*******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 1:40 AM EDT...
Satellite Image from 0500Z (red indicates tropical cyclone or remnants of tropical cyclone...with P marking Paulette...R marking Rene...and 19 marking tropical depression 19. 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 0000Z:
See Tropical Storm Paulette...Tropical Storm Rene...and tropical depression nineteen sections below for the three currently active tropical cyclones in the Atlantic basin. See area of interest sections below for multiple disturbances being monitored for tropical cyclone formation.
TROPICAL STORM PAULETTE...While in the open central Atlantic...Paulette has been able to restrengthen and reach 65 mph maximum sustained winds (and as of 11 PM EDT 70 mph) as the shearing cold core upper vorticity that spans the Caribbean Sea to the northeastern Atlantic has begun to weaken while cut off from higher latitude cold air...and also possibly from Paulette’s thunderstorm latent heat release. My updated forecast track below is shifted northeast of my previous one 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 72 hours as the surface ridge currently over the Great Lakes region of the United States passes north of Paulette...with the tropical storm rounding the southwest side of that ridge. By 120 hours...the current western United States upper trough will have merged with another upper trough from western Canada...with the merger creating a frontal zone moving into the northwestern Atlantic. The models now agree with my previous assessment that this front will sweep Paulette northward and northeastward...with my forecast continuing to show Paulette’s center passing just east of Bermuda...and the model guidance either showing Paulette passing over or east of Bermuda. The models that show a further west position directly over Bermuda show the aforementioned upper vorticity breaking up into two lobes...with the southwestern lobe coaxing Paulette more westward in the shorter term. 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 to negate the steering effects of the southwestern lobe...explaining my forecast remaining further east. The northeast track may slow by 120 hours as the frontal zone leaves behind Paulette...with a surface ridge behind the front acting as a blocking mechanism for northeastward acceleration (but Paulette will be embedded in upper southwesterly flow that will likely still keep a northeast direction going as Paulette will likely remain strong/tall during the entire forecast period to be steered by upper winds).
I have raised my intensity forecast to show a peak strength of category 2...in general agreement with the National Hurricane Center...as Paulette has already been able to strengthen despite the shearing upper vorticity only beginning to weaken. The fastest intensification rate is shown between 48 to 72 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 pause the strengthening at 96 hours as Paulette may incur some southwesterly shear from the upper trough associated with the northwestern Atlantic frontal zone expected by that time...with weakening shown by 120 hours as the southwesterly upper winds move closer to Paulette.
While there remains some uncertainty in whether or not the center of Paulette passes over or just east of Bermuda...either scenario will bring at least damaging tropical storm force winds...heavy rain...and coastal storm surge by Monday. Therefore I recommend interests here to begin preparing for tropical storm and possible strong hurricane force conditions.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 11)...65 mph maximum sustained wind tropical storm centered at 24.6N-53.7W
IOH 24 Hr Forecast (1800Z Sep 12)...70 mph maximum sustained wind tropical storm centered at 27.5N-57W
IOH 48 Hr Forecast (1800Z Sep 13)...80 mph maximum sustained hurricane centered at 30N-60W
IOH 72 Hr Forecast (1800Z Sep 14)...100 mph maximum sustained wind hurricane centered just east of Bermuda at 32N-63W
IOH 96 Hr Forecast (1800Z Sep 15)...100 mph maximum sustained wind hurricane centered at 36N-61W
IOH 120 Hr Forecast (1800Z Sep 16)...85 mph maximum sustained wind hurricane centered at 39N-58W
TROPICAL STORM 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 sheared tropical storm during the last 48 hours with thunderstorm bursts tending to be west of the center instead of symmetrically distributed around the center...a pattern of light easterly shear that has caused Rene to weaken to 40 mph maximum sustained winds (it’s possible that the shear wafted in dry Saharan air to the east...explaining some of the brief episodes when Rene totally loses thunderstorms). I could understand if Rene was under easterly shear if the tropical storm 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 continues to state in their 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.
My updated forecast track points are nudged northeastward due to the current position of the tropical storm. I forecast Rene to soon curve more northward in track in response to a surface ridge weakness to be induced by Paulette and also a piece of energy from the current eastern Canada upper trough that will soon generate a surface frontal cyclone in the north Atlantic. The steering picture gets a little murky by 48+ hours as the current surface ridge over the Great Lakes region of the United States will be passing north of Rene which will try to push Rene more westward. At the same time the energy that ejects from 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 to eastern North America (to be driven by the currently amplified western United States upper trough and also an upper trough from western Canada merging with it). Since it is probable that Rene will still be tall enough to also be steered by upper-level winds...it is possible that the back western side of the northeast Atlantic upper trough tries to turn Rene more eastward. For now my solution is to have Rene continue its northwest direction from 48 to 72 hours but with a slower forward motion while the surface ridge and upper trough try to move Rene in opposite directions...an effect that will likely shear the storm as well. By 96 and 120 hours I vote Rene will continue northwestward toward the low pressure field of Paulette and around the southwest side of the surface ridge as the northeast Atlantic upper trough moves away.
Regarding intensity...my forecast is significantly lower than my previous given Rene’s failure to intensify. I forecast some possible strengthening in the next 24 hours as Rene has been able to generate bursts of thunderstorms near the center in its current light shear environment. I flatten the intensity by 48 hours and show weakening to a remnant low pressure by day 3 due to likely northerly shear from the northeast Atlantic upper trough mentioend in the previous paragraph. I show possible regenesis into a tropical depression by day 5 as the upper trough and its shear shifts eastward...allowing for the vast northwest Atlantic upper ridge to overspread Rene with lower shear and upper outflow. If Paulette does become a major hurricane...it’s possible it’s low level inflow and upper outflow would dominate over Rene given the close proximity of both storms...so it’s also possible Rene dissipates by day 3 and then never regenerates.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 11)...40 mph maximum sustained wind tropical storm centered at 20.7N-41.1W
IOH 24 Hr Forecast (1800Z Sep 12)...50 mph maximum sustained wind tropical storm centered at 23N-43W
IOH 48 Hr Forecast (1800Z Sep 13)...50 mph maximum sustained wind tropical storm centered at 28N-46W
IOH 72 Hr Forecast (1800Z Sep 14)...Remnant low pressure centered at 30N-47W
IOH 96 Hr Forecast (1800Z Sep 15)...Remnant low pressure centered at 31N-49W
IOH 120 Hr Forecast (1800Z Sep 16)...30 mph maximum sustained wind tropical depression centered at 33N-55W
AREA OF INTEREST #1...The tropical wave of low pressure behind Tropical Storm Rene has only slowly organized as it accelerates west across the eastern tropical Atlantic...due to competition with the nearby wave to the east mentioned in area of interest #2. My updated outlook forecast points are shifted westward as the low pressure spin of the wave is further west near 12N-28W. I continue to lean with a steady west track along 12N latitude as an ongoing fujiwhara interaction with area of interest #2 will initially prevent a northward bend in track...and the surface ridge weakness associated with Paulette and Rene likely to minimally influence the track with Rene remaining a compact and weak system while Paulette will already be lifting northward by 96 and 120 hours. An ever so slight northward bend I have at 120 hours is a reflection of the ridge weakness...which might slightly influence the track of this system especially if Paulette strengthens more than forecast and also expands in size. I still have high 90% odds of tropical cyclone formation given the good structure of the low pressure spin in this wave and forecast track which keeps this system below a tropical upper ridge axis with low shear and upper outflow...but have delayed these high odds for a bit longer given the wave has stopped organizing further over the last day.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 12)...50% chance of tropical cyclone formation (tropical Atlantic near 12N-33W)
IOH 48 Hr Outlook (1800Z Sep 13)...70% chance of tropical cyclone formation (tropical Atlantic near 12N-38W)
IOH 72 Hr Outlook (1800Z Sep 14)...90% chance of tropical cyclone formation (tropical Atlantic near 12N-43W)
IOH 96 Hr Outlook (1800Z Sep 15)...90% chance of tropical cyclone formation (tropical Atlantic near 12N-48W)
IOH 120 Hr Outlook (1800Z Sep 16)...90% chance of tropical cyclone formation (tropical Atlantic near 12.5N-53W)
AREA OF INTEREST #2...A tropical wave of low pressure that was over interior Western Africa has arrived to the west African coast in vicinity of 13N-15W...and has become notably less organized while competing with the better organized wave in area of interest #1. My outlook forecast points are adjusted westward due to the current position of this wave. A northward angle in track is shown thru 48 hours due to the ongoing fujiwhara interaction with area of interest #1 and also as the eastern divergence zone of a northeast Atlantic upper trough potentially weakens the east side of the steering surface subtropical ridge in the Atlantic (this upper trough is forecast to originate from energy to eject from the current eastern Canada upper trough...see Rene section above for details). I also slow the forward speed a bit between 24 and 48 hours due to the surface ridge weakness. After 48 hours....the upper trough shifts east which allows the Atlantic surface subtropical ridge to recover...so I bend the track on a more westward angle for the middle and end of the 5 day forecast period. I have lowered my 5-day peak odds of development to 15% due to the decreased organization of this wave...and as the south side of the northeast Atlantic upper trough may impart some disrupting westerly shear in the middle part of the forecast period. The shear will also persist later in the forecast as a chunk of the northeast Atlantic upper trough breaks westward and stays in sync with this wave under the influence of the northwest Atlantic upper ridge.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 12)...0% chance of tropical cyclone formation (east of the Republic of Cabo Verde Islands near 14N-20W)
IOH 48 Hr Outlook (1800Z Sep 13)...5% chance of tropical cyclone formation (over the Republic of Cabo Verde Islands at 15N-24W)
IOH 72 Hr Outlook (1800Z Sep 14)...10% chance of tropical cyclone formation (tropical Atlantic near 15.5N-29W)
IOH 96 Hr Outlook (1800Z Sep 15)...15% chance of tropical cyclone formation (tropical Atlantic near 16N-34W)
IOH 120 Hr Outlook (1800Z Sep 16)...15% chance of tropical cyclone formation (tropical Atlantic near 16.5N-39W)
AREA OF INTEREST #3 (RECENTLY UPGRADED TO TROPICAL DEPRESSION NINETEEN)...The tropical low pressure crossing the western Bahamas has strengthened into tropical depression nineteen while now nearing south Florida. Computer models have agreed that the other low pressure to the west...area of interest #4...will increasingly curve the track of this system toward the Florida panhandle/Alabama/Mississippi coastal region after crossing south Florida tonight and tomorrow. This northward turn is also anticipated from a surface ridge weakness to be created by the eastern divergence zone of the upper troughs over the western US and western Canada as the troughs merge. Thus my forecast track is adjusted northward in lieu of the model consensus. After 48 hours the track should bend west toward southern Louisiana as the surface ridge weakness passes. The troubling thing is the models 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...this system could become tall enough to also be slowed by the upper westerly winds of the cut off upper trough. A slight bend to the north is shown by 120 hours when a shortwave upper trough over southern Canada creates more of a ridge weakness and near the US/Canada border region. The slow track could mean a significant rainfall flooding problem across southern Louisiana and southern Mississippi Monday through Wednesday if it verifies.
The depression has been under westerly shear induced by upper vorticity near the Carolina coast which has kept thunderstorm bands to the east of the center. Going forward the upper-level wind outlook for this system will improve as follows. The north-south axis of upper vorticity currently off to the west in the Gulf of Mexico is expected to tilt southwest to northeast...with the northeastern part shifting east toward the aforementioned vorticity at the Carolina coast...and the southwestern part retrograding southwestward and away under the influence of warm upper ridging that has built in the warm sector of the unusual snowstorm that affected Colorado and Wyoming a few days ago (snowstorm that was supported by the sharp upper trough currently over the western United States). As the Gulf upper vorticity tilts and then breaks into two parts that move away from each other...this will leave a favorable low shear and upper outflow environment in between and directly over this system as it traverses the Gulf of Mexico warm waters...so I forecast strengthening to a strong tropical storm by 48 hours. It would not surprise me if this system became a minimal hurricane in between the 48 and 72 hour forecast point before landfall with the Louisiana coastline. I show weakening from land interaction...but the rate of weakening is kept slow with the center remaining at the coast rather than fully inland...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 start 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 11)...35 mph maximum sustained wind tropical depression centered just offshore of southeast Florida at 25.4N-79W
IOH 24 Hr Forecast (1800Z Sep 12)...45 mph maximum sustained wind tropical storm centered just west of the Florida peninsula at 26.5N-82.5W
IOH 48 Hr Forecast (1800Z Sep 13)...60 mph maximum sustained wind tropical storm centered offshore of the Florida panhandle at 28N-86W
IOH 72 Hr Forecast (1800Z Sep 14)...65 mph maximum sustained wind tropical storm centered over southeastern Louisiana at 28.5N-90W
IOH 96 Hr Forecast (1800Z Sep 15)...50 mph maximum sustained wind tropical storm centered over southern coastal Louisiana at 28.5N-91.5W
IOH 120 Hr Forecast (1800Z Sep 16)...45 mph maximum sustained wind tropical storm centered over coastal southwestern Louisiana at 29N-93W
AREA OF INTEREST #4...The surface trough of low pressure in the Gulf of Mexico currently near 27.5N-87.5W 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
depression nineteen to dominate over this surface trough terms of low-level inflow in the region. Instead of a typical 5W longitude per day westward track around the surface ridge over the Great Lakes...I forecast only an increasingly slowing west-southwest track while it undergoes a fujiwhara type interaction with the depression and gets pulled toward its increasing low-level inflow as it strengthens...followed by absoprtion into the depression in 24 hours. Therefore at this time...I am forecasting a 0% chance of tropical cyclone formation from this surface trough.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 12)...0% chance of tropical cyclone formation (absorbed by tropical depression nineteen while located in the Gulf of Mexico near 26.5N-89W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...For Tropical Storm Paulette...passes over Bermuda as a strong hurricane in 78 hours...located at 39N-52.5W at 120 hours. For Tropical Storm Rene...located at 23N-52.5W as a remnant low pressure at 120 hours. For area of interest #1...moves northward while absorbing area of interest #2...located at 25N-35W at 120 hours as a tropical cyclone. For area of interest #2...no development shown while absorbed by area of interest #1. For area of interest #3...possible compact tropical cyclone formation along Alabama coast at 78 hours...stays stationary through 120 hours. For area of interest #4...no development shown while slowing drifting southwest into the southwestern Gulf of Mexico through 120 hours.
1200Z ECMWF Model Run....For Tropical Storm Paulette...passes over Bermuda as a strong hurricane in 72 hours...located at 37.5N-57.5W at 120 hours. For Tropical Storm Rene...located at 25N-52.5W as a remnant trough at 120 hours. For area of interest #1...located at 13.5N-50W at 120 hours as an intense tropical cyclone. For area of interest #2...located at 22N-37W as a tropical cyclone. For area of interest #3...possible compact tropical cyclone formation just offshore of the Florida panhandle at 72 hours...moves slowly west into southeastern Louisiana by 120 hours. For area of interest #4...no development shown while slowing drifting southwest into the southwestern Gulf of Mexico through 120 hours.
1200Z GFS Model Run... For Tropical Storm Paulette...passes over Bermuda as a strong hurricane in 84 hours...located at 37.5N-59W at 120 hours. For Tropical Storm Rene...located at 27.5N-52.5W as a remnant low pressure at 120 hours. For area of interest #1...located at 15N-52W at 120 hours as an intense tropical cyclone. For area of interest #2...located at 24N-32.5W as a compact tropical cyclone at 120 hours. For area of interest #3..moves west-northwest into southern Louisiana by 120 hours without tropical cyclone formation. For area of interest #4...no development shown while slowing drifting southwest into the southwestern Gulf of Mexico through 120 hours.
1200Z NAVGEM Model Run...For Tropical Storm Paulette...passes just east of Bermuda as a strong hurricane in 84 hours...located at 37.5N-55W at 120 hours. For Tropical Storm Rene...absorbed into southeast side of Paulette at 108 hours. For area of interest #1...no tropical cyclone formation. For area of interest #2...located at 22.5N-33W as a weak broad tropical low pressure. For area of interest #3...possible tropical cyclone formation just offshore of Florida panhandle at 54 hours...moves slowly west-northwest into coastal Mississippi by 120 hours. For area of interest #4...no development shown while slowing drifting southwest into the southwestern Gulf of Mexico through 120 hours.