MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #122
*******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.**********
...WEDNESDAY SEPTEMBER 16 2020 10:07 PM EDT...
See remnants of Paulette...Hurricane Sally...Hurricane Teddy...and Tropical Storm Vicky for an update on all current named systems in the Atlantic basin. See area of interest sections below for multiple disturbances being monitored for tropical or subtropical cyclone formation. Note that the Paulette...Teddy...and Vicky forecasts were completed earlier in the afternoon and thus have 1200Z timestamps instead of 1800Z...and there have been no changes to these systems tonight for me to change my forecasts from the 1200Z versions.
REMNANTS OF PAULETTE...Hurricane Paulette has transitioned into a hurricane force frontal cyclone supported by upper divergence on the east side of an upper trough that exited eastern North America. The arrival of the front’s upper trough and cooler North Atlantic waters are repsponsible for the transition to non tropical status.
Expect Paulette to retain hurricane force winds as a frontal cyclone through 24 hours while aligning with the robust upper divergence maximum of the incoming upper trough. After that time the upper trough should amplify into a cold core cut-off upper vortex due to the cold surface northerly flow on the west side of Paulette...and also due to amplification of adjacent upper ridging located upstream (to the west) in the warm sector of another frontal system developing over Canada. This will cause Paulette's remnant to whirl directly beneath the cut-off upper vortex center where there is a lack of divergence...thus where it begins to weaken as a classical post-mature non-tropical cyclone. During this weakening phase all models are in agreement that the remnant of Paulette and upper vortex will swing southward to the west of the Azores in the northerly flow to set up between the amplifying upper ridging to the west and northeast Atlantic upper trough to the east. This will take the remnant of Paulette back toward increasingly warmer waters...but at this time I do not anticipate regenesis into a subtropical cyclone through 72 hours as the remnant will still be over waters below 26 deg C and the upper vortex in the 0600Z GFS model run measures too warm (around 1215 to 1220 dekameters in height at 200 mb instead of a colder 1200) to aid in generation of instability of thunderstorms at those water temperatures. By 96 hours the southward track approaches the 26 deg C sea surface temperature isotherm...but I am not very confident at this time in calling for Paulette to regenerate as the upper vortex could become stretched southwest-to-northeast by the upper ridging to the northwest such that there is a possiblity that Paulette's remnant gets caught in unfavorable upper convergence between the northwest side of the upper vortex and southeast of the upper ridging.
Given this outlook however...I am continuing to forecast Paulette through 5 days even though Paulette will not be tropical for much of that time. This is because I believe the odds that Paulette regenerates as a subtropical cyclone by days 4 and 5 are not zero either...so the forecast points after 24 hours for now are only serving as preparation for me to begin probability outlooks on the remnant of Paulette in my next blog post.
0 Hr Position (1200Z Sep 16)...85 mph maximum sustained wind non-tropical remnant gale centered at 43.3N-45.2W
IOH 24 Hr Forecast (1200Z Sep 17)...Hurricane force non-tropical remnant gale centered at 45N-35W
IOH 48 Hr Forecast (1200Z Sep 18)...Non-tropical remnant gale centered at 42N-34W
IOH 72 Hr Forecast (1200Z Sep 19)...Non-tropical remnant gale centered at 37.5N-35W
IOH 96 Hr Forecast (1200Z Sep 20)...Non-tropical remnant gale centered at 34N-35W
IOH 120 Hr Forecast (1200Z Sep 21)...Non-tropical remnant gale centered at 32.5N-33W
HURRICANE TEDDY...Teddy has continued to intensify while in a favorable low shear and upper outflow environment beneath a tropical upper ridge axis....and rapidly as it is already a category 2 hurricane as of this morning. My updated forecast track is nudged northward once again due to the current position of Teddy...as a northward bend in the track (slightly sharper than I previously anticipated) has begun in response to how strong the north Atlantic upper ridge is....with the ridge sending a chunk of the upper trough in the northeast Atlantic southwestward to Teddy. Because Teddy is a strong/tall hurricane...it has bent northward in track while dragged by the approaching upper trough. I forecast a little weakening in the next 24 hours as the approaching upper trough potentially blocks the western outflow of Teddy and maybe even shears Teddy...and I think this disruption will be too much for Teddy to hold category 2 status. After 24 hours I ramp up the intensity as the upper trough dissipates...and as Teddy approaches the western Atlantic upper vorticity currently north of the Caribbean Islands which will also be gradually fading. I forecast category 4 intensity by 72 hours due to the unanimous model consensus that Teddy will be a powerful hurricane by that time...and as Teddy will be in a low shear and upper outflow environment to the northeast of the western Atlatnic upper vorticity. This upper vorticity will keep Teddy moving northwest through 72 hours...albeit I bend the track more westward around 96 and especially by 120 hours as Teddy rounds the north side of the upper vorticity. On a final note regarding intensity at days 3 to 5 (72 to 120 hours)...with Teddy remaining undisturbed by shear while over warm 28 to 29 deg C waters....I forecast a mid-range category 4 hurricane for the entire three day duration...but it is impossible to know whether or not and when the intensity will fluctuate up and down from my forecast intensity due to structural events such as eye wall replacement cycles.
One concern with Teddy is whether or not it will affect Bermuda not long after Paulette already struck the island. Because Teddy will likely be a powerful hurricane tall enough to be coupled to the upper-level steering flow...at some point Teddy will be recurved northward in track due to the approach of a large upper trough which will be a merger between an upper trough currently moving across Canada and the cut-off upper trough over the southern United States. Unfortunately the long range mode runs have shifted westward closer to Bermuda as the large upper trough is now shown to split into two halves...with the north half passing too far from Teddy to recurve the hurricane and the southern cut off half not approaching Teddy till later on which delays the northward turn. Based on the upper trough evolution shown in the 0600Z GFS...I see that Teddy could pass southwest of Bermuda. What is clear is that Bermuda will see at least coastal sea swells by Sunday and Monday as Teddy passes nearby. With Teddy possibly passing west of Bermuda and being a much stronger hurricane than Paulette...this opens the door for the United States east coast to experience sea swells and rip currents as well by early next week.
0 Hr Position (1200Z Sep 16)...100 mph maximum sustained wind hurricane centered at 16.5N-49.7W
IOH 24 Hr Forecast (1200Z Sep 17)...95 mph maximum sustained wind hurricane centered at 18.5N-52W
IOH 48 Hr Forecast (1200Z Sep 18)...120 mph maximum sustained wind major hurricane centered at 21N-54W
IOH 72 Hr Forecast (1200Z Sep 19)...145 mph maximum sustained wind major hurricane centered at 24N-58W
IOH 96 Hr Forecast (1200Z Sep 20)...145 mph maximum sustained wind major hurricane centered southeast of Bermuda at 28N-62.5W
IOH 120 Hr Forecast (1200Z Sep 21)...145 mph maximum sustained wind major hurricane centered southwest of Bermuda at 29N-66.5W
TROPICAL STORM VICKY...While a weak and shallow tropical storm...Vicky appears to have been turned directly westward while bumping into the northeast Atlantic surface ridge...and my track forecast below shows this current motion continuing for the next 24 hours. This direction of travel is more opposed to the upper westerly shearing winds produced by upper vorticity cut off from the northeast Atlantic upper trough...thus increasing the shear further. The shear has also increased as this upper vorticity is moving closer to Vicky. The shear increase has weakened Vicky to a minimal tropical storm...and as the forecast track takes Vicky closer to the shearing upper vorticity I forecast dissipation to a remnant low pressure at 24 hours.
0 Hr Position (1200Z Sep 16)...40 mph maximum sustained wind tropical storm centered at 21.5N-34.7W
IOH 24 Hr Forecast (1200Z Sep 17)...Remnant low pressure centered at 21.5N-38W
Satellite Image of Hurricane Sally making a landfall as a category 2 hurricane earlier today at 1011Z:
TROPICAL STORM SALLY...The surface ridge to the north of Hurricane Sally finally weakened enough from the divergence zones of the upper troughs over the southern US and Canada such that Sally’s center was finally able to move northward into the Alabama and Florida Panhandle coasts this morning. Also the approach of the southern US upper trough which gradually increased the southwesterly upper steering flow has aided in Sally’s now northeastward motion. Before landfall Sally intensified at the last minute to a category 2 with 105 mph maximum sustained winds. I previously anticipated that Sally would not regain category 2 status due to light westerly shear from the aforementioned upper southwesterlies and because its previous very slow motion was likely upwelling cooler waters. Perhaps then Sally has not upwelled cooler waters and the core had completed an eye replacement cycle that allowed for strengthening...as an eye became better defined than it ever has with Sally on satellite imagery and also on Doppler Radar.
Now that landfall has occurred Sally has weakened to a tropical storm...and there is evidence that southwesterly shear from the southern US upper trough is also weakening the storm as the heaviest thunderstorms are lopsided to the northeast of the center. Once Sally weakens to a remnant low further inland over the southeastern United States...it will become entangled with a surface frontal zone to be generated in the eastern divergence zone of the the merger between the Canadian upper trough and southern US cut-off upper trough. Models simply vary on whether or not Sally's remnant retains its identity along this front as it moves into the western Alantic.
Wind damage in the western Florida panhandle and southern Alabama will remain a concern until Sally weakens below tropical storm force. Rainfall flooding has also been a problem due to Sally’s slow motion...with the threat shifting eastward as Sally has tracked further east of previous forecasts. Flash flood watches and warnings are in effect across northern and western Georgia...southeast Alabama...the Florida panhandle...much of the Carolinas...and southeast Virginia for Sally and what is forecast to be its remnant frontal low pressure.
0 Hr Position (1800Z Sep 16)...60 mph maximum sustained wind tropical storm centered over southern Alabama at 31.2N-86.8W
IOH 24 Hr Forecast (1800Z Sep 17)...Remnant low pressure centered over northern Georgia at 33.5N-83.5W
AREA OF INTEREST #1...The surface low pressure spin in the southwestern Gulf of Mexico has drifted further south to 22N latitude while pushed by a fujiwhara interaction with Tropical Storm Sally to the northeast. Intermittently the thunderstorm activity has show signs of organization followed by a collapse and disorganization...likely from the competition for low level inflow with Sally. The National Hurricane Center surface analysis has the center at 22N-94W...but based on the latest satellite imagery which shows an increasingly organizing spin further west I estimate the center of circulation is currently at 22N-95.5W. This has caused me to shift my forecast points southwestward for the updated outlook shown below. This surface low pressure should initially drift northeastward toward Sally’s low pressure field. Then a surface ridge to then build under the western convergence zone of an upper trough swinging across Canada should then turn this system westward by days 2 and 3. Given the slowly improving organization of this system...I have nudged my peak 5-day odds of tropical cyclone formation upward to 70%. At 72 to 96 hours I dip the odds downward to 50% as the cut-off upper trough over the southern US will slowly pass just to the north and increase wind shear. I raise the odds back up to 60% by 120 hours as the upper trough begins to pass which will reduce the shear. On a final note...because this system is more likely to become a tropical cyclone coupled to upper level steering...I have adjusted the long term forecast points to be much slower to the west while dragged by the upper westerly flow of the passing upper trough...and as the eastern divergence zone of the upper trough potentially creates a pocket of low pressure to the northeast that slows the track of this system around the surface ridge (also the western convergence zone of the upper trough will extend the surface ridge southwestward...with the southwestern lobe of the ridge also blocking west progress)
IOH 24 Hr Outlook (1800Z Sep 17)...70% chance of tropical cyclone formation (southwestern Gulf of Mexico near 22N-94W)
IOH 48 Hr Outlook (1800Z Sep 18)...70% chance of tropical cyclone formation (western Gulf of Mexico near 24N-94W)
IOH 72 Hr Outlook (1800Z Sep 19)...50% chance of tropical cyclone formation (western Gulf of Mexico near 24N-95W)
IOH 96 Hr Outlook (1800Z Sep 20)...50% chance of tropical cyclone formation (western Gulf of Mexico near 24N-95.5W)
IOH 120 Hr Outlook (1800Z Sep 21)...60% chance of tropical cyclone formation (western Gulf of Mexico near 24N-95.5W)
AREA OF INTEREST #2...The tropical wave of low pressure in the eastern tropical Atlantic has continued to become slowly better organized while in a low shear and upper outflow environment beneath a tropical upper ridge axis and to the south of the northeast Atlantic upper trough. Therefore I have raised the odds of tropical cyclone formation to an 80% peak for the next five days. The high odds are for the short term...with odds tapering down by 3 to 5 days as some of the northeast Atlantic upper vorticity retrogrades southwest toward this tropical wave and potentially shears it. Also this upper vorticity may bend the longer term track of the wave north as shown in the outlook below...especially if it becomes a strong enough tropical cyclone that couples with upper level steering flow.
IOH 24 Hr Outlook (1800Z Sep 17)...70% chance of tropical cyclone formation (tropical Atlantic near 10N-28W)
IOH 48 Hr Outlook (1800Z Sep 18)...80% chance of tropical cyclone formation (tropical Atlantic near 10N-33W)
IOH 72 Hr Outlook (1800Z Sep 19)...60% chance of tropical cyclone formation (tropical Atlantic near 11N-39W)
IOH 96 Hr Outlook (1800Z Sep 20)...40% chance of tropical cyclone formation (tropical Atlantic near 13N-42W)
IOH 120 Hr Outlook (1800Z Sep 21)...40% chance of tropical cyclone formation (tropical Atlantic near 15N-45W)
AREA OF INTEREST #3...The northern portion of the upper trough in the northeastern Atlantic has amplified into a quiet cold cut-off upper vortex...courtesy of southward cold air tranpsort on the west side of the northeastern Atlantic surface cyclone its supporting with its eastern divegence...and due to adjacent amplification of high latitude upper ridging in the warm sector of a strong frontal cyclone southwest of Greenland and supported by the current upper trough that recently exited eastern North America. Despite being centered at 39N-19W and over 22 deg C waters...pockets of showers and thunderstorms continue around the surface cyclone's center due to the destabilizing effect of the rather cold upper vortex.
Going forward the surface cyclone should follow the path of the cold core upper vortex while it weakens as a classical post-mature non-tropical system as there is a lack of divergence beneath the upper vortex center. As such the upper vortex (and hence the surface cyclone) should first be pushed southeastward by the expanding block of high latitude upper ridging that passes to the north...followed by an east-northeast drift toward the direction of Portugal as the upper trough southwest of Greenland tries to drag the upper vortex as it passes north of the vortex. I have shifted my forecast points south and east due to the current position of the surface cyclone. I forecast a 40% chance of subtropical cyclone formation due to the current amounts of shower and thunderstorm activity for the next 24 hours. After that time I gradually drop the odds as the upper vortex gradually warms to 1200 dekameters in height at 200 mb by 72 hours and as this system moves back toward the cooler 20 deg C sea surface temperature isotherm (would like to see water temperatures of 22 deg C if the upper vortex warms to 1200 dekameters).
IOH 24 Hr Outlook (1800Z Sep 17)...40% chance of subtropical cyclone formation (northeastern Atlantic near 38N-17W)
IOH 48 Hr Outlook (1800Z Sep 18)...10% chance of subtropical cyclone formation (west of Portugal near 40N-14W)
IOH 72 Hr Outlook (1800Z Sep 19)...0% chance of subtropical cyclone formation (west of Portugal near 40N-14W)
AREA OF INTEREST #4...Another tropical wave of low pressure over Western Africa is producing concentrated thunderstorm activity...therefore I am adding it as another area of interest for possible tropical cyclone formation on this blog...as it will have a period of favorable low shear and upper outflow in the eastern tropical Atlantic over the next 3 to 4 days before reaching the less favorable upper vorticity that will also shear area of interest #2. However I have not yet begun a specific outlook with probabilities of development to avoid further delays in putting together and publishing this post.
...COMPUTER MODEL SUMMARY...
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