*******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 23 2021 2:30 AM EDT...
The Atlantic tropics remain very active with multiple systems to watch:
**Note that although Peter has recently decayed into a remnant low… interests in Bermuda should be aware that Peter could re-organize and relocate closer to the island by Friday or Saturday m… or alternatively could be affected by a new subtropical or tropical low that takes Peter’s place. See remnants of Peter section below for more details.
**See Tropical Depression Rose and Eighteen sections below for an update on the two currently active storms in the Atlantic basin. Eighteen is expected to strengthen into a major Hurricane and could approach the northern Lesser Antilles after 5 days
**See Odette (area of interest #1) section below for an update on the remnant cyclone in the open north Atlantic which may re-acquire tropical character
**See area of interest #2 section below for info on yet another tropical wave of low pressure that could develop to the east of Tropical Depression Eighteen in the days ahead.
Elsewhere…the tail end of the cold front moving across eastern North America has reached the western Gulf of Mexico. Once the upper trough associated with the front shifts northeast in the next couple of days… wind shear in the Gulf may relax enough to allow for possible tropical development along the tail end of the front. Anything that does develop is likely to drift west toward east-central Mexico… Veracruz and vicinity… under the influence of the central US surface ridge as that ridge later settles over the eastern US.
TROPICAL DEPRESSION PETER (RECENTLY DOWNGRADED TO REMNANTS OF PETER)… While located due east of the eastern Bahamas… Peter has degenerated into a remnant low pressure without a defined center due to increased southwesterly shear caused by upper vorticity deposited by the North Atlantic upper trough and near Peter. However the future of Peter’s remnants after 24 hours may be interesting to watch. One possibility is that Peter reforms to the northeast close to Bermuda as a subtropical or tropical low in an upper divergence maximum and lower shear region to develop once the upper vorticity weakens to an upper vortex. The other possibility… championed by the GFS…is that Peter stays southwest of the upper vortex while a new subtropical or tropical surface low develops closer to Bermuda. The GFS scenario could get more complicated if Peter makes a comeback in upper anticyclonic flow with lower shear/upper outflow which may develop in the environment southwest of the upper vortex… which may result in Peter and the separate subtropical or tropical low orbiting each other in a fujiwhara type of interaction.
Any future developments that occur with Peter’s remnants or any subtropical-to-tropical low that supersedes Peter may be handled with an area of interest section in future updates… and also on the home page bulletins of this site should impacts to Bermuda become a concern. Any gusty winds… heavy rains… etc from this situation could reach the island by Friday or Saturday.
TROPICAL DEPRESSION ROSE… Rose continues its northwest track into the open eastern Atlantic while tracking toward Odette’s surface ridge weakness. My forecast track is nudged a tad west due to Rose’s current position. After having its thunderstorms sheared off to the east by the current northeast Atlantic upper vorticity… Rose has not developed any new thunderstorms despite arriving to a lower shear environment beneath a mid-latitude central Atlantic upper ridge. The result is Rose is a cloud swirl surface low about to lose its qualification as a tropical depression. The NHC 11 PM EDT advisory sites dry air as a probable cause for the lack of thunderstorms… perhaps an artifact of the dry Saharan air layer to the east which often surges into this region? Or perhaps the dry air is a result of the western upper convergence zone of the northeast Atlantic upper vorticity? My forecast assumes Rose will weaken to a remnant low shortly due to its continued lack of thunderstorms.
Of note… the remnant low of Rose is expected to undergo a fujiwhara interaction with the remnants of Odette (area of interest #1) which would send Rose to the northeast. Some models indicate Rose’s remnants making a comeback in the eastern divergence zone of the upper vortex tied to the remnants of Odette. However Rose would be positioned east of Odette in such an interaction… where waters are cooler and thus Rose would likely not be tropical in nature in this comeback scenario.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official
forecast***********
0 Hr Position (0000Z Sep 23)… 35 mph maximum sustained wind tropical depression centered at 24.6N-40.9W
IOH 24 Hr Forecast (0000Z Sep 24)… Remnant low centered at 28N-44W
TROPICAL DEPRESSION EIGHTEEN…The tropical wave of low pressure in the eastern tropical Atlantic has continued to evolve essentially into a compact swirl of thunderstorms associated with its ongoing low pressure center… and is now the eighteenth tropical depression of this busy Atlantic Hurricane Season. Over the next five days the forecast track keeps this system under low shear and upper outflow beneath the tropical upper ridge axis in the region. And given the organized compact nature of this system… it could undergo rapid changes in intensity. Thus by day 5 I forecast a category 4 hurricane. And this is after slightly lowering the intensity forecast to account for the fact this system has not yet made it to tropical storm status as shown in my prior forecast.
Regarding track… my updated track forecast is nudged south and east to account for the current position of the depression relative to the prior forecast. The west speed of this system has slowed down while encountering the surface ridge weakness caused by Rose and Odette. However this weakness is not expected to be strong enough to recurve this system north… as both Rose and Odette are expected to weaken (see Rose and Odette sections for the factors expected to weaken each system). The additional slow down and increase in north angle at 3+ days is a reflection of Peter’s remnants contributing to the weakness. Just after day 5… it appears whatever is left of Pete will be swept north by the currently developing frontal low over the eastern US… and the frontal low will also be located toward the north as it enters the Atlantic. This would allow the western Atlantic surface ridging to recover and potentially turn this system more west. As a result… interests in the northern Lesser Antilles should closely monitor this system’s progress for possible impacts after day 5.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Sep 23)…35 mph maximum sustained wind tropical storm centered at 10.2N-35W
IOH 24 Hr Forecast (0000Z Sep 24)… 50 mph maximum sustained wind tropical storm centered at 10.8N-40W
IOH 48 Hr Forecast (0000Z Sep 25)… 70 mph maximum sustained wind tropical storm centered at 11.2N-45W
IOH 72 Hr Forecast (0000Z Sep 26)… 90 mph maximum sustained wind hurricane centered at 13N-48W
IOH 96 Hr Forecast (0000Z Sep 27)… 105 mph maximum sustained wind hurricane centered at 15N-51W
IOH 120 Hr Forecast (0000Z Sep 28)… 130 mph maximum sustained wind major hurricane centered at 17N-54W
AREA OF INTEREST #1 (REMNANTS OF ODETTE)… As of 0000Z… the remnant frontal cyclone of Odette was centered in the open North Atlantic at 44N-41W. Odette’s strength as a remnant frontal cyclone has been facilitated by the upper divergence zone of the amplifying upper trough in the north Atlantic. The amplification of the upper trough is being caused by adjacent amplification of warm northwestern Atlantic upper ridging to the west in the warm sector of the frontal system now over eastern North America. The upper trough has recently amplified further into an upper vortex to the west of Odette… which has caused Odette to whirl north. A northwest whirl into the center of the upper vortex is expected
In the next 24 hours. By 48+ hours… a portion of the upper trough associated with the eastern North America frontal system is then expected to kick the upper vortex and Odette southeastward into the open eastern Atlantic. At present… all models now agree that Odette after reaching the east Atlantic will arc more south in track while undergoing a fujiwhara interaction with what is to be left of Tropical Depression Rose expected to approach from the southeast. By 120 hours the convergence zone of the west side of the upper vortex is expected to build a surface ridge to the north that turns Odette west into the central Atlantic.
Traditionally for subtropical development over water temps below 26 deg C… I look for the upper vortex being at 1200 dekameters in height or lower to indicate that the vortex is cold enough to aid in thunderstorm generation. The upper vortex is not currently nor is forecast to become this cold… and interestingly a ring of showers has developed around Odette’s center. I cannot ignore this observation and therefore have raised short term odds of subtropical cyclone formation to 10%. I have also raised longer term odds to 50% as the models now all agree on a more south track into 26+ deg C waters in a fujiwhara interaction with Rose. The fujiwhara interaction would push Odette toward the west side of the overhead upper vortex where northerly shear or upper convergence could be factors in limiting subtropical development… therefore I do not have development odds above 50% at this time. If Odette does not developed a thunderstorm latent heat release warm core tropical upper outflow before 120 hours… it will be considerably weaker due to prolonged exposure to the upper convergence zone of the upper vortex. This is why I drop odds of development back down to 10% by then.
On a final note… coastal sea swells may return to the shores of Newfoundland in the next 24 hours as Odette whirls back to the northwest.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Sep 24)…10% chance of subtropical cyclone formation (north Atlantic near 44N-44W)
IOH 48 Hr Outlook (0000Z Sep 25)… 10% chance of subtropical cyclone formation (northeast Atlantic near 40N-39W)
IOH 72 Hr Outlook (0000Z Sep 26)… 50% chance of subtropical cyclone formation (eastern Atlantic near 35N-39.5W)
IOH 96 Hr Outlook (0000Z Sep 27)… 50% chance of subtropical cyclone formation (eastern Atlantic near 31N-39.5W)
IOH 120 Hr Outlook (0000Z Sep 28)… 10% chance of subtropical cyclone formation (central Atlantic near 30N-43W)
AREA OF INTEREST #2… Cyclonic curvature of clouds among thunderstorm activity near 10N-5E over central Africa suggest a tropical wave of low pressure is located here. Multiple models suggest this wave could develop under the low shear and outflow of the eastern tropical Atlantic upper ridge axis after emerging from Africa in 4 to 5 days. As a result… I have added this tropical wave as a new area of Interest for tropical development. I assign low 10% odds of development at day 5 as it is rare for tropical development to occur just offshore of Africa. However in future updates… these development odds could be increased as the 5-day forecast window will have more offshore forecast points.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Sep 24)…0% chance of tropical cyclone formation (central Africa near 10N-0W)
IOH 48 Hr Outlook (0000Z Sep 25)… 0% chance of tropical cyclone formation (central Africa near 10N-5W)
IOH 72 Hr Outlook (0000Z Sep 26)… 0% chance of tropical cyclone formation (western Africa near 10N-10W)
IOH 96 Hr Outlook (0000Z Sep 27)… 0% chance of tropical cyclone formation (west coast of Africa near 10N-15W)
IOH 120 Hr Outlook (0000Z Sep 28)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-20W)
...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 Tropical Depression Peter… weakens to a broad remnant low near 23.5N-68W at 42 hours… passes east of Bermuda as a remnant trough by 120 hours
**For Tropical Depression Rose… weakens to a remnant trough near 25N-42W at 18 hours… soon after the trough regains a closed circulation but not tropical cyclone status… turns east then northeast in a fujiwhara interaction with ex-Odette and reaches 30N-32.5W at 120 hours
**For Tropical Depression Eighteen…located at 14.5N-54.5W at 120 hours as a compact hurricane
**For Area of Interest #1 (Ex-Odette)…cyclonically loops northwest to 45N-41.5W thru 30 hours…dives southeast and loses identity near 36N-39W during a fujiwhara interaction with Rose’s remnant low
**For Area of Interest #2… emerges from west coast of Africa by 108 hours… tropical cyclone formation suggested near 8N-43W at 168 hours
**Additional tropical wave emerges from Africa at 144 hours… tropical cyclone formation suggested near 10N-11.5W at 168 hours
**Tail end of eastern North America cold front leaves behind a tropical low in the southwest Gulf of Mexico at 66 hours… tropical low approaches Veracruz province in Mexico as an elongated feature by 120 hours
0000Z Sept 22 ECMWF Model Run (1200Z Run not available at above mentioned source)...
**For Tropical Depression Peter… weakens to a remnant low south of Bermuda at 72 hours near 28N-65W… passes just east of Bermuda at 96 hours… currently developing eastern US frontal low moves into northwest Atlantic and absorbs Peter’s remnants by 120 hours
**For Tropical Depression Rose… weakens to remnant low in next 24 hours near 25N-40.5W… turns east in a fujiwhara interaction with ex-Odette and reaches 30N-32W at 120 hours
**For Tropical Depression Eighteen…located at 14N-51.5W at 120 hours as a compact hurricane
**For Area of Interest #1 (Ex-Odette)…cyclonically loops northwest to 45N-42.5W thru 48 hours…dives southeast and loses identity near 34N-36W during a fujiwhara interaction with Rose’s remnant low
**For Area of Interest #2…no development shown
1800Z GFS Model Run...
** For Tropical Depression Peter… weakens to remnant trough east of the eastern Bahamas by 15 hours
** For Tropical Depression Rose… weakens to a remnant trough at 12 hours while located near 26N-42W… swings northeast while re-gaining a closed circulation in a fujiwhara interaction with ex-Odette and reaches waters just south of the Azores by 120 hours
**For Tropical Depression Eighteen… compact hurricane at 15N-51W by 120 hours
**For Area of Interest #1 (Ex-Odette)…cyclonically loops northwest to 45N-41W thru 36 hours…accelerates south to 30N-45W by 120 hours
** For Area of Interest #2…emerges from west coast of Africa by 120 hours… tropical low suggested near 9N-25W at 168 hours
**Possible subtropical or tropical low east of Bermuda at 48 hours that cyclonically whirls west to waters just northeast of Bermuda by 81 hours…currently developing eastern US frontal low moves into northwest Atlantic and absorbs this system by 120 hours
1200Z NAVGEM Model Run...
**For Tropical Depression Peter… weakens to remnant trough located northeast of the eastern Bahamas by 48 hours
**For Tropical Depression Rose… weakens to a remnant low that undergoes a fujiwhara interaction with ex-Odette… reaches 34N-31W at 120 hours
**For Tropical Depression Eighteen… reaches 16N-52W at hurricane strength at 120 hours
**For Area of Interest #1 (Ex-Odette)…cyclonically loops northwest to 45N-42W thru 24 hours…accelerates south and reaches 32N-40.5W at 120 hours
**For Area of Interest #2… Tropical wave emerges from Africa at 114 hours… tropical cyclone formation suggested just southeast of the Republic of Cabo Verde Islands at 132 hours