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 #129

Updated: Oct 6

*******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 OCTOBER 5 2021 1:06 AM EDT...

The Atlantic tropics remain busy with multiple systems to watch:

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

**See area of interest #1 section below for an update on the tropical wave of low pressure in the eastern Atlantic

**See area of Interest #2 section below for an update on the potential for subtropical to tropical development in the vicinity of the Bahamas in the days ahead.


HURRICANE SAM... On its continued northeast journey into the far North Atlantic… Hurricane Sam is slowly but surely losing tropical characteristics due to cooler water temperatures and as the hurricane has wrapped in cooler dry air associated with the nearby upper trough to the west its interacting with… resulting in the loss of thunderstorms on the south side of its circulation tonight. The remaining northern thunderstorm activity and tropical characteristics has lasted as long as it has likely due to the supportive eastern divergence zone of the cold core upper trough which is only increasing as the upper trough rapidly amplifies from the southward push of cold air caused by the west side of Sam’s strong surface circulation. A gradual decline in the remaining thunderstorms and tropical characteristics is expected in the next 24 hours due to even cooler water temps that Sam is heading rapidly into… and also as the hurricane ingests more cool dry air. As Sam completes transition to a non-tropical frontal cyclone… the upper trough will have amplified into a vortex which will cyclonically whirl in Sam which is a typical event in a post-mature frontal cyclonic system. The 24-hour forecast point is based on the position of the upper vortex in the GFS model. After 24 hours… the upper vortex and Sam will shift northeast toward the waters between Greenland and Iceland… with Sam executing another cyclonic loop beneath the upper vortex at this location later this week.


Given these forecast updates:

**The Atlantic Canada coast… eastern Greenland… and Iceland will see coastal sea swells generated by Sam’s eventual remnant frontal cyclone over the next few days.

**Sea swells will continue to be particularly vigorous for coastal Newfoundland through tomorrow… and be particularly vigorous for the east coast of Greenland and Iceland later this week... as Sam passes nearby.


I plan this to be my final statement on Sam on this blog as transition to non-tropical is expected in the next 24 hours. Future updates regarding Sam’s impact to land areas will be carried on the home page bulletins of this site.

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

0 Hr Position (0000Z Oct 5)… 85 mph maximum sustained wind hurricane centered at 47.7N-40.2W

IOH 24 Hr Forecast (0000Z Oct 6)… 75 mph maximum sustained wind frontal cyclone centered at 54N-38.5W


AREA OF INTEREST #1 ... The tropical wave of low pressure in the eastern tropical Atlantic has continuously become better defined while the western side of its broad scattered thunderstorm field has developed a rotation near 7.5N-37.5W with increasingly defined banding features. In addition the wave remains under tropical upper ridging with low shear and upper outflow… therefore I am maintaining this tropical wave as an area of interest for tropical development. However my peak 5-day odds of development have been slightly lowered to 20% as the wave has not developed much in the last 24 hours… and now has an increasingly narrower window of time to develop further before it encounters less favorable southerly shearing upper winds being generated by the east side of the current axis of central Atlantic upper vorticity. The central Atlantic upper vorticity will tend to drift west into the Lesser Antilles while trying to link up with the current west Atlantic axis of upper vorticity as the west Atlantic axis shifts northeast in the mid-latitude westerlies. The central Atlantic upper vorticity will also continuously weaken while remaining cut-off from high-latitude cold air. The west drift and continuous weakening of the upper vorticity suggests a possible relaxation of the shear in the long range… however the GFS is too slow to weaken the upper vorticity such that I have very low odds of development for this wave by day 5. It is interesting to note the GFS develops another disturbance to the east of this wave in the eastern divergence zone of the weakening upper vorticity… so it will be interesting to see if in fact we end up with yet another disturbance to monitor in the days ahead. It is also interesting to see the NAVGEM try and develop this wave by 5+ days… perhaps as it sees the upper vorticity weakening fast enough to allow this wave to develop. Taking the NAVGEM idea with a grain of salt at this time as it tends to be a less reliable model when it comes to tropical development.


The forecast track is due west with a north angle toward the Lesser Antilles… with the north angle induced by the ridge weakness associated with area of interest #2… especially towards day 5. Regardless of whether or not this wave develops into a tropical cyclone… heavy rains maybe possible in the Lesser Antilles by day 5… especially if the eastern divergence zone of the weakening upper vorticity aids in generating thunderstorm activity during that timeframe.

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

IOH 24 Hr Outlook (0000Z Oct 6)… 10% chance of tropical cyclone formation (central tropical Atlantic near 8.5N-42.5W)

IOH 48 Hr Outlook (0000Z Oct 7)… 20% chance of tropical cyclone formation (central tropical Atlantic near 9N-47.5W)

IOH 72 Hr Outlook (0000Z Oct 8)… 15% chance of tropical cyclone formation (central tropical Atlantic near 10N-52.5W)

IOH 96 Hr Outlook (0000Z Oct 9)… 10% chance of tropical cyclone formation (east of the southern Lesser Antilles near 11.5N-57.5W)

IOH 120 Hr Outlook (0000Z Oct 10)… 5% chance of tropical cyclone formation (Lesser Antilles near 13N-61.5W)


AREA OF INTEREST #2...The eastern divergence zone of amplified upper vorticity in the west Atlantic continues to produce an area of disturbed weather featuring a surface trough of low pressure near the southeastern Bahamas and numerous squalls of showers and thunderstorms. The west Atlantic upper vorticity is expected to shift northeast in the flow ahead of another round of upper vorticity to be left behind over the eastern US by the current central US upper trough. This disturbance is therefore expected to shift northwest through 96 hours while evolving into a feature supported by the eastern divergence zone of the eastern US upper vorticity. The forecast track below is initially based on the location of the divergence maximum of the west Atlantic upper vorticity… followed by the location of the divergence maximum to be generated by the eastern US upper vorticity. The track is likely to shift east or northeast by day 5 as the eastern US upper vorticity pushes into the western Atlantic. I maintain peak 5-day development odds of 30% for this disturbance as most models agree on keeping a well-defined disturbance in the days ahead with an amplified surface trough signature.


Of note… models have stopped showing the development of a second western Caribbean disturbance to the south. Instead some model runs suggest another disturbance that forms southeast of this area of interest and north of the Bahamas with the support of the eastern divergence zone of the eastern US upper vorticity once that vorticity moves into the west Atlantic after day 5. It is possible the remnants of Victor become involved in the genesis of this second disturbance if the remnants survive long enough. It is also possible that instead of having a second disturbance… this area of interest instead reforms to the southeast after day 5 and takes another stab at trying to develop.


With these forecast updates:

**The heavy rain risk appears to have reduced for the Bahamas as the disturbance has become concentrated more toward the north. However heavy rain episodes from the southern outer bands of this disturbance cannot be ruled out.

**Coastal sea swells are possible for the Bahamas… southeast United States coast… and mid-Atlantic United States coast should tropical or subtropical cyclone development occur. Even without the formation of a cyclone… surface pressures are expected to be rather low offshore of the southeast US in the days ahead due to the upper divergence pattern to be induced by the eastern US upper vorticity. In addition surface pressures over the northeast US coast will be elevated due to the western convergence zone of the current west Canada upper trough as that trough later shifts east. The pressure difference between the northeast US coastal high pressure and surface low pressure offshore of the southeast US will drive strong onshore winds that will kick up the surf across the mid-Atlantic US coast in the days ahead.

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

IOH 24 Hr Outlook (0000Z Oct 6)… 0% chance of tropical cyclone formation (north of the eastern Bahamas near 24N-72.5W)

IOH 48 Hr Outlook (0000Z Oct 7)… 10% chance of tropical cyclone formation (north of the central Bahamas near 26N-75W)

IOH 72 Hr Outlook (0000Z Oct 8)… 20% chance of tropical cyclone formation (offshore of southeastern US near 29.8N-76W)

IOH 96 Hr Outlook (0000Z Oct 9)… 30% chance of tropical cyclone formation (offshore of southeastern US near 30.2N-77.5W)

IOH 120 Hr Outlook (0000Z Oct 10)… 30% chance of tropical cyclone formation (offshore of southeastern US near 32.5N-75W)


...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 Hurricane Sam...as a remnant frontal cyclone it cyclonically loops between Iceland and Greenland from 72 to 96 hours… stationary from 96 to 120 hours

**For area of interest #1... no development shown

**For area of interest #2...by 156 hours develops a closed circulation just offshore of the North Carolina Outer Banks… possible subtropical to tropical development suggested in long range as track drifts eastward


1200Z ECMWF Model Run...

** For Hurricane Sam...as a remnant frontal cyclone it cyclonically loops between Iceland and Greenland from 72 to 120 hours

**For area of interest #1... no development shown

**For area of interest #2... surface trough located just offshore of Georgia and South Carolina from 72 to 120 hours


1800Z GFS Model Run...

**For Hurricane Sam... as a remnant frontal cyclone it cyclonically loops between Iceland and Greenland from 66 to 120 hours

**For area of interest #1... no development shown

**For area of interest #2... surface trough becomes better defined near 24.5N-72.5W at 24 hours… stalls offshore of the southeast US from 60 hours to 93 hours… drifts southeast to 30N-74W from 96 to 120 hours… drifts west back toward the southeast US coast through 168 hours


1200Z NAVGEM Model Run...

**For Hurricane Sam... as a remnant frontal cyclone it cyclonically loops between Iceland and Greenland from 66 to 120 hours

**For area of interest #1... tropical wave amplifies into a tropical low over the Lesser Antilles at 126 hours… tropical low approaches south coast of Dominican Republic by 168 hours

**For Area of Interest #2...surface trough located just offshore of Carolinas by 84 hours… moves southwest into Georgia coast at 126 hours… loses definition to a more potent surface trough to the southeast after that time.

**Another and more potent surface trough develops over the southeast Bahamas at 138 hours… evolves into possible subtropical to tropical cyclone near 31N-76.5W by 168 hours

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