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 #139

*******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 4 2020 5:00 PM EDT...

Satellite Image from 2030 (red indicates tropical cyclone or remnants of tropical cyclone...with G marking Gamma. Green indicates an area of interest in the National Hurricane Center Tropical Weather Outlook. Yellow indicates an area of interest not in the National Hurricane Center Tropical Weather Outlook.):

NHC TAFB Surface Analysis 1200Z:


GFS Model Upper Level Winds 1200Z:

See Tropical Storm Gamma section below for the only currently active tropical cyclone in the Atlantic basin. See area of interest sections below for multiple areas being monitored for tropical cyclone formation.


TROPICAL STORM GAMMA...Slow-moving Tropical Storm Gamma has drifted northward to a position in the southern Gulf of Mexico and offshore of the northern Yucatan peninsula. This is the net effect of the central US surface ridge (supported by the western convergence zone of the central North America upper trough) trying to push Gamma westward...the Mexico surface ridge (supported by the southwest extent of the upper trough’s convergence zone) blocking the westward progress of Gamma...and the upper southwesterly flow ahead of the central North America upper trough trying to drag Gamma northeastward. The tropical storm weakened to a minimum of 50 mph maximum sustained winds overnight due to the southwesterly upper shear and land interaction with the Yucatan...but has since re-strengthened to 60 mph while moving back over water.

My updated forecast track points in the short-term are adjusted northeastward due to the current position of Gamma. As the central North America upper trough fractures into a southwestern and northeastern half...with both halves moving away from the region....a westward turn is still anticipated as the steering upper southwesterly flow induced by the upper trough diminishes...and as the southwestern half of the upper trough and its Mexico surface ridge retrograde westward and away...clearing the way for the central US surface ridge to finally take over the steering. However the west track will be slow as the eastern divergence zone of a second upper trough to dive into central North America weakens the central US surface ridge. Although the surface ridge recovers again in 96 hours once the second upper trough exits stage right...Gamma will likely be prevented from making a home run westward into Mexico while more likely tugged on a southwest drift by the vigorous tropical wave in area of interest #1 which has developed more of a spin...which has increased the chances that Gamma will undergo a fujiwhara interaction with the incoming tropical wave. My longer term track forecast is adjusted southwestward and with a slower forward speed to reflect the fujiwhara interaction.

Regarding intensity...the short term forecast has been raised slightly as Gamma is already recovering from the land interaction with the Yucatan peninsula...and as the shear will relax once the current central North America upper trough fractures in half. However my longer term intensity has been adjusted downward to category 1 hurricane strength due to increased competition for low level inflow and upper outflow with the incoming tropical wave in area of interest #1 which has developed more than I anticipated...with some weakening reflected at 96 hours as I estimate the slow

motion of Gamma will eventually upwell cooler waters. The weakening by 120 hours is a reflection of possible land interaction with the northern Veracruz coast of east-central Mexico.

Over the next few days we can expect slow-moving Tropical Storm Gamma to produce coastal sea swells for the shores of the Yucatan....Campeche....Tabasco...Veracruz... and Tamaulipas provinces of Mexico. Although there remains some forecast track uncertainty...the highest chance of wind and rain impacts I estimate will be for the northern Veracruz region by day 5.

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

0 Hr Position (1800Z Oct 4)...60 mph maximum sustained wind tropical storm centered just offshore of the northern Yucatan peninsula at 22.4N-88W

IOH 24 Hr Forecast (1800Z Oct 5)...70 mph maximum sustained wind tropical storm centered over the southern Gulf of Mexico at 22.8N-89W

IOH 48 Hr Forecast (1800Z Oct 6)...80 mph maximum sustained wind hurricane centered over the southern Gulf of Mexico at 22.5N-91W

IOH 72 Hr Forecast (1800Z Oct 7)...80 mph maximum sustained wind hurricane centered over the southwestern Gulf of Mexico at 22N-93.5W

IOH 96 Forecast (1800Z Oct 8)...75 mph maximum sustained wind hurricane centered over the southwestern Gulf of Mexico at 21N-95W

IOH 120 Forecast (1800Z Oct 9)...65 mph maximum sustained wind tropical storm centered over the north Veracruz Mexico coast at 21N-97W


AREA OF INTEREST #1 (RECENTLY UPGRADED TO POTENTIAL TROPICAL CYCLONE TWENTY-SIX)...The vigorous tropical wave of low pressure crossing the central Caribbean Sea has been fighting cold core upper vorticity that has been retrograding southwestward around the western Atlantic upper ridge and toward the wave. This is evidenced by ongoing strong thunderstorm activity initially triggered a few days ago by the upper divergence on the southeast side of the vorticity...with the thunderstorm latent heat release washing out the cold core upper vorticity...and today’s 0600Z GFS model run indicates the upper vorticity fading away just after 24 hours. Due to the increased surface spin with the wave...I have notably raised my odds of tropical cyclone formation. In the next 24 hours I have my odds at a lower 30% as the lingering upper vorticity may still disrupt the organization of the thunderstorms...followed by 50% odds thereafter once the upper vorticity dissipates. I do not have odds above 50% just yet as I anticipate this wave still being too close to Tropical Storm Gamma such that Gamma’s low level inflow and upper outflow will dominate over this yet-to-develop wave...but note if this wave does develop more and in a hurry then this would change the situation.

Regarding track...in the short term I show a slight northward angle in the surface east-southeasterly steering flow setup between mid-latitude surface ridging and east side of Tropical Storm Gamma...with a notable northward shift in track in the long range as this tropical wave now has more of a surface spin that will allow it to undergo a fujiwhara interaction with Gamma...especially as Gamma and this wave will be trapped together in the Gulf of Mexico thanks to a recovering central US surface ridge to be bolstered in 96 hours by the western convergence zone of a second upper trough to dive into central North America. By 120 hours the southwest fracture of the current first North America upper trough will swing back eastward from Mexico and (along with the northern outflow of Gamma) increase the southwesterly shear in the region...so I drop my odds of development by then. On this current forecast track and outlook...I mention Jamaica...the Cayman Islands...the northeastern Yucatan peninsula (Quintana Roo province)...and western Cuba in regards to this tropical wave on the home page bulletins of this site. I am not mentioning potential impacts to the northern US Gulf coast just yet as I am waiting to see how the wave fares when it begins it’s fight with Tropical Storm Gamma.

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

IOH 24 Hr Outlook (1800Z Oct 5)...30% chance of tropical cyclone formation (western Caribben Sea near 17N-81W)

IOH 48 Hr Outlook (1800Z Oct 6)...50% chance of tropical cyclone formation (western Caribben Sea near 19N-85W)

IOH 72 Hr Outlook (1800Z Oct 7)...50% chance of tropical cyclone formation (southeastern Gulf of Mexico near 22N-87W)

IOH 96 Hr Outlook (1800Z Oct 8)...50% chance of tropical cyclone formation (northern Gulf of Mexico near 26.5N-90W)

IOH 120 Hr Outlook (1800Z Oct 9)...25% chance of tropical cyclone formation (northwestern Gulf of Mexico offshore of Texas and Louisiana near 28N-94W)

AREA OF INTEREST #2...A tropical wave of low pressure that has recently entered the the eastern tropical Atlantic from Western Africa is seeing thunderstorms limited to its southern half....as the outflow in the north half had been choked off by upper vorticity from the east-central Atlantic being pushed southward toward the tropical wave by the current north Atlantic upper ridge. Therefore I keeps odds of development at 0% until 48 hours when the wave reaches the eastern tropical Atlantic upper ridge where outflow is more symmetrical. In the longer range..the current central North America upper trough and another larger second one to dive southeast in its place will increase the mid-latitude westerly flow across the Atlantic by late in the forecast period...sending a chunk of the central Atlantic upper vorticity southeastward toward this tropical wave. This vorticity may suppress the northern outflow of the tropical wave thru 96 hours...another reason I keeps odds of development on the lower side thru that time. I slightly raise odds to 20% by 120 hours when the wave will likely move past the upper vorticity.

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

IOH 24 Hr Outlook (1800Z Oct 5)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-24W)

IOH 48 Hr Outlook (1800Z Oct 6)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-29W)

IOH 72 Hr Outlook (1800Z Oct 7)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-34W)

IOH 96 Hr Outlook (1800Z Oct 8)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-39W)

IOH 120 Hr Outlook (1800Z Oct 9)...20% chance of tropical cyclone formation (central tropical Atlantic near 11N-44W)

AREA OF INTEREST #3...The tropical wave of low pressure in the open central Atlantic has lost thunderstorm activity over the last day. Therefore I have lowered its odds of development to 10%...with odds also kept low as a chunk of central Atlantic upper vorticity will retrograde southwestward around the west Atlantic upper ridge and into the Caribbean...with the east side of this chunk potentially inducing southerly shear across this tropical wave. However with the vorticity chunk expected to continuously retrograde westward ahead of the tropical wave thru 120 hours...I keep odds above 0% as the wave would stay east of the worst of the shear.

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

IOH 24 Hr Outlook (1800Z Oct 5)...0% chance of tropical cyclone formation (central tropical Atlantic near 11.5N-51.5W)

IOH 48 Hr Outlook (1800Z Oct 6)...10% chance of tropical cyclone formation (central tropical Atlantic east of the Lesser Antilles near 12N-56.5W)

IOH 72 Hr Outlook (1800Z Oct 7)...10% chance of tropical cyclone formation (just west of the Lesser Antilles near 12.5N-61.5W)

IOH 96 Hr Outlook (1800Z Oct 8)...10% chance of tropical cyclone formation (eastern Caribbean sea near 13N-66.5W)

IOH 120 Hr Outlook (1800Z Oct 9)...10% chance of tropical cyclone formation (central Caribbean sea near 13.5N-71W)

AREA OF INTEREST #4...The surface trough of low pressure in the open central Atlantic will continue westward toward Bermuda while steered by the North Atlantic surface ridge. A large upper trough that will dive southeastward into central North America will create a surface ridge weakness in the northwestern Atlantic that should slow the track and then bend the track of the surface trough northward...which I estimate will keep the trough just east of Bermuda. I keep odds of development at 0% in the next 24 hours as the surface trough reaches a zone of suppressing upper convergence between the west side of the current central Atlantic upper vorticity and east side of the western Atlantic upper ridge. I then have a window of very low 5% odds of tropical development assuming the surface trough survives and reaches the low shear and upper outflow of the west Atlantic upper ridge. The large North American upper trough expected by 96 hours should increase westerly shear across the northwest Atlantic...with my forecast position taking the surface trough into the shear and thus I drop my odds of development back to 0% by then.

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

IOH 24 Hr Outlook (1800Z Oct 5)...0% chance of tropical cyclone formation (central Atlantic near 28N-57W)

IOH 48 Hr Outlook (1200Z Oct 6)...5% chance of tropical cyclone formation (western Atlantic to the east-southeast of Bermuda near 28N-61W)

IOH 72 Hr Outlook (1200Z Oct 7)...5% chance of tropical cyclone formation (southeast of Bermuda near 29N-63W)

IOH 96 Hr Outlook (1200Z Oct 8)...0% chance of tropical cyclone formation (east of Bermuda near 31N-63W)

...COMPUTER MODEL SUMMARY...

Temporarily cancelled to avoid further delays in creating and publishing the post.

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