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 #121

Updated: Sep 27

*******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 25 2021 11:54 AM EDT...

More recent satellite image of the Atlantic tropics as of 1510Z:

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

**See Hurricane Sam section below for an update on the only currently active storm in the Atlantic basin.

**See Teresa section below for info on the subtropical cyclone near Bermuda that has already weakened and is on its way to becoming a remnant low pressure

**See Ex-Odette (area of interest #1) section below for an update on the remnant cyclone in the open North Atlantic.

**See area of interest #2 section below for info on a tropical wave of low pressure that could develop well east of Hurricane Sam in the days ahead.

**See area of interest #3 section below for info on possible southwest Gulf of Mexico tropical activity in the days ahead.

**As the upper vortex that triggered Teresa continues to weaken… it’s shearing effect on the remnants of Peter has relaxed which has allows the remnants to develop thunderstorms closer to its surface low pressure area. Ex-Peter as a result has only recently been introduced into the NHC outlook with an assigned 20% chance of redeveloping into a tropical cyclone. It’s recent introduction into the NHC outlook is why it is not marked as an area of interest in this update when it now is. Will address developments with ex-Peter in my next update.


Elsewhere… two additional areas of interest may emerge in the Atlantic tropics in the coming days:

(1) A tropical wave of low pressure currently over central Africa to the east of area of interest #2 which has become better defined… which may develop in tandem with area of interest #2

(2) The latest model runs show that the current central Canada upper trough will enter the western Atlantic in a more amplified state. The amplified trough could leave behind an upper vortex that induces subtropical cyclone development just offshore of the US east coast over the next few days.


HURRICANE SAM (RECENTLY UPGRADED TO MAJOR HURRICANE SAM)…Satellite image of Hurricane Sam taken as of 1510Z:

As Sam continues to traverse west-northwest across the central tropical Atlantic… it established a pinhole eye and strengthened further into a category 2 hurricane. As of 5 AM EDT… Sam was knocking on the door of category 3 while estimated to have 110 mph maximum sustained winds… and as of 11 AM EDT is now a category 3 with 120 mph maximum sustained winds.


Over the next few days the forecast track keeps Sam under low shear and upper outflow beneath the tropical upper ridge axis in the region. The last 48 hours have reminded us that small systems can change their 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 in the short-term… and assumes Sam will become a category 4 hurricane by tonight before undergoing an eye wall replacement cycle. It appears the outer bands around Sam’s core have struggled with dry Saharan air that the outer circulation of the hurricane has pulled in closer and now ingested. I project this dry air will cause Sam to more slowly ramp up to a second category 4 peak from 48 to 96 hours. The latest model runs show that the current central Canada upper trough will enter the western Atlantic in a more amplified state and thus have a harder time lifting out… and the southeast side of the large upper trough may start to lightly shear or block the western outflow of the hurricane by 120 hours. As a result… I forecast Sam to weaken at that time.


Regarding track… my updated track forecast is nudged north once again to account for the current position of Sam relative to the prior forecast. Sam is beginning to slow down and have more of a north angle in track as the Atlantic surface ridging has been weakened by the Rose-Odette complex to the northeast and Teresa-Peter complex to the northwest. Although the current eastern Canada frontal low is expected to sweep up Teresa to the north…the northwestern ridge weakness is expected to linger as the current upper trough over central Canada amplifies more than previously shown in the modeling and struggles to lift out of the western Atlantic… with the eastern divergence zone of the upper trough maintaining the weakness. Thus the slow northwest track toward the weakness is likely to continue through day 5… which bodes well for the northern Lesser Antilles as the islands would avoid a direct hit from the hurricane… but this track could result in impacts to Bermuda down the road.


Given these forecast updates:

**The Lesser Antilles are unlikely to receive rain or wind from Sam… however coastal sea swells are quiet likely sometime after this weekend.

**Bermuda should monitor the progress of Sam for possible impacts after day 5.

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

0 Hr Position (0000Z Sep 25)… 100 mph maximum sustained wind hurricane centered at 12.4N-45.9W

IOH 24 Hr Forecast (0000Z Sep 26)… 140 mph maximum sustained wind major hurricane centered at 14.4N-49W

IOH 48 Hr Forecast (0000Z Sep 27)… 115 mph maximum sustained wind major hurricane centered at 16.4N-52W

IOH 72 Hr Forecast (0000Z Sep 28)… 125 mph maximum sustained wind major hurricane centered at 18.4N-55W

IOH 96 Hr Forecast (0000Z Sep 29)… 145 mph maximum sustained wind major hurricane centered northeast of the Lesser Antilles at 20.2N-57.5W

IOH 120 Hr Forecast (0000Z Sep 30)… 115 mph maximum sustained wind major hurricane centered at 22N-61W


SUBTROPICAL STORM TERESA (RECENTLY DOWNGRADED TO SUBTROPICAL DEPRESSION TERESA)… A subtropical Storm has been able to quickly form north-northeast of Bermuda as of Friday with the support of the eastern divergence zone of the upper vortex in the region. We are down to the next name on the list of names for this season… Teresa. The blocking northwest Atlantic surface ridge has pushed Teresa’s surface swirl westward and away from the divergence zone of the upper vortex and showers and thunderstorms. As of 11 AM EDT… Teresa has weakened to a subtropical depression … and will likely be downgraded to a remnant low pressure soon while now located northwest of Bermuda. What a short-lived named storm this will be.


A north turn is forecast in the next 24 hours as the current upper trough over central Canada moves into the west Atlantic and erodes the blocking northwest Atlantic surface ridge with its eastern divergence zone. This divergence zone may help Teresa produce some thunderstorms around 24 hours from now… however the forecast below assumes a remnant low well before 24 hours given the current observations in the prior paragraph… and by the time these thunderstorms may develop it could be sheared to the northeast of the center in the flow ahead of the upper trough… and they could be short-lived as Teresa heads into cooler waters soon after 24 hours. Thus the forecast assumes these thunderstorms… if they develop… will not be enough to re-classify Teresa’s remnant low as a subtropical cyclone.


I plan this to be my final statement on Teresa on this blog… unless somehow Teresa manages to stay a subtropical cyclone for longer than currently expected.


With these forecast updates:

**Any coastal sea swells generated by Teresa to Bermuda should diminish as Teresa is weakening.

**The remnants of Teresa are no longer forecast by model runs to strengthen over Nova Scotia… Newfoundland and Labrador as the shape of the incoming upper trough in the latest runs positions it’s upper divergence maximum over the eastern Canada frontal low… which will favor the frontal low over Teresa.

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

0 Hr Position (0000Z Sep 25)…45 mph maximum sustained wind subtropical storm centered north of Bermuda at 34.3N-65.4W

IOH 24 Hr Forecast (0000Z Sep 26)…Remnant low centered at 37N-65W


AREA OF INTEREST #1 (REMNANTS OF ODETTE)…The current North Atlantic upper trough has kicked the parent upper vortex of ex-Odette to the southeast… and while tied to the vortex ex-Odette has also moved southeast. Ex-Odette will soon arc more south in track while undergoing a fujiwhara interaction with the remnants of Rose located to the south. The fujiwhara interaction will likely push Odette toward the west side of the overhead upper vortex where northerly shear and upper convergence will be factors in limiting subtropical development. And given ex-Odette has already lost its ring of shower activity… subtropical development is no longer expected. This is my final statement on ex-Odette on this blog as it is no longer an area of interest for subtropical development.

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

IOH 24 Hr Outlook (0000Z Sep 26)… 0% chance of subtropical cyclone formation (eastern Atlantic near 35N-40W)


AREA OF INTEREST #2… The tropical wave of low pressure that has recently moved into Western Africa from central Africa briefly had an increase in thunderstorms late Friday… before becoming less defined and absent of thunderstorms as it has become as of this late morning. As a result I have not raised peak odds of development above 40%… and also have lowered short-term development odds. However I have not lowered the peak odds below 40% either as most models still develop this wave under the low shear and outflow of the eastern tropical Atlantic upper ridge axis after emerging from Africa.


Note that as the wave exits Africa… a strong eastern Atlantic surface ridge is forecast due to the western convergence zone of the current North Atlantic upper trough as that trough later moves in the northeast Atlantic and amplifies. However despite the presence of this ridge the models now slow down the speed of the wave instead of increasing it… apparently due to forecast entanglement with another tropical wave of low pressure to the east that is currently over central Africa. Some models such as the CMC and NAVGEM also want to develop the wave to the east… which could result in a fujiwhara interaction between this wave and the adjacent wave to the east. In such an interaction… this wave would stay suppressed to the south. My updated longer-term forecast track therefore stays toward the south… but now has a slower forward speed to reflect the latest model guidance.

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

IOH 24 Hr Outlook (0000Z Sep 26)… 0% chance of tropical cyclone formation (western Africa near 10N-10W)

IOH 48 Hr Outlook (0000Z Sep 27)… 0% chance of tropical cyclone formation (west coast of Africa near 10N-15W)

IOH 72 Hr Outlook (0000Z Sep 28)… 0% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-20W)

IOH 96 Hr Outlook (0000Z Sep 28)… 20% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-23W)

IOH 120 Hr Outlook (0000Z Sep 29)… 40% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-26W)

AREA OF INTEREST #3… The tail end of the surface front moving slowly into the western Atlantic is in the central Gulf of Mexico waters just offshore of the northwestern Yucatan peninsula. Once the upper trough currently over central Canada shifts northeast … wind shear in the Gulf is expected relax… therefore will still watch 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 southwest quadrant of the southeastern US surface ridge. Should a tropical disturbance develop… heavy rains could spread into east-central and/or northeast Mexico by late this weekend or early next week. I currently project low 5% odds of tropical cyclone formation as model support showing such development is absent… and the tail end of the front of interest has not yet seen an increase in thunderstorms.

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

IOH 24 Hr Outlook (0000Z Sep 26)…0% chance of tropical cyclone formation (southwest Gulf of Mexico near 21.2N-93W)

IOH 48 Hr Outlook (0000Z Sep 27)… 5% chance of tropical cyclone formation (offshore of the Tamaulipas/Veracruz border near 22.5N-96W)

IOH 72 Hr Outlook (0000Z Sep 28)… 0% chance of tropical cyclone formation (Inland over Tamaulipas near 24.8N-98W)


...COMPUTER MODEL SUMMARY (AS OF 1800Z LAST EVENING)...

Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)


1200Z CMC Model Run...

**For Hurricane Sam…located just northeast of the Lesser Antilles at 18.5N-60W at 120 hours.

**For Subtropical Storm Teresa… weakens to a broad remnant low near 35.5N-64W at 42 hours

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 31N-40W at 48 hours during a fujiwhara interaction with Rose’s remnant low

**For Area of Interest #2… tropical cyclone formation suggested at 8.5N-33.5W at 156 hours

**For area of interest #3… no development shown

**Additional tropical wave emerges from Africa at 84 hours… tropical cyclone formation suggested near 9.8N-22W at 126 hours


1200Z ECMWF Model Run...

**For Hurricane Sam…located just east of the northern Lesser Antilles at 17.5N-58W at 120 hours.

** For Subtropical Storm Teresa… while nearing the Maine/Canada border becomes absorbed by approaching frontal low from eastern Canada between 48 and 72 hours.

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 32.5N-40W at 48 hours during a fujiwhara interaction with Rose’s remnant low

**For Area of Interest #2…emerges from west Africa at 72 hours… tropical cyclone formation suggested near 8N-27W at 144 hours

**For Area of Interest #3…no development shown


1800Z GFS Model Run...

**For Hurricane Sam… located at 19.5N-57W by 120 hours

**For Subtropical Storm Teresa… absorbed by approaching frontal low from eastern Canada at 42 hours

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 32.5N-41.5W at 45 hours during a fujiwhara interaction with Rose’s remnant low

** For Area of Interest #2…tropical cyclone formation suggested near 7.5N-21.2W at 120 hours

**For Area of Interest #3… no development shown

**Frontal low that absorbs Teresa leaves behind cold front offshore of northeastern US where a new frontal low forms by 120 hours… performs a fujiwhara interaction with Hurricane Sam thru 168 hours which takes it south to waters offshore of North Carolina where it becomes a subtropical cyclone.


1200Z NAVGEM Model Run...

**For Hurricane Sam… reaches 20.2N-57W at 120 hours

**For Subtropical Storm Teresa… absorbed by approaching frontal low from eastern Canada at 42 hours

**For Area of Interest #1 (Ex-Odette)…dives south and loses closed circulation near 35N-38W at 42 hours during a fujiwhara interaction with Rose’s remnant low

**For Area of Interest #2… no development shown

**For Area of Interest #3… no development shown

**Additional tropical wave emerges from Africa at 90 hours… tropical cyclone formation suggested just offshore of Africa at 9.9N-17W at 102 hours… rapidly intensifies to near-hurricane strength at 10.5N-19.5W by 120 hours

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