atlantic. The swells are forecast to transfer northeastward with the track of this system and into the north and northeastern Atlantic... including the shores of the Azores islands... over the next 72 hours. As noted above... the long range intensity of the remnant cyclone beyond 72 hours is uncertain which increases the uncertainty in how long the north Atlantic sea swells will last.

****** forecast. Visit (hurricanes dot gov) for official forecast***********

0 Hr Position (1800Z Dec 7)... strong subtropical low centered at 28.5N-54W

IOH 24 Hr Forecast (1800Z Dec 8)... 60 mph maximum sustained wind subtropical storm centered at 30.5N-48.5W

IOH 48 Hr Forecast (1800Z Dec 9)... 70 mph maximum sustained wind subtropical storm centered at 36N-44W

IOH 72 Hr Forecast (1800Z Dec 10)... Remnant frontal cyclone centered at 41N-39W


Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (

1200Z (Dec 7) CMC Model Run...

**For area of interest #48... subtropical low moves east-northeast to 31.8N-43W through 42 hours where it becomes more elongated/less tropical... remnant frontal cyclone reaches 45N-31.5W by 120 hours.

1200Z (Dec 7) ECMWF Model Run...

**For area of interest #48... subtropical low moves east-northeast to 30.5N-44W through 36 hours where it becomes more elongated/less tropical... remnant frontal cyclone reaches 45N-36.2W by 120 hours.

1200Z (Dec 7) GFS Model Run...

**For area of interest #48... subtropical low moves east-northeast to 31.5N-44.5W through 42 hours where it becomes more elongated/less tropical... remnant frontal cyclone reaches 46.5N-38.8W by 120 hours.

1200Z (Dec 7) NAVGEM Model Run...

**For area of interest #48... subtropical low moves east-northeast to 29.2N-48.2W through 30 hours where it strengthens further into a potential hurricane... as a remnant frontal cyclone over cooler waters reaches 45.8N-32.5W by 120 hours.

**Frontal low currently over the eastern Great Lakes region of North America moves into the Atlantic from coastal Maine at 18 hours... cold air advection on west side of frontal low breaks off portion of current North American cold core upper vortex resulting in the formation of an upper trough just west of the frontal low (near 39.5N-63W) through 36 hours... upper trough dives south on east side of amplified Gulf of Mexico upper ridge while itself becoming more amplified with increasing divergence on east side of the amplified trough strengthening the surface low into a subtropical cyclone (east-northeast of Bermuda near 33N-60W) through 84 hours... subtropical cyclone continues east-southeast into the open central Atlantic to 29.8N-47W through 120 hours.

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