*******Note that forecasts 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 SEPTEMBER 6 2020 11:45 PM EDT...
The following is a special update on the two organized tropical waves of low pressure in the eastern Atlantic...one of which has already become a tropical depression in the open Atlantic...the other (Invest 93L) which is approaching the Republic of Cabo Verde Islands while appearing to already become a tropical depression as well. The tropical waves are being referred to with their invest numbers instead of an area of interest number as this is a special update without a birdseye view chart that has area of interest numbers.
TROPICAL WAVE INVEST 92L (RECENTLY UPGRADED TO TROPICAL DEPRESSION SEVENTEEN)...The tropical wave of low pressure in the eastern tropical Atlantic featuring a large spin with spiral thunderstorm bands has recently been upgraded to a tropical depression. My updated positions in the forecast below are adjusted westward given the current position of the depression with respect to my previous forecast for this system. The depression is currently moving only slowly westward...perhaps from low surface pressures that are weakening the south side of the steering Atlantic subtropical ridge...being created by divergence zones of cut-off upper upper vorticity in the northeast Atlantic and in the western Atlantic. Thus I show a slow forward speed for the next 72 hours...with some increase in forward speed after 72 hours as the south side of the Atlantic subtropical ridge recovers once the cut off upper vorticity to the northeast and west begin to weaken. I have some northward angle in track at 24+ hours as this system likely becomes strong/tall enough to be dragged by the upper vorticity to the west.
For the next 48 hours this system will be below a tropical upper ridge axis with low shear and upper outflow...so I forecast steady intensification to a strong tropical storm by the end of 48 hours. The GFS model continues to show less separation between the upper vorticity to the northeast and west...so this system may get sheared when it runs into upper southwesterly winds being generated by both lobes of upper vorticity at 72 hours...so I only forecast minimal hurricane strength at that time. At 96+ hours a widening gap between both lobes of upper vorticity as they weaken may allow for reduced shear...so I forecast category 2 hurricane strength by the end of the 5 day forecast period. On this current forecast...a direct strike to the Lesser Antilles is not expected...but some sea swells from this system may reach the islands especially if it becomes a hurricane.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
0 Hr Position (0000Z Sep 7)...35 mph maximum sustained wind tropical depression centered at 17N-41.5W
IOH 24 Hr Forecast (0000Z Sep 8)...50 mph maximum sustained wind tropical storm centered at 17.5N-43W
IOH 48 Hr Forecast (0000Z Sep 9)...65 mph maximum sustained wind tropical storm centered at 18.5N-45.5W
IOH 72 Hr Forecast (0000Z Sep 10)...75 mph maximum sustained wind hurricane centered at 19.5N-48W
IOH 96 Hr Forecast (0000Z Sep 11)...95 mph maximum sustained wind hurricane centered at 20.5N-52.5W
IOH 120 Hr Forecast (0000Z Sep 12)...105 mph maximum sustained wind hurricane centered at 22N-57W
TROPICAL WAVE INVEST 93L...The tropical wave of low pressure that was over interior Western Africa has become much better organized with spiral thunderstorm bands while exiting the west African coast and heading toward the Republic of Cabo Verde Islands. Therefore I have begun a tropical cyclone formation forecast as outlined below. My updated forecast track is a bit further west of my previous given the current position of the low pressure center of the tropical wave. I forecast a slight north bend in track in the next 24 hours in response to lowering surface pressure on the southeast side of the Atlantic subtropical ridge...induced by the upper divergence zone of the upper vortex in the northeast Atlantic. I then bend the track more westward after 24 hours as this system moves past the northeast Atlantic surface low pressure field. My forecast points are also adjusted southward overall as the GFS shows the northeast Atlantic upper vortex not reaching as far south...reducing its potential to drag this system northward as this system strengthens and becomes taller. The forward speed between 24 and 72 hours is kept slower as the aforementioned northeast Atlantic upper vortex and also upper vorticity in the west Atlantic have weakened the south side of the steering Atlantic subtropical ridge...see tropical depression seventeen section above. I accelerate the forward speed after 72 hours as the Atlantic subtropical ridge recovers while the upper vorticity weakens.
Regarding intensity...thru the next 5 days this system will remain in low shear and upper outflow environment beneath a tropical upper ridge axis. Therefore I forecast major hurricane strength by days 4 and 5. This is higher than my intensity forecast for tropical depression seventeen as I expect thru day 5 that this system will stay east of the shearing upper southwesterly winds that will affect seventeen. I recommend interests in the Republic of Cabo Verde Islands to quickly prepare for tropical storm conditions (gusty winds...heavy rains...and coastal sea swells) which could arrive by Monday night.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
0 Hr Position (0000Z Sep 7)...Tropical low pressure centered at 15N-19W
IOH 24 Hr Forecast (0000Z Sep 8)...45
mph maximum sustained wind tropical storm centered over the Republic of Cabo Verde Islands at 16N-23W
IOH 48 Hr Forecast (0000Z Sep 9)....65 mph maximum sustained wind tropical storm centered just west of the Republic of Cabo Verde Islands at 16.5N-26W
IOH 72 Hr Forecast (0000Z Sep 10)....90 mph maximum sustained wind hurricane centered at 17N-29W
IOH 96 Hr Forecast (0000Z Sep 11)...115 mph maximum sustained wind major hurricane centered at 17.5N-34W
IOH 120 Hr Forecast (0000Z Sep 12)...125 mph maximum sustained wind major hurricane centered at 18N-39W
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