MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #112
*******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.**********
...WEDNESDAY SEPTEMBER 15 2021 7:25 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. Elsewhere… a tropical wave of low pressure in the central Atlantic has seen some increase on thunderstorms. However this wave is not likely to develop as area of interest #2 to the east is already more developed… and will likely dominate the surface inflow in the region needed for further development.
AREA OF INTEREST #1... The shower and thunderstorm activity northeast of the central and eastern Bahamas continues to be supported by the eastern divergence zone of the upper vortex in the region. This activity has continued to become gradually better organized while the surface trough has developed into a more defined low pressure spin seen in the low-clouds via satellite animation. At the moment the shower and thunderstorm bands are lopsided to the east side of the spin due to light shear induced by the aforementioned upper vortex. However as the upper vortex fades while remaining cut-off from high-latitude cold air... the upper air pattern will become more conducive for tropical development in the next 24 hours.
Short-term forecast positions in the outlook are nudged east due to the current position of the surface low...which I estimate to be near 27.5N-73.8W. This system will soon track more north into a narrow surface ridge weakness to be induced by the eastern divergence zone of southeastern US upper vorticity to be left behind by the current central Canada upper trough. By day 3... the next frontal system after that 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 4+ days... the upper trough of this next 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 50% due to the continually improving organization of this system. At this time I have not established higher odds like the NHC has as the surface circulation exhibits multiple cloud swirls instead of a well-defined single center. Plus the window of time for favorable conditions is narrow as the approach of southeast US upper vorticity after 24 hours... and an amplified upper trough by 4+ days... will increase the shear and potentially make this system less tropical... which is why I taper down development odds after 24 hours. Models are split on the scenario at 4+ days. One soltuion is for a less amplified upper trough that pulls this system north over cooler waters and transitions this system to non-tropical. The second scenario is the upper trough is more amplified and is almost an upper vortex which captures this system at a further south location...with lower shear and higher upper divergence on the east side of the upper vortex strengthening this system into a subtropical storm... essentially a hybrid system supported non-tropically by the upper vortex’s upper divergence while located south enough over warm water to still have thunderstorms and warm core outflow in the environment just east of the upper vortex. Today's models that champion the second scenario... the GFS and ECMWF... have delayed the formation of a subtropical storm to beyond 5 days and at a further east-northeast location while having the upper trough taking longer to amplify. Therefore I have lowered the odds of subtropical cyclone formation in the 4 to 5 day window to 20% and also have adjusted those forecast points a little more toward the north.
Because this system has not developed into a tropical cyclone thus far...the potential for coastal sea swells in the Bahamas is lowering. The potential remains however for the US east coast within the next couple of days and requires this system to become a tropical cyclone. Gusty winds and heavy rains may reach the North Carolina Outer Banks in about 2 to 3 days (late tomorrow or Friday).
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 16)... 50% chance of tropical cyclone formation (offshore of the southeastern US near 31N-75W)
IOH 48 Hr Outlook (1800Z Sep 17)... 40% chance of tropical cyclone formation (just offshore of Cape Hatteras North Carolina near 35N-74W)
IOH 72 Hr Outlook (1800Z Sep 18)... 20% chance of tropical cyclone formation (offshore of the northeastern US near 37.5N-70W)
IOH 96 Hr Outlook (1800Z Sep 19)... 20% chance of subtropical cyclone formation (northwestern Atlantic near 39N-61W)
IOH 120 Hr Outlook (1800Z Sep 20)...20% chance of subtropical cyclone formation (north-central Atlantic near 40N-50W)
AREA OF INTEREST #2... The tropical wave of low pressure in the eastern Atlantic in the last has over the last 24 hours seen a gradual increase in the coverage and organization of thunderstorm activity in association with its low pressure spin. The wave remains in a favorable upper outflow and low shear environment under a stout tropical upper ridge in the region. With these observations I have begun a tropical cyclone formation forecast as outlined below. The forecast assumes this system will need about another 24 hours to mix out the dry Saharan air in its circulation… thus projecting a tropical cyclone by 48+ hours. With the GFS being the only model today to develop this system within 5 days… my intensity forecast is on the conservative side and keeps this system below hurricane force through day 5.
The current northwest Atlantic upper trough is re-enforcing the current central Atlantic upper vorticity near the northern Lesser Antilles. Even though the upper vorticity may weaken by days 4 and 5 while becoming cut-off from additional upper troughs… 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 4 and 5… and a pause in intensification at day 4 is a reflection of the possible shear from the upper southwesterly flow. Additional strengthening is shown by day 5 as the shear relaxes when the upper vorticity fades further.
Of note… an additional factor that could increase the north angle in the track at days 4 and 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 the latest model runs that keep AOI #1 as a further south subtropical storm are doing so at a further north location… which will tend to keep the surface ridge weakness on the weak side. The forecast track is also adjusted faster to the west to reflect the faster forward speed this system has displayed… but shows a slow down by the end of the forecast period due to the surface ridge weakness. 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 15)... Tropical low pressure centered at 12N-30W
IOH 24 Hr Forecast (1800Z Sep 16)... Tropical low pressure centered at 12.5N-37W
IOH 48 Hr Forecast (1800Z Sep 17)... 45 mph maximum sustained wind tropical storm centered at 13N-44W
IOH 72 Hr Forecast (1800Z Sep 18)... 60 mph maximum sustained wind tropical storm centered at 13.5N-50W
IOH 96 Hr Forecast (1800Z Sep 19)... 60 mph maximum sustained wind tropical storm centered at 16N-54W
IOH 120 Hr Forecast (1800Z Sep 20)...70 mph maximum sustained wind tropical storm centered east of the northern Lesser Antilles at 17.5N-58W
AREA OF INTEREST #3... Another tropical wave of low pressure featuring an impressive thunderstorm complex on its west side…currently over western Africa near 10N-6W… is forecast to enter the eastern tropical Atlantic by day 3. The ECMWF model has continued to suggest the wave could develop with the stout presence of low shear and upper outflow supplied by the eastern tropical Atlantic upper ridge. I have raised development odds due to the impressive thunderstorm complex… but only to 30% as the wave only has model support from the ECMWF... and the recent waves that have emerged from Africa have struggled with dry Saharan air. The ECMWF is also a northern outlier in track while assuming this wave will quickly develop offshore of Africa to become tall enough to be tugged by upper vorticity that is currently being left behind by the southwest Europe upper trough. As a compromise between the ECMWF and the rest of the models... I assume slower development and a smaller increase in the north angle in the track by day 3. I project this system to then escape the upper vorticity and continue more west by days 4 and 5. There is some chance this wave brings heavy rain and gusty winds to the Republic of Cabo Verde Islands this weekend.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 16)... 0% chance of tropical cyclone formation (western Africa near 10N-12W)
IOH 48 Hr Outlook (1800Z Sep 17)... 0% chance of tropical cyclone formation (just offshore of Western Africa near 10.5N-17W)
IOH 72 Hr Outlook (1800Z Sep 18)... 20% chance of tropical cyclone formation (southeast of the Republic of Cabo Verde Islands near 12N-22W)
IOH 96 Hr Outlook (1800Z Sep 18)... 30% chance of tropical cyclone formation (southwest of the Republic of Cabo Verde Islands near 13N-27W)
IOH 120 Hr Outlook (1800Z Sep 18)... 30% chance of tropical cyclone formation (eastern tropical Atlantic near 13.5N-32W)
...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... transitions to large and strong non-tropical frontal cyclone centered just southeast of Newfoundland by 120 hours.
**For area of interest #2... tropical wave amplifies into a tropical low pressure spin just north of the northeastern Caribbean Islands at 120 hours.
**For area of interest #3... organizes into a broad tropical low pressure that tracks northwest from 72 to 102 hours at a location just southwest of the Republic of Cabo Verde Islands...tropical low located at 18.5N-30W at 126 hours.
1200Z ECMWF Model Run...
**For area of interest #1...transitions to large and strong subtropical storm centered at 40.2N-54W by 144 hours.
**For area of interest #2... tropical wave amplifies into a tropical low pressure spin northeast of the northeastern Caribbean Islands at 120 hours.
**For area of interest #3... tropical wave departs Africa at between 24 and 48 hours... passes just east of the Republic of Cabo Verde Islands at 72 hours as a tropical storm... strengthens further to near hurricane strength by 120 hours while turning north and reaching 25.5N-24W.
1200Z GFS Model Run...
**For area of interest #1... transitions to a strong non-tropical frontal cyclone centered southeast of Newfoundland by 120 hours.
**For area of interest #2... located at 16.2N-56W a compact hurricane by 120 hours.
**For area of interest #3... tropical wave departs Africa at 48 hours... no development shown afterwards
1200Z NAVGEM Model Run...
**For area of interest #1...transitions to an elongated non-tropical frontal low near 37.8N-58.5W by 120 hours.
**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