*******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.**********
...FRIDAY JULY 14 2023 1:35 PM EDT...
Surface low pressure cyclone in the open central Atlantic has become Subtropical Storm Don... and is expected to meander in the region in erratic steering currents over the next few days... see Don section below for more information. Elsewhere... the tropical belt of the Atlantic remains calm as the Caribbean is covered by suppressing upper vorticity and the central and eastern tropical Atlantic has a swath of dry Saharan air being wafted across from Africa.
SUBTROPICAL STORM DON... Colorized infrared satellite image taken at 0200Z earlier today as Subtropical Storm Don formed overnight:
The central Atlantic surface low pressure area developed a well-defined center and persistent organized thunderstorm activity overnight and was upgraded to Subtropical Storm Don with 50 mph maximum sustained winds as of 5 AM EDT. The system is designated as subtropical as it was aided both in a non-tropical manner by the eastern divergence zone of the upper vorticity in the region and in a tropical manner as the latent heat release of the thunderstorms created a layer of warm core upper ridging and outflow. Don is located a little west of my previous forecast track and my updated one is adjusted accordingly... especially considering that the more west initial position will now allow Don to move into the easterly flow on the north side of the upper vorticity which should carry Don more west and less north in the next 24 hours. Beyond that time the current western Atlantic upper trough... which recently seperated from the current North America upper vortex... and additional upper vorticity to also seperate from the upper vortex will finally break through the northwest Atlantic upper ridge and approach Don. These incoming features will combine with the upper vorticity currently over Don to make an eastward-moving supporting upper trough... resulting in an eastward turn of Don. I expect the remnant low pressure system of Don... after it sheds tropical characteristics over cooler waters at around 48 hours... to maintain gale force strength for some time due to the supportive eastern divergence zone of the upper trough.
In the long range (through 5 days)... models agree that the warm core northwest Atlantic upper ridge will rebuild in the warm sectors of additional surface frontal systems to be generated by the ongoing North American upper vortex... with the remnant system of Don (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 of Don regains tropical characteristics... however there is some uncertainty in this idea and that is why my forecast below simply ends at the 48-hour mark. For example it is also possible the remnant surface low of Don slips into the suppressive convergent northwestern side of the upper trough and southeast side of the re-amplified northwest Atlantic upper ridge during its southward dive and dissipates. Noting that as of this writing the NHC offical forecast has a different strategy and keeps Don as a subtropical storm through 5 days... but does acknowledge in their forecast discussions that Don could lose tropical characteristics in the middle of the 5-day window. I think the loss of the tropical characteristics before day 5 is quiet likely as the upper vorticity over Don is not particularly cold enough to help generate instability and thunderstorms over waters below 26 deg C... and Don's thunderstorm activity has already waned as it begins to lift northward away from the 26 deg C sea surface temperature isotherm. For intensity... noting that the initial intensity of the subtropical storm is less than my previous forecast with Don recently weakening from 50 mph to 45 mph maximum sustained winds as of the more recent 11 AM EDT NHC advisory... and my updated intensity forecast is lowered. Before the supporting upper trough materializes in Don's environment in the 48+ hour window... some additional weakening should commence as the more west initial position of Don noted in the prior paragraph puts it further away from the supporting eastern divergence zone of the overhead upper vorticity and as the thunderstorm activity continues to weaken over cooler waters along the forecast track.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Jul 14)... 50 mph maximum sustained wind subtropical storm centered at 32.9N-46.8W
IOH 24 Hr Forecast (1200Z Jul 15)... 40 mph maximum sustained wind subtropical storm centered at 36N-48W
IOH 48 Hr Forecast (1200Z Jul 16)... Non-tropical remnant cyclone centered at 38N-47.5W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************
5-Day Position (1200Z Jul 19)... 45 mph maximum sustained wind subtropical storm centered at 33.7N-38.6W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z (Jul 14) CMC Model Run...
**For Subtropical Storm Don... curves north to 38N-46W through 72 hours where it maintains some strength as a potentially less tropical system over cooler waters... the persisting and gradually weakening surface low then curves southeast to 32N-41.5W through 120 hours
0000Z (Jul 14) ECMWF Model Run...
**Not available at above-mentioned source
0600Z (Jul 14) GFS Model Run...
**For Subtropical Storm Don... curves north to 39N-45.5W through 63 hours where it maintains strength as a potentially as a less tropical system over cooler waters... the persisting surface low then curves southeast to 35.5N-39W through 102 hours... performs a cyclonic loop to 36N-38.5W through 120 hours under the influence of an upper vortex that develops in the region.
**Slow moving eastern Atlantic tropical wave of low pressure evolves into a tropical low near 12.5N-31W by 168 hours
0600Z (Jul 14) NAVGEM Model Run...
**For Subtropical Storm Don...curves north to 38.8N-45.5W through 60 hours where it maintains strength as a potentially as a less tropical system over cooler waters... the persisting surface low then curves southeast to 34N-36W through 120 hours
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