*******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.**********
...SUNDAY JULY 16 2023 4:30 AM EDT...
Subtropical Storm Don is expected to meander in the open central Atlantic over the next few days in erratic steering currents... 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... The subtropical storm in the open central Atlantic has continued northwestward into cooler waters while steered by the northeastern side of the upper vorticity in the region. The effects of the cooler waters are pronounced by a continual decrease in showers and thunderstorms in the circulation... and as of this writing Don is barely a subtropical storm while having only a small area of thunderstorms just northeast of its center. Over the next couple of days the current western Atlantic upper trough and upper vorticity currently over southeastern Canada are expected to merge with the upper vorticity over Don... with the merger resulting in an eastward-moving supporting upper trough in Don’s environment. This will result in a reversal of Don’s northwest track into a northeastward and then eastward heading. Because Don is holding on to subtropical status over 24 deg C waters... my updated forecast below now holds onto the subtropical status until Don moves eastward into below 24 deg C waters. Even after shedding tropical characteristics... I expect the remnant low pressure system of Don to maintain gale force strength for some time due to the supportive eastern divergence zone of the upper trough.
In the longer range (48+ hours)... 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 trough to materialize in Don’s environment is not forecast to be particularly cold enough to help generate instability and thunderstorms over waters below 26 deg C.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Jul 16)... 40 mph maximum sustained wind subtropical storm centered at 38.2N-48.7W
IOH 24 Hr Forecast (0000Z Jul 17)... 40 mph maximum sustained wind subtropical storm centered at 40N-47.5W
IOH 48 Hr Forecast (0000Z Jul 18)... Non-tropical remnant cyclone centered at 38N-41W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 PM EDT***************************
5-Day Position (0000Z Jul 21)... 40 mph maximum sustained wind subtropical storm centered at 34.2N-42.8W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (Jul 15) CMC Model Run...
**For Subtropical Storm Don... curves north to 39.5N-46.5W through 30 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 32.5N-40W through 102 hours... the surface low then drifts north to 33N-40.5W through 120 hours
1200Z (Jul 15) ECMWF Model Run...
**Not available at above-mentioned source
1800Z (Jul 15) GFS Model Run...
**For Subtropical Storm Don... curves north to 40N-47W through 27 hours where it maintains strength as a potentially as a less tropical system over cooler waters... persisting surface low then curves southeast to 34N-40.5W through 78 hours... the persisting surface low begins to weaken while drifting southwest to 33N-42.2W through 120 hours
1800Z (Jul 15) NAVGEM Model Run...
**For Subtropical Storm Don... curves north to 39N-46W through 24 hours where it maintains strength as a potentially as a less tropical system over cooler waters... the persisting surface low then curves southeast to 35N-40W through 78 hours... the persisting surface low begins to weaken while drifting south-southwest to 33N-41W through 120 hours.