MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #116 (Weekend Edition)
*******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.**********
...MONDAY SEPTEMBER 20 2021 1:05 AM EDT...
Satellite image as of 1830Z. 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 line are in the NHC 2-day tropical weather outlook:
NHC TAFB Surface Analysis 1200Z (Sept 19):
GFS Model Upper-Level Winds 1200Z (Sept 19):
The Atlantic tropics remain active with multiple
systems to watch:
**See Tropical Storm Peter and Rose sections below for an update on the two currently active storms in the Atlantic basin… Peter is expected to pass just north of the Caribbean Islands and Bahamas in the coming days
**See Odette (area of interest #1) section below for an update on the remnant cyclone offshore of Atlantic Canada which may re-acquire tropical character
**See area of interest #2 section below for an update on the latest tropical wave of low pressure that has entered the eastern tropical Atlantic and is now being monitored for development.
TROPICAL STORM PETER… In the last 24 hours… bursts of thunderstorms helped to intensify tropical depression sixteen in the central tropical Atlantic into Tropical Storm Peter in the face of shear being generated by a nearby upper vortex to the west. Although these bursts waned in the late morning and early afternoon hours… they have returned in the late afternoon and through now to help maintain Peter as a tropical storm. Regarding Peter’s forecast track… Odette and its supporting upper vortex is expected to stall at a further east location… which will help western Atlantic surface ridging stay more intact and thus help Peter in tracking more westward. Therefore my updated forecast track is adjusted westward… especially with Peter also a little west of my previous forecast track. And although Peter is also a little north of the previous forecast track… I have opted for a less of a north angle in the updated track forecast due to the more robust western Atlantic surface ridge now expected. However some north angle in track is still expected due to the drag of the upper southwesterly shearing flow. Even though the shearing upper vortex 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 2 that would prolong the upper southwesterly flow. A turn more toward the north is projected by day 5 while Peter turns toward the ridge weakness associated with the frontal system to approach from its current central North American position. The 120Z GFS run also suggested that the eastern divergence zone of some of the upper vorticity in the region could generate a subtropical surface low to the northeast that could also aid in Peter’s north turn.
Peter has recently strengthened a little further to 50 mph max sustained winds. My short-term intensity forecast is nudged downward for the next few days as the westward adjusted track keeps Peter closer to the shearing upper vortex to the west. That means I forecast Peter to weaken back down to 45 mph max winds in the short-term. Even though this shear may be prolonged by upper vorticity deposited by the upper trough/vortex to be tied to Odette… the GFS does hint that this round of upper vorticity will also weaken in Peter’s immediate environment (but linger to the north) while remaining cut-off from high-latitude cold air… but has delayed this weakening by one day. Therefore my updated forecast delays the timeframe for additional strengthening.
Given the latest forecast track:
**Interests in the northern Lesser Antilles will see coastal sea swells tomorrow.
**The track shown below is closer to Puerto Rico… Virgin Islands… the Dominican Republic north coast… and the Bahamas. Coastal sea swells will also be possible here in the coming days.
**The updated track below has shifted further away from Bermuda. However Bermuda should monitor future updates on Peter for any additional shifts in the forecast track.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 19)... 45 mph maximum sustained wind tropical storm centered at 18.4N-57.8W
IOH 24 Hr Forecast (1800Z Sep 20)... 45 mph maximum sustained wind tropical storm centered north of the northern Lesser Antilles at 20N-61.5W
IOH 48 Hr Forecast (1800Z Sep 21)... 45 mph maximum sustained wind tropical storm centered north of Puerto Rico at 21.5N-65W
IOH 72 Hr Forecast (1800Z Sep 22)… 45 mph maximum sustained wind tropical storm centered east-northeast of the eastern Bahamas at 23N-69.5W
IOH 96 Hr Forecast (1800Z Sep 23)… 45 mph maximum sustained wind tropical storm centered north of the eastern Bahamas at 24N-71.5W
IOH 120 Hr Forecast (1800Z Sep 24)… 50 mph maximum sustained wind tropical storm centered at 26.5N-73.5W
TROPICAL DEPRESSION SEVENTEEN (RECENTLY UPGRADED TO TROPICAL STORM ROSE)… The vigorous tropical wave of low pressure in the eastern Atlantic early on Sunday intensified into the seventeenth tropical depression of the 2021 Atlantic Hurricane Season… and then the seventeenth named storm of the season as it became Tropical Storm Rose as of 5 PM EDT on Sunday. Rose’s track has been more north and less west… perhaps from the tug of westerly surface inflow into the neighboring tropical wave to the east (area of interest #2). Also the outflow of the neighboring wave appears to have sheared the thunderstorms of Rose to the west side of the circulation. Needless to say… my updated forecast track is more north and less west over the previous. This adjustment also makes sense as Odette is now forecast to stall at a further east location… toward 45W longitude… and Rose will be tracking into Odette’s ridge weakness. In addition… Rose is already nearing the upper southwesterly flow on the north side of tropical upper ridging in the region and out ahead of North Atlantic upper vorticity that has been deposited by the current upper trough moving into Europe. The drag of this flow will help keep Rose’s track on a north angle in the short-term and before Rose feels the weakness induced by Odette.
As the direction of shear switches from easterly (caused by the outflow of the neighboring tropical wave) to westerly (out ahead of the aforementioned North Atlantic upper vorticity)… I forecast the thunderstorms will shift over the center for a period of time to allow the tropical storm to strengthen in the next 24 hours. Weakening to a tropical depression is then shown by 48 hours from the westerly shear. Rose may pass through the axis of the shearing upper vorticity and move into a haven of lower shear at 72 and 96 hours… and also the upper vorticity is forecast to weaken while remaining cut-off from high-latitude cold air. Thus my forecast for now calls for Rose to survive and perhaps re-strengthen by 96 hours. By 120 hours Rose will near the upper vortex tied to Odette… thus getting sheared yet again. Thus I once again bring down the forecast intensity at that time. The conclusion of Rose might get interesting after day 5… that is assuming Odette’s forecast does not change and Rose does survive the next five days to near Odette. It is possible Odette and Rose undergo a fujiwhara dance under the upper vortex… perhaps turning into a contest to see which system dominates if any.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official
0 Hr Position (1800Z Sep 19)...40 mph maximum sustained wind tropical storm centered at 14.3N-29.9W
IOH 24 Hr Forecast (1800Z Sep 20)... 50 mph maximum sustained wind tropical storm centered at 17.8N-33W
IOH 48 Hr Forecast (1800Z Sep 21)... 35 mph maximum sustained wind tropical depression centered at 21N-36W
IOH 72 Hr Forecast (1800Z Sep 22)… 35 mph maximum sustained wind tropical depression centered at 24N-39W
IOH 96 Hr Forecast (1800Z Sep 23)…45 mph maximum sustained wind tropical storm centered at 27.5N-42.5W
IOH 120 Hr Forecast (1800Z Sep 24)… 35 mph maximum sustained wind tropical depression centered at 29N-45W
AREA OF INTEREST #1 (REMNANTS OF ODETTE)… As of 1800Z… the remnant frontal cyclone of Odette was centered offshore of Atlantic Canada at 42.5N-57.5W on the NHC TAFB surface analysis. ASCAT-B and ASCAT-C ascending passes suggest 45 knot (50 mph) maximum sustained winds southeast of the center. I am continuing forecasts on Odette as it could re-acquire tropical characteristics in the coming days. Only recently the NHC outlook introduced Odette’s remnant cyclone to also acknowledge its potential re-acquisition of tropical characteristics… which is why in the above charts it is marked as not being in the NHC outlook when it now is.
Odette’s strength as a remnant frontal cyclone is facilitated by the ever-increasing upper divergence zone of the approaching amplifying upper trough ejecting from eastern Canada. The amplification of the upper trough is being caused by adjacent amplification of warm eastern North America upper ridging to the west in the warm sector of the frontal system now over central North America. Models generally agree on the upper trough amplifying into a cut-off upper vortex around 45W longitude (with the exception of the GFS which remains an eastern outlier). My updated forecast track is shifted east to align with the model consensus… and in the short-term is nudged north due to Odette’s current position. For the longer-term part of the forecast where Odette and the upper vortex are expected to drift south under the influence of the upper ridging to the west… some models have shifted further south and so my updated longer range forecast track does not have a northward adjustment. By 120 hours some recent model runs show the upper vortex shift further south with Odette becoming whirled west-northwest in the north side of the vortex. The forecast track below shows a similar west-northwest motion in that timeframe.
The intensity forecast below ramps up through 24 hours under the eastern divergence zone of the amplifying upper trough… however is nudged downward due to Odette’s current strength relative to the prior forecast. Once Odette whirls beneath the center of the upper vortex where upper divergence is lacking (2+ days)… gradual weakening is projected. The further east forecast track means Odette will be over a slightly cooler part of the Atlantic where it will have to drift south to at least 39N latitude to reach the 26 deg C isotherm. As such I have delayed forecast transition to a subtropical storm by 24 more hours. Subtropical status is maintained through 120 hours when Odette moves west-northwest as the track floats not too far away from the 26 deg C isotherm.
On the updated forecast track… Odette will soon move eastward enough to lower the coastal sea swell potential on the coastal northeastern US. Coastal sea swells could linger on the shores of Atlantic Canada for a few more days… in particular toward Newfoundland… until Odette begins to weaken as forecast below.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 19)... 50 mph maximum sustained frontal cyclone centered at 42.5N-57.5W
IOH 24 Hr Forecast (1800Z Sep 20)... 65 mph maximum sustained frontal cyclone centered at 43.5N-47.5W
IOH 48 Hr Forecast (1800Z Sep 21)... 60 mph maximum sustained frontal cyclone centered at 41N-45W
IOH 72 Hr Forecast (1800Z Sep 22)… 50 mph maximum sustained frontal cyclone centered at 40N-45W
IOH 96 Hr Forecast (1800Z Sep 23)… 45 mph maximum sustained subtropical storm centered at 39N-45W
IOH 120 Hr Forecast (1800Z Sep 24)… 40 mph maximum sustained subtropical storm centered at 40N-47W
AREA OF INTEREST #2…The tropical wave of low pressure that was nearing the west coast of Africa has emerged into the far eastern tropical Atlantic with some signs of cyclonic curvature in its clouds and scattered thunderstorms. Based on the latest satellite imagery… the lowest pressure center (center of cyclonic curvature in the low clouds) appears to be near 10.2N-18.5W. Various model runs suggest this wave will develop due to the stout presence of the tropical upper ridge in the region providing low shear and upper outflow… and the NHC has added this wave into their 5-day outlook and more recently into their 2-day outlook. The tropical wave is expected to continue west as the surface ridge weakness to be induced by Rose and Odette is expected to be too weak to bend the track of anything that develops from this wave to the north… as both Rose and Odette are expected to weaken in the longer range (see above Rose and Odette sections for the factors expected to weaken each system). I agree with the NHC outlook ramping up their odds of development to 50% by day 5 due to the mix of model runs that do and don’t develop this system. However in the short-term… I forecast low development odds as the wave is not yet very organized and will need some time to gather organization.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 20)... 0% chance of tropical cyclone formation (south of the Republic of Cabo Verde Islands near 11N-23.5W)
IOH 48 Hr Outlook (1800Z Sep 21)... 10% chance of tropical cyclone formation (eastern tropical Atlantic near 11.5N-28.5W)
IOH 72 Hr Outlook (1800Z Sep 22)… 30% chance of tropical cyclone formation (eastern tropical Atlantic near 12N-33.5W)
IOH 96 Hr Outlook (1800Z Sep 23)… 50% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-38.5W)
IOH 120 Hr Outlook (1800Z Sep 23)… 50% chance of tropical cyclone formation (central tropical Atlantic near 13N-43.5W)