BIRDSEYE VIEW POSTS

Since 2012 on the Weather Underground (www.wunderground.com) 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 via Disqus on Weather Underground at www.wunderground.com/cat6. You can see my Disqus feed at this link for my latest comments. Feel free to reply to me with your disqus account or e-mail at IOHurricanes@outlook.com 

 
 
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MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #112

*******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 SEPTEMBER 4 2020 12:39 PM EDT...

See Tropical Depression Omar section below for the only currently active tropical cyclone in the Atlantic basin. See area of interest sections below for all areas of interest being monitored for tropical cyclone formation.


TROPICAL DEPRESSION OMAR...Omar continues to hang on as a tropical cyclone while blasted by northerly wind shear on the west side of a shortwave upper trough...and a recent thunderstorm burst just southeast of the cloud swirl center has caused National Hurricane Center to keep Omar as a re-strengthened 35 mph maximum sustained wind tropical depression in their 11 AM EDT advisory after weakening it to 30 mph maximum winds in the overnight. Omar has been stuck beneath the west side of this trough while continuing to be gravitated toward the frontal low pressure to the east-northeast that has been supported by the eastern divergence zone of the trough. This frontal low pressure has recently been introduced into the National Hurricane Center tropical weather outlook for potential acquisition of tropical characteristics...see area of interest #4 section below for details. I have nudged my forecast points southwestward as satellite animation shows Omar stalling as it runs into the west side of the Atlantic subtropical surface ridge...which has delayed an inevitable turn to the northeast around this ridge. My forecast track keeps Omar over 26+ deg C waters for the next 24 hours...so I wait till 48 hours to make Omar a dissipating system. In the next 24 hours the shear direction will switch from northerly to southwesterly as the upper trough currently over central Canada finally approaches while the shortwave upper trough to the east that has been shearing Omar moves away. However my 24 hour forecast point is just south of the southwesterly shearing winds...so it wouldn't surprise me if Omar briefly re-strengthens by 24 hours before the southwesterly shear hits it after that time.

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

0 Hr Position Hr Forecast (1200Z Sep 4)...35 mph maximum sustained wind tropical depression centered in the northwestern Atlantic at 35.2N-57.8W

IOH 24 Hr Forecast (1200Z Sep 5)...35 mph maximum sustained wind tropical depression centered in the northwestern Atlantic at 39N-56W

IOH 48 Hr Forecast (1200Z Sep 6)...Absorbed by cold front while passing 44N-52W


AREA OF INTEREST #1...The tropical wave of low pressure in the far eastern tropical Atlantic continues to merge with another low pressure immediately to the west which was recently downgraded to a tropical wave. This merger is creating a large area of curved thunderstorm bands to the southwest of the Republic of Cabo Verde Islands...with the curvature in the thunderstorm bands suggesting the lowest pressure maybe near 14N-26W. My updated outlook below is a continuity of the previous one as the computer models and current satelltie animation suggest not much has changed...albeit I have nudged my forecast points northeastward given that I estimate the lowest pressure to be a bit further northeast of my previous outlook. The models agree that this disturbance and tropical low pressure in area of interest #2 will orbit each other in a fujiwhara type of interaction...keeping area of interest #2 generally stationary while this system tracks more northward in its westward track. My forecast track in the outlook below in the next 24 hours bends the track of this system northward as it undergoes a fujiwhara interaction with area of interest #2...followed by a more westward track resuming after the intraction is complete. I maintain peak 5-day odds of tropical cyclone formation to 60%...which is not quiet as high as the National Hurricane Center's peak odds in their tropical weather outlook. This is because there is a batch of dry saharan air just north of this system...and I have some uncertainty as to how much this dry air will affect this system as this hurricane season has had a recent history of no development in the eastern Atlantic from the dry saharan air layer. My short-term odds are at a low 10% to 30% for the next 48 hours give time for this large broad disturbance to potentially absorb area of interest #2...and then consolidate after the potential absorption. This also gives time for any dry saharan air intrusions to be mixed out. On a final note...this disturbance is forecast to be beneath rather favorable upper winds by 96 to 120 hours while in a vast area of low shear and upper outflow between a fragment of the current western Atlantic upper vorticity and a large upper vortex to cut-off from the south side of the current northeast Atlantic upper trough...so if this system develops it could become a strong tropical cyclone.

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

IOH 24 Hr Outlook (1200Z Sep 5)...10% chance of tropical cyclone formation (tropical Atlantic near 15N-31.5W)

IOH 48 Hr Outlook (1200Z Sep 6)...30% chance of tropical cyclone formation (tropical Atlantic near 16.5N-36.5W)

IOH 72 Hr Outlook (1200Z Sep 7)...50% chance of tropical cyclone formation (tropical Atlantic near 17N-41.5W)

IOH 96 Hr Outlook (1200Z Sep 8)...60% chance of tropical cyclone formation (tropical Atlantic near 17.5N-46.5W)

IOH 120 Hr Outlook (1200Z Sep 9)...60% chance of tropical cyclone formation (tropical Atlantic near 18N-51.5W)



AREA OF INTEREST #2...A tropical low pressure spin has persists near 12N-37.5W after switching from an east-northeastward to southwestward drift. It appears the change in motion is due to the beginnings of a fujiwhara interaction with the much larger disturbance in area of interest #1. The thunderstorms continue to be sheared off to the northwest quadrant of the low pressure spin due to easterly upper winds on the south side of the tropical upper ridge in the region...normally shear does not occur here during the peak of the hurricane season in September as tropical systems tend to move westward around the Atlantic surface subtropical ridge and thus keep up with the upper easterly winds. Instead this low pressure swirl has been having a rather slow motion caused by its gravitation toward the much larger low pressure system mentioned in area of interest #1. Because this system is the smaller of the two disturbances and is getting sheared due to its wandering motion...I have only low 10% odds of tropical cyclone formation while assuming the larger disturbance in area of interest #1 will absorb it before it has a chance to develop. Forward drifting motion in the outlook below is shown to shift from its current southwest direction to east-southeast as it continues a fujiwhara interaction with area of interest #1 as area of interest #1 passes to the north.

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

IOH 24 Hr Outlook (1200Z Sep 5)...10% chance of tropical cyclone formation (tropical Atlantic near 11N-36.5W)

IOH 48 Hr Outlook (1200Z Sep 6)...0% chance of tropical cyclone formation (absorbed by area of interest #1 near 11N-36W)


AREA OF INTEREST #3...Computer models continue to agree a well-organized tropical wave of low pressure will emerge from western Africa in the timframe that is now about two days from now...possibly developing into a tropical cyclone not long after exiting the west African coast. Based on the model initializations and satellite imagery...it appears the wave is currently over eastern Mauritania in the vicinity of 19N-7W...with clusters of thunderstorms on the west side of the wave over western Mauritania. The forecast positions in the outlook below are based on the model consensus that shows a west-southwestward track in the short-term around the southeast side of the Atlantic subtropical ridge. In the medium and long term...the south side of the northeast Atlantic upper trough is forecast to cut-off into a large vortex thanks to amplification of adjacent high latitude upper ridging to be kikced off by warm air advection ahead of the frontal cyclone currently over eastern Canada. The eastern divergence zone of this vortex will create a pocket of low pressure on the southeast side of the Atlantic subtropical ridge that potnetially draws this system more northward....and if this system indeed does quickly develop as some models show it could become strong/tall enough to be guided more northward by the southerly upper flow ahead of the vortex. Given the ongoing model projections...we could be in for an unusal tropical cyclone event that passes directly over or northeast of the Republic of Cabo Verde Islands instead of south of the islands as we usually see...therefore I am continuing statements on the home page bulletins of this site for the Republic of Cabo Verde Islands in regards to this system. I have nudged my 5-day peak odds of tropical cyclone formation downward to 40%. This is because the thunderstorm activity of this tropical wave has not become any better organized and the current and forecast positions of the tropical wave are closer to the dry saharan air layer currently in the eastern Atlantic.

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

IOH 24 Hr Outlook (1200Z Sep 5)...0% chance of tropical cyclone formation (western Mauritania near 18N-12W)

IOH 48 Hr Outlook (1200Z Sep 6)...0% chance of tropical cyclone formation (just offshore of Mauritania near 17.5N-17W)

IOH 72 Hr Outlook (1200Z Sep 7)...15% chance of tropical cyclone formation (northeast of the Republic of Cabo Verde Islands near 18.5N-22.5W)

IOH 96 Hr Outlook (1200Z Sep 8)...30% chance of tropical cyclone formation (tropical Atlantic near 19.8N-25W)

IOH 120 Hr Outlook (1200Z Sep 9)...40% chance of tropical cyclone formation (tropical Atlantic near 20.5N-29W)


AREA OF INTEREST #4...The National Hurricane Center in their tropical weather outlook is monitoring a frontal low pressure located east-northeast of Tropical Depression Omar (located at 39.8N-51W) for acquisition of tropical characteristics...therefore I have added this feature as an area of interest on this blog. The frontal low pressure has formed over the last couple of days due to the eastern divergence zone of a shortwave upper trough that ejected from the longwave trough over central Canada. A few days ago on August 31st the GFS previously forecasted that this frontal low pressure would become a subtropical cyclone as a second shortwave trough would merge with the current one...re-enforcing it and amplifying it into a cut-off upper vortex with a stronger eastern divergence zone that strengthens the frontal low pressure. By September 1st the GFS showed the second shortwave become diverted northeastward over eastern Canada instead of merging with the first shortwave...which is actually what has happened. This has kept the shortwave weaker with less eastern divergence...thus the surface frontal low pressure has been on the weaker side...and in birdseye view post #109 on September 1st (in the tropical depression fifteen section) I decided not to consider this feature an area of interest given that the two shortwaves would not merge.


The frontal low pressure is already crossing the 26 deg C isotherm and moving toward cooler waters while moving north-northeast around the west side of the Atlantic surface subtropical ridge. The frontal low pressure is now directly below the shortwave upper trough axis where there is a lack of divergence...so going forward the low pressure will weaken. Given that upper air temperatures are also warm as indicated by the 200 mb pressure height being above 1248 dekameters...there will not be enough instablity for this system to fire strong thunderstorm at its core as it moves into waters below 26 deg C. Therefore I give a 0% chance of subtropiacl cyclone formation from this system.

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

IOH 24 Hr Outlook (1200Z Sep 5)...0% chance of subtropical cyclone formation (north Atlantic near 45N-49W)


AREA OF INTEREST #5...Thunderstorms in the central Caribbean Sea to the south of Hispaniola (Haiti and the Dominican Republic) have significantly increased and show some signs of organization this morning and early afternoon. These thunderstorms are being triggered by the eastern divergence zone of upper vorticity that stretches from the Caribbean to the western Atlantic. A study of the 0600Z GFS model upper-wind forecast shows the upper vorticity breaking into two halves as it continues to decay while starved of higher latitude colder air while cut-off along the south side of the western Atlantic upper ridge. The latent heat release of the thunderstorms may aid in the breakup of this upper vorticity...thus dropping the wind shear associated with this vorticity. As a result I have added this disturbance as an area of interest for tropical development. I will note that the 1200Z NAVGEM model run from September 2nd predicted this disturbance and forecasted it to gradually develop.


The outlook below assumes this disturbance will be initially stationary while it merges with the tropical wave currently entering the eastern Caribbean Sea...with the disturbance then continuing west-northwestward around the southwest side of the Atlantic surface subtropical ridge which will soon be rebuilt by the western convergence zone of the central Canadian upper trough once the trough accelerates across the north Atlantic. My peak 5-day odds of tropical development are at a low 25% as today's model guidance does not develop this system. I lower the odds from this peak by 96 and 120 hours as the disturbance catches up to the south side of the western fracture of the upper vorticity...potentially disrupting this system. Thus the home page bulletins of this site only mentions Jamaica in relation to this disturbance as the upper winds may not be as favorable for development once the disturbance nears the Cayman Islands.

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

IOH 24 Hr Outlook (1200Z Sep 5)...5% chance of tropical cyclone formation (central Caribbean Sea near 14N-71W)

IOH 48 Hr Outlook (1200Z Sep 6)...15% chance of tropical cyclone formatino (central Caribbean Sea near 14N-72W)

IOH 72 Hr Outlook (1200Z Sep 7)...25% chance of tropical cyclone formation (southeast of Jamaica near 15.5N-76W)

IOH 96 Hr Outlook (1200Z Sep 8)...10% chance of tropical cyclone formation (west of Jamaica and south of the Cayman Islands near 17N-80W)

IOH 120 Hr Outlook (1200Z Sep 9)...10% chance of tropical cyclone formation (northwestern Caribbean Sea to the west of the Cayman Islands near 18.5N-84W)


...COMPUTER MODEL SUMMARY...

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


0000Z CMC Model Run...For Tropical Depression Omar...merges with front from eastern Canada frontal cyclone in 54 hours while located near 36N-57W...perists as a remnant frontal low pressure that drifts northward...remnant frontal low dissipates just south of eastern Newfoundland in 120 hours. For area of interest #1...tropical cyclone formation suggested near 16.5N-30W in 84 hours...drifts slowly north-northeastward while absorbing area of interest #3 and is located just northwest of the Republic of Cabo Verde Islands at 120 hours. For area of interest #2...generally stationary while absorbed into the west side of area of interest #1 by 60 hours. For area of interest #3...absorbed into northeast side of area of interest #1 in 108 hours. For area of interest #4...opens into a surface trough located near 45N-48W at 36 hours.


0000Z ECMWF Model Run...For Tropical Depression Omar...loses identity while absorbed by cold front from eastern Canada frontal cyclone in 48 hours. For area of interest #1...tropical cyclone formation suggested near 17.5N-47.5W at 96 hours...located at 17.5N-48W at 120 hours. For area of interest #2...generally stationary through 144 hours while areas of interest #1 and #3 pass to the north. For area of interest #3...tropical cyclone formation suggested just north of the Republic of Cabo Verde Islands at 96 hours...located at 19N-30W at 120 hours. For area of interest #4...opens to a surface trough near 42.5N-48W in 24 hours.


0600Z GFS Model Run...For Tropical Depression Omar...merges with cold front from eastern Canada frontal cyclone in 48 hours while located near 40.5N-54W...continues northeastward into the northeast Atlantic as a frontal low pressure...remnant frontal low pressure loses identity along front while passing near 49N-35W just after 84 hours. For area of interest #1...tropical cyclone formation suggested near 16N-42.5W in 84 hours...located at 15N-47W at 120 hours. For area of interest #2...generally stationary while absorbed into the south side of area of interest #1 by 66 hours. For area of interest #3...tropical cyclone formation suggested just east of the Republic of Cabo Verde Islands in 84 hours...becomes a strengthening tropical cyclone at 17N-30W at 120 hours. For area of interest #4...opens to a surface trough near 44N-50W in 24 hours.


0600Z NAVGEM Model Run...For Tropical Depression Omar...opens to a surface trough near 39N-56.5W in 36 hours. For area of interest #1...tropical cyclone formation suggested near 17N-31.5W at 48 hours...becomes a large and intense tropical cyclone at 17.5N-35W at 120 hours. For area of interest #2...shown to be generally stationary and then absorbed into the southwest side of area of interest #1 at 18 hours. For area of interest #3...no tropical cyclone formation suggested while area of interest #1 dominates to the west. For area of interest #4...opens into a surface trough at 39.5N-52.5W in 6 hours.

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