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 #120

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


...FRIDAY SEPTEMBER 24 2021 9:35 AM EDT...

The Atlantic tropics remain very active with multiple systems to watch:

**See remnants of Rose section for a statement on the former tropical cyclone

**See Hurricane Sam section below for an update on the only currently active storm in the Atlantic basin. Sam is expected to strengthen into a major Hurricane and pass near northern Lesser Antilles after 5 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 info on yet another tropical wave of low pressure that could develop to the east of Hurricane Sam in the days ahead.

**See area of interest #3 section below for info on the developing subtropical disturbance located just east of Bermuda

**See area of interest #4 section below for info on possible southwest Gulf of Mexico tropical development in the days ahead.


REMNANTS OF ROSE… Rose continues its northwest track across the open Atlantic while tracking toward Odette’s surface ridge weakness. After failing to fire thunderstorms near its surface swirl…it was downgraded to a remnant low early on Thursday. The remnant low pressure swirl of Rose… currently located near 27.5N-42.5W… has finally been able to redevelop thunderstorm flares. However the new activity have become sheared off to the east of the swirl as Rose nears the upper westerly jet on the south side of the upper vortex tied to ex-Odette. As the jet and Rose get into closer proximity… the shear is likely to worsen and thus Rose is not likely to be re-upgraded to a tropical depression.


Of note… the remnant low of Rose is expected to undergo a fujiwhara interaction with ex-Odette which would send Rose to the northeast. Some models indicate Rose’s remnants making a comeback in the eastern divergence zone of the upper vortex tied to ex-Odette. However Rose would be positioned east of ex-Odette in such an interaction… where waters are cooler and thus Rose would likely not be tropical in nature in this comeback scenario. This is my final statement on Rose as it is no longer tropical cyclone.


TROPICAL STORM SAM (RECENTLY UPGRADED TO HURRICANE SAM)…Compact tropical depression eighteen in the eastern tropical Atlantic has rapidly intensified into a hurricane within the last 30 hours. The upgrade to hurricane strength came at 5 AM EDT. The next name on the list is Sam… and Sam wants to impress it seems as it developed an eye-type feature as soon as it became named Thursday morning. The eye feature has been replaced by a small central thunderstorm canopy… with the outer bands less ragged in appearance. Sam’s maximum sustained winds are estimated to be 75 mph as of the NHC 5 AM EDT advisory.


Over the next five days the forecast track keeps Sam under low shear and upper outflow beneath the tropical upper ridge axis in the region. The last 24 hours have reminded us that small systems can change there intensity rapidly… for example the pressure gradient is tight between the center and outside of a compact storm such that a small change in central pressure more easily changes the wind speed. The intensity forecast below is upped from yesterday given Sam’s current intensity… and calls for a category 4 hurricane sooner. The oscillations in intensity shown at days 3 to 5 is a reflection of possible eye wall replacement cycles often seen in intense hurricanes.


Regarding track… my updated track forecast is nudged north and west to account for the current position of Sam relative to the prior forecast. The surface ridge weakness caused by the remnants of Rose and Odette is not enough to turn Sam north… this is already evident by Sam’s ongoing westward heading. The slow down and increase in north angle at 2+ days is a reflection of area of Interest (AOI) #3 contributing to the ridge weakness. Towards the latter part of the forecast period… the current eastern US frontal low is expected to sweep up AOI #3 to the north. However the GFS indicates the ridge weakness associated with these features will linger as the current upper trough over western North America arrives into the west Atlantic and is slow to lift out… with the eastern divergence zone of the upper trough prolonging the ridge weakness and a more north track of Sam after day 5. The ECMWF lifts out the upper trough and hence closes the ridge weakness faster… resulting in a more west track of Sam after day 5. Interests in the northern Lesser Antilles should closely monitor Sam’s progress for possible impacts after day 5 as the exact track during that timeframe is a bit uncertain.

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

0 Hr Position (0000Z Sep 24)… 70 mph maximum sustained wind tropical storm centered at 11.2N-40.9W

IOH 24 Hr Forecast (0000Z Sep 25)… 95 mph maximum sustained wind hurricane centered at 11.8N-46W

IOH 48 Hr Forecast (0000Z Sep 26)… 115 mph maximum sustained wind major hurricane centered at 13.8N-49W

IOH 72 Hr Forecast (0000Z Sep 27)… 135 mph maximum sustained wind major hurricane centered at 15.8N-52W

IOH 96 Hr Forecast (0000Z Sep 28)… 125 mph maximum sustained wind major hurricane centered at 17.8N-55W

IOH 120 Hr Forecast (0000Z Sep 29)… 145 mph maximum sustained wind major hurricane centered northeast of the Lesser Antilles at 19.5N-57.5W


AREA OF INTEREST #1 (REMNANTS OF ODETTE)… As of 0000Z very early today… the remnant frontal cyclone of Odette was centered in the open North Atlantic at 44N-42W. Odette has whirled northwest toward the center of the upper vortex. Going forward… the upper trough fast approaching from northeast Canada will kick the upper vortex and Odette southeastward into the open eastern Atlantic. After reaching the east Atlantic… Odette is expected to arc more south in track while undergoing a fujiwhara interaction with the remnants of Rose expected to approach from the southeast. By 96 hours the convergence zone of the west side of the upper vortex is expected to build a surface ridge to the north that turns Odette west into the central Atlantic. The updated forecast track below is a reflection of the above expectations… and is nudged east due the current position of Odette.


Traditionally for subtropical development over water temps below 26 deg C… I look for the upper vortex being at 1200 dekameters in height or lower to indicate that the vortex is cold enough to aid in thunderstorm generation. The upper vortex is not currently nor is forecast to become this cold… and the ring of showers around the center of Odette has actually weakened in the last 24 hours. Therefore I have lowered short term development odds to 5%. With Odette not having much of a head start with showers and thunderstorms before reaching warmer waters… I have lowered longer term odds of development to 40%. The fujiwhara interaction with Rose’s remnants will likely push Odette toward the west side of the overhead upper vortex where northerly shear or upper convergence could be factors in limiting subtropical development… therefore I prefer having odds no higher than 40% at present. As of 8 AM EDT… the NHC has also recently lowered their development odds to 20%. If Odette does not developed a thunderstorm latent heat release warm core tropical upper outflow before 96 hours… it will be considerably weaker due to prolonged exposure to the upper convergence zone of the upper vortex. This is why I drop odds of development back down to 0% by then.


On a final note… any coastal sea swells on the shores of Newfoundland generated by Odette should subside in the next 24 hours as Odette begins to accelerate south-southeast and away.

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

IOH 24 Hr Outlook (0000Z Sep 25)… 5% chance of subtropical cyclone formation (northeast Atlantic near 40N-38W)

IOH 48 Hr Outlook (0000Z Sep 26)… 40% chance of subtropical cyclone formation (eastern Atlantic near 35N-39W)

IOH 72 Hr Outlook (0000Z Sep 27)… 40% chance of subtropical cyclone formation (eastern Atlantic near 31N-39W)

IOH 96 Hr Outlook (0000Z Sep 28)… 0% chance of subtropical cyclone formation (central Atlantic near 30.5N-42.5W)


AREA OF INTEREST #2… Cyclonic curvature of clouds near 10N-0W over central Africa suggest a tropical wave of low pressure is located here. Multiple models suggest this wave could develop under the low shear and outflow of the eastern tropical Atlantic upper ridge axis after emerging from Africa in 3 to 4 days. I agree with the NHC’s 40% odds of development by day 5 as the GFS and ECMWF… usually the two reliable global models… want to develop this wave in the 5 to 6 day window. Although these odds are on the low side… they are still quiet high for a wave that is still well inland over Africa… especially as the wave lacks concentrated thunderstorm masses at the moment. These odds could be raised in future updates if the model support continues… and/or the wave develops persisting thunderstorms and organizes later on.


Note that as the wave exits Africa… a strong eastern Atlantic surface ridge is forecast due to the western convergence zone of the current northeast Canada upper trough as that trough later moves in the northeast Atlantic and amplifies. This strong ridge will likely result in a track suppressed to the south with an increase in forward speed by day 5.

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

IOH 24 Hr Outlook (0000Z Sep 25)… 0% chance of tropical cyclone formation (central Africa near 10N-5W)

IOH 48 Hr Outlook (0000Z Sep 26)… 0% chance of tropical cyclone formation (western Africa near 10N-10W)

IOH 72 Hr Outlook (0000Z Sep 27)… 0% chance of tropical cyclone formation (west coast of Africa near 10N-15W)

IOH 96 Hr Outlook (0000Z Sep 28)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-20W)

IOH 120 Hr Outlook (0000Z Sep 28)… 40% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-26W)


AREA OF INTEREST #3… The western Atlantic upper vorticity string that sheared apart tropical cyclone Peter has weakened into an upper vortex located over Bermuda. The southwest side of the upper vortex is bombarding the remnants of Peter with northwesterly shear. Meanwhile to the north of ex-Peter… a spot of low shear and upper divergence on the east side of the vortex is triggering an increasingly defined subtropical surface low pressure area with a comma shaped region of thunderstorms just due east of Bermuda. This subtropical surface low pressure is expected to drift north in the flow on the east side of the upper vortex while the northwest Atlantic surface ridge blocks the system from accelerating north. The current upper trough over western North America is the expected to move into the west Atlantic after 48 hours and erode the blocking surface ridge… in the process taking over as the supporting upper trough for the current eastern US frontal low and driving that frontal low toward this system. Thus by 72 hours this system is expected to accelerate north toward Nova Scotia in the flow ahead of the frontal low and upper trough. Depending on the shape of the upper trough… which affects the nature and location of its upper divergence maximum… some model runs show this system become an intensifying non-tropical frontal cyclone passing through Atlantic Canada.


Previously the GFS was the only model to forecast the formation of this system… but now all global models are onboard. This… combined with the improving organization of the comma shaped cloud mass causes me to already project 70% odds of subtropical cyclone formation in the next 48 hours… notably higher than the NHC outlook. By 72 hours this system will likely shed any tropical characteristics it has while accelerating north across cooler water which is when I drop subtropical development odds to 0%. As alluded to in the previous paragraph… this system could maintain strength with the support of the incoming upper trough even after losing tropical character.


With these forecast updates:

**The track of this developing subtropical system is expected to pass just east of Bermuda in the next 24 hours. However coastal sea swells are possible if this system strengthens to a subtropical cyclone

**A reminder again that the non-tropical remnants of this system could strengthen across Nova Scotia… Newfoundland… and Labrador with the support of an incoming upper trough… resulting in possible gusty winds and coastal sea swells by Sunday and Monday.

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

IOH 24 Hr Outlook (0000Z Sep 25)… 70% chance of subtropical cyclone formation (northeast of Bermuda near 35N-62.5W)

IOH 48 Hr Outlook (0000Z Sep 26)… 70% chance of subtropical cyclone formation (northwest Atlantic near 37N-62.5W)

IOH 72 Hr Outlook (0000Z Sep 27)… 0% chance of subtropical cyclone formation (offshore of Nova Scotia near 43.5N-62.5W)


AREA OF INTEREST#4… The tail end of the cold front moving across the eastern US has reached the central Gulf of Mexico waters just offshore of the northwestern Yucatan peninsula. Once the upper troughs currently over the eastern US and western North America shift northeast in the next couple of days… wind shear in the Gulf may relax enough to allow for possible tropical development along the tail end of the front. Anything that does develop is likely to drift west then northwest toward east-central or northeast Mexico… Veracruz and Tamaulipas provinces and vicinity… under the influence of the south-central US surface ridge as that ridge later settles over the southeastern US and steers this system with its southwestern quadrant. Should development occur… heavy rains… gusty winds… and coastal sea swells would reach east-central and/or northeast Mexico by late this weekend or early next week. I currently project low 10% odds of tropical cyclone formation as only the CMC model at this time shows some possible development.

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

IOH 24 Hr Outlook (0000Z Sep 25)…0% chance of tropical cyclone formation (just northwest of the Yucatan Peninsula near 21.2N-91W)

IOH 48 Hr Outlook (0000Z Sep 26)…0% chance of tropical cyclone formation (southwest Gulf of Mexico near 21.2N-93W)

IOH 72 Hr Outlook (0000Z Sep 27)… 10% chance of tropical cyclone formation (offshore of the Tamaulipas/Veracruz border near 22.5N-96W)

IOH 96 Hr Outlook (0000Z Sep 28)… 0% chance of tropical cyclone formation (Inland over Tamaulipas near 24.8N-98W)

...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 Sam…located at 17N-56.5W at 120 hours as a compact hurricane

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 32.5N-42.5W at 72 hours during a fujiwhara interaction with Rose’s remnant low

**For Area of Interest #2… emerges from west coast of Africa by 42 hours… located at 8.8N-41W as a tropical low at 168 hours

**For Area of Interest #3… surface low forms east-southeast of Bermuda near 30.5N-60.5W at 12 hours… whirls west and passes just north of Bermuda at 36 hours as a subtropical or tropical cyclone… current eastern US frontal low moves into northwest Atlantic and absorbs this system by 78 hours.

**For area of interest #4… tropical low becomes defined in the southwest Gulf of Mexico at 42 hours… makes landfall at the Tamaulipas/Veracruz border at 96 hours

**Additional tropical wave emerges from Africa at 108 hours… tropical cyclone formation suggested near 10.5N-26W at 168 hours


1200Z ECMWF Model Run...

**For Tropical Storm Sam…located at 16.5N-54W at 120 hours as a compact hurricane

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 32.5N-40W at 72 hours during a fujiwhara interaction with Rose’s remnant low

**For Area of Interest #2…emerges from west Africa at 96 hours… tropical cyclone formation suggested near 9N-26.2W at 144 hours

**For Area of Interest #3…surface low forms just east of Bermuda at 32N-63.5W at 24 hours… while nearing Nova Scotia becomes absorbed by approaching frontal low from eastern US between 72 and 96 hours

**For Area of Interest #4… no development shown


1800Z GFS Model Run...

**For Tropical Storm Sam… compact hurricane at 17N-53W by 120 hours

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 31.5N-41W at 72 hours during a fujiwhara interaction with Rose’s remnant low

** For Area of Interest #2…emerges from west coast of Africa by 84 hours… tropical cyclone formation suggested near 9.5N-21W at 120 hours

** For Area of Interest #3…surface low forms east of Bermuda at 31.8N-62.5W at 12 hours… passes just northeast of Bermuda at 33 hours as a subtropical or tropical cyclone… makes landfall at Nova Scotia as a less tropical low pressure at 81 hours… transforms into an intensifying non-tropical frontal cyclone over Labrador at 102 hours… located between Labrador and Greenland by 120 hours

**For Area of Interest #4… no development shown


1200Z NAVGEM Model Run...

**For Tropical Storm Sam… reaches 18.5N-52.5W at intense hurricane strength at 120 hours

**For Area of Interest #1 (Ex-Odette)…dives south and dissipates near 31N-45W at 102 hours

**For Area of Interest #2… Tropical wave emerges from Africa at 48 hours… tropical cyclone formation suggested at 10N-36W at 168 hours

**For Area of Interest #3…Subtropical or tropical cyclone suggested just northeast of Bermuda at 36 hours… becomes absorbed by approaching frontal low from eastern US at 96 hours while located in open northwest Atlantic

**For Area of Interest #4… no development shown

**Additional tropical wave emerges from Africa at 120 hours… tropical cyclone formation suggested just offshore of Africa at 9.9N-16W at 132 hours

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