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 #124

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


...THURSDAY SEPTEMBER 30 2021 12:30 AM EDT...

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

**See Hurricane Sam and Tropical Storm Victor sections below for an update on the two currently active tropical cyclones in the Atlantic basin.

**See area of interest #2 and #3 sections below for information on low pressure areas west and east of Victor being monitored for development

**See area of Interest #1 section below for an update on the remnants of Peter.


MAJOR HURRICANE SAM...While passing northeast of the Lesser Antilles islands… Sam has remained a category 4 hurricane with maximum sustained winds fluctuating between 130 and 140 mph maximum sustained winds. As of 5 PM EDT aircraft recon spotted two concentric eye walls instead of a single eye wall… an indication another eye wall replacement cycle may be starting. In addition with the approach of the large upper trough approaching from the west… which will block the western outflow of the hurricane and begin to shear it… the forecast below calls for weakening going forward. The weakening rate will tend to be gradual as the east side of the upper trough is divergent as we always see with a mid-latitude upper trough which will aid Sam. 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 48 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 then northeast turn into the surface ridge weakness induced by the eastern divergence zone of the incoming upper trough is expected. The short-term forecast track below is the same as previous...but the long term GFS-ECMWF model guidance shows a slower turn to the northeast while the upper trough is shown to be less amplified than before. Therefore my longer term track has a slower forward speed. The ECMWF has significantly slowed its long range forward speed for Sam while showing a low enough amplitude in the trough that it leaves behind Sam… resulting in Sam idling around in the central Atlantic as a hurricane near 35N latitude (where waters are sufficiently warm) until the current upper trough over western North America picks up Sam several days later. The long range forecast track below splits the difference between the GFS and ECMWF. Note that if the GFS is correct… Sam would reach cooler

water transition into a non-tropical frontal cyclone supported by the incoming upper trough sooner than indicated below.


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 48+ hours.


Update as of 11 PM EDT… Sam has abruptly strengthened to 145 mph maximum sustained winds after completing the eye wall replacement cycle that started at 5 PM EDT. For now I am maintaining the intensity forecast below as the approaching upper trough is expected to begin impacting shear by 24 hours as discussed above.

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

0 Hr Position (0000Z Sep 30)… 140 mph maximum sustained wind major hurricane centered northeast of the Lesser Antilles at 20.4N-58W

IOH 24 Hr Forecast (0000Z Oct 1)... 120 mph maximum sustained wind major hurricane centered at 24N-61.5W

IOH 48 Hr Forecast (0000Z Oct 2)… 105 mph maximum sustained wind hurricane centered east-southeast of Bermuda at 30N-61.5W

IOH 72 Hr Forecast (0000Z Oct 3)… 95 mph maximum sustained wind hurricane centered at 35N-59W

IOH 96 Hr Forecast (0000Z Oct 4)… 90 mph maximum sustained wind hurricane centered at 37.5N-55W

IOH 120 Hr Forecast (0000Z Oct 5)… 80 mph maximum sustained wind hurricane centered at 38.5N-52.5W


TROPICAL STORM VICTOR... The latest tropical wave of low pressure in the far eastern Atlantic has continued to organize… becoming tropical depression twenty this past morning and then Tropical Storm Victor in the afternoon hours. Despite the presence of the eastern Atlantic surface ridge...Victor is expected to have on a north angle in its westward track... initially from a fujiwhara interaction with area of interest #3... followed by the influence of southerly flow on the east side of the central Atlantic upper vortex as Victor nears the vortex. The upper southerly flow is expected to play a role in the steering as this Victor will more likely than not become a stronger/taller system able to couple with the upper flow. By the end of the five day forecast period… the surface ridge weakness induced by Hurricane Sam will also contribute to the north angle of Victor’s track. The updated forecast track is the same as the previous as Victor has remained on track.


Regarding intensity… I project brisk strengthening into a hurricane through 48 hours due to the supportive outflow and low shear of the eastern tropical Atlantic upper ridge in progress... and also due to the initial healthy structure that Victor is developing. Weakening is shown at 72 and 96 hours as Victor moves into southerly upper winds on the east side of the central Atlantic upper vortex which will shear this system. The vortex by 120 hours is forecast to weaken and become jettisoned northeast and away due to the flow ahead of the upper trough to approach from the western Atlantic. This will cause the shear to relax… and I forecast some re-strengthening of Victor during that time.

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

0 Hr Position (0000Z Sep 30)… 40 mph maximum sustained wind tropical storm centered at 8.3N-26W

IOH 24 Hr Forecast (0000Z Oct 1)… 70 mph maximum sustained wind tropical storm centered at 9.5N-29W

IOH 48 Hr Forecast (0000Z Oct 2)… 95 mph maximum sustained wind hurricane centered at 10.7N-34W

IOH 72 Hr Forecast (0000Z Oct 3)… 85 mph maximum sustained wind hurricane centered at 13.5N-37.5N

IOH 96 Hr Forecast (0000Z Oct 4)… 60 mph maximum sustained wind tropical storm centered at 17.8N-40.5W

IOH 120 Hr Forecast (0000Z Oct 5)… 75 mph maximum sustained wind hurricane centered at 22N-43W


AREA OF INTEREST #1 (REMNANTS OF PETER)... The remnant low pressure of what was Tropical Storm Peter continues northeast into the open northwest Atlantic in the flow ahead of the current northwest Atlantic frontal low centered offshore of the northeast US. Despite moving into cooler waters… there was a period of time on Wednesday when Ex-Peter generated renewed thunderstorms near its center. This was likely with the aid of the divergence zone of the upper trough associated with the aforementioned frontal low…. thus Ex-Peter has lingered as an area of interest in the NHC outlook product. The shearing upper southwesterly flow ahead of the upper trough has not allowed these thunderstorms to organize… and as of now Ex-Peter has lost this activity and has been assigned 0% odds of development in the NHC outlook product as of 8 PM EDT. Ex-Peter will likely soon lose its identity within the larger-scale surface low pressure field of the aforementioned frontal low… this is my final statement on Ex-Peter on this blog.


AREA OF INTEREST #2… The tropical wave of low pressure currently over western Africa appears to be located near 7.5N-8W based on a general increase in thunderstorms that has occurred in this region. 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 Victor will remain in place in the days ahead. Therefore I am maintaining this tropical wave as an area of interest for tropical development. I have slightly raised odds of development to 15% by day 5 due to the recent increase in thunderstorms associated with this wave. An additional increase in development odds in future updates will be possible should model support develop… or if this tropical wave develops signs of organization.

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

IOH 24 Hr Outlook (0000Z Oct 1)...0% chance of tropical cyclone formation (western Africa near 7.5N-12.5W)

IOH 48 Hr Outlook (0000Z Oct 2)… 0% chance of tropical cyclone formation (offshore of Africa near 8N-17.5W)

IOH 72 Hr Outlook (0000Z Oct 3)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-22W)

IOH 96 Hr Outlook (0000Z Oct 4)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 10.5N-26W)

IOH 120 Hr Outlook (0000Z Oct 5)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-30W)


AREA OF INTEREST #3… The tropical low pressure area between Victor and Sam has begun a fujiwhara interaction with Victor located nearby to the east… resulting in this system staying toward the south while located near 7.5N-37.5W as of 0000Z. A slow westward track located toward the south is expected to continue through 72 hours until Victor turns far north enough and away to end the fujiwhara interaction. An increase in forward speed is therefore probable by 96 and 120 hours. I have dropped peak odds of development to 20% as the thunderstorm activity of this feature has been on the decline due to the dominance of Victor’s surface inflow which is stealing much of the fueling low-level moisture in the region needed for this system to develop. This system in the longer range is expected to drift west into the atmosphere south of the central Atlantic upper vortex where westerly shear is possible. However the GFS suggests the upper vortex will not reach as far to the south as previously shown… which may result in less shear and thus my longer-term development odds are nudged upward. However I still taper down to a low 10% chance of development by day 5 as the GFS subtly suggests some of the upper vorticity associated with the vortex may be left behind at a location over this system where it can suppress its upper outflow.

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

IOH 24 Hr Outlook (0000Z Oct 1)… 20% chance of tropical cyclone formation (eastern tropical Atlantic near 7.5N-39.5W)

IOH 48 Hr Outlook (0000Z Oct 2)… 20% chance of tropical cyclone formation (eastern tropical Atlantic near 7.5N-42W)

IOH 72 Hr Outlook (0000Z Oct 3)… 20% chance of tropical cyclone formation (central tropical Atlantic near 7.5N-45W)

IOH 96 Hr Outlook (0000Z Oct 4)… 15% chance of tropical cyclone formation (central tropical Atlantic near 8.5N-49W)

IOH 120 Hr Outlook (0000Z Oct 5)… 10% chance of tropical cyclone formation (central tropical Atlantic near 9.5N-53W)


...COMPUTER MODEL SUMMARY...

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


1200Z CMC Model Run...Not available at above-mentioned source

1200Z ECMWF Model Run...

**For Hurricane Sam...passes east of Bermuda and located at 35N-55W at 120 hours

**For Tropical Storm Victor... weakens and becomes compact while reaching 20.5N-44.5W at 120 hours

**For area of interest #1 (Ex-Peter)... absorbed by northwest Atlantic frontal low in next 24 hours

**For area of interest #2… no development shown

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


1800Z GFS Model Run...

**For Hurricane Sam...passes east of Bermuda and located at 38.5N-51.5W at 120 hours

**For Tropical Storm Victor... Hurricane strength suggested at 24 hours near 10N-28W… weakens to a tropical storm near 17N-34W at 81 hours… located at 27N-39W as a remnant low at 120 hours

**For area of interest #1 (Ex-Peter)... absorbed by northwest Atlantic frontal low in next 24 hours

**For area of interest #2… no development shown

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

**North end of tropical wave currently at 42.5W longitude evolves into central Bahamas disturbance at 135 hours


1200Z NAVGEM Model Run...

**For Hurricane Sam...makes landfall over Newfoundland at 108 hours as a large and intense frontal cyclone… begins to shift east into the North Atlantic by 126 hours

**For Tropical Storm Victor... located at 23.5N-39.5W as a Hurricane at 120 hours

**For area of interest #1 (Ex-Peter)... absorbed by northwest Atlantic frontal low in next 24 hours

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

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

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