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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 2023 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #94

Updated: Oct 8, 2023

*******Note that forecast 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.*********


...SATURDAY OCTOBER 7 2023 1:05 PM EDT...

Continuing to watch a deep-layer cyclone northwest of the Azores that could acquire tropical characteristics in the days ahead... see area of interest #43 section below for more details. See area of interest #44 section below for information on a tropical wave of low pressure that has recently moved offshore from western Africa which is at high risk of tropical cyclone formation... any tropical cyclone that develops here is likely to move northwestward into the open central Atlantic in the long range.


For the Gulf of Mexico... within the next five days a tropical disturbance is likely to develop in the southwestern Gulf with the disturbance and heavy rainfall likely spreading northeastward toward southeastern Louisiana... southern Mississippi... southern Alabama... southern Georgia... and northern Florida by the end of the 5-day window:

(1) At the surface an eastern Pacific tropical low has developed over the last couple of days in divergent upper northeasterly flow between the southeast quadrant of the Mexico upper ridge and west side of persisting western Caribbean upper vorticity. A surface cold front has also arrived to the southwestern Gulf of Mexico while driven by the amplified upper trough over eastern North America. The eastern Pacific feature is at risk of tropical cyclone formation (refer to the official NHC eastern Pacific outlook at hurricanes.gov)... any eastern Pacific tropical cyclone that develops is likely to lift northward across southeastern Mexico and toward the southwestern Gulf of Mexico thanks to an upper trough that pivots into western North America from the Pacific. As such the current southwest Gulf surface front and remnants of the eastern Pacific feature would provide the surface low pressure field/spin to trigger a southwestern Gulf tropical disturbance.

(2) Southward cold air transport on the west side of the currently developing western Atlantic frontal cyclone will cause the cold core eastern North America upper trough to amplify into a southward-shifting vortex... and combined with the arrival of the second upper trough into western North America the Mexico upper ridge will be forced southeastward into the northwestern Caribbean. Divergence on the northwest side of the Caribbean upper ridge will be aiding in the development of a southwestern Gulf tropical disturbance.

(3) Any disturbance that does develop in the southwestern Gulf will be accelerating northeastward toward the above-mentioned land areas of the US Gulf coastal region thanks to deep-layer southwesterly flow on the northwest side of the Caribbean upper ridge and out ahead of the western North America upper trough and surface frontal low pressure it will be generating. The strength of the upper southwesterly winds will likely mean excess wind shear for tropical cyclone formation... and the tropical disturbance could quickly transition into a non-tropical one should the cold front from western North America quickly arrive and overspread the disturbance. Therefore not declaring a tropical area of interest for the Gulf of Mexico in this update.


AREA OF INTEREST #43... The surface frontal cyclone north-northwest of the Azores has moves northeastward to 47.5N-32.5W through 1200Z today while pivoting around the northeast side of its oblong parent upper vortex. Warm core upper ridging is expected to build to the west of this system due to surface warm southerly flow ahead of the currently developing west Atlantic frontal cyclone... and over the next 48 hours the oblong upper vortex is forecast to become more circular and drift southwest under the influence of the upper ridging. The frontal cyclone is expected to whirl southwestward into the core of the upper vortex to make a deep-layer cyclone... and the deep-layer cyclone will be undergoing a gradual decay phase as the surface frontal cyclone slowly weakens below the core of the upper vortex where upper divergence is lacking. By 72 hours the latest model runs indicate that a north fragment of the current eastern North American upper trough skips over the north side of the upper ridging and drags the deep-layer cyclone eastward... with the upper trough fragment then leaving behind the deep-layer cyclone at 96 hours. However the latest model runs show that the deep-layer cyclone will not be stalled for long while now showing the next high-latitude upper trough (to approach from Greenland) having more of a southward reach and already pulling the deep-layer cyclone northeastward between 96 and 120 hours. My updated forecast track below is adjusted based on the current position of the surface frontal cyclone while incorporating the above ideas through 120 hours.


Regarding odds of subtropical cyclone formation... the updated forecast track has this system arriving to 20 to 22 deg C water temps by 48 hours. Also the overhead upper vortex is expected to remain rather cold over the next five days to support instability and possible development of core thunderstorms and tropical characteristics (200 mb heights of the upper vortex are currently at around 1170 dekameters... only warming to 1185 dekameters through day 5 as the upper vortex remains isolated from fresh cold air injections... and looking for 1200 dekameters or less for tropical development in the 20 to 22 deg C sea surface temperature range). Because the core of the surface cyclone has not yet developed much shower and thunderstorm activity... I am not raising peak odds of subtropical cyclone formation above 30% in this update cycle... and reserve those peak odds for when the cyclone sees its warmest water temps. Odds of subtropical development will be adjusted accordingly in future updates when we see how the cyclone actually reacts when it arrives to the 20 to 22 deg C water temps. Between 72 and 96 hours the forecast track still keeps this system over 20 deg C water waters... however I begin to lower odds of subtropical development from the peak as the surface layer of the cyclone will have decayed below the lack of divergence at the upper vortex core as described in the prior paragraph if it has not undergone subtropical development by then. By 120 hours the increase in the forecast track's north angle positions this system at the 20 deg C isotherm or even cooler water... and odds of subtropical development are lowered even further.


Regardless of whether or not the deep-layer cyclone acquires tropical characteristics... expect a prolonged period of coastal surf for the Azores over the next four days. Coastal surf will likely decrease after day 4 as the surface layer of the cyclone decays.

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

IOH 24 Hr Outlook (1200Z Oct 8)... 10% chance of subtropical cyclone formation (northeast Atlantic near 45N-36W)

IOH 48 Hr Outlook (1200Z Oct 9)... 30% chance of subtropical cyclone formation (northeast Atlantic near 42.5N-37.5W)

IOH 72 Hr Outlook (1200Z Oct 10)... 30% chance of subtropical cyclone formation (northeast Atlantic near 42.5N-35W)

IOH 96 Hr Outlook (1200Z Oct 11)... 20% chance of subtropical cyclone formation (northeast Atlantic near 42.5N-31W)

IOH 120 Hr Outlook (1200Z Oct 12)... 10% chance of subtropical cyclone formation (northeast Atlantic near 43N-27.5W)

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT***************************

Not in the official outlook


AREA OF INTEREST #44... Overnight a tropical wave of low pressure has recently emerged into the far eastern tropical Atlantic from the west coast of Africa. The wave still appears elongated east-to-west with the maximum rotation toward the east side of the thunderstorm activity (near 7.5N-16W as of 1200Z)... however this system appears a little better organized while the thunderstorm activity shows signs of developing curved banding features around the area of maximum rotation. For the next 24 hours the wave should more sluggishly toward the west as the wave deals with the surface ridge weakness caused by area of interest (AOI) #43. The models then show the surface ridge over Europe building southwestward into the eastern Atlantic as the south part of the upper vortex associated with AOI #43 becomes a shortwave upper trough whose western convergence zone helps the surface ridge expansion. The eastern Atlantic surface ridge then remains intact for some time as the outflow over this tropical wave clashes with the outflow of the central Atlantic mid-latitude upper ridge to create a zone of upper convergence. As such a steady westward track around the surface ridge is shown from 24 to 72 hours. By 96+ hours some of the energy associated with the current eastern North American upper trough shifts toward the central Atlantic... with the divergence zone of this energy creating a surface ridge weakness that this system will move northwestward toward in the long range. Also noting some of the latest model runs show the central Atlantic mid-latitude upper ridge now having enough strength to break off a good chunk of the upper vortex tied to AOI #43 into an eastern Atlantic upper trough that dives southeastward toward this system in 72+ hours. Should this system become strong/tall enough to couple with upper winds in the short-term... it could be dragged northward earlier by this upper trough and therefore I have steepened the northward angle of my forecast track for the October 10 to 11 timeframe.


Regarding odds of tropical cyclone formation... upper winds are conducive for short-term tropical cyclone formation with this system tucked under the low shear and outflow of the eastern tropical Atlantic upper ridge. Coupled with the increase in organization noted in the prior paragraph and the fact the global models insist on developing this wave... I agree with the NHC in raising peak odds of tropical cyclone formation well above 50%... and I have selected peak odds slightly higher than the NHC 8 AM EDT outlook. However for the next 48 hours I have development odds at or below 50% to give time for the tropical wave to consolidate its current east-west elongated structure into a more circular structure. Due to the long-term northwest forecast track... it was always inevitable this system would run into increasing westerly shear on the north side of the tropical upper ridge. With the development of an eastern Atlantic upper trough possible in the 72+ hour window noted in the prior paragraph... the increase in shear could occur even earlier... therefore by 96 and 120 hours I begin to trim down development odds from the peak.

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

IOH 24 Hr Outlook (1200Z Oct 8)... 20% chance of tropical cyclone formation (eastern tropical Atlantic near 8N-18.5W)

IOH 48 Hr Outlook (1200Z Oct 9)... 50% chance of tropical cyclone formation (eastern tropical Atlantic near 9N-23W)

IOH 72 Hr Outlook (1200Z Oct 10)... 80% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-28W)

IOH 96 Hr Outlook (1200Z Oct 11)... 70% chance of tropical cyclone formation (eastern tropical Atlantic near 11.5N-32W)

IOH 120 Hr Outlook (1200Z Oct 12)... 60% chance of tropical cyclone formation (eastern tropical Atlantic near 14N-35W)

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT***************************

Formation chance through 48 hours... 20%

Formation chance through 7 days... 70%


...COMPUTER MODEL SUMMARY...

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


0000Z (Oct 7) CMC Model Run...

**For area of interest #43... frontal cyclone swings south in a cyclonic loop and reaches 41.5N-34W at 66 hours while potentially acquiring tropical characteristics... subsequently weakens to an elongated frontal low that drifts east to 41N-30W through 120 hours

**For area of interest #44... tropical cyclone formation suggested near 10.5N-32.5W at 102 hours... while moving northwestward approaches hurricane strength and reaches 20N-39.5W at 168 hours

**Through 96 hours upper ridge over Mexico shifts east into western Caribbean under influence of current eastern North American upper trough that evolves into an upper vortex and next upper trough that moves into western North America from the Pacific... tropical low develops in the southwestern Gulf of Mexico and just offshore of northern Veracruz at 96 hours due to divergence on the northwest side of the Caribbean upper ridge... while moving northeastward ahead of the western North America upper trough the tropical low strengthens but it also sheared while reaching 25N-92.5W through 120 hours... through 168 hours the tropical low transitions into a frontal low supported by the divergence ahead of the upper trough while moving across northern Florida and southern Georgia.


0000Z (Oct 7) ECMWF Model Run...

**For area of interest #43... frontal cyclone swings south in a cyclonic loop and reaches 42N-35W at 66 hours while potentially acquiring tropical characteristics... subsequently weakens to an elongated frontal low that drifts east to 42.5N-29W through 120 hours

**For area of interest #44... tropical wave organizes into a tropical low near 12.5N-30.5W at 120 hours... through 168 hours the northwestward-moving tropical low develops into a weak tropical cyclone that reaches 18.8N-35.5W


0600Z (Oct 7) GFS Model Run...

**For area of Interest #43... frontal cyclone swings south in a cyclonic loop and reaches 42N-35W at 63 hours while potentially acquiring tropical characteristics... frontal cyclone weakens to a frontal low while moving northeastward to 45N-30.5W through 120 hours

**For area of interest #44... tropical wave organizes into a tropical low near 8.8N-26W at 72 hours... compact tropical cyclone formation suggested near 10.5N-28.5W at 93 hours... compact tropical cyclone moves northwestward while staying weak and reaches 13.5N-34.5W through 120 hours... tropical cyclone weakens to a remnant low through 168 hours while continuing northwestward to 18.5N-41W


0600Z (Oct 7) NAVGEM Model Run...

**For Area of Interest #43... frontal cyclone swings south in a cyclonic loop and reaches 42N-34W at 60 hours while potentially acquiring tropical characteristics... frontal cyclone weakens to a frontal low while moving northeastward to 44N-31.5W through 120 hours

**For area of interest #44... no development shown

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