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 #128 (Weekend Edition)

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


...MONDAY OCTOBER 4 2021 7:30 AM EDT...

Satellite image as of 0140Z. 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 (Sunday Oct 3):

GFS Model Upper-Level Winds 1200Z (Sunday Oct 3):

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 southeast of Victor.

**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...Sam is moving northeast into the open North Atlantic while steered by the nearby upper trough to its west. Just as it appeared Sam was weakening faster than my previous intensity forecast… the hurricane’s storm shield became more symmetric around the center and a well-defined eye redeveloped on satellite pictures. In fact Sam is estimated to have re-strengthened to 105 mph maximum sustained winds as of the 11 PM EDT NHC advisory… which aligns with my previous intensity forecast that called for more gradual weakening. The temporary halt in weakening appears to be from multiple factors… (1) Sam is moving faster to the northeast which may have allowed it to catch up to the upper southwesterly wind speed… effectively reducing the upper southwesterly shear over the hurricane… (2) the jet of strongest winds on the east side of the upper trough appears to have lifted northward which may have also contributed to reduced shear… (3) the jet appears to be enhancing the northern outflow of the hurricane… (4) the hurricane is also embedded in the eastern divergence zone of the upper trough which is also aiding the hurricane’s surface low pressure field. My updated intensity forecast is the same as previous… which calls for weakening to re-commence and a loss of tropical characteristics once Sam moves into waters below 26 deg C in the next 24 hours. Even though Sam will lose tropical character… the list of the above otherwise favorable atmospheric conditions will allow Sam to be a vigorous frontal cyclone in the days ahead. My 24 hour forecast point is based on the location of the upper divergence maximum of the upper trough’s east side in the 18Z GFS model run… which is where the remnant frontal cyclone will initially be. After 24 hours… the frontal cyclone will whirl cyclonically toward the central axis of the upper trough as a typical post-mature frontal cyclone does.


Given these forecast updates:

**The 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 on Monday and Tuesday… and be particularly vigorous for the east coast of Greenland and Iceland by the middle of this week... as Sam passes nearby.

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

0 Hr Position (0000Z Oct 4)… 105 mph maximum sustained wind hurricane centered at 38.9N-52W

IOH 24 Hr Forecast (0000Z Oct 5)… 90 mph maximum sustained wind remnant frontal cyclone centered at 46N-41W


TROPICAL DEPRESSION VICTOR… Victor has not yet weakened to a remnant low pressure as it continues to fire thunderstorms flares in the face of southerly shear being generated by the axis of central Atlantic upper vorticity. The bulk of this upper vorticity is shifting northeast and away in the flow ahead of the west Atlantic upper trough interacting with Sam… which could allow the shear to relax. Despite this… none of the models hang on to Victor as a tropical cyclone for much longer… probably as some of the upper vorticity will be left behind in Victor’s near-term path which will suppress Victor’s upper outflow once Victor slides beneath the residual upper vorticity. Thus I agree with the NHC and models calling for Victor’s dissipation in the next 24 hours. Despite Victor being a shallow tropical Depression not being steered by the upper southerly shearing winds… the west track has had some north angle and is likely due to Sam’s surface ridge weakness… and so the 24-hour forecast point is a reflection of that. After that time… whatever is left of Victor will likely bend more west in track between 20N and 25N latitude as Sam lifts north and its surface ridge weakness closes. 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 after passing through the suppressing upper vorticity

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

0 Hr Position (0000Z Oct 4)… 35 mph maximum sustained wind tropical depression centered at 17.8N-42.5W

IOH 24 Hr Forecast (0000Z Oct 5)…Remnant low centered at 19N-47.5W


AREA OF INTEREST #1 ... The tropical wave of low pressure in the eastern tropical Atlantic has continued to become better defined while developing a large and broad area of rotating thunderstorms. Due to the broad nature of the wave… it is difficult to fix an exact center… and as of 0000Z I estimated the center of rotation to be near 9N-27.5W. However the west side of the wave appeared to become better organized near 9N-32.5W for a period of time. 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 remains in place. Therefore I am maintaining this tropical wave as an area of interest for tropical development. I have raised my peak 5-day development odds for this wave to 25% due to the increased thunderstorm activity. The forecast track points below are also nudged west when compared to my previous forecast due to the current position of the wave… and assume the wave will coalesce toward the more organized activity on its west side.


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. This upper vorticity should prevent any waves behind this wave (to the east) from developing. Meanwhile this wave should escape the influence of this upper vorticity... especially with its more west position compared to previous estimates. I begin to lower development odds by days 4 and 5 as the wave nears southerly shearing upper flow to be induced by central Atlantic upper vorticity to linger in the region.

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

IOH 24 Hr Outlook (0000Z Oct 5)… 5% chance of tropical cyclone formation (eastern tropical Atlantic near 9N-35W)

IOH 48 Hr Outlook (0000Z Oct 6)… 15% chance of tropical cyclone formation (central tropical Atlantic near 9.5N-40W)

IOH 72 Hr Outlook (0000Z Oct 7)… 25% chance of tropical cyclone formation (central tropical Atlantic near 10N-45W)

IOH 96 Hr Outlook (0000Z Oct 8)… 20% chance of tropical cyclone formation (central tropical Atlantic near 10.5N-50W)

IOH 120 Hr Outlook (0000Z Oct 9)… 10% chance of tropical cyclone formation (central tropical Atlantic near 11.5N-55W)


AREA OF INTEREST #2...The eastern divergence zone of amplified upper vorticity that has been left behind in the west Atlantic by the current northwest Atlantic upper trough has triggered an area of disturbed weather featuring a surface trough of low pressure over and near the southeastern Bahamas and numerous squalls of showers and thunderstorms. Since this disturbance already has its own surface trough… the north end of the current Caribbean Sea tropical wave of low pressure is unlikely to contribute to seeding a surface low pressure area. 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 North American upper trough. This disturbance is therefore expected to shift northwest with time while evolving into a feature supported by the eastern divergence zone of the eastern US upper vorticity. The forecast track below is largely the same as the previous and 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. I have raised development odds of this disturbance to 30% as model support among the CMC and ECMWF is increasing while the GFS and NAVGEM continue to promote this disturbance.


Of note… some model runs 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. These solutions are not yet consistent for me to declare another area of interest to the south of this one… however will closely monitor future observations and model runs in the days ahead for a possible second disturbance.


With these forecast updates:

**Heavy rains are possible for the southeastern Bahamas over the next 24 hours… with the rainfall potential to shift west across the remainder of the Bahamas later this week

**Coastal sea swells are possible for the Bahamas and southeast United States coast should tropical or subtropical development occur.

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

IOH 24 Hr Outlook (0000Z Oct 5)… 0% chance of tropical cyclone formation (southeastern Bahamas near 22N-72.5W)

IOH 48 Hr Outlook (0000Z Oct 6)… 10% chance of tropical cyclone formation (central Bahamas near 22.5N-74W)

IOH 72 Hr Outlook (0000Z Oct 7)… 20% chance of tropical cyclone formation (western Bahamas near 26N-78W)

IOH 96 Hr Outlook (0000Z Oct 8)… 20% chance of tropical cyclone formation (offshore of southeastern US near 29.8N-79W)

IOH 120 Hr Outlook (0000Z Oct 9)… 30% chance of tropical cyclone formation (offshore of southeastern US near 30.2N-79W)

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