BIRDSEYE VIEW POSTS

Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at IOHurricanes@outlook.com 

 
 
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MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #117

Updated: Sep 22

*******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.**********


...TUESDAY SEPTEMBER 21 2021 3:46 AM EDT...

The Atlantic tropics remain active with multiple

systems to watch:

**See Tropical Storm Peter and Rose sections below for an update on the two currently active storms in the Atlantic basin… Peter is expected to pass just north of the Caribbean Islands and Bahamas in the coming days

**See Odette (area of interest #1) section below for an update on the remnant cyclone in the open north Atlantic which may re-acquire tropical character

**See area of interest #2 section below for an update on the latest tropical wave of low pressure in the eastern tropical Atlantic… it’s potential to develop into a strong tropical cyclone is high.


TROPICAL STORM PETER… While passing just north of the Lesser Antilles and Virgin Islands… Peter has maintained strength while firing bursts of thunderstorms in the face of shear being generated by the nearby upper vortex to the west. Regarding Peter’s forecast track… Odette and its supporting upper vortex are expected to stall at a further east location… which will help western Atlantic surface ridging stay more intact and thus help Peter in tracking more westward. Some north angle in track is still ongoing due to the drag of the upper southwesterly shearing flow. Even though the shearing upper vortex is slated to fade away while cut-off from high-latitude cold air… the upper trough/vortex to be tied with Odette is expected to soon deposit upper vorticity in the west Atlantic. The eastern divergence zone of this upper vorticity is expected to lower surface pressures in the vicinity of Bermuda… and most models except the GFS agree on turning or reforming Peter toward Bermuda in the longer range. The GFS has preferred a more southwest track and assumes that a separate subtropical or tropical low will form near Bermuda instead. My updated forecast track is a blend of these two ideas… at first maintaining the more south and west forecast track from my previous update since that track has done well… but then shifting the longer range forecast track toward Bermuda since most models are adamant about a reformation or turn towards the northeast.


Peter has maintained 50 mph maximum sustained winds… and my updated forecast intensity shows Peter holding this strength thru 24 hours as the shear potentially wanes a bit when the upper vortex to the west fades. The shear is expected to increase by 48 hours once the upper vorticity to be deposited by the upper trough/vortex tied with Odette takes hold. Even though the south part of the upper vorticity is expected to weaken… the updated forecast track keeps Peter entangled with its lingering north part and thus the shear will continue. Even as this upper vorticity potentially accelerates to the east and away by day 5… Peter would still be embedded in upper westerly shearing flow out ahead of an upper trough to be left behind over the eastern US by the current central North American frontal system. Simply put… my updated intensity forecast tapers Peter down to a minimal tropical storm after day 2 due to the negative shear outlook.


With these forecast updates:

**Interests in the northern Lesser Antilles… Virgin Islands… Puerto Rico… the Dominican Republic… and the Bahamas could see coastal sea swells from Peter in the next couple of days.

**The updated track below has shifted closer to Bermuda. So far however Peter is expected to be a weak tropical cyclone on its approach toward Bermuda due to unfavorable upper shearing winds.

******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********

0 Hr Position (0000Z Sep 21)… 50 mph maximum sustained wind tropical storm centered north of the northern Lesser Antilles at 20N-62.2W

IOH 24 Hr Forecast (0000Z Sep 22)… 50 mph maximum sustained wind tropical storm centered northwest of Puerto Rico at 21.7N-66W

IOH 48 Hr Forecast (0000Z Sep 23)… 45 mph maximum sustained wind tropical storm centered east-northeast of the eastern Bahamas at 23.2N-70W

IOH 72 Hr Forecast (0000Z Sep 24)… 40 mph maximum sustained wind tropical storm centered at 24.5N-70W

IOH 96 Hr Forecast (0000Z Sep 25)… 40 mph maximum sustained wind tropical storm centered at 28N-67.5W

IOH 120 Hr Forecast (0000Z Sep 26)… 40 mph maximum sustained wind tropical storm centered south of Bermuda at 30N-65W


TROPICAL STORM ROSE… Rose continues its northwest track into the open eastern Atlantic due to the drag of upper southwesterly flow out ahead of upper vorticity in the northeast Atlantic. This track will also continue as Rose tracks toward Odette’s surface ridge weakness. The track by 96 hours is shifted east as the latest model guidance shows Odette accelerating east or southeast by that time.

Regarding intensity… as forecast yesterday the tropical storm intensified to 50 mph maximum sustained winds while it passed below the low shear/upper outflow environment of the eastern tropical Atlantic upper ridge axis. Things are about to get ugly for Rose in the next 24 hours as the tropical storm tracks right into southwesterly shearing upper winds out ahead of the northeast Atlantic upper vorticity that will displace the strongest thunderstorms northeast of Rose’s center. It is interesting that thunderstorms near the center have already faded and have increased in a complex well to the northeast… with this complex aided by the eastern divergence zone of the upper vorticity. I project Rose to weaken to a depression in the next 24 hours from the shear. Once Rose moves past the axis of the shearing upper vorticity and move into a haven of lower shear at 48 to 72 hours… I project Rose to regain storm strength. By 96 hours Rose will near an upper westerly jet on the south side of the upper vortex tied to Odette… thus I project that Rose will weaken to a remnant low by that time as the shear from the jet will likely be too much for Rose to handle.

******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official

forecast***********

0 Hr Position (0000Z Sep 21)… 50 mph maximum sustained wind tropical storm centered at 18.7N-35W

IOH 24 Hr Forecast (0000Z Sep 22)… 35 mph maximum sustained wind tropical depression centered at 22.5N-37.5W

IOH 48 Hr Forecast (0000Z Sep 23)… 40 mph maximum sustained wind tropical storm centered at 25N-40W

IOH 72 Hr Forecast (0000Z Sep 24)… 40 mph maximum sustained wind tropical storm centered at 28N-43W

IOH 96 Hr Forecast (0000Z Sep 25)… Remnant low centered at 31N-41W


AREA OF INTEREST #1 (REMNANTS OF ODETTE)… As of 0000Z… the remnant frontal cyclone of Odette was centered in the open North Atlantic east-southeast of Newfoundland at 44N-46W. A recent ASCAT-A ascending passes showed greater than 50 knot (60 mph) winds on the west side of the center… therefore I assume the current intensity of Odette is at 65 mph maximum sustained winds. I am continuing forecasts on Odette as it could re-acquire tropical characteristics in the coming days. The NHC outlook also mentions Odette’s remnant cyclone to also acknowledge its potential to re-acquire of tropical characteristics.


Odette’s strength as a remnant frontal cyclone is facilitated by the upper divergence zone of the amplifying upper trough in the northwest Atlantic. The amplification of the upper trough is being caused by adjacent amplification of warm eastern North America upper ridging to the west in the warm sector of the frontal system now over central North America. Models have come into notable agreement about Odette’s future with a somewhat different solution than recent days…having the upper trough amplify further into an upper vortex to the northwest of Odette instead of nearby. This causes Odette to move east-southeast in the next 24 hours… then whirl northwest into the upper vortex thru 72 hours. By 96+ hours… a portion of the upper trough associated with the central North America frontal system then kicks the upper vortex and Odette east or southeast depending on which model run is picked. The updated forecast track below reflects the scenario in the latest model consensus… and lies a bit south of the GFS as that model over the last days… and even as of now… has been north or east of the consensus.


Because the upper trough is not yet a vortex… and Odette will not be whirling beneath the expected vortex till at least after 24 hours per the latest model guidance… I hold onto Odette’s current strength for 24 more hours. Once Odette whirls beneath the center of the upper vortex where upper divergence is lacking… gradual weakening is projected. The latest forecast track is east of the previous and toward the northeast Atlantic where waters are cooler. Therefore the only way Odette becomes subtropical at this point is for the upper vortex to become cold enough (200 mb height at or less than 1200 dekameters) to aid in thunderstorm generation. The GFS did show the vortex get a shot of colder air from the upper trough energy that ejects from the central North America frontal system… but only for a short time at 96 hours. This is when I now forecast Odette to become a subtropical Storm… followed by loss of tropical character at 120 hours when the upper vortex begins to warm back up.


Coastal sea swells could linger on the shores of Newfoundland thru 72 hours… after which time Odette is expected to weaken and shift east and away in track.

******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********

0 Hr Position (0000Z Sep 21)… 65 mph maximum sustained wind frontal cyclone centered at 44N-46W

IOH 24 Hr Forecast (0000Z Sep 22)… 65 mph maximum sustained wind frontal cyclone centered at 40N-41W

IOH 48 Hr Forecast (0000Z Sep 23)… 60 mph maximum sustained wind frontal cyclone centered at 43N-41W

IOH 72 Hr Forecast (0000Z Sep 24)… 50 mph maximum sustained wind frontal cyclone centered at 43N-44W

IOH 96 Hr Forecast (0000Z Sep 25)… 45 mph maximum sustained wind subtropical storm centered at 40N-39W

IOH 120 Hr Forecast (0000Z Sep 26)… 40 mph maximum sustained wind remnant low centered west of the Azores at 37.5N-36W


AREA OF INTEREST #2…The tropical wave of low pressure in the eastern tropical Atlantic has become well organized in the last 24 hours while displaying a low pressure spin and spiral thunderstorm bands. Over the next five days the forecast track keeps this system under low shear and upper outflow beneath the tropical upper ridge axis… and all models at this point forecast tropical cyclone formation. Therefore I have begun a tropical cyclone formation forecast as outlined below… with specific track and intensify forecast points.


I have adjusted the short-term forecast west due to the current speed of this system… but do slow down the west speed and increase the north angle in the track by 120 hours to avoid leaving behind the model consensus. A slow down in track makes sense as the Atlantic surface ridge will have a weakness caused by Rose and Odette. However this weakness is not expected to be strong enough to recurve this system north thru 4 days… as both Rose and Odette are expected to weaken in the longer range (see above Rose and Odette sections for the factors expected to weaken each system). The additional slow down and increase in north angle at day 5 is to reflect the possibility of Peter nearing Bermuda and contributing to the ridge weakness. Regarding intensity… unlike the last three storms (Odette… Rose… Peter) this system will be staying away from upper-level shearing winds thru day 5. And with the healthy initial structure of the wave… I forecast a strong hurricane by day 5. I keep this system below tropical storm force in the next 24 hours as the thunderstorm intensity has waned a bit as of late.

******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********

0 Hr Position (0000Z Sep 21)… Tropical low pressure centered south of the Republic of Cabo Verde Islands at 10N-24.8W

IOH 24 Hr Forecast (0000Z Sep 22)… 35 mph maximum sustained wind tropical depression centered at 10.5N-31W

IOH 48 Hr Forecast (0000Z Sep 23)…50 mph maximum sustained wind tropical storm centered at 11N-36W

IOH 72 Hr Forecast (0000Z Sep 24)… 70 mph maximum sustained wind tropical storm centered at 11.5N-41W

IOH 96 Hr Forecast (0000Z Sep 25)… 85 mph maximum sustained wind hurricane centered at 12N-46W

IOH 120 Hr Forecast (0000Z Sep 26)… 100 mph maximum sustained wind hurricane centered at 14N-49W

...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 Peter… located at 27.5N-69W at 120 hours as a remnant low

**For Tropical Storm Rose… located at 27.5N-37.5W at 120 hours as a remnant low

**For Area of Interest #1 (Ex-Odette)… moves southeast to 38N-40.5W at 36 hours… cyclonically loops northwest to 45N-42W thru 78 hours while re-intensifying… accelerates rapidly southeast while weakening and reaches 37.5N-36W at 120 hours

**For Area of Interest #2…tropical cyclone formation shown at 10.5N-37W at 90 hours… compact and strong tropical storm at 13N-42W at 120 hours


1200Z ECMWF Model Run...

**For Tropical Storm Peter… weakens to a remnant low… while turning north and passing over Bermuda between 96 and 120 hours redevelops into a broad tropical storm

**For Tropical Storm Rose… turns in a large anticyclonic (clockwise) loop and reaches 27.5N-33W as a weaker tropical depression by 120 hours

**For Area of Interest #1 (Ex-Odette)…moves southeast to 40N-44W at 24 hours…cyclonically loops north to 46N-44W thru 72 hours while re-intensifying… north part of cold front ejecting from central North America absorbs ex-Odette at a location northwest of the Azores by 120 hours

**For Area of Interest #2…tropical cyclone formation shown at 10.5N-37W at 72 hours…becomes a compact hurricane near 14N-46W by 120 hours


1800Z GFS Model Run...

** For Tropical Storm Peter… weakens to a remnant trough that reaches waters northeast of the Bahamas by 120 hours

** For Tropical Storm Rose… weakens to a remnant trough that becomes absorbed by ex-Odette’s outer circulation at 105 hours while located near 30N-40W

**For Area of Interest #1 (Ex-Odette)…moves east-southeast to 41N-40.5W at 24 hours…cyclonically loops north to 45N-44W thru 72 hours while re-intensifying…accelerates southeast to 39N-40W by 120 hours

**For Area of Interest #2…tropical cyclone formation shown at 10N-35W at 75 hours… becomes a compact hurricane near 14N-42.5W by 120 hours

**Possible subtropical or tropical cyclone east of Bermuda at 87 hours…generally stationary through 120 hours


1200Z NAVGEM Model Run...

**For Tropical Storm Peter… weakens to remnant low that reaches 26N-70W by 120 hours

**For Tropical Storm Rose… reaches 27.5N-38.5W as a strong tropical storm by 120 hours

**For Area of Interest #1 (Ex-Odette)…moves southeast to 37N-41.5W at 48 hours… turns north to 45N-40W thru 96 hours while re-intensifying… north part of cold front ejecting from central North America kicks ex-Odette eastward 45N-30W by 120 hours

**For Area of Interest #2…located at 13N-39W as a tropical storm by 120 hours

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