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 #127 (Weekend Edition)

Updated: Oct 4

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


...SUNDAY OCTOBER 3 2021 12:10 AM EDT...

Satellite image as of 1750Z on Saturday just before sunset. Areas of interest circled in yellow are not mentioned in the NHC tropical weather outlook. Areas circled in a green dashed line are in the NHC 5-day tropical weather outlook. Areas circled in a solid green line are in the NHC 2-day tropical weather outlook:

NHC TAFB Surface Analysis 1200Z (Saturday Oct 2):

GFS Model Upper-Level Winds 1200Z (Saturday Oct 2):

The Atlantic tropics remain busy with multiple systems to watch:

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

**See area of interest #1 section below for an update on the tropical wave of low pressure east of Victor.

**See area of Interest #2 section below for the potential for heavy rains and possible formation of a subtropical to tropical disturbance over the Dominican Republic and the Bahamas after this weekend.


MAJOR HURRICANE SAM...At long last… as Sam enters the open northwest Atlantic waters after passing east of Bermuda… southerly shear induced of the nearby upper trough to the west has finally been able weaken the hurricane. The previously well-defined eye has collapsed with the remaining shield of thunderstorms biased to the north side of the circulation due to the shear. Sam has also been moving across marginally warm 26 deg C waters...which are cooler than the 28 deg C water Sam was previously over. This has also been a likely factor is Sam’s weakening. The weakening rate has been and will continue to be gradual as the east side of the aforementioned 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. My updated intensity forecast is unchanged as Sam has been following the previous forecast thus far.


Regarding track... Sam has continued north into the surface ridge weakness induced by the eastern divergence zone of the upper trough. Once the upper trough shifts east Sam’s track will likewise bend northeast. The updated forecast below has a slightly higher forward speed as Sam has continued to move faster… and the model guidance also agrees with a higher speed. Transition to a non-tropical frontal cyclone supported by the eastern divergence zone of the upper trough is anticipated by 48 hours once Sam reaches waters below 26 deg C.


Given these forecast updates:

**Bermuda... the mid-Atlantic US coast...northeast US coast...Atlantic Canada coast… eastern Greenland… and Iceland will see coastal sea swells generated by Hurricane Sam and its eventual remnant frontal cyclone over the next few days.

**Sea swells will be particularly vigorous for coastal Newfoundland after this weekend… and be particularly vigorous for the east coast of Greenland and Iceland by the middle of next week... as Sam passes nearby.

**Sam’s tropical storm wind field passed east of Bermuda… the tropical storm warning was lifted from the island as of 5 AM EDT on Saturday

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

0 Hr Position (1800Z Oct 2)… 125 mph maximum sustained wind major hurricane centered at 35.4N-58.2W

IOH 24 Hr Forecast (1800Z Oct 3)… 105 mph maximum sustained wind hurricane centered at 38N-55W

IOH 48 Hr Forecast (1800Z Oct 4)… 90 mph maximum sustained wind remnant frontal cyclone centered at 42N-44W


TROPICAL STORM VICTOR (RECENTLY DOWNGRADED TO TROPICAL DEPRESSION VICTOR)... Victor continues to weaken while southerly shear induced by the east side of the central Atlantic upper vortex pushes the thunderstorms to the north of the center… and also due to dry Saharan air surrounding the tropical cyclone to the east… west… and north. Victor as of 5 PM EDT Saturday afternoon has recently weakened to a tropical depression… and with the current negative conditions not expected to abate in the next 24 hours I now forecast Victor to become a remnant low soon. The track has leaned more west and less north as the weakening cyclone has become more shallow… thus feeling less of a drag from the upper southerly shearing winds. My updated forecast track below is adjusted accordingly. This is my planned final statement on Victor on this blog unless (1) Victor still hangs on as a tropical cyclone by my next update… or (2) the remnants of Victor make a comeback as the shear may relax after 24 hours once the current central Atlantic upper vortex shifts northeast and away in the flow ahead of the west Atlantic upper trough interacting with Sam.

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

0 Hr Position (1800Z Oct 2)… 35 mph maximum sustained wind tropical depression centered at 13.7N-38W

IOH 24 Hr Forecast (1800Z Oct 3)… Remnant low centered at 17.5N-41.5W


AREA OF INTEREST #1 ... The tropical wave of low pressure in the eastern tropical Atlantic has become a little better defined while developing a rotation with outer cloud bands… and the rotation is centered near 9N-21W as of tonight. The thunderstorm activity at present remains limited as the wave appears embedded within a recent surge of dry Saharan air. Although none of the models develop this tropical wave... the tropical upper ridging with low shear and upper outflow that aided in the development of Victor a few days ago will remain in place in the days ahead. Therefore I am maintaining this tropical wave as an area of interest for tropical development. Because the wave has not been able to develop sustained thunderstorms… I am continuing low peak 5-day development odds of 10%. The forecast track points below are also nudged east when compared to my previous forecast due to the current position of the wave.


What will be the warm sector of Sam’s remnant frontal cyclone will amplify upper ridging in the northeast Atlantic... which will cause the upper vorticity currently in the east Atlantic to shift south by days 2 to 4. This upper vorticity should prevent any waves behind this wave (to the east) from developing. Meanwhile this wave should just escape the influence of this upper vorticity... and it will be interesting to see if split flow upper divergence between the west side of the upper vorticity and south side of the tropical upper ridging manifests itself over this wave. Such a split flow upper divergence regime could aid in this wave’s development. This favorable upper air pattern will subside by day 5 as the wave nears southerly and potentially shearing upper flow to be induced by central Atlantic upper vorticity to be left behind by the large upper vortex currently in the region. This is also another reason I have low development odds on day 5.

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

IOH 24 Hr Outlook (1800Z Oct 3)… 0% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-25W

IOH 48 Hr Outlook (1800Z Oct 4)… 5% chance of tropical cyclone formation (eastern tropical Atlantic near 10.5N-29W)

IOH 72 Hr Outlook (1800Z Oct 5)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-34W)

IOH 96 Hr Outlook (1800Z Oct 6)… 10% chance of tropical cyclone formation (central tropical Atlantic near 11.5N-39W)

IOH 120 Hr Outlook (1800Z Oct 7)… 10% chance of tropical cyclone formation (central tropical Atlantic near 12N-44W)


AREA OF INTEREST #2...The north end of a currently inactive eastern Caribbean tropical wave of low pressure currently passing 62.5W longitude has potential to interact with amplified upper vorticity to be left behind by the current west Atlantic upper trough and also the upper trough currently approaching from western North America in the days ahead. This could result in the development of a subtropical to tropical disturbance in the vicinity of the Dominican Republic and Bahamas after this weekend... especially if the upper vorticity is amplified enough to keep shear levels lower and upper divergence elevated. The GFS and NAVGEM models continue to champion this idea… and for now my development odds for a subtropical or tropical cyclone are kept at a low 20% peak over the next five days until either additional models come on board… and/or until a disturbance forms with signs of organization. It is worth noting that already bands of showers and thunderstorms are brewing north of the Caribbean Islands under the supportive eastern divergence zone of the southern base of the west Atlantic upper trough… and this will continue once the trough leaves behind upper vorticity as mentioned above. Therefore it appears a disturbance is likely to emerge in the region in the days ahead.


The updated forecast track below is nudged west due to the current position of the tropical wave which is further west of my previous forecast. The positions are based on the divergence maximum of the upper vorticity to be left behind by the sequence of upper troughs as mentioned in the above paragraph. The GFS and now the NAVGEM also suggest another tropical low pressure area forming further south in the western Caribbean as the upper vorticity enhances the poleward (northbound) upper outflow in that part of the Caribbean. If these model trends continue… will consider adding an additional area of interest section in future updates to address that situation.

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

IOH 24 Hr Outlook (1800Z Oct 3)…0% chance of tropical cyclone formation (southeast of the Dominican Republic near 17.5N-68W)

IOH 48 Hr Outlook (1800Z Oct 4)… 0% chance of tropical cyclone formation (northwest coast of the Dominican Republic near 20N-71.5W)

IOH 72 Hr Outlook (1800Z Oct 5)… 10% chance of tropical cyclone formation (central Bahamas near 22.5N-74W)

IOH 96 Hr Outlook (1800Z Oct 6)… 20% chance of tropical cyclone formation (western Bahamas near 26N-78W)

IOH 120 Hr Outlook (1800Z Oct 7)… 20% chance of tropical cyclone formation (offshore of southeastern US near 29.8N-79W)


...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...located southwest of Iceland as an intense frontal cyclone by 120 hours.

**For Tropical Depression Victor... located at 25N-54W at 120 hours as a dissipating remnant surface trough

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

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


1200Z ECMWF Model Run...

**For Hurricane Sam...located southwest of Iceland as an intense frontal cyclone by 120 hours.

**For Tropical Depression Victor... located at 25N-55W at 120 hours as a dissipating remnant surface trough

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

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


1800Z GFS Model Run...

**For Hurricane Sam... located southwest of Iceland as an intense frontal cyclone by 120 hours.

**For Tropical Depression Victor... weakens to a surface trough which dissipates near 23N-51.5W at 84 hours

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

**For area of interest #2... north end of wave currently at 61.5W longitude evolves into amplified surface trough just north of the Bahamas by 69 hours...trough lifts north to a location just offshore of the Carolinas by 120 hours


1200Z NAVGEM Model Run...

**For Hurricane Sam... located just southwest of Iceland as an intense frontal cyclone by 120 hours.

**For Tropical Depression Victor... located at 25N-55W at 120 hours as a dissipating remnant surface trough

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

**For Area of Interest #2...north end of wave currently at 61.5W longitude evolves into a surface low between south Florida and the western Bahamas at 168 hours

**Tropical wave currently at 61.5W produces additional tropical low in western Caribbean near 18N-84.8W at 168 hours

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