MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #148
*******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 29 2021 4:55 PM EDT...
Satellite image as of 1640Z. Areas of interest circled in yellow are not mentioned in the NHC tropical weather outlook. Areas circled in a green dashed line are in the NHC 5-day tropical weather outlook. Areas circled in a solid green are in the NHC 2-day tropical weather outlook:
NHC TAFB Surface Analysis 0600Z:
GFS Model Upper-Level Winds 0600Z:
See area of interest section below for multiple areas of interest being monitored for tropical development in the Atlantic basin:
**Area of interest #1...northern Atlantic surface cyclone being monitored for acquisition of tropical characteristics
**Area of interest #2...tail end of cold front currently crossing the western Caribbean Sea...expected to reach the southern Caribbean Sea in an upper wind environment potentially favoring tropical development within the next few days.
**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 in the open northwestern Atlantic has continued east into the open north Atlantic while moving in tandem with its parent upper vortex. Coastal sea swells affecting the eastern North American coast from the northeastern US to Atlantic Canada are expected to diminish over the next day or so as the eastward track takes the cyclone further offshore. Despite its current position over water temps in the low 20s of deg C...the surface cyclone continues to feature bands of showers and thunderstorms due to the cold de-stabilizing temperatures of the upper vortex.
In the next 24 hours... the current northeast Atlantic upper trough will be amplifying due to the amplified eastern Canada 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 strong 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 strong upper ridge is also forecast 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. Given this ongoing forecast and current shower and thunderstorm bands with the surface cyclone... I have extended elevated 60% odds of subtropical cyclone formation through 48 hours. The southeast track of the surface cyclone will cause it to near the warm 26 deg isotherm at 72 hours which will aid in keeping instability as the upper vortex finally begins to warm while becoming cut-off from high-latitude cold air. This is a factor in keeping elevated 60% odds of subtropical development during that time.
By 72 hours...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 72 and 96 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 is likely to shift faster to the east from the influence of the current eastern North America upper trough and another large-scale North American upper vortex to shift towards the Atlantic. This will likely cause the loss of stacking between the surface cyclone and upper vortex by 96 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 96 hours are trimmed down to 30%. Between 96 and 120 hours...the surface cyclone is likely to turn north in the flow between the surface ridging to the east and current eastern North America upper trough to approach from the west. Because this upper trough is likely to approach the surface cyclone in an amplified state...it has potential to increase the upper divergence over the surface cyclone and drop shear levels. However the upper trough is not forecast to be cold enough for thunderstorm generation and subtropical development as the surface cyclone lifts north away from the warm 26 deg C sea surface temperature isotherm. As a result my current 120-hour forecast point drops subtropical development odds to 0%.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 30)... 60% chance of subtropical cyclone formation (north Atlantic near 38.5N-46W)
IOH 48 Hr Outlook (1800Z Oct 31)... 60% chance of subtropical cyclone formation (central Atlantic near 34N-43.5W)
IOH 72 Hr Outlook (1800Z Nov 1)... 60% chance of subtropical cyclone formation (central Atlantic near 31N-42.5W)
IOH 96 Hr Outlook (1200Z Nov 2)... 30% chance of subtropical cyclone formation (central Atlantic near 31N-40W)
IOH 120 Hr Outlook (1200Z Nov 3)...0% chance of subtropical cyclone formation (central Atlantic near 35N-40W)
AREA OF INTEREST #2...The current highly amplified cold core upper trough moving across eastern North America is expected to drive a surface cold front from its current position in the northwestern Caribbean Sea and into the south-central Caribbean Sea over the next 48 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 3 and 4 as only some model runs show the formation of a defined tropical low pressure... and even those runs do not show tropical cyclone formation. Anything that does develop will likely drift west toward Nicaragua by days 3 and 4 as a steering surface ridge builds over North America in the wake of the amplified upper trough. Odds of development are dropped to 0% by day 5 due to forecast landfall over Nicaragua.
IOH 24 Hr Outlook (1800Z Oct 30)...0% chance of tropical cyclone formation (central Caribbean Sea near 15N-80W)
IOH 48 Hr Outlook (1800Z Oct 31)...5% chance of tropical cyclone formation (south-central Caribbean Sea near 12.5N-78W)
IOH 72 Hr Outlook (1800Z Nov 1)...10% chance of tropical cyclone formation (south-central Caribbean Sea near 12.5N-80W)
IOH 96 Hr Outlook (1800Z Nov 2)...10% chance of tropical cyclone formation (just offshore of the Nicaragua coast near 12.5N-83W)
IOH 120 Hr Outlook (1800Z Nov 3)...0% chance of tropical cyclone formation (western Nicaragua near 12.5N-86W)
AREA OF INTEREST #3...Shower and thunderstorm activity continues to be widespread in the far eastern tropical Atlantic with the aid of outflow supplied by 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. At the surface...the tropical wave of low pressure that was previously near 35W longitude has evolved into a tropical low pressure spin. I had previously estimated that an additional tropical wave was located further east and closer to the coast of Africa...and this wave has been added to the NHC TAFB surface analysis near 30W longitude. This is further west than I previously estimated...and throughout the last several hours this wave axis and spin near 35W longitude have been merging...resulting in a better-defined and larger spin near 7.5N-32W as of this writing. This rotation is located toward the west edge of the thunderstorm activity and closer to the hostile westerly shearing winds lurking to the northwest. Meanwhile an additional low pressure spin more embedded in the thunderstorm activity appears to be developing near 7.5N-23W. My updated outlook below assumes this developing rotation will become the dominant due to its position well within the thunderstorm activity and furhter away from the westerly shear.
The core upper vorticity generating the shear in the region 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 in the short-term. In addition other models have joined the GFS in showing the formation of a tropical depression in the days ahead. Despite this...I have kept my peak odds of tropical cyclone formation at 40% as the 7.5N-23W initialization of my updated forecast is west of my previous forecast...which results in this system reaching shearing upper southwesterly flow of the upper vortex tied to AOI #1 sooner. The forecast track is west-northwest to northwest toward the surface ridge weakness associated with AOI #1. The upper southwesterly flow may increase the north angle of this system's track in the longer range...especially if this system indeed strengthens (becomes taller to be guided by upper winds). Odds of development are dropped to 0% by 96 hours as this system moves incresaingly into the upper southwesterly shearing flow.
IOH 24 Hr Outlook (1800Z Oct 30)...15% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-26W)
IOH 48 Hr Outlook (1800Z Oct 31)...40% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-29W)
IOH 72 Hr Outlook (1800Z Nov 1)...35% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-33W)
IOH 96 Hr Outlook (1800Z Nov 2)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 15N-36W)
...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 turns southeast into the open central Atlantic and reaches 31.5N-41.5W at 102 hours while potentially gaining tropical characteristics...turns north-northeast and reaches 36N-38.5W at 126 hours.
**For area of interest #2...tail end of front develops into a southern Caribbean tropical low pressure near 12.5N-80W at 84 hours...drifts southwest to waters just offshore of the Costa Rica/Nicaragua border at 11N-83W by 126 hours.
**For area of interest #3...surface low consolidates near 9.5N-22.5W at 24 hours... evolves into possible tropical depression near 11.2N-26W at 48 hours... weakens to a remnant trough near 13.5N-31.5W at 72 hours
0000Z ECMWF Model Run...
**For area of interest #1... frontal cyclone turns southeast into the open central Atlantic and reaches 32.5N-43.5W at 96 hours while potentially gaining tropical characteristics...turns north-northeast and reaches 35N-40.5W 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 48 hours...tropical low pressure gradually loses defintion thereafter and no development forecast
**For area of interest #3...surface trough becomes better defined near 13N-28.5W at 72 hours...continues northwest and dissipates just after 96 hours.
1200Z GFS Model Run...
**For area of interest #1... frontal cyclone accelerates east-southeast and reaches to 36N-45W at 42 hours while developing into a subtropical storm...continues southeast and reaches 32.5N-41.5W at 90 hours...subtropical storm later turns north-northeast and reaches 40N-40W by 120 hours.
**For area of interest #2...tail end of front develops into a southern Caribbean tropical low pressure near 13.5N-77.5W at 27 hours...drifts west-southwest to 11.5N-81.5W through 69 hours...drifts east to 10.5N-78.8W at 123 hours.
**For area of interest #3...surface low consolidates near 10N-24.5W at 33 hours...develops into a tropical depression west of the Republic of Cabo Verde Islands near 16N-29W at 81 hours... weakens to a remnant trough by 120 hours while passing 20N-32.5W.
1200Z NAVGEM Model Run...
**For area of interest #1...frontal cyclone accelerates southeast to 34N-41W by 84 hours while potentially acquiring tropical characteristics... turns north-northeast and reaches 38.8N-38.5W at 120 hours.
**For area of interest #2...tail end of front develops into a southern Caribbean tropical low pressure near 13.5N-77.5W at 36 hours... tropical low pressure gradually loses defintion through 120 hours.
**For area of interest #3...large broad low pressure circulation develops near 9.5N-31W at 18 hours... circulation consolidates into a tropical depression near 10.2N-33W at 78 hours... weakens to a remnant low near 20N-32.5W at 108 hours... remnant low reaches 22N-34W at 120 hours.