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 #126

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

...FRIDAY OCTOBER 1 2021 10:33 PM EDT...

The Atlantic tropics remain busy 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 #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 by early next week.


MAJOR HURRICANE SAM...While en route to pass just east of Bermuda… aircraft reconnaissance has shown that Sam has maintained an incredible category 4 strength strength with 150 mph maximum sustained winds for much of today. This is a little stronger than yesterday’s 145 mph maximum sustained winds. This strength is ongoing despite southerly shear generated by the east side of the nearby upper trough to the west which is keeping the ring of thunderstorms around the eye lopsided toward the north… and the intensity of the thunderstorms around the eye on colorized infrared satellite has not been very intense as typically seen in such a strong hurricane. In fact as of the 11 AM EDT advisory… the NHC stated that infrared satellite intensity estimates such as the Dvorak technique suggest a lower strength than aircraft recon has been finding. What we probably have here is the thunderstorm latent heat release levels and resultant warm core upper outflow levels needed for a typical category 3 hurricane… with the jet of stronger upper southerly winds lurking just north of the hurricane boosting its northern outflow such that the central surface pressure is being kept very low such that we have a category 4 hurricane instead. In addition no new eye wall replacement cycles have occurred which would have otherwise disrupted the hurricane.


In lieu of the above notes... my intensity forecast for Sam is once again raised. Weakening is inevitable once Sam moves directly into the stronger jet of southerly upper winds which would allow these winds to more strongly shear the hurricane instead of aiding it. In fact the weakening trend may be starting as shear finally appears to be eroding the thunderstorms on the southwest side of the eye. Note the weakening rate will tend 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. Sam has also begun moving into marginally warm 26 deg C waters east of Bermuda...which are cooler than the 28 deg C water Sam was previously over. This should also help start the weakening of Sam.


Regarding track... a classic north turn into the surface ridge weakness induced by the eastern divergence zone of the upper trough has occurred. Once the upper trough shifts east Sam’s track will likewise bend northeast. Sam has arrived to 30N latitude a little sooner than I previously projected… so the updated forecast track below is largely based on the 0000Z forecast points from previous post #125 but having Sam arrive to those points a little sooner at 1800Z. The short-term forecast points are also nudged westward due to Sam’s current position… with the longer-range points the same or nudged northeast as the latest model guidance shows a faster northeast track than in prior runs. The models show a more amplification of the steering upper trough which results in a faster track. This makes sense as Sam arrived further north to 30N latitude sooner and Sam is also stronger than previously forecast.. with the further north position placing Sam closer to the upper trough with the higher strength allowing Sam’s west side to pull more colder air associated with the upper trough southward… thus making the upper trough more amplified. Transition to a non-tropical frontal cyclone supported by the eastern divergence zone of the amplifying upper trough is anticipated by 72 hours once Sam reaches waters below 26 deg C.


Given these forecast updates:

**Bermuda... the mid-Atlantic US coast...northeast US coast...and Atlantic Canada coasts will see sea swells generated by Hurricane Sam

**Sea swells will be particularly vigorous for Bermuda over the next several hours... and be vigorous for coastal Newfoundland by early next week... as Sam passes nearby.

**Sam’s tropical storm wind field is on track to pass east of Bermuda… the tropical storm warning will likely be dropped in the coming hours.

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

0 Hr Position (1800Z Oct 1)… 150 mph maximum sustained wind major hurricane centered east-southeast of Bermuda at 30N-61.8W

IOH 24 Hr Forecast (1800Z Oct 2)… 125 mph maximum sustained wind major hurricane centered at 35N-59.5W

IOH 48 Hr Forecast (1800Z Oct 3)… 105 mph maximum sustained wind hurricane centered at 37.5N-55W

IOH 72 Hr Forecast (1800Z Oct 4)… 90 mph maximum sustained wind remnant frontal cyclone centered at 41N-45W


TROPICAL STORM VICTOR... Victor has peaked in intensity and is now likely weakening while southerly shear induced by the east side of the central Atlantic upper vortex which is pushing the thunderstorms to the north of the center… and also due to dry Saharan air surrounding the tropical storm to the east… west… and north. Despite the ongoing presence of a strong eastern Atlantic surface ridge... Victor has maintained a north angle in track due to the southerly shearing upper flow which Victor is still strong/tall enough to remain coupled to. The model guidance has shifted more west and less north track-wise in the long range as the surface ridge weakness induced by Sam is forecast to close more quickly as Sam as now forecast to accelerate more rapidly to the northeast. A less north and more west track also makes sense as Victor has not reached hurricane strength… and thus will weaken under the shear to a shallow tropical depression sooner which decouples from the upper southerly winds that have otherwise been dragging Victor into a north angle. The GFS has once again shifted to a faster northeast track for the central Atlantic upper vortex which allows for the shear to somewhat relax once the vortex moves away… and with the long range track having a less north angle Victor may dodge the worst of the shearing upper winds on the south side of the upper trough to approach from the west Atlantic. As a result my updated forecast hangs on to Victor as a tropical depression through day 5.

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

0 Hr Position (1800Z Oct 1)… 65 mph maximum sustained wind tropical storm centered at 12N-34.3W

IOH 24 Hr Forecast (1800Z Oct 2)… 45 mph maximum sustained wind tropical storm centered at 15N-37.5N

IOH 48 Hr Forecast (1800Z Oct 3)… 35 mph maximum sustained wind tropical depression centered at 18.5N-40.5W

IOH 72 Hr Forecast (1800Z Oct 4)… 35 mph maximum sustained wind tropical depression centered at 20.5N-44W

IOH 96 Hr Forecast (1800Z Oct 5)… 35 mph maximum sustained wind tropical depression centered at at 22N-49W

IOH 120 Hr Forecast (1800Z Oct 6)… 35 mph maximum sustained wind tropical depression centered at at 22.5N-54W


AREA OF INTEREST #1 ... The tropical wave of low pressure that was over Western Africa is now over the eastern tropical Atlantic in the vicinity of 7.5N-17.5W. The wave features outer curved cloud bands in particular on its south side… with the thunderstorm activity at present 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 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. Because the wave has seen an overall decline in thunderstorms in the last 24 hours… I have trimmed down my peak 5-day development odds to 10%.


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 3 to 5. 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.

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

IOH 24 Hr Outlook (1800Z Oct 2)… 0% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-22W)

IOH 48 Hr Outlook (1800Z Oct 3)… 0% chance of tropical cyclone formation (eastern tropical Atlantic near 10.5N-26W)

IOH 72 Hr Outlook (1800Z Oct 4)… 5% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-30W)

IOH 96 Hr Outlook (1800Z Oct 5)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 11.5N-35W)

IOH 120 Hr Outlook (1800Z Oct 6)… 10% chance of tropical cyclone formation (central tropical Atlantic near 12N-40W)


AREA OF INTEREST #2...The north end of a currently inactive tropical wave of low pressure currently passing 55W 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 by early next week. This could result in the development of a subtropical to tropical disturbance in the vicinity of the Dominican Republic and Bahamas by early next week... 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… with the NAVGEM now explicitly showing the formation of a subtropical cyclone in the long range. As a result I have raised development odds by day 5… but modestly to 20% as the NAVGEM model is not usually the most reliable model for tropical development.

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

IOH 24 Hr Outlook (1800Z Oct 2)… 0% chance of tropical cyclone formation (just east of the Lesser Antilles near 15N-60W)

IOH 48 Hr Outlook (1800Z Oct 3)…0% chance of tropical cyclone formation (just southeast of Puerto Rico near 16.5N-65W)

IOH 72 Hr Outlook (1800Z Oct 4)… 0% chance of tropical cyclone formation (Dominican Republic near 19N-70.5W)

IOH 96 Hr Outlook (1800Z Oct 5)… 10% chance of tropical cyclone formation (southeastern Bahamas near 22N-72.5W)

IOH 120 Hr Outlook (1800Z Oct 6)… 20% chance of tropical cyclone formation (western Bahamas near 25N-77.5W)


...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...passes east of Bermuda and located in the open North Atlantic as an intense frontal cyclone by 120 hours.

**For Tropical Storm Victor... located at 28N-45W at 120 hours as a remnant low

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

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


1200Z ECMWF Model Run...

**For Hurricane Sam...passes east of Bermuda and located in the open North Atlantic as an intense frontal cyclone by 120 hours.

****For Tropical Storm Victor... located at 25N-52.5W 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 GFS Model Run...

**For Hurricane Sam...passes east of Bermuda and located in the open North Atlantic as an intense frontal cyclone by 120 hours.

**For Tropical Storm Victor... located at 24N-54W 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 55W longitude evolves into amplified surface trough over western Bahamas by 96 hours...trough lifts north and evolves into a surface low near 29N-77W at 120 hours.


1200Z NAVGEM Model Run...

**For Hurricane Sam... located just east of Newfoundland as a cyclonically looping intense frontal cyclone at around 90 hours...cyclone accelerates east into the open north Atlantic by 120 hours.

**For Tropical Storm Victor... located at 25N-51W at 120 hours as a remnant low

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

**For Area of Interest #2...north end of wave currently at 55W longitude evolves into amplified surface trough north of the Bahamas by 102 hours...subtropical or tropical cyclone suggested near 32N-71W at 126 hours.

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