*******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 SEPTEMBER 12 2020 11:45 PM EDT...
Satellite Image from 0300Z (red indicates tropical cyclone or remnants of tropical cyclone...with P marking Paulette...R marking Rene...S marking Sally...and 20 marking tropical depression twenty. 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 Paulette...Tropical Depression Rene...Tropical Storm Sally...and tropical depression twenty sections below for the four currently active tropical cyclones in the Atlantic basin. See area of interest sections below for multiple disturbances being monitored for tropical cyclone formation. Elsewhere...another tropical wave of low pressure currently over interior Western Africa may need to be added as another area of interest for tropical development....especially if it stays below favorable upper ridging to persist south of an upper trough regime in the northeast Atlantic.
Also note that Paulette was just upgraded to a minimal hurricane with 75 mph maximum sustained winds as this post was being published...therefore this is why Paulette is referred to as a tropical storm in the text above and below. The next full post I am scheduling for Monday morning. However on Sunday I will likely have special updates on Paulette and Sally as those storms are threatening land areas.
TROPICAL STORM PAULETTE...While in the open central Atlantic...Paulette has been able to slowly strengthen and reach 70 mph maximum sustained winds as the shearing cold core upper vorticity that spans the western and northeastern Atlantic has continued to weaken while cut off from higher latitude cold air...and also possibly from Paulette’s thunderstorm latent heat release. My updated forecast track points below are shifted northwest of my previous forecast due to Paulette’s current position. The initial part of the forecast shows Paulette continuing northwestward under the combined influences of the Atlantic surface subtropical ridge to the north trying to push the storm westward and the upper southwesterly shearing winds trying to pull the storm northward. A westward component is kept in the track through 48 hours as the surface ridge currently over the northeastern United States passes north of Paulette...with Paulette rounding the southwest side of that ridge. After 48 hours...the current western United States upper trough will have merged with the current one over western Canada...with the merger creating a frontal zone moving into the northwestern Atlantic. This front will sweep Paulette northward and northeastward...with my northwest adjusted forecast showing Paulette’s center passing very close to and just east of Bermuda. It is possible the center moves a bit further west and directly over Bermuda as the aforementioned upper vorticity breaks into two lobes...with the southwestern lobe coaxing Paulette more westward. However I am not yet convinced about the southwestern lobe of the upper vorticity having weight as the remaining northeastern lobe may also be near enough...and also reenforced by the upper trough exiting eastern Canada...to negate the steering effects of the southwestern lobe...explaining my forecast leaning further east. I show a faster northeast acceleration at 72+ hours compared to my previous forecast as the 1200Z GFS shows a south fracture of the incoming upper trough moving closer to Paulette which will increase the southwesterly upper steering flow.
I continue to forecast a peak strength of category 2...but not quiet as strong as the NHC forecast...as Paulette has not become much better organized thus far. The fastest intensification rate is shown between 24 to 48 hours when Paulette moves into the gap between splitting halves of the upper vorticity. Paulette could strengthen more than forecast and into a major hurricane (category 3+) depending on the size of the gap...with a smaller gap allowing the upper vorticity to interefere with Paulette's outflow and dampening the intensification...and a larger gap allowing for enhanced outflow channels into both halves of the upper vorticity and thus a stronger storm. I show weakening occurring sooner...now as early as September 15...as the south fracture of the incoming upper trough may move closer to Paulette and not only accelerate the storm faster as mentioned in the prior paragraph...but also increase the shear across the storm. By 120 hours Paulette should reach cooler waters...but slowly weaken as it transitions into a strong non-tropical remnant low pressure supported by divergence on the east side of the south fracture upper trough and northwest side of the northwest Atlantic upper ridge....which will have shifted east into the North Atlantic ahead of the upper trough.
Interests in Bermuda should continue preparing for damaging high winds...coastal storm surge...and heavy rain as it appears likely the center will pass over or very close to the island on Monday as a category 2 hurricane.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position Forecast (1800Z Sep 12)...70 mph maximum sustained wind tropical storm centered at 28.4N-58.5W
IOH 24 Hr Forecast (1800Z Sep 13)...80 mph maximum sustained hurricane centered southeast of Bermuda at 31N-61W
IOH 48 Hr Forecast (1800Z Sep 14)...100 mph maximum sustained wind hurricane centered just northeast of Bermuda at 33N-64W
IOH 72 Hr Forecast (1800Z Sep 15)...85 mph maximum sustained wind hurricane centered at 37N-60W
IOH 96 Hr Forecast (1800Z Sep 16)...75 mph maximum sustained wind hurricane centered at 40N-54W
IOH 120 Hr Forecast (1800Z Sep 17)...70 mph maximum sustained wind non-tropical remnant gale centered at 42N-45W
TROPICAL DEPRESSION RENE...Despite being below a vast tropical upper ridge axis at the 200 mb upper layer of the atmosphere... Rene has had the appearance of a struggling tropical cyclone during the last 3 days with thunderstorm bursts that do not last and at times were leaned to the west of the center...a pattern of light easterly shear that has caused Rene to weaken to once again weaken to a tropical depression. It is possible that the shear wafted in dry Saharan air to the east...explaining the recent lack of thunderstorms. I could understand if Rene was under easterly shear if it was south of the 200 mb upper ridge axis instead of directly under it while also moving slower to the west than it currently has been...which would allow the easterly winds on the south side of the upper ridge axis to rake the thunderstorms westward from the center...but this is not the case. The National Huricane Center has stated in multiple Rene advisory discussion packages that there is a mid-level easterly jet "undercutting an otherwise favorable outflow layer"...presuming that outflow layer refers to the outflow being generated by the 200 mb tropical upper ridge axis. Perhaps then the mid-level African Easterly Jet that generates surface African tropical waves of low pressure...located below 200 mb and above the surface...is what has been shearing Rene.
For the next 24 hours my updated forecast track points are nudged northeastward due to the current position of Rene. The depression has curved more northward in track in response to a surface ridge weakness induced by Paulette and also from the current upper trough exiting eastern Canada whose eastern divergence is generating a surface frontal cyclone in the north Atlantic. By 48+ hours the current surface ridge over the northeastern United States will be passing north of Rene which will try to push Rene westward. At the same time the eastern Canada upper trough is forecast to amplify in the northeast Atlantic in response to the northwestern Atlantic upper ridge amplifying in the warm sector of a frontal zone arriving the northwest Atlantic (to be driven by the currently amplified western United States upper trough and also an upper trough from western Canada merging with it). With the tropical cyclone now much weaker and shallower...I have adjusted the 48 hour forecast track westward around the surface ridge to pass to the north...with the northwesterly upper winds on the back west side of the northeast Atlantic upper trough to shear Rene more strongly and deal a death blow to the system. As the upper trough moves away and the northwest Atlantic upper ridge moves eastward closer to Rene the shear will relax...but I am not continuing to forecast beyond 48 hours till we see how Rene deals with the impeding northwesterly shear burst first.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 12)...35 mph maximum sustained wind tropical depression centered at 24.3N-45.6W
IOH 24 Hr Forecast (1800Z Sep 13)...35 mph maximum sustained wind tropical depression centered at 29N-47
IOH 48 Hr Forecast (1800Z Sep 14)...Remnant low pressure centered at 29N-51W
TROPICAL DEPRESSION TWENTY...The tropical wave of low pressure southeast of Tropical Depression Rene has organized into the twentieth tropical depression of this busy 2020 Atlantic Hurricane Season while continuing west across the eastern tropical Atlantic. In the short term...I continue to lean with a steady west track along 12N latitude as an ongoing fujiwhara interaction with the tropical wave in area of interest #1 could initially prevent a northward bend in track (I also show a slightly slower speed than before thru the next 72 hours due to the interaction which has become stronger as the area of interest has moved closer than I previously anticipated)...and the surface ridge weakness associated with Paulette and Rene is likely to minimally influence the track of this system as Rene remains a compact and weak system while Paulette will soon be lifting northward. I forecast only gradual development initially as the depression remains broad and will need time to consolidate...and it is possible there maybe competition with area of interest #1.
For later in the forecast period...model guidance has shifted northward with the forecast track. This is in response to how strong the latest guidance makes the northwest Atlantic upper ridge which will soon move into the North Atlantic...with the ridge sending a chunk of upper troughing in the northeast Atlantic southwestward to this system. And assuming this system strengthens into a decent and tall tropical storm...it will be bent northward by the approaching upper troughing. I have not yet bent my forecast track as sharply north in case the model guidance chafes again and in case the fujiwhara interaction with area of interest #1 keeps this system further south. But I have flattened the intensity forecast as the upper troughing could negatively shear this system...but I do not show weakening either as the 1200Z GFS had the upper troughing on the weaker side.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 12)...35 mph maximum sustained wind tropical depression centered at 11.4N-33.5W
IOH 24 Hr Forecast (1800Z Sep 13)...40 mph maximum sustained wind tropical storm centered at 12N-38W
IOH 48 Hr Forecast (1800Z Sep 14)...45 mph maximum sustained wind tropical storm centered at 12N-42W
IOH 72 Hr Forecast (1800Z Sep 15)...60 mph maximum sustained wind tropical storm centered at 12N-46W
IOH 96 Hr Forecast (1800Z Sep 16)...70 mph maximum sustained wind tropical storm centered at 14N-48.5W
IOH 120 Hr Forecast (1800Z Sep 17)...70 mph maximum sustained wind tropical storm centered at 16N-51W
AREA OF INTEREST #1...A tropical wave of low pressure that was over the west African coast has briskly moved west northwest faster than I anticipated...toward the strengthening low level inflow on the north side of tropical depression twenty as it undergoes a fujiwhara interaction with the depression. This has taken the wave already across the Republic of Cabo Verde Islands...and the wave has also become much better organized while now spinning up just west of the northern islands near 16.5N-26W. I therefore sharply increase short term odds of tropical cyclone formation to 75%. The outlook below after 24 hours is written as if this system is shallow and still not a tropical cyclone...pulled briskly west-northwest by the surface inflow into the northeast side of depression twenty. As such I sharply drop the odds of development as this track will take this wave into shearing upper westerlies on the south side of northeast Atlantic upper troughing and into possible absorption into depression twenty by 72 hours. But if the wave goes on to develop in the next 24 hours...the 48 and 72 hour positions will need to be adjusted northeastward as it would become tall enough to be pulled by the troughing...albeit it would likely be a weak tropical cyclone in this scenario as the troughing still shears this system.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 13)...75% chance of tropical cyclone formation (tropical Atlantic near 17.5N-31W)
IOH 48 Hr Outlook (1800Z Sep 14)...10% chance of tropical cyclone formation (tropical Atlantic near 18.8N-36W)
IOH 72 Hr Outlook (1800Z Sep 15)...0% chance of tropical cyclone formation (tropical Atlantic near 19.5N-41W)
TROPICAL STORM SALLY...Depression nineteen strengthened into Tropical Storm Sally while the center began departing the southwest Florida peninsula coast earlier today. The low pressure to the west...area of interest #2...will increasingly curve the track of Sally northward toward the Florida panhandle/Alabama/Mississippi coastal region. This northward bend is also anticipated from a surface ridge weakness/frontal zone created by the eastern divergence zone of the upper troughs over the western US and western Canada as the troughs merge. After 24 hours the track should bend west toward southern Louisiana as the surface ridge weakness passes. The troubling thing is the models still show a slowing westward track...apparently from a south fracture of the western US upper trough expected to cut off while becoming stretched east to west across the southern US...with the divergence zone of this feature inducing a faint surface ridge weakness to the northeast that could slow the track. Assuming strengthening occurs...Sally will also become tall enough to also be slowed by the upper westerly winds of the cut off upper trough. A slow turn to the north is shown by 96 and 120 hours when a shortwave upper trough over southern Canada creates more of a ridge weakness and near the US/Canada border region...but with this weakness will also be weak and pass quickly northeast of Sally such that the track is slow. The slow track could mean a significant rainfall flooding problem across southern Louisiana and southern Mississippi Monday through Wednesday...maybe even longer if the circulation still pulls in Gulf moisture as it moves more inland....if it verifies. The slow track shown below may have to be adjusted more eastward in agreement with the latest NHC forecast and some of the modeling as those models suggest the upper westerly flow may actually bend the track more eastward earlier should Sally be taller/stronger. As such this may involve the southern Alabama and Florida panhandle region in the rainfall flood problem.
Sally has been under light westerly shear induced by upper vorticity to the north which has kept thunderstorm bands to the east of the center. Going forward the upper-level wind outlook for this system will improve as Sally moves away from this upper vorticity...and as the upper vorticity in the western Gulf of Mexico continues to shift westward out of the way such that shear in the Gulf of Mexico will remain light as Sally moves further into the Gulf. So I forecast strengthening to a strong tropical storm by 24 hours. It would not surprise me if Sally became a minimal hurricane in between the 24 and 48 hour forecast point before landfall with the Louisiana coastline. I then show weakening from land interaction...but the rate of weakening is kept slow with the center remaining at the coast rather than fully inland thru 96 hours on my forecast track...with some of the weakening also possibly being caused by some light westerly shear from the forecast southern US cut off upper trough mentioned in the previous paragraph.
Given the current outlook on this system...interests across the coastal regions of the Florida panhandle... Alabama... Mississippi.... and southeastern Louisiana should continue making preparations for possible strong tropical storm conditions (coastal storm surge....gusty winds with some damage possible...and heavy rain). Hurricane conditions maybe possible along coastal southeast Louisiana should this system indeed become a minimal hurricane.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 12)...40 mph maximum sustained wind tropical storm centered just west of the Florida peninsula at 26N-82.5W
IOH 24 Hr Forecast (1800Z Sep 13)...60 mph maximum sustained wind tropical storm centered offshore of the Florida panhandle at 28N-86W
IOH 48 Hr Forecast (1800Z Sep 14)...65 mph maximum sustained wind tropical storm centered over southeastern Louisiana at 28.5N-90W
IOH 72 Hr Forecast (1800Z Sep 15)...50 mph maximum sustained wind tropical storm centered over southern coastal Louisiana at 28.5N-91.5W
IOH 96 Hr Forecast (1800Z Sep 16)...45 mph maximum sustained wind tropical storm centered over coastal southwestern Louisiana at 29N-93W
IOH 120 Hr Forecast (1800Z Sep 17)...Remnant low pressure over Louisiana at 30.5N-92.5W)
AREA OF INTEREST #2...The surface trough of low pressure in the Gulf of Mexico currently near 27N-92.5W remains as an area of interest on this blog as it remains a feature in the NHC tropical weather outlook. Due to the lack of thunderstorms with this surface trough...I forecast Tropical Storm Sally to dominate over this surface trough terms of low-level inflow in the region. A southwest drift is forecast to be caused by the west side of Sally’s surface circulation...and this time I am forecasting a 0% chance of tropical cyclone formation from this surface trough as Sally should continue prevent the accumulation of inflow and thunderstorms with this surface trough as it becomes the dominant system in the region. I plan this to be my final statement on this disturbance unless it remains a feature in the NHC tropical weather outlook.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 13)...0% chance of tropical cyclone formation (Gulf of Mexico near 26N-93W)
...COMPUTER MODEL SUMMARY...
Due to the currently high amount of activity in the Atlantic basin...this section is temporarily halted to shorten the time needed to put this post together.