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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.**********

...THURSDAY JULY 13 2023 3:40 PM EDT...

The thunderstorm activity associated with the surface low pressure area in the open central Atlantic... tagged as area of interest #13 in full update #29 earlier today... continues to become stronger and better organized. The system is also expected to strengthen further and develop a well-defined center within the next 24 hours... therefore I have begun a track and intensity forecast for this feature... see area of interest #13 section below for more information.

AREA OF INTEREST #13... Satellite imagery of subtropical surface low pressure area in the open central Atlantic as of 1830Z... left is visible and right is colorized infrared:

The central Atlantic surface low pressure area as expected has transited eastward while steered by the south side of upper vorticity in the region... and over the next 24 hours is expected to strengthen further into a cyclone while now having arrived into the supportive eastern divergence zone of the vorticity. Although visible satellite imagery shows that this system does not yet have a well-defined center... with multiple swirls appearing at times... the models are in agreement that the upper vorticity in the region will consolidate such that a single upper divergence maximum will end up resulting in the formation of a well-defined center. The well-defined center is likely to also become established toward the northeast side of the circulation where a continuously expanding area of stronger thunderstorms has been in progress over the last few hours on colorized infrared satellite imagery... as the latent heat release of this thunderstorm activity is likely developing a layer of warm core upper ridging and outflow that will also be helping to drop surface pressures. The hybrid nature of this system... where both the thunderstorm activity and the upper divergence of the upper vorticity are helping in development... means that this system is subtropical in nature. As such I have begun a subtropical cyclone formation forecast with specific track and intensity forecast points as outlined below. ASCAT passes ( suggest the southeast quadrant of this system is reaching storm force (35 knots or 40 mph)... particularly on the descending ASCAT-B and ASCAT-C passes. Based on this initial intensity and the healthy burst of thunderstorm activity in the northeast quadrant... I forecast a moderate to strong subtropical storm to be in progress by 24 hours. Beyond that time I expect this system to whirl closer to the core of the overhead upper vorticity where there is less upper divergence... and reduced sea-surface temps along the forecast track should cause a weakening in the thunderstorm activity... therefore I forecast a gradual decay after 24 hours.

Through 48 hours there is a westward slant in the northward forecast track as this system cyclonically arcs around the east side of the upper vorticity. Beyond that time the track is expected to then take an eastward slant as some of the upper vorticity from the current eastern North America upper vortex finally breaks through the northwest Atlantic upper ridge and moves toward this system and associated central Atlantic upper vorticity... with all the upper vorticity combining into an eastward-moving supporting upper trough in the environment of this system. I expect the remnant low pressure system... after it sheds tropical characteristics over cooler waters at around 72 hours... to maintain gale force strength for some time due to the supportive eastern divergence zone of the upper trough. In the long range (beyond 5 days)... models are increasingly suggesting that the warm core northwest Atlantic upper ridge rebuilds in the warm sectors of additional surface frontal systems to be generated by the ongoing North American upper vortex... with the remnant system (surface low and upper trough) taking a southward dive back toward warmer waters under the influence of the re-amplifying ridge. Therefore it is possible that the remnant system regains tropical characteristics... however there is some uncertainty in this idea and that is why my forecast below simply ends at the 72-hour mark. For example it is also possible the remnant surface low slips into the convergent northwestern side of the upper trough and southeast side of the re-amplified northwest Atlantic upper ridge during its southward dive and dissipates.

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

0 Hr Position (1800Z Jul 13)... Surface low pressure located at 31.5N-47.5W

IOH 24 Hr Forecast (1800Z Jul 14)... 60 mph maximum sustained wind subtropical storm centered at 33N-46W

IOH 48 Hr Forecast (1800Z Jul 15)... 50 mph maximum sustained wind subtropical storm centered at 37N-47W

IOH 72 Hr Forecast (1800Z Jul 16)... Non-tropical remnant cyclone centered at 39N-45W

******National Hurricane Center ( official outlook as of 2 PM EDT***************************

Formation chance through 48 hours...60%

Formation chance through 7 days (168 hours)...60%

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