MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #113

*******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 SEPTEMBER 16 2021 11:50 PM EDT...

The Atlantic tropics remain active with three areas of interest to watch. Each area of interest is detailed in its own section below.

AREA OF INTEREST #1... The surface low pressure area that was north of the central Bahamas has drifted north into the waters offshore of North Carolina while moving into a narrow surface ridge weakness generated by the eastern divergence zone of upper vorticity over the southeastern United States. This system has some signs of organization while its eastern thunderstorm activity has retained curved banding features. Meanwhile activity over and west of the low pressure area is suppressed by lingering western Atlantic upper vorticity and also westerly shear induced by the upper vorticity over the southeastern US.

Although the surface low pressure is centered a little north of the previous forecast… at 33.5N-75W as of 1800Z… my updated forecast track is the same in the short-term as this system will tend to be located below the supportive eastern divergence zone of the southeast US upper vorticity… and the forecast location of this divergence zone has not changed. By 48 hours… the frontal system over central Canada will push the southeast US upper vorticity toward this system and also create a surface ridge weakness in the northwest Atlantic which should attract this system on a northeast track out to sea. By 3+ days... the upper trough of the central Canadian frontal system is forecast to merge with the upper vorticity ejecting from the southeast US... resulting in an amplified upper trough that could transition this system into a strengthening non-tropical or subtropical system... more on that in the next paragraph below.

I have raised odds of development to 60% as the surface spin does not show multiple swirls as it did yesterday… indicating a slightly more consolidated system. However I do not project 70% odds as the NHC shows as of this writing as the surface spin does not seem very consolidated either. In addition… the window of time for favorable upper winds will only last 24 hours once the upper vorticity over the center dissipates as it has remained cut-off from high-latitude cold air… and before the southeast US upper vorticity nears and increases the shear to more hostile levels. I drop the odds of development to 30% by 48 hours from the increase in shear and as the forecast track takes this system at the border between warm 26+ deg C Gulf stream waters and cooler northwest Atlantic waters. For the 3+ day window… once an amplified upper trough develops near this system as noted in the prior paragraph… models have remarkably converged on forecasting transition to a subtropical system. This transformation is to be facilitated by further amplification of the upper trough into a vortex as adjacent warm upper ridging to the west amplifies in the warm sector of a central North American frontal system. The longer term forecast tracks in the models are a little more north and notably more west than my previous forecast… so my updated longer range forecast track is adjusted accordingly. Odds of subtropical

cyclone formation in the 3 to 4 day window are kept at a low 30% as the upper vortex will not be quiet cold enough to help in thunderstorm generation while this system is located over waters below 26 deg C. However by day 5… the adjacent amplified upper ridging to the west could push this system further south toward the 26 deg C isotherm… and so I raise development odds to 40%. If the models remain in agreement on the current forecast scenario… future long range odds of subtropical development may need to be increased.

The westerly shear induced by the upper vorticity over the southeastern US is keeping this system’s rainfall offshore of the North Carolina Outer Banks.. and if tropical cyclone formation were to occur the gusty winds would

likewise most likely stay offshore. However this system may still produce coastal sea swells here and the rest of the US mid-Atlantic and northeast US coast. In the long range…. coastal sea swells are likely for the Atlantic Canada coast as the tremendous eastern divergence zone of the forecast upper vortex will strengthen this system into a vigorous surface cyclone even if it never acquires subtropical characteristics.

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

IOH 24 Hr Outlook (1800Z Sep 17)... 60% chance of tropical cyclone formation (just offshore of Cape Hatteras North Carolina near 35N-74W)

IOH 48 Hr Outlook (1800Z Sep 18)... 30% chance of tropical cyclone formation (offshore of the northeastern US near 37.5N-70W)

IOH 72 Hr Outlook (1800Z Sep 19)... 30% chance of subtropical cyclone formation (northwestern Atlantic near 41N-59W)

IOH 96 Hr Outlook (1800Z Sep 20)...30% chance of subtropical cyclone formation (northwestern Atlantic near 42N-55W)

IOH 120 Hr Outlook (1800Z Sep 21)...40% chance of subtropical cyclone formation (northwestern Atlantic near 40.5N-55W)

AREA OF INTEREST #2... The tropical wave of low pressure in the eastern Atlantic in the last 24 hours has become a little less organized and has also seen some decrease in thunderstorm activity due to dry Saharan air in the region. An additional factor that may have contributed to the recent stunted development is competition for surface inflow with the nearby central tropical Atlantic wave to the west which was in the process of trying to develop its own surface low pressure spin and thunderstorms… and also possible competition with area of interest #3 to the east which has accelerated westward closer to this system. I am continuing tropical cyclone formation forecasting as this system is moving into the central tropical Atlantic where dry Saharan air concentrations are currently lower… and as this system has absorbed whatever tropical low pressure area the adjacent central Atlantic wave was trying to generate which should eliminate surface inflow competition to the west.

Given the above observations… my updated forecast delays tropical cyclone formation by 24 hours and has a lower intensity projection. The current central Atlantic upper vortex will be pushed south near the northern Lesser Antilles as the warm western Atlantic upper ridge amplifies in the warm sector of the current central Canada frontal system. Even though the upper vortex may weaken by days 3 to 5 while cut-off from high-latitude cold air... there may be enough upper southwesterly flow on its east side to deflect the track of this system north... especially if it goes on to develop a stronger/taller structure as shown in the forecast below. The north angle in the forecast track is thus increased by days 3 to 5... and slow intensification is shown at day 3 from possible shear from the upper southwesterly flow. Additional strengthening is shown by day 4 as the shear relaxes when the upper vortex fades further. However no intensification is shown at day 5 as this system goes very close to the upper vortex and becomes sheared again.

Of note... an additional factor that will increase the north angle in the track at days 3 to 5 is the surface ridge weakness to be induced by area of interest (AOI) #1. However the potential for this system to recurve completely north out to sea with this weakness is low as AOI #1 is forecast to be toward the north… which will tend to keep the surface ridge weakness on the weak side. In the long range after day 5… the only two models that develop this system today are the GFS and CMC… and the CMC keeps this system further west toward the waters southwest of Bermuda as the west part of the Atlantic surface ridge gradually weakens from the approach of a frontal system from central North America. The GFS has a sharper north toward the waters southeast of Bermuda while a shortwave upper trough on the southwest side of the upper vortex to be tied to area of interest #1 is flung toward this system. This means that the long range forecast lies with how the upper vortex tied to area of interest #1 will be shaped… will it be circular or oblong with shortwave troughs on its outer edge? It is too early to know at this time.

Given the current forecast track... interests in the northern Lesser Antilles could see coastal sea swells by Monday.

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

0 Hr Position (1800Z Sep 16)... Tropical low pressure centered at 12.5N-38W

IOH 24 Hr Forecast (1800Z Sep 17)... Tropical

low pressure centered at 13N-45W

IOH 48 Hr Forecast (1800Z Sep 18)... 40 mph maximum sustained wind tropical storm centered at 14N-51W

IOH 72 Hr Forecast (1800Z Sep 19)... 45 mph maximum sustained wind tropical storm centered at 16N-55W

IOH 96 Hr Forecast (1800Z Sep 20)...60 mph maximum sustained wind tropical storm centered east of the northern Lesser Antilles at 18N-59W

IOH 120 Hr Forecast (1800Z Sep 21)… 60 mph maximum sustained wind tropical storm centered north of the northern Lesser Antilles at 20N-62W

AREA OF INTEREST #3... The vigorous western Africa tropical wave of low pressure has accelerated westward and is already at the coastline entering the eastern tropical Atlantic. Although the thunderstorm activity of the wave has declined in coverage and intensity due to the presence of dry Saharan air… it has become better defined while featuring a large broad area of cyclonic turning. Due to the reduced thunderstorm activity and the ECMWF model still being the only model showing development… I have nudged development odds back down to 20%… and the NHC outlook product has remained parked at 20% odds over the last couple of days.

Due to the recent westward acceleration of the wave… the short-term forecast track is accelerated westward. Part of the westward shift in the track is that the wave will now miss becoming entangled with the southwest Europe upper trough because it has not developed a strong/tall enough structure to be influenced track-wise by upper winds… and also the early arrival of the wave to the west coast of Africa means the southward-digging base of the upper trough will miss the wave. I slow the forecast speed of the wave by 3+ days to avoid leaving behind the model consensus… and this also makes sense as the wave will approach the surface ridge weakness associated with area of interest #1. The north angle in the track is also increased in that timeframe due to the weakness. This track by day 5 will place this system near upper vorticity to be left behind by the current east Canada upper trough as that trough crosses the Atlantic… resulting in possible westerly shear induced by the south side of the upper vorticity. My development odds are trimmed down to 15% at day 5 as a result.

The upper outflow and thus the thunderstorm activity on the north side of the wave is being suppressed by the southwest Europe upper trough. Therefore it is now unlikely the Republic of Cabo Verde Islands will see heavy rainfall from this wave.

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

IOH 24 Hr Outlook (1800Z Sep 17)... 0% chance of tropical cyclone formation (south of the Republic of Cabo Verde Islands near 12N-23W)

IOH 48 Hr Outlook (1800Z Sep 18)... 0% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-29W)

IOH 72 Hr Outlook (1800Z Sep 19)… 20% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-33W)

IOH 96 Hr Outlook (1800Z Sep 20)… 20% chance of tropical cyclone formation (eastern tropical Atlantic near 14N-37W)

IOH 120 Hr Outlook (1800Z Sep 21)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 16N-40.5W)

...COMPUTER MODEL SUMMARY...

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

1200Z CMC Model Run...

**For area of interest #1... located at 40.2N-53W at 120 hours as a large and possible subtropical storm.

**For area of interest #2... located east of the eastern Bahamas at 22.5N-68.5W as a tropical depression

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

1200Z ECMWF Model Run...

**For area of interest #1...located at 41.2N-53W at 120 hours as a large and possible subtropical storm.

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

**For area of interest #3...tropical depression suggested near 17.8N-32.5W at 96 hours...located at 23N-34W at 120 hours.

1200Z GFS Model Run...

**For area of interest #1... located at 40.8N-54W at 120 hours as a large and possible subtropical storm.

**For area of interest #2... tropical cyclone formation suggested at 15N-51W at 54 hours...passes just north of the northern Lesser Antilles as a tropical storm by 120 hours.

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

0600Z NAVGEM Model Run...

**For area of interest #1...located at 39.5N-58.2W at 120 hours as a large and possible subtropical storm.

**For area of interest #2... tropical wave amplifies into a tropical low pressure spin just northeast of the northeastern Caribbean Islands at 120 hours.

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