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 #147

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


...THURSDAY OCTOBER 28 2021 2:40 PM EDT...

See area of interest section below for multiple areas of interest being monitored for tropical development in the Atlantic basin:

**Area of interest #1...northwestern Atlantic surface cyclone being monitored for acquisition of tropical characteristics

**Area of interest #2...tail end of cold front currently crossing the Gulf of Mexico...expected to reach the Caribbean Sea in an upper wind environment potentially favoring tropical development.

**Area of interest #3...broad tropical low pressure area developing in the eastern tropical Atlantic offshore of Africa and to the south of the Republic of Cabo Verde Islands.


AREA OF INTEREST #1...The surface frontal cyclone that was previously parked offshore of the United States east coast has finally accelerated eastward into the open northwestern Atlantic as it parent upper trough continues its eastward march. Coastal sea swells affecting the eastern North American coast from the mid-Atlantic US to Atlantic Canada are expected to diminish over the next day or so as the eastward track takes the cyclone further offshore. The upper trough is amplifying further into a cold core upper vortex due to the cold air pulled southward by the frontal cyclone's west side and also due to adjacent amplification of upper ridging over eastern North America in the warm southerly flow ahead of the current central US frontal low. Pockets of showers and thunderstorms have started to develop near the surface cyclone's center with the instability provided by the cold temperatures of the developing upper vortex combined with the north wall of warm Gulf stream waters. Even as this system continues east toward slighlty cooler waters in the low 20s of deg C in the next 24 hours...the upper vortex is forecast to remain cold enough to aid in generating instability. Given that the pockets showers and thunderstorms as of this early afternoon have merged into banding features...I have raised my short-term odds of subtropical cyclone formation to 60%.


After 24 hours... the current northeast Atlantic upper trough will be amplifying due to the amplified eastern North America upper ridge to shift east into the northwest Atlantic. The flow between the northeast Atlantic upper trough and northwest Atlantic upper ridge is expected to push this system southeast into the open central Atlantic. Models have continued to trend with a stronger northwestern Atlantic upper ridge...resulting in more separation between the upper vortex of this system and the northeastern Atlantic upper trough... keeping the upper vortex more circular instead of elongated while trying to link with the northeast Atlantic upper trough. The more amplified upper ridge is also shown to push some of the cold air associated with the northeastern Atlantic upper trough southwestward into the upper vortex. The less elongated and more circular upper vortex means less wind shear over the surface cyclone. The colder upper vortex means more instability. Even the GFS model which had previously shown a more elongated and warmer upper vortex is now trending toward colder and less elongated. Therefore I have raised medium and longer term odds of subtropical cyclone formation to 50%. This is not quiet as high as the short-term 60% in case the upper vortex does indeed elongate or become a little warmer than currently forecast.


By 96 hours... the southeast track of the surface cyclone will cause it to near the warm 26 deg C isotherm which will aid in keeping instability as the upper vortex finally begins to warm while becoming cut-off from high-latitutde cold air. As a result I keep 40% subtropical development odds at that timeframe. It is also during this timeframe that the current warm central Atlantic upper ridge will have shifted into the eastern Atlantic while supported by the warm southerly surface flow generated by the east side of the surface cyclone. In addition the northwest Atlantic upper ridge will be making its way into the northeastern Atlantic. Between 96 and 120 hours... the eastern convergence zone of both upper ridges is expected to produce strong surface ridges to the east that will dampen the eastward progress of the surface cyclone. At the same time... the upper vortex may shift faster to the east from the influence of the current central North America upper trough and another large-scale North American upper vortex to shift towards the Atlantic. This may result in the loss of stacking between the surface cyclone and upper vortex by 120 hours...with the west side of the upper vortex potentially shearing the surface cyclone or producing upper convergence over the surface cyclone. As a result...my subtropical development odds by 120 hours are trimmed down to 30%.

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

IOH 24 Hr Outlook (1200Z Oct 29)... 60% chance of subtropical cyclone formation (north Atlantic near 40N-56W)

IOH 48 Hr Outlook (1200Z Oct 30)... 50% chance of subtropical cyclone formation (north Atlantic near 39.5N-47.5W)

IOH 72 Hr Outlook (1200Z Oct 31)... 50% chance of subtropical cyclone formation (central Atlantic near 34.5N-44W)

IOH 96 Hr Outlook (1200Z Nov 1)... 50% chance of subtropical cyclone formation (central Atlantic near 31N-42.5W)

IOH 120 Hr Outlook (1200Z Nov 2)... 30% chance of subtropical cyclone formation (central Atlantic near 31N-40W)


AREA OF INTEREST #2...The current highly amplified cold core upper trough moving across central North America is expected to drive a surface cold front as far south as the Caribbean Sea over the next 72 hours. The tail end of this front may evolve into a tropical disturbance supported by the outflow and low shear of the ongoing upper ridge in the Caribbean. I assign low 10% odds of tropical cyclone development by days 4 and 5 as only some model runs show the formation of a defined tropical low pressure... and even those runs do not show tropical cyclone formation. I have shifted my 72 hour forecast point south and east in accordance with the consensus seen in the 0600Z GFS and 0000Z CMC model runs from earlier today. Anything that does develop will likely drift west toward Nicaragua by days 4 and 5 as a steering surface ridge builds over North America in the wake of the amplified upper trough.

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

IOH 24 Hr Outlook (1200Z Oct 29)...0% chance of tropical cyclone formation (northwestern Caribbean Sea near 19N-85W)

IOH 48 Hr Outlook (1200Z Oct 30)...0% chance of tropical cyclone formation (central Caribbean Sea near 15N-80W)

IOH 72 Hr Outlook (1200Z Oct 31)...5% chance of tropical cyclone formation (south-central Caribbean Sea near 12.5N-78W)

IOH 96 Hr Outlook (1200Z Nov 1)...10% chance of tropical cyclone formation (south-central Caribbean Sea near 12.5N-80W)

IOH 120 Hr Outlook (1200Z Nov 2)...10% chance of tropical cyclone formation (just offshore of the Nicaragua coast near 12.5N-83W)


AREA OF INTEREST #3...Based on a general increase in thunderstorms in the vicinity of 17.5W just offshore of Africa...I continue to estimate that a tropical wave of low pressure is in the far eastern tropical Atlantic. The NHC TAFB surface analysis also shows another tropical wave near 35W longitude with satellite showing an associated curl of clouds. Big picture...it appears the tropical wave offshore of Africa and the additional wave near 35W are merging into a broad tropical low pressure area featuring an increase in thunderstorms over the last 24 hours. The increase in thunderstorm activity is driven by the outflow of low-latitude tropical upper ridging...located south of hostile upper westerly shearing winds being generated by the string of upper vorticity located in the mid-latitudes of the central and eastern Atlantic. This cold core upper vorticity is forecast to diminish over the next few days...becoming replaced by amplifying warm upper ridging induced by the warm southerly surface flow to occur on the east side of area of interest (AOI) #1. This upper ridging will promote lower wind shear...and combined with today's increase in thunderstorms and the ongoing computer model support from the GFS and hint of support in the NAVGEM model showing development...I have initiated an area of interest in the eastern tropical Atlantic.


Anything that does develop in the eastern tropical Atlantic will likely track west-northwest to northwest toward the surface ridge weakness associated with AOI #1. Regarding tropical cyclone formation odds...I already assign a peak as high as 40% due to the favorable upper wind forecast...today's increase in thunderstorm activity... and vigorous model support shown in the GFS which is one of the two respected global computer models. The other respected model...the ECMWF...does not show development at the present time. I begin to taper down odds of development after 96 hours as this system likely tracks into shearing upper southwesterly flow to be generated by the east side of the upper vortex associated with AOI #1. This upper southwesterly flow may also bend the track of this system more northward by the end of the 5-day forecast period...especially if this system strengthens (becomes taller in structure). However I do not turn this system as sharply north as the GFS as the GFS curiously turns this system north before it reaches the surface ridge weakness of AOI #1 and before it reaches the upper southwesterly flow.

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

IOH 24 Hr Outlook (1200Z Oct 29)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 7.5N-20W)

IOH 48 Hr Outlook (1200Z Oct 30)...15% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-22W)

IOH 72 Hr Outlook (1200Z Oct 31)...30% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-25W)

IOH 96 Hr Outlook (1200Z Nov 1)...40% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-29W)

IOH 120 Hr Outlook (1200Z Nov 2)...35% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-33W)


...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 area of interest #1... frontal cyclone accelerates east and then southeast into the open central Atlantic and reaches 31N-42W at 120 hours while potentially gaining tropical characteristics

**For area of interest #2...tail end of front develops into a southern Caribbean tropical low pressure near 12.5N-78W at 78 hours...drifts southwest to 11.5N-80W by 120 hours.

**For area of interest #3...no development shown


0000Z ECMWF Model Run...

**For area of interest #1... accelerates east and then southeast into the open central Atlantic while potentially acuqiring tropical characteristics...reaches 33N-43.5W by 120 hours.

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

**For area of interest #3...no development shown


0600Z GFS Model Run...

**For area of interest #1... accelerates east and develops a circular and potentially tropical core over the next 24 hours while reaching 40.5N-57.5W... afterwards dives southeast as a non-tropical and more elongated low pressure and reaches 33N-41W at 120 hours.

**For area of interest #2...tail end of front develops into a southern Caribbean tropical low pressure near 12.5N-78W at 75 hours...tropical low pressure gradually loses defintion thereafter and no development forecast

**For area of interest #3...tropical low pressure consolidates near 8.5N-21.5W at 54 hours...compact tropical cyclone formation suggested near 10N-22W at 66 hours...continues north-northwest and passes just west of the Republic of Cabo Verde Islands through 120 hours.


0600Z NAVGEM Model Run...

**For area of interest #1...accelerates east and then southeast into the open central Atlantic while potentially acuqiring tropical characteristics...reaches 33N-42.5W by 120 hours

**For area of interest #2...tail end of front develops into a southern Caribbean tropical low pressure near 12.5N-81.5W at 102 hours...loses consolidated circulation thereafter and redevelops a new circulation furhter southeast near 10.5N-76W at 126 hours.

**For area of interest #3...develops a low pressure center just southwest of the Republic of Cabo Verde Islands at 6 hours...accelerates west and degenerates into a remnant surface trough near 14N-39W by 120 hours.

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