*******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 JULY 17 2023 2:00 PM EDT...
Subtropical Storm Don has transitioned to a fully tropical depression over the last 36 hours... and is expected to meander in the open central Atlantic over the next few days in erratic steering currents... its future intensity is uncertain... 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.
TROPICAL DEPRESSION DON... The subtropical storm in the open central Atlantic has undergone two changes in the last 36 hours... (1) it initially weakened to a subtropical depression due to an ongoing lack of thunderstorm activity... (2) upper vorticity from southeastern Canada... an upper trough over the western Atlantic... and the pre-existing upper vorticity that was over Don have merged into a southeastward-moving supporting upper trough in Don's environment... with the eastern divergence zone of the upper trough helping Don re-build showers and thunderstorms despite the 23 deg C sea surface temperature environment. The renewed shower and thunderstorm activity is concentrated enough near Don's center such that the NHC as of 11 AM EDT has now deemed that Don is a fully tropical depression. Since Don is currently over the lowest sea surface temperature environment it will see over the next five days and has still survived as a tropical entity... I have now extended my track and intensity forecast through day 5 assuming that Don now has a decent chance of staying a tropical entity over the next few days.
The warm core deep-layer ridge in the northwest Atlantic is in the process of re-building due to warm air being wafted northward along the east side of the current broad frontal low over eastern Canada. Don and the upper trough in Don's environment will be influenced by the deep-layered ridge over the next five days as the ridge slowly shifts east as a typical amplified mid-latitude feature. While the northeastern part of the upper trough is forecast to also drift eastward... the southwestern part is forecast to move westward around the south side of the deep-layer ridge as a cut-off upper vortex and eventually become streteched north-south once it moves around the southwest side of the ridge. Don itself is currently moving southeast as the re-building deep-layer ridge is pushing it and the upper trough in that direction... with the forecast track for Don then showing a turn toward the south... southwest... west... and then northwest while rounding the south side of the passing deep-layer ridge. Regarding strengthening potential... the max sea surface temperatures Don should see in the forecast period is 25 deg C at 48+ hours while upper air temps are not forecast to be particularly cold for instability over these water temps (typically would like to see 26+ deg C for tropical development). If Don stays further south such as shown in the CMC and NAVGEM examples in the computer model summary below... it could end up being suppressed by upper convergence on the northwest side of the upper trough and southeast side of the deep-layer ridge... and then in the longer range suffer from a lack of divergence within the north side of the cut-off upper vortex. Meanwhile the GFS is robust in showing Don strengthening over the next five days while keeping Don a little further north in a supporting split flow upper divergence zone to ensue between the west side of the upper trough and southeast corner of the deep-layer ridge... and in the longer range still keeping Don aligned a little further north in split flow upper divergence between the north side of the cut-off upper vortex and southwest side of the deep-layer ridge. It is hard to say whether or not the GFS-type camp or CMC/NAVGEM-type camp is correct regarding strength... so I split the difference and keep Don as a steady-state minimal tropical storm through day 5. The NHC has a similar strategy of keeping Don as a steady-state low-end tropical storm over the next five days as well. A conservative intensity forecast also seems reasonable for now given the lackluster thermodynamic picture (25 deg C water... etc) noted above.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Jul 17)... 35 mph maximum sustained wind tropical depression centered at 38.6N-42.9W
IOH 24 Hr Forecast (1200Z Jul 18)... 40 mph maximum sustained wind tropical storm centered at 35.5N-40W
IOH 48 Hr Forecast (1200Z Jul 19)... 40 mph maximum sustained wind tropical storm centered at 33N-40.5W
IOH 72 Hr Forecast (1200Z Jul 20)... 40 mph maximum sustained wind tropical storm centered at 32.5N-44W
IOH 96 Hr Forecast (1200Z Jul 21)... 40 mph maximum sustained wind tropical storm centered at 33.5N-47.5W
IOH 120 Hr Forecast (1200Z Jul 22)... 40 mph maximum sustained wind tropical storm centered at 37.5N-52W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************
Peak Strength (1200Z Jul 21)... 50 mph maximum sustained wind tropical storm centered at 35.8N-45.5W
5-Day Position (1200Z Jul 22)... 45 mph maximum sustained wind tropical storm centered at 39.5N-49.0W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z (Jul 17) CMC Model Run...
**For Tropical Depression Don... curves southeast to 35N-40W through 36 hours and then southwest to 32.5N-41.5W through 84 hours... subsequently curves northwest to 36N-45W through 120 hours while weakening to a remnant low
0000Z (Jul 17) ECMWF Model Run...
**Not available at above-mentioned source
0600Z (Jul 17) GFS Model Run...
**For Tropical Depression Don... curves southeast to 35.5N-40W through 30 hours while strengthening to a tropical storm... curves southwest to 33.5N-42W through 78 hours while gaining additional strength... while maintaining strength accelerates northwest to 38.5N-50.5W through 120 hours
**Tropical wave currently on west coast of Africa slows down its westward track near 10N-34W by 102 hours due to surface ridge weakness created by Don... after the surface ridge weakness closes (due to Don's northwest and away acceleration) the tropical wave regains a westward heading and reaches 12N-45W through 168 hours while evolving into a tropical low
0600Z (Jul 17) NAVGEM Model Run...
**For Tropical Depression Don... curves southeast to 35N-41W through 30 hours and then south to 31.5N-40.5W through 60 hours... subsequently accelerates north-northwest to 37N-44W through 120 hours while weakening to a remnant low