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 #123

*******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 28 2021 3:34 PM EDT...

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 area of interest #1... #3... and #4 sections below for info on a train of tropical waves of low pressure spanning from the eastern Atlantic and into Africa being monitored for possible development in the days ahead.

**See area of Interest #2 section below for an update on the remnants of Peter.


Elsewhere...the upper trough entering the western Atlantic from eastern North America is forecast to generate a frontal low pressure offshore of the northeastern United States in the next 24 hours. The upper trough however is no longer forecast to amplify enough to whirl this frontal low toward the south and cause potential subtropical development of the frontal low. Therefore subtropical development offshore of the eastern United States is not anticipated.


MAJOR HURRICANE SAM...The eye wall replacement cycle that Sam underwent through much of yesterday had more of a negative effect on the hurricane's strength...with the hurricane dipping to category 3 strength with 120 mph maximum sustained winds. A new eye has slowly become re-established with Sam and the hurricane has recovered to a category 4 with 130 mph maximum sustained winds. With Sam remaining over warm 28 deg C waters and under a tropical upper ridge where low shear and upper outflow is ongoing...my intensity forecast below calls for Sam to strengthen further in the next 24 hours...but has a lower intensity than I previously showed as the prior eye wall replacement cycle had more of a negative effect as mentioned above. By 48 hours I forecast another round of weakening from a second eye wall replacement cycle... followed by additional weakening as Sam begins to interact with the east side of the upper trough currently emerging from eastern North America. This side of the upper trough is expected to block the western outflow of the hurricane and also shear it...but the weakening rate will tend to be gradual as this side of the upper trough is divergent as we always see with a mid-latitude upper trough. Sam is also likely to grow in size during this weakening phase due to the size of the upper divergence region which will lower surface pressures over a wide area. Weakening after 72 hours will also be facilitated by the marginally warm 26 deg C waters east of Bermuda...which are cooler than the 28 deg C water Sam is currently over.


Regarding track... a classic north turn in the surface ridge weakness induced by the eastern divergence zone of the incoming eastern North America upper trough is expected. The short-term forecast track is similar to my previous...but the long term guidance has shifted east as the upper trough is not expected to be quiet as amplified as shown in earlier model runs which allows the trough to move more easily east. Therefore my updated longer-range track is also shifted east to reflect the latest model guidance. On this updated track...Sam will stay east of Bermuda instead of directly striking the island.


Given these forecast updates:

**The Lesser Antilles and Bermuda will see coastal sea swells generated by Sam over the next few days. Because Sam is forecast to expand in size as dicussed above...coastal sea swells will likely spread out farther and reach the mid-Atlantic US coast...northeast US coast...and Atlantic Canada coasts by 72+ hours.

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

0 Hr Position (1200Z Sep 28)... 130 mph maximum sustained wind major hurricane centered at 17.5N-54.3W

IOH 24 Hr Forecast (1200Z Sep 29)... 140 mph maximum sustained wind major hurricane centered northeast of the Lesser Antilles at 19.5N-56.5W

IOH 48 Hr Forecast (1200Z Sep 30)... 120 mph maximum sustained wind major hurricane centered at 22N-60W

IOH 72 Hr Forecast (1200Z Oct 1)... 110 mph maximum sustained wind hurricane centered southeast of Bermuda at 27.5N-62W

IOH 96 Hr Forecast (1200Z Oct 2)... 100 mph maximum sustained wind hurricane centered east of Bermuda at 32.5N-60W

IOH 120 Hr Forecast (1200Z Oct 3)... 95 mph maximum sustained wind hurricane centered at 37.5N-55W


AREA OF INTEREST #1... The latest tropical wave of low pressure to emerge from Africa is showing signs organization as its thunderstorm banding features increase around a low pressure center at 7.5N-21W. The tropical wave struggled to organize yesterday as most of the thunderstorm activity was west of the rotation which was positioned just offshore of the Africa coast at the time... however with a spin now well-embedded in the thunderstorms and the above-mentioned organizational trends I have begun a tropical cyclone formation forecast as outlined below. Despite the presence of the eastern Atlantic surface ridge...the latest models agree on a north angle in this system's westward track... initially from a fujiwhara interaction with area of interest #4...followed by the arrival of upper southerly flow on the east side of the eastern Atlantic upper vortex as that vortex gets pushed closer to this system courtesy of the northwest Atlantic upper ridge. The upper southerly flow is expected to play a role in the steering as this system will more likely than not be a stronger/taller system in the coming days able to couple with the upper flow. I project brisk strengthening into a hurricane through 72 hours due to the supportive outflow and low shear of the tropical Atlantic upper ridge in progress... and also due to the initial healthy structure that this system is developing. Slower strengthening is projected at 96 hours... followed by weakening by 120 hours... as this system nears the southerly upper winds on the east side of the upper vortex which will shear this system.

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

0 Hr Position (1200Z Sep 28)... Tropical low centered at 7.5N-21W

IOH 24 Hr Forecast (1200Z Sep 29)... 35 mph maximum sustained wind tropical depression centered at 8.5N-25W

IOH 48 Hr Forecast (1200Z Sep 30)... 60 mph maximum sustained wind tropical storm centered at 9.5N-29W

IOH 72 Hr Forecast (1200Z Oct 1)... 90 mph maximum sustained wind hurricane centered at 10.5N-33W

IOH 96 Hr Forecast (1200Z Oct 2)... 100 mph maximum sustained wind hurricane centered at 12.5N-37W

IOH 120 Hr Forecast (1200Z Oct 3)... 75 mph maximum sustained wind hurricane centered at 16N-40W


AREA OF INTEREST #2 (REMNANTS OF PETER)... The remnant low pressure of what was Tropical Storm Peter continues northeast across the central Atlantic in the flow ahead of the eastern Canada frontal low supported by the upper trough emerging from eastern North America. The divergence zone of this upper trough is forecast to produce another frontal low offshore of the northeastern US in the next 24 hours...which will cause the northeast track of Ex-Peter to continue. My updated forecast track below is adjusted south and west due to the current position of Ex-Peter. The shear produced by the approaching upper trough had not been too bad on Ex-Peter... which through this morning allowed Ex-Peter to fire an organized flare of thunderstorms over and near its swirl center. However as of late it appears the shear is increasing as the upper trough is nearing... with the thunderstorms now well-removed from the swirl. And with Ex-Peter heading to waters below 26 deg C by 24 hours... odds that Peter becomes a tropical cyclone again are decreasing. My odds of development in the next 12 hours are halved from my previous update's 70%... and are now at 35%. I have 12-hour odds above 0% in case Peter fires another thunderstorm flare over or near the center and makes another run at becoming a short-lived tropical cyclone. However my 24-hour odds are 0% due to the cooler water temps at the forecast location. This is my planned final statement on Ex-Peter on this blog unless it indeed becomes a tropical cyclone... or if for some reason Ex-Peter is still an area of interest in the NHC outlook product by my next update.

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

IOH 12 Hr Outlook (0000Z Sep 29)...35% chance of tropical cyclone formation (northwest Atlantic near 35.5N-57W)

IOH 24 Hr Outlook (1200Z Sep 29)... 0% chance of tropical cyclone formation (northwest Atlantic near 37.5N-55.5W)


AREA OF INTEREST #3... The tropical wave of low pressure that was over central Africa appears to now be over western Africa near 6N-2.5W based on the cloudiness and loosely-packed thunderstorm clusters in the region. This position is east of my prior forecast for this system...thus increasing the separation between this wave and area of interest (AOI) #1. Therefore this system will no longer undergo a fujiwhara interaction with AOI #1. Although none of the models develop this tropical wave as it moves offshore... the tropical upper ridging with low shear and upper outflow that has aided in the development of area of interest #1 will remain in place. Therefore I am maintaining this tropical wave as an area of interest for tropical development. I assign low 10% odds of development by day 5 until model support develops or until the tropical wave develops increasing thunderstorms.

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

IOH 24 Hr Outlook (1200Z Sep 29)...0% chance of tropical cyclone formation (western Africa near 6.5N-7.5W)

IOH 48 Hr Outlook (1200Z Sep 30)...0% chance of tropical cyclone formation (western Africa near 7N-12.5W)

IOH 72 Hr Outlook (1200Z Oct 1)...0% chance of tropical cyclone formation (offshore of Africa near 7.5N-17.5W)

IOH 96 Hr Outlook (1200Z Oct 2)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-22W)

IOH 120 Hr Outlook (1200Z Oct 3)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-26W)


AREA OF INTEREST #4...The area of disturbed weather west of area of interest (AOI) #1 and east of Hurricane Sam...induced by a pair of tropical waves of low pressure at the surface and the divergent nature of the flow aloft induced by the tropical Atlantic upper ridge... has become better defined toward the east as the eastern of the two waves has developed a low pressure spin with thunderstorm banding features. The low pressure spin appears to be near 8N-32W as of this writing. The more eastward position of this system places it closer to AOI #1. As a result...the updated forecast track below...as well as the latest models...now show a slower westward track suppressed to the south as this system will now undergo a fujiwhara interaction with AOI #1. Because AOI #1 has a head start in its organization over this system...it is likely to produce dominating surface inflow and upper outflow in the region... therefore I have lowered my peak odds for this system's development from 50% in my last update to now 40% in this update. The odds are continuously trimmed down from the 40% through 120 hours as I forecast the dominance of AOI #1 to increase quickly. By 96 hours I have development odds as low as 10% as westerly shear on the south side of the approaching eastern Atlantic upper vortex likely begins to negatively affect this system. As AOI #1 bends north in track and away...the fujiwhara interaction is likely to end after 96 hours which may allow for an increase in forward speed by day 5. My odds of development are kept at a low 10% at day 5 due to the aforementioned likelihood of westerly shear.

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

IOH 24 Hr Outlook (1200Z Sep 29)...40% chance of tropical cyclone formation (eastern tropical Atlantic near near 7.5N-34W)

IOH 48 Hr Outlook (1200Z Sep 30)...20% chance of tropical cyclone formation (eastern tropical Atlantic near near 7.5N-37W)

IOH 72 Hr Outlook (1200Z Oct 1)...20% chance of tropical cyclone formation (eastern tropical Atlantic near near 7.5N-39.5W)

IOH 96 Hr Outlook (1200Z Oct 2)...10% chance of tropical cyclone formation (central tropical Atlantic near near 7.5N-42.5W)

IOH 120 Hr Outlook (1200Z Oct 2)...10% chance of tropical cyclone formation (central tropical Atlantic near near 7.5N-46.5W)


...COMPUTER MODEL SUMMARY...

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


0000Z CMC Model Run...

**For Hurricane Sam...passes east of Bermuda and located at 36.5N-57.5W at 120 hours

**For Area of Interest #1... located at 16N-39W at 120 hours as a tropical cyclone

**For area of interest #2 (Ex-Peter)... absorbed by frontal low offshore of the northeastern US at 36 hours

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

**For area of interest #4... no development shown


0000Z ECMWF Model Run...

**For Hurricane Sam...passes east of Bermuda and located at 33N-59.5W at 120 hours

**For Area of Interest #1... located at 15N-35W at 120 hours as a tropical cyclone

**For area of interest #2 (Ex-Peter)... absorbed by frontal low offshore of the northeastern US at 48 hours

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

**For area of interest #4... no development shown


0600Z GFS Model Run...

**For Hurricane Sam...passes east of Bermuda and located at 35.5N-55W at 120 hours

**For Area of Interest #1... located at 18N-37.5W at 120 hours as a tropical cyclone

**For area of interest #2 (Ex-Peter)... absorbed by frontal low offshore of the northeastern US at 39 hours

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

**For area of interest #4... no development shown

**Tropical wave currently at 45W longitude evolves into central Caribbean broad tropical low near 12.5N-77.5W at 168 hours


0600Z NAVGEM Model Run...

**For Hurricane Sam...passes east of Bermuda and located at 39.5N-60.5W at 120 hours

**For Area of Interest #1... located at 18N-37.5W at 20N-35W as a tropical cyclone

**For Area of Interest #2 (Ex-Peter)...transitions into a frontal low that moves rapidly east across the north Atlantic...intensifies into a frontal cyclone near the British Isles at 120 hours.

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

**For Area of Interest #4...Stationary while in a fujiwhara interaction with Area of Interest #1 and is absorbed by Area of Interest #1 at 78 hours.

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