*******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.**********
...SATURDAY OCTOBER 3 2020 7:45 PM EDT...
Satellite Image from 2310Z (red indicates tropical cyclone or remnants of tropical cyclone...with G marking Gamma. Green indicates an area of interest in the National Hurricane Center Tropical Weather Outlook. Yellow indicates an area of interest not in the National Hurricane Center Tropical Weather Outlook.):
NHC TAFB Surface Analysis 1800Z:
GFS Model Upper Level Winds 1800Z:
See Tropical Storm Gamma section below for the only currently active tropical cyclone in the Atlantic basin. See area of interest sections below for multiple areas being monitored for tropical cyclone formation.
TROPICAL STORM GAMMA...In the last 24 hours the twenty-fifth tropical depression has undergone steady intensification to Tropical Storm Gamma...and Gamma is already just below hurricane force as it makes landfall on the northeast Yucatan Peninsula coast.
My updated forecast track is again adjusted northward due to the current position of Gamma....with the northward angle in track being caused by upper southwesterly flow ahead of an upper trough over central North America. As the upper trough fractures into a southwestern and northeastern half...with both halves moving away from the region....a more westward track is anticipated after Gamma exits the northern Yucatan and moves into the southern Gulf of Mexico after 24 hours. At this time I do not anticipate a southward bend in track induced by the tropical wave to approach from the east...area of interest #1...as I still think the chances for that tropical wave to develop are fairly low...see area of interest #1 section below for more details on that wave. A slow track is shown throughout the forecast period as the western convergence zone of the aforementioned upper trough will reach far south enough to produce surface ridging over Mexico that blocks the westward progress of Gamma...with this ridging persisting through 48 hours once the southwest part of the upper trough breaks away from the northeastern part and persists over Mexico as a cut-off upper vortex. Even though by 72 hours the Mexico surface ridging will likely weaken once the cut-off upper vortex continues retrograding westward and away...a slow forward speed is still expected as the eastern divergence zone of a second upper trough to dive into central North America weakens the central United States surface ridge trying to push Gamma westward. I show a slightly faster westward track by 96+ hours than previously shown...as the 0600Z GFS shows the second longwave upper trough being weaker and also exiting the region...allowing for a quicker recovery of the steering central United States surface ridge. On this track Gamma could be over the northern Veracruz or southern Tamaulipas provinces of Mexico by 120 hours...and also be near the east side of the aforementioned cut-off upper vortex over Mexico. However I do not show a north bend in track under the influence of the vortex by that time as the vigorous tropical wave in area of interest #1 could try to bend the track of Gamma southward at the same time even if it does not become another tropical cyclone.
Regarding intensity...due to the current low shear and upper outflow of the western Caribbean upper ridge...warm waters...and very well organized structure of Gamma...Gamma was already able to reach a strength just below hurricane force before making landfall in the northeastern Yucatan. Due to the higher strength and also updated track which reduces the amount of time over the Yucatan...I have raised my overall intensity forecast. The forecast still shows weakening for the next 24 hours due to the land interaction and increasing shear as Gamma nears the central North America upper trough. Once the upper trough fractures into two halves...the shear should relax and provide an opportunity for Gamma to strengthen once it arrives over the warm southern Gulf of Mexico waters. Therefore at this time I am forecasting category 2 hurricane strength by 96 hours....with weakening by 120 hours due to potential landfall with the northern Veracruz and southern Tamaulipas regions of Mexico and possible light southerly shear from the cut-off upper vortex forecast to be over Mexico.
Damaging gusty winds and coastal sea swells will continue over Quintana Roo and Yucatan provinces over the next 24 hours until Gamma clears the area. Due to the slow motion of Gamma...heavy rainfall lasting for long periods may cause flash flooding. Weather south of these areas has improved as Gamma is becoming increasingly sheared with the heavier weather becoming lopsided to the north side of the circulation. I encourage interests in northern Veracruz and southern Tamaulipas to monitor the progress of Gamma as it has potential to become a strong hurricane in the southwestern Gulf of Mexico.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Oct 3)...70 mph maximum sustained wind tropical storm centered on the eastern Yucatan peninsula coast at 20.4N-87.6W
IOH 24 Hr Forecast (1800Z Oct 4)...45 mph maximum sustained wind tropical storm centered just offshore of the northern Yucatan peninsula at 21.5N-89.2W
IOH 48 Hr Forecast (1800Z Oct 5)...65 mph maximum sustained wind tropical storm centered over the southern Gulf of Mexico at 22N-90.2W
IOH 72 Hr Forecast (1800Z Oct 6)...85 mph maximum sustained wind hurricane centered over the southwestern Gulf of Mexico at 22N-92.2W
IOH 96 Hr Forecast (1800Z Oct 7)...105 mph maximum sustained wind hurricane centered over the southwestern Gulf of Mexico at 22N-95W
IOH 120 Forecast (1800Z Oct 8)...75 mph maximum sustained wind hurricane centered just inland over east-central Mexico at 22N-99W
AREA OF INTEREST #1...The tropical wave of low pressure now entering the central Caribbean Sea is fighting cold core upper vorticity that has been retrograding southwestward around the western Atlantic upper ridge and toward the wave. This is evidenced by strong thunderstorm activity initially triggered by the upper divergence on the southeast side of the vorticity...with the ongoing thunderstorm latent heat release trying to wash out the cold core upper vorticity. I keep odds of development at 0% through 24 as the upper vorticity will likely continue shearing the tropical wave. The 0600Z GFS model run indicates the upper vorticity fading away by 48+ hours due to the aforementioned thunderstorm latent heat warming....therefore I have raised my peak odds of tropical cyclone formation to 20%. This is lower than the latest NHC outlook as by the time the 0600Z GFS shows the upper vorticity being completely gone...this tropical wave will be nearing Tropical Storm Gamma which could dominate the low-level inflow in the region if it intensifies into a strong hurricane as I currently forecast.
Regarding track...I show a slight northward angle in the surface east-southeasterly steering flow to setting up between mid-latitude surface ridging and east side of Tropical Storm Gamma...with a slow down in forward speed shown at 96 hours as the steering flow weakens once the central United States surface ridge weakens under the influence of a longwave upper trough forecast to dive into central North America. By 120 hours...the central United States surface ridge is forecast to recover...so I forecast an acceleration across the Yucatan peninsula and toward Gamma. This is also when I drops the odds of development back to 0%...due to land interaction and potential for competition with Gamma if it had not develop into a tropical cyclone before then. The track forecast is near or over Jamaica...the Cayman Islands...and Yucatan peninsula...so those land areas are mentioned in regards to this tropical wave on the home page bulletins of this site.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 4)...0% chance of tropical cyclone formation (central Caribben Sea near 15.8N-76W)
IOH 48 Hr Outlook (1800Z Oct 5)...5% chance of tropical cyclone formation (western Caribben Sea near 17N-81W)
IOH 72 Hr Outlook (1800Z Oct 6)...15% chance of tropical cyclone formation (western Caribben Sea near 18N-86W)
IOH 96 Hr Outlook (1800Z Oct 7)...20% chance of tropical cyclone formation (east coast of the Yucatan peninsula near 18.8N-87.8W)
IOH 120 Hr Outlook (1800Z Oct 8)...0% chance of tropical cyclone formation (offshore of the west coast of the Yucatan peninsula near 19.8N-91W)
AREA OF INTEREST #2...A tropical wave of low pressure over interior western Africa is producing a thick curved band of thunderstorms on its north side and another thinner band of activity on its southeast side as it nears the west coast of Africa. Upper vorticity from the east-central Atlantic and also from a trough that has recently moved into western Europe is getting pushed southward toward the tropical wave by the eastern tropical Atlantic upper ridge and also the current north Atlantic upper ridge...therefore upper level winds will become less favorable for tropical development over the next 48 hours. After that time I forecast a low 10% chance of development as the tropical wave moves into an upper outflow environment beneath the eastern tropical Atlantic upper ridge. The current central North America upper trough and another larger second one to dive southeast in its place will increase the mid-latitude westerly flow across the Atlantic by late in the forecast period...sending a chunk of the central Atlantic upper vorticity southeastward toward this tropical wave. This vorticity may suppress the northern outflow of the tropical wave by 120 hours...another reason I keeps odds of development on the lower side.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 4)...0% chance of tropical cyclone formation (offshore of western Africa near 11N-19W)
IOH 48 Hr Outlook (1800Z Oct 5)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-24W)
IOH 72 Hr Outlook (1800Z Oct 6)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-29W)
IOH 96 Hr Outlook (1800Z Oct 7)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-34W)
IOH 120 Hr Outlook (1800Z Oct 8)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-39W)
AREA OF INTEREST #3...A tropical wave of low pressure that was southwest of the Republic of Cabo Verde Islands appears to have dominated over the wave that was southeast of the islands....as it has developed an expansive and slowly organizing field of thunderstorms while the other wave has disappeared on satellite pictures. This tropical wave has been added to the NHC tropical weather outlook...and due to its large size I only slowly ramp up the odds of development as it will take time for the wave to consolidate and develop. I stop increasing odds of development after 72 hours as this is when a chunk of central Atlantic upper vorticity will retrograde southwestward around the west Atlantic upper ridge and into the Caribbean...with the east side of this chunk potentially inducing southerly shear across this tropical wave. I have begun to mention the Lesser Antilles in regards to this tropical wave in the home page bulletins of this site.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 4)...10% chance of tropical cyclone formation (central tropical Atlantic near 11.5N-46.5W)
IOH 48 Hr Outlook (1800Z Oct 5)...20% chance of tropical cyclone formation (central tropical Atlantic near 11.5N-51.5W)
IOH 72 Hr Outlook (1800Z Oct 6)...30% chance of tropical cyclone formation (central tropical Atlantic east of the Lesser Antilles near 12N-56.5W)
IOH 96 Hr Outlook (1800Z Oct 7)...30% chance of tropical cyclone formation (just west of the Lesser Antilles near 12.5N-61.5W)
IOH 120 Hr Outlook (1800Z Oct 8)...30% chance of tropical cyclone formation (eastern Caribbean sea near 13N-66.5W)
AREA OF INTEREST #4...The long lasting remnant low pressure of what was Tropical Storm Paulette appeared over the last several days to retrograde slowly southwest into the open central Atlantic while under the influence of a strengthening North Atlantic surface ridge bolstered by the upper convergence zone of the current west to North Atlantic upper ridge. A surface trough of low pressure with thunderstorms currently in the central Atlantic is supported by the southeastern divergence zone of the large scale upper vorticity this has been in this region...but is not being considered as Paulette’s remnant low pressure in the NHC outlook perhaps as it earlier weakened to a surface trough with no defined spin that could be considered Paulette.
The surface trough will continue westward toward Bermuda while still steered by the North Atlantic surface ridge. A large upper trough that will dive southeastward into central North America will create a surface ridge weakness in the northwestern Atlantic that should slow the track and then bend the track of the surface trough northward...which I estimate will keep the trough just east of Bermuda. On this track...the surface trough will remain over 26 deg C warm water while passing beneath the axis of the central Atlantic upper vorticity where there is less shear...thus I forecast some chance of subtropical cyclone formation in the next 24 hours. I then drop the odds of development to 0% as the surface trough reaches a zone of suppressing upper convergence between the west side of the upper vorticity and east side of the western Atlantic upper ridge. I then have a window of very low 5% odds of tropical development assuming the surface trough survives and reaches the low shear and upper outflow of the west Atlantic upper ridge. The large North American upper trough expected by 120 hours should increase westerly shear across the northwest Atlantic...with my forecast position taking the surface trough into the shear and thus I drop my odds of development back to 0% by then.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 4)...10% chance of subtropical cyclone formation (central Atlantic near 28N-53W)
IOH 48 Hr Outlook (1800Z Oct 5)...0% chance of tropical cyclone formation (central Atlantic near 28N-57W)
IOH 72 Hr Outlook (1200Z Oct 6)...5% chance of tropical cyclone formation (western Atlantic to the east-southeast of Bermuda near 28N-61W)
IOH 96 Hr Outlook (1200Z Oct 7)...5% chance of tropical cyclone formation (southeast of Bermuda near 29N-63W)
IOH 120 Hr Outlook (1200Z Oct 8)...0% chance of tropical cyclone formation (east of Bermuda near 31N-63W)
...COMPUTER MODEL SUMMARY...
Temporarily cancelled to avoid further delays in creating and publishing the post.