*******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.**********
...MONDAY SEPTEMBER 7 2020 3:08 PM EDT...
See Tropical Storm Paulette and Tropical Depression Eighteen sections below for the two currently active tropical cyclones in the Atlantic basin. See area of interest sections below for multiple areas of interest being monitored for tropical cyclone formation. Elsewhere...the eastern divergence zone of a cut-off upper vortex in the northeastern Atlantic is producing a surface trough of low pressure south of the Azores with some shower activity. Although this system is over 26 deg C waters only marginal for tropical development...the cold temperatures of this upper vortex may aid in producing instability for thunderstorms...and will consider add this feature as an area of interest for subtropical development should thunderstorm activity increase and become better organized.
TROPICAL STORM PAULETTE...Since special update #113A last night Tropical Depression Seventeen has developed stronger and more widespread thundertorms at its core...which has allowed it to intensify into Tropical Storm Paulette...the earliest sixteenth named storm in the Atlantic basin on record as it beats Philippe of 2005 which was named on September 17. Thus the record-setting pace of the 2020 Atlantic Hurricane Season continues! My updated forecast track is a continuity of the one in update #113A. Paulette 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 48 hours...with some increase in forward speed after 48 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 as Paulette will be strong/tall enough to be dragged by the upper vorticity to the west.
For the next 36 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 48 hours. The GFS model continues to show less separation between the upper vorticity to the northeast and west...so Paulette will likely get sheared when it runs into upper southwesterly winds being generated by both lobes of upper vorticity at 48 hours. In the latest GFS model run...the shear is forecast to persist for longer as a portion of the northeast Atlantic upper vorticity retrogrades into the western Atlantic upper vorticity...thus my updated intensity forecast below no longer shows Paulette becoming a hurricane. Instead I forecast the shear to now weaken Paulette between 72 and 96 hours. I cautiously begin to raise the intensity again by 120 hours when the latest GFS finally shows a gap developing between both lobes of upper vorticity and over Paulette as the vorticity weakens. On this current forecast...a direct strike to the Lesser Antilles is not expected...and the chances of Paulette producing sea swells to the islands is less with the latest intensity forecast that keeps Paulette below hurricane force.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 7)...40 mph maximum sustained wind tropical storm centered at 17.2N-42.2W
IOH 24 Hr Forecast (1200Z Sep 8)...60 mph maximum sustained wind tropical storm centered at 18.2N-44.5W
IOH 48 Hr Forecast (1200Z Sep 9)...70 mph maximum sustained wind tropical storm centered at 19.2N-47.5W
IOH 72 Hr Forecast (1200Z Sep 10)...70 mph maximum sustained wind tropical storm centered at 20N-51.5W
IOH 96 Hr Forecast (1200Z Sep 11)...60 mph maximum sustained wind tropical storm centered at 21.2N-55W
IOH 120 Hr Forecast (1200Z Sep 12)...65 mph maximum sustained wind tropical storm centered at 24N-58W
TROPICAL DEPRESSION EIGHTEEN...The tropical wave of low pressure that recently emerged from western Africa has continued to become better organized with spiral thunderstorm bands...and the wave was upgraded to tropical deprssion eighteen this morning. My updated forecast track is a continuity of the previous one from special update #113A last night. 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 stay south of the 0600Z GFS and are more in agreement with the 0000Z ECMWF 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. What makes the GFS shoot this system more northward in track is I think it overdoes the size of Paulette...which causes Paulette to swing this system north in its solutions. The forward speed for the next 48 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 Storm Paulette section above. I accelerate the forward speed after 48 hours as the Atlantic subtropical ridge recovers while the upper vorticity weakens. I show a north bend in track at 120 hours as upper vorticity to eject from the current central Canadian upper trough ejects into the north Atlantic and weakens the middle of the steering Atlantic surface subtropical ridge by generating a frontal cyclone. However I do not anticipate a complete turn to the north from this point as the ridge weakness is shown to be narrow such that I think this system would turn more westward beyond 120 hours once the ridge weakness passes.
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...which seems to jive with the 0000Z ECMWF model run. This is much higher than my intensity forecast for Tropical Storm Paulette as I expect thru day 5 that this system will stay east of the shearing upper southwesterly winds that will affect Paulette. My intensity forecast does not go above category 3 for the next five days as this system will remain east of the very warm 28+ deg C waters. Interests in the Republic of Cabo Verde Islands should continue quickly preparing for tropical storm conditions (gusty winds...heavy rains...and coastal sea swells) which will arrive by tonight as this depression strengthens further into a tropical storm in the near term.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 7)...35 mph maximum sustained wind tropical depression centered east of the Republic of Cabo Verde Islands at 15.7N-21.2W
IOH 24 Hr Forecast (1200Z Sep 8)....50 mph maximum sustained wind tropical storm centered over the western Republic of Cabo Verde Islands at 16.2N-24W
IOH 48 Hr Forecast (1200Z Sep 9)....75 mph maximum sustained wind hurricane centered at 16.8N-27W
IOH 72 Hr Forecast (1200Z Sep 10)...100 mph maximum sustained wind hurricane centered at 17.3N-32W
IOH 96 Hr Forecast (1200Z Sep 11)...120 mph maximum sustained wind major hurricane centered at 17.8N-37W
IOH 120 Hr Forecast (1200Z Sep 12)...125 mph maximum sustained wind major hurricane centered at 19N-41.5W
AREA OF INTEREST #1...The tropical wave moving across the central Caribbean Sea has seen an increase in thunderstorms just west of Jamaica and heading toward the Cayman Islands as it interacts with the eastern divergence zone of upper vorticity in the western Caribben Sea. This system will continue west-northwest to northwest toward the southeastern Gulf of Mexico over the next 48 hours while steered around the southwest side of the surface ridge exiting the eastern United States...and then bend more westward in track after that time under the influence of another surface ridge to build behind the current frontal zone over the central United States. A slow down in the westward track is possible by 120 hours once the surface ridge to the north weakens while a sharp fragment of cut-off upper vorticity to be left behind by the current upper trough over central Canada merges with an upper trough from western Canada...with the eastern divergence zone of the merger weakening the surface ridge. I only have odds of development at 10% as this system will remain below the aforementioned upper vorticity which could constrict this system's upper outflow. I do not have odds at 0% despite the unfavorable upper wind outlook as the latent heat release of this system's thunderstorms could waeken the cold core upper vorticity...especially considering this upper vorticity will remain weak as it remains starved of higher latitude cold air while remaining cut-off from mid-latitude cold core upper troughs. I finally drop odds of development to 0% by 120 hours as the forecast track moves this system into land by that point.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Sep 8)...10% chance of tropical cyclone formation (northwestern Caribbean Sea to the west of the Cayman Islands and south of western Cuba near 20N-84W)
IOH 48 Hr Outlook (1200Z Sep 9)...10% chance of tropical cyclone formation (southeastern Gulf of Mexico near 23N-86W)
IOH 72 Hr Outlook (1200Z Sep 10)...10% chance of tropical cyclone formation (south-central Gulf of Mexico near 24N-90W)
IOH 96 Hr Outlook (1200Z Sep 11)...10% chance of tropical cyclone formation (southwestern Gulf of Mexico near 24N-95W)
IOH 120 Hr Outlook (1200Z Sep 12)...0% chance of tropical cyclone formation (just inland over northeastern Mexico near 24N-98W)
AREA OF INTEREST #2...Over the last couple of days...thunderstorms that were sheared southward from what was Tropical Depression Omar persisted in a zone of split flow upper divergence between a western Atlantic upper ridge to the west and western Atlantic upper vorticity to the southeast. This resulted in the formation of a surface trough of low pressure that has gradually organized into a weak surface low pressure spin currently southwesat of Bermuda at 30N-66.5W...and the National Hurricane Center has introduced this surface low pressure into their tropical weather outlook within the last 24 hours. This system is expected to begin moving west-northwest toward the southeastern United States coast under the influence of the surface ridge currently exiting the eastern United States...and toward the low pressure field of a cold front to be left behind by the current central Canadian frontal cyclone and move into the southeastern United States. This track will take this system beneath more favorable low shear and upper outflow beneath the western Atlantic upper ridge axis. However my peak odds of development are only at 20%..lower than the National Hurricane Center's current outlook...as this system will only have a breif window of low shear before potentially dealing with southerly shear from upper vorticity to cut-off from the main upper trough over central Canada and park itself over the southeastern United States. It is also possible this upper vorticity stays weak enough to not really shear this system such that I may have to raise my odds of development later on. By 96 hours I drop the odds of developmetn down to 0% as this system makes landfall over the southeastern United States and becomes absorbed by the aformentioend trailing cold front to be in the region by that timeframe.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Sep 8)...15% chance of tropical cyclone formation (western Atlantic near 30N-69.5W)
IOH 48 Hr Outlook (1200Z Sep 9)...20% chance of tropical cyclone formation (western Atlantic near 31N-74.5W)
IOH 72 Hr Outlook (1200Z Sep 10)...20% chance of tropical cyclone formation (just offshore of South Carolina near 32N-79.5W)
IOH 96 Hr Outlook (1200Z Sep 11)...0% chance of tropical cyclone formation (absorbed by cold front while over eastern South Carolina near 33N-80W)
AREA OF INTEREST #3...Computer models insist that the next tropical wave to emerge from western Africa behind tropical depression eighteen will have a high chance at tropical cyclone formation...and as a result the National Hurricane Center has introduced it into their 5-day tropical weather outlook. The above visible satellite image of western Africa taken from late this morning shows a cloud swirl lacking thunderstorms at the present time....which identifies the current position of this tropical wave to be near 12.5N-0W. Given the models correctly identified the formation of tropical depression eighteen...the current organization of this tropical wave with a swirl already present...and the model consensus on this tropical wave...I agree with the National Hurricane Center's medium 50% odds of development by day 5. If the thunderstorm activity increases and this system maintains a swirl...I would agree in increasing the odds to a high number above 50% in my next update.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Sep 8)...0% chance of tropical cyclone formation (western Africa near 12.5N-5W)
IOH 48 Hr Outlook (1200Z Sep 9)...0% chance of tropical cyclone formation (western Africa near 12.5N-10W)
IOH 72 Hr Outlook (1200Z Sep 10)...0% chance of tropical cyclone formation (west coast of Africa near 12.5N-15W)
IOH 96 Hr Outlook (1200Z Sep 11)...40% chance of tropical cyclone formation (southeast of the Republic of Cabo Verde Islands near 12.5N-20W)
IOH 120 Hr Outlook (1200Z Sep 12)...50% chance of tropical cyclone formation (just south of the Republic of Cabo Verde Islands near 13N-25W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z CMC Model Run...For Tropical Storm Paulette...located at 22N-57.5W at 120 hours. For tropical depression eighteen...passes over the Republic of Cabo Verde Islands by 30 hours...weakens to a tropical wave near 38W at 102 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...surface low opens to a surface trough southwest of Bermuda in 24 hours...absorbed into cold front arriving from central United States by 84 hours as surface trough reaches the North Carolina coast. For area of interest #3...emerges from western Africa near 12N-15W at 84 hours...tropical cyclone formation suggested near 13N-20W at 102 hours...located just southwest of the Republic of Cabo Verde Islands at 120 hours.
0000Z ECMWF Model Run....For Tropical Storm Paulette...located at 23N-57.5W at 120 hours. For tropical depression eighteen...located over the Republic of Cabo Verde Islands at 24 hours...located at 18N-44W at 120 hours as an intense tropical cyclone. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...surface low opens to a surface trough southwest of Bermuda in 24 hours...merges with cold front from central United States at 120 hours while offshore of the southeastern United States. For area of interest #3...emerges from western Africa near 11.5N-15W at 72 hours...tropical cyclone formation suggested near 11.5N-27.5W at 120 hours.
0600Z GFS Model Run...For Tropical Storm Paulette...located at 20.5N-55W at 120 hours. For tropical depression eighteen...located over the Republic of Cabo Verde Islands at 24 hours...Paulette shown to be a large weak tropical cyclone such that Paulette swings eighteen northwestward...eighteen located at 27.5N-42W at 120 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...surface low opens to a surface trough near 30N-70W at 42 hours...merges with cold front from central United States at 90 hours while making landfall on Carolina coast. For area of interest #3...emerges from western Africa near 12.5N-15W at 72 hours...tropical cyclone formation suggested near 14N-27.5W at 120 hours.
0600Z NAVGEM Model Run...For Tropical Storm Paulette...located at 27.5N-55W at 120 hours as a large and intense hurricane. For tropical depression eighteen...passes over the Republic of Cabo Verde Islands in 24 hours...located at 16.5N-37.5W at 120 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...surface low opens to surface trough southwest of Bermuda in 24 hours...merges with cold front from central United States at 96 hours while over the North Carolina coast. For area of interest #3...emerges from western Africa near 14N-17W in 90 hours...located just west of the Republic of Cabo Verde Islands as a weak surface low at 126 hours.