MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #114
Updated: Sep 19
*******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 SEPTEMBER 17 2021 9:45 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… and are:
**Area of Interest #1… tropical low pressure offshore of the United States east coast recently upgraded to Tropical Storm Odette
**Area of interest #2… tropical wave of low pressure in the central tropical Atlantic forecast to pass just north of the Lesser Antilles by Monday
**Area of interest #3… tropical wave of low pressure in the eastern tropical Atlantic
AREA OF INTEREST #1 (RECENTLY UPGRADED TO TROPICAL STORM ODETTE)... The surface low pressure area that has been offshore of Virginia and North Carolina displayed a southwest-northeast elongated circulation and sheared off thunderstorms to the northeast as southwesterly shearing upper winds continue ahead of the southeast US upper vorticity and on the north side of upper ridging in the western Atlantic. In the afternoon hours the northeast side of the elongated circulation displayed a circular consolidated spin next to the thunderstorms… and this spin is now considered the center of what is now Tropical Storm Odette as of 5 PM EDT. The NHC has set the initial intensity to 40 mph max sustained winds which are easily seen in a broad area northeast of the center in ASCAT passes and within the sheared off thunderstorms. The system is considered tropical as this system owes a lot of its current strength to the warm core outflow of the sheared off thunderstorms in the north side of the west Atlantic upper ridging… with the latent heat release of the thunderstorms re-enforcing the warm upper ridging and resultant outflow.
By 24 hours... the frontal system over central Canada will push the southeast US upper vorticity toward Odette and also create a surface ridge weakness in the northwest Atlantic which should attract Odette on a northeast track. I forecast only a little strengthening as the shear will worsen as the upper vorticity nears… and have nudged the short-term track to the north and east due to Odette’s current center fix with respect to the previous forecast. By 48+ hours… 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. The upper trough is expected to amplify as adjacent warm upper ridging to the west amplifies in the warm sector of a central North American frontal system. After all the models came into agreement yesterday about having the upper trough amplify into a stationary upper vortex… the GFS has now returned back to a less amplified upper trough that accelerates Odette quickly east-northeast across the North Atlantic while the others still hold on to a stalled upper vortex scenario. The difference in both scenarios is related to how the models vary in forecasting the warm sector of the frontal system to be over central North America… which affects how amplified the warm upper ridging to the west of Odette gets and thus how amplified the upper trough interacting with Odette gets. If you compare the GFS to the stalled upper vortex scenario in the other models… the difference in the forecast positions by day 5 spans several hundred miles! Needless to say the forecast track confidence by day 5 is unusually low.
My long term track forecast strategy is to split the difference between the two possible scenarios… by still forecasting a stalled upper vortex scenario but having the location of the stalled upper vortex further east which could certainly happen if the upper trough takes longer to amplify into a cut-off upper vortex. The intensity forecast is ramped through 72 hours as the tremendous eastern divergence zone of the amplifying upper trough aids Odette… but Odette will likely lose tropical character during that time while tracking over cooler water. A south drift is assumed at days 4 and 5 as the upper vortex is pushed south by the adjacent amplified upper ridging to the west. During this time I forecast the intensity to taper down as Odette whirls beneath the center of the upper vortex where upper divergence is lacking… and assume the south drift toward warmer 26 deg C waters will allow Odette to re-acquire some tropical characteristics by day 5.
Odette will bring coastal sea swells to the mid-Atlantic US… northeastern US and Atlantic Canada shorelines over the next few days. When these coastal sea swells will end is uncertain due to the unusual uncertainty in the long range track forecast.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 17)... 40 mph maximum sustained wind tropical storm centered offshore of the United States east coast near 36.7N-71.8W
IOH 24 Hr Forecast (1800Z Sep 18)... 45 mph maximum sustained wind tropical storm centered at 38N-68W
IOH 48 Hr Forecast (1800Z Sep 19)... 60 mph maximum sustained frontal cyclone centered at 41N-58W
IOH 72 Hr Forecast (1800Z Sep 20)... 70 mph maximum sustained frontal cyclone centered at 42.5N-50W
IOH 96 Hr Forecast (1800Z Sep 21)... 65 mph maximum sustained frontal cyclone centered at 40N-49.5W
IOH 120 Hr Forecast (1800Z Sep 22)… 60 mph maximum sustained subtropical storm centered at 39N-49W
AREA OF INTEREST #2... The tropical wave of low pressure that has moved into the central tropical Atlantic from the eastern Atlantic has been gradually organizing as thunderstorms increase with its low pressure spin while moving into lower concentrations of dry Saharan air. I am continuing tropical cyclone formation forecasting on this system as a result.
Given the above observation of continuous improvement in the organization… my timeline for tropical cyclone formation is unchanged and is now 24 hours away. The intensity projection however is lowered a bit as due to my updated forecast track which is adjusted north and west due to the current position of this system… closer to the upper vorticity in the central Atlantic which will increase the shear. At this time I expect the shear levels will be too high for additional strengthening after day 3. The model spread in the forecast track is wide due to the uncertainty with Odette’s forecast track. As discussed in the above Odette (area of interest #1) section… some models have Odette stalled at a further southwest location while it becomes trapped under an upper vortex… while the GFS thrusts Odette into the far North Atlantic. The GFS scenario would result in a further south track for this system as the surface ridge weakness associated with Odette quickly closes… while a more southwest-positioned Odette would promote a stronger surface ridge weakness and a more north track for this system. My updated forecast track is in the middle of the model spread… resulting in a north angle in track thru 5 days. It does make sense to have a north angle in the track over the next few days due to a myriad of factors besides Odette’s surface ridge weakness… as the western end of the Atlantic surface ridge will be weakened further with the approach of a frontal system from central North America by day 5 and upper southwesterly flow ahead of the central Atlantic upper vorticity as this system is likely to become a strong/tall enough tropical cyclone to be guided by upper winds. And even though this upper vorticity is slated to fade away while cut-off from high-latitude cold air… the upper trough/vortex to be tied with Odette could leave behind upper vorticity in the west Atlantic by day 5 that continues the upper southwesterly flow. The shearing nature of the prolonged upper southwesterly flow is another consideration for keeping the intensity forecast below flat through day 5.
Given the current forecast track... interests in the northern Lesser Antilles could see coastal sea swells by Monday. Interests in Bermuda should be aware of this system due to the uncertainty in this system’s track… caused by the uncertainty in Odette’s forecast track.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 17)...Tropical low pressure centered at 14N-46W
IOH 24 Hr Forecast (1800Z Sep 18)... 40 mph maximum sustained wind tropical storm centered at 15N-52W
IOH 48 Hr Forecast (1800Z Sep 19)... 45 mph maximum sustained wind tropical storm centered at 17.5N-56W
IOH 72 Hr Forecast (1800Z Sep 20)... 50 mph maximum sustained wind tropical storm centered northeast of the northern Lesser Antilles at 19.5N-60W
IOH 96 Hr Forecast (1800Z Sep 21)... 50 mph maximum sustained wind tropical storm centered north of the northern Lesser Antilles at 21.5N-63W
IOH 120 Hr Forecast (1800Z Sep 22)… 50 mph maximum sustained wind tropical storm centered east-northeast of the eastern Bahamas at 23.5N-67.5W
AREA OF INTEREST #3... The vigorous tropical wave of low pressure that recently entered the eastern Atlantic from Africa has continued to display a broad area of cyclonic turning. Shower and thunderstorm bands have become a little stronger and better organized with this system despite the presence of dry Saharan air lurking just the north… therefore I have raised my peak development odds for this wave to 40% (the NHC has also recently increased development odds to 30%).
My updated forecast track below is nudged west due to the current position of this wave. I slow the forecast speed of the wave after 24 hours to avoid leaving behind the model consensus... and this also makes sense as the wave will approach the surface ridge weakness associated with Odette. The north angle in the track is also increased with time due to the weakness. This track by day 4 will place this system near upper vorticity to be left behind by the current north Atlantic upper trough…resulting in possible westerly shear induced by the south side of the upper vorticity. My development odds are trimmed down to 30% at day 4 as a result. Even though the GFS currently forecasts this upper vorticity to fade by day 5 as it remains cut-off from high-latitude cold air… I do not raise development odds at that time to see if the GFS remains consistent in its forecast of the upper vorticity.
On a final note… if Odette accelerates east-northeast into the far North Atlantic as the GFS today forecasted… then the associated surface ridge weakness would close. This would require some westward adjustment to this system’s forecast track in the longer range.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 18)... 0% chance of tropical cyclone formation (eastern tropical Atlantic near 12N-30W
IOH 48 Hr Outlook (1800Z Sep 19)... 30% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-34W
IOH 72 Hr Outlook (1800Z Sep 20)... 40% chance of tropical cyclone formation (eastern tropical Atlantic near 13.5N-38W
IOH 96 Hr Outlook (1800Z Sep 21)... 30% chance of tropical cyclone formation (central tropical Atlantic near 15N-41W)
IOH 120 Hr Outlook (1800Z Sep 22)… 30% chance of tropical cyclone formation (central tropical Atlantic near 17.5N-43W)
...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 40.5N-56.5W at 120 hours as a possible subtropical storm.
**For area of interest #2... located east-northeast of the eastern Bahamas at 23.5N-68.5W as a tropical storm at 120 hours
**For area of interest #3... no development shown
1200Z ECMWF Model Run...
**For area of interest #1...located at 41.5N-54W at 120 hours as a large and possible subtropical storm.
**For area of interest #2...tropical wave amplifies into a tropical low near 25.5N-69W at 120 hours
**For area of interest #3...no development shown
**Tropical wave emerges from Africa at 72 hours...tropical wave develops into a tropical low near 11.5N-43W at 168 hours.
1200Z GFS Model Run...
**For area of interest #1... located in the north Atlantic as a non-tropical frontal cyclone near 55N-37W at 120 hours.
**For area of interest #2... tropical cyclone formation suggested near 17.5N-56W at 51 hours...weakens to a remnant low while passing just north of the Lesser Antilles by 90 hours...remnant low located just offshore of the northern Dominican Republic at 90 hours.
**For area of interest #3... no development shown
1200Z NAVGEM Model Run...
**For area of interest #1...located at 39N-48W at 120 hours as a large and possible subtropical storm.
**For area of interest #2... tropical cyclone formation suggested near 16.2N-53.5W at 42 hours...while gradually strengthening into a strong tropical storm curves north and reaches 25N-61W by 126 hours.
**For area of interest #3... no development shown