*******Note that forecast 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 NOVEMBER 20 2023 2:10 PM EDT...
The frontal low in the vicinity of the Azores has weakened without developing tropical characteristics... and is expected to not gain tropical characteristics going forward... see area of interest #55 section below for more information.
Elsewhere... the complex frontal low that was offshore of the southeastern United States early Saturday has since bifurcated into one frontal low that accelerated rapdily north-northeast across Atlantic Canada and into southwestern Greenland... and another frontal low that has moved eastward into the waters south of Bermuda. This system has triggered two Atlantic tropical areas of interest as follows:
(1) The frontal low just south of Bermuda is forecast to move east-southeast into the open central Atlantic waters sufficiently warm for possible tropical transition of this feature... see area of interest #54 section below for details.
(2) A lengthy cold front/surface trough extends southward from the frontal low south of Bermuda into the central Caribbean Sea. A tropical low pressure spin being monitored for signs of development has materialized along the trough and near 14N-74W... see area of interest #56 section below for details.
AREA OF INTEREST #54... The frontal low that materialized near the east coast of the Florida peninsula late last week has swung east into the waters just south of Bermuda while following the supporting eastern divergence zone of its parent upper trough. Global models have increasingly indicated that this feature has another shot at acquiring tropical characteristics while zooming east-southeast into the open central Atlantic in the coming days... therefore the frontal low has been re-introduced as an Atlantic tropical area of interest. Because the frontal low was last tagged as area of interest #54 in birdseye view post #124... it will use this number in this and future posts going forward.
For the next 72 hours... the warm sector of the currently developing surface frontal system over the United States will amplify the current Caribbean upper ridge cell across the western Atlantic... the eastern convergence zone of which will cause the eastward shift of the current eastern North America surface ridge into the western Atlantic. The end result is a warm core western Atlantic deep-layer ridge that pushes this area of interest and a southern frgament of its upper trough east-southeastward into the open central Atlantic. The cut-off upper trough fragment is expected to be amplified while squashed between the western Atlantic deep-layer ridge and current deep-layer ridge heading toward the Azores. The amplified nature of the cut-off upper trough is expected to result in higher divergence and lower shear on its east side conducive for thunderstorm generation and tropical characteristics of the surface frontal low... particularly as this system will be moving across the part of the open central Atlantic where water temps are still 26+ deg C. By 96+ hours... the upstream frontal system from the United States and its upper trough breaks through the western Atlantic deep-layer ridge... with the cut-off upper trough of this area of interest merging with the upstream upper trough to make a northwest-southeast tilted upper trough that lifts this system north-northeastward in the long range.
Regarding odds of subtropical cyclone formation... I agree with the NHC in already issuing peak 30% odds due to the current global model consensus on this system acquiring tropical characteristics. Higher odds of subtropical cyclone formation do not seem reasonable yet as it is also possible for this system and its supportive upper divergence region to become too elongated for cyclone status should the southern cut-off upper trough remain too entangled with the northern upper trough fragment through the next 72 hours... and as the cut-off upper trough then merges with the upstream upper trough to eject from the United States late in the forecast period.
Update as of 1 PM EDT... the NHC has raised peak odds of subtropical cyclone formation to 40%
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Nov 21)... 0% chance of subtropical cyclone formation (central Atlantic near 31N-56w)
IOH 48 Hr Outlook (1200Z Nov 22)... 0% chance of subtropical cyclone formation (central Atlantic near 29N-51W)
IOH 72 Hr Outlook (1200Z Nov 23)... 30% chance of subtropical cyclone formation (central Atlantic near 26.5N-47.5W)
IOH 96 Hr Outlook (1200Z Nov 24)... 30% chance of subtropical cyclone formation (central Atlantic near 27.5N-46W)
IOH 120 Hr Outlook (1200Z Nov 25)... 30% chance of subtropical cyclone formation (central Atlantic near 29N-45W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 7 AM EDT***************************
Formation chance through 48 hours... 0%
Formation chance through 7 days... 30%
AREA OF INTEREST #55... The surface frontal low that spent this past weekend moving into and across the Azores is currently centered just south of the eastern Azores as of 1200Z. The south part of the parent upper trough has developed into a cut-off upper vortex due to the amplifying Atlantic deep-layer ridge to the west. Despite the surface feature being positioned over 20 deg C waters... it has produced pockets of shower activity near and around its center due to the instability provided by the rather cold temperatures of the overhead upper vortex (200 mb heights currently near 1180 dekameters). The shower activity has not been impressive enough for acquisiton of tropical characeristics... a sign that the surface system has likely already weakened underenath the lack of divergence below the upper vortex's core. Therefore despite this system being forecast to accelerate southwestward into warmer waters over the next 48 hours (under the steering influence of the deep-layer ridge)... it is not expected to have enough surface convergence to build thunderstorms and tropical characteristics despite the potentially unstable thermal profile... and the global models agree on dissipating the surface system by 48 hours. This is my planned final statement on this area of interest on this blog.
Note that gusty winds and coastal surf across the Azores going forward will be more associated with the strength of the surface layer of the regional deep-layer ridge.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Nov 21)... 0% chance of subtropical cyclone formation (eastern Atlantic near 33N-31.5W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 7 AM EDT***************************
Not in the official outlook
AREA OF INTEREST #56... The northern part of the elongated tropical low that was over Jamaica and eastern Cuba this past weekend became absorbed by cold front driven by the frontal low south of Bermuda (more information on the absorbing frontal low is covered in the above area of interest #54 section). The southern part of the elongated tropical low has persisted as a trough in the central Caribbean waters southeast of Jamaica with the aid of the outflow of an upper ridge cell that has moved into the Caribbean from Mexico. More recently this trough has intensified into a small tropical low with thunderstorm burst near 14N-74W. Meanwhile toward the west... an additional surface low has spent late last week and this weekend diving southeastward across the Gulf of Mexico and western Caribbean... and has arrived into the waters southwest of Jamaica. The tropical low near 14N-74W is quasi-stationary while in conflicting steering between the ridge weakness associated with area of interest #54 and approaching surface ridge from eastern North America. For the next 24 hours I forecast the tropical low to edge slightly westward toward 75W longitude while merging with the surface low southwest of Jamaica... it is possible this system becomes elongated east-west during the merger which is why my 24-hour development odds are below peak. Beyond 24 hours... as the eastern North America surface ridge moves into the western Atlantic... this system should be pushed west by the ridge. Between 48 and 72 hours a frontal system ejecting from the United States should be passing to the north... however its ridge weakness should be too narrow to recurve this system north. Instead a west-northwest track should ensue for the 48+ hour period... with the ongoing west angle in track caused by the surface ridge that builds behind the passing frontal system and while the north slant in the track is caused by a secondary frontal low that develops across the southern Gulf of Mexico and heads across the Florida peninsula (the secondary frontal low develops with the support of an upper trough fragment left behind by the frontal system ejecting from the United States).
Regarding odds of tropical cyclone formation... global models do not develop this system and the NHC continues to assign low 10% odds of cyclone formation while citing dry air encroaching from the north in association with the back side of the frontal system tagged in area of interest #54. However through 72 hours this system should remain under a vast upper ridge cell with low shear and upper outflow... thus I assign higher peak odds of development... at 30%... as the ouflow of the upper ridge cell may help this system develop enough thunderstorms to fight off dry air. By 96 and 120 hours the upper trough fragment tied to the aforementioned southern Gulf secondary frontal low increases westerly shear across this system... thus I taper down development odds below the peak. There is also a possiblity of the westerly shear not being excessive as the upper trough fragment potentially passes far north enough... thus I keep long-term developemnt odds above 0% through day 5.
Should this system indeed persist through day 5... the most likely impact to land areas would be rainfall across the Cayman Islands from moisture sheared-off from this system.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Nov 21)... 20% chance of tropical cyclone formation (central Caribbean Sea near 14N-75W)
IOH 48 Hr Outlook (1200Z Nov 22)... 30% chance of tropical cyclone formation (central Caribbean Sea near 14N-77.5W)
IOH 72 Hr Outlook (1200Z Nov 23)... 30% chance of tropical cyclone formation (central Caribbean Sea near 15N-80W)
IOH 96 Hr Outlook (1200Z Nov 24)... 20% chance of tropical cyclone formation (northeast of eastern Honuduras near 16N-82.5W)
IOH 120 Hr Outlook (1200Z Nov 25)... 20% chance of tropical cyclone formation (offshore of Honduras near 17N-85W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 7 AM EDT***************************
Formation chance through 48 hours... 10%
Formation chance through 7 days... 10%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).
0000Z (Nov 20) CMC Model Run...
**For area of interest #54... frontal low currently positioned just south of Bermuda moves east-southeast across the open central Atlantic and evolves into a possible subtropical cyclone near 28.5N-38.5W at 96 hours... possible subtropical cyclone then accelerates northeastward to 32.5N-34W through 120 hours
**For area of interest #55... under influence of deep-layer ridge passing to the north the frontal low currenlty over the eastern Azores accelerates west-southwest while weakening and dissipates near 35N-35W
**For area of interest #56... current central Caribbean tropical low quasi-stationary until 36 hours when current eastern North America surface ridge moves offshore and begins to push this system west... tropical low weakens to a trough and makes landfall at the Nicaragua/Honduras border at 114 hours... trough positioned over far eastern Honduras at 120 hours...
0000Z (Nov 20) ECMWF Model Run...
**For area of interest #54... frontal low currently positioned just south of Bermuda moves east-southeast across the open central Atlantic and evolves into a possible subtropical cyclone near 30N-55W at 42 hours... possible subtropical cyclone then continues southeast to 24.5N-48.5W through 84 hours... subtropical cyclone then moves east-northeast to 26N-40W through 126 hours
**For area of interest #55... under influence of deep-layer ridge passing to the north the frontal low currently over the eastern Azores accelerates west-southwest while weakening and dissipates near 33.5N-36W at 54 hours
**For area of interest #56... current central Caribbean tropical low quasi-stationary until 36 hours when current eastern North America surface ridge moves offshore and begins to push this system west... dissipates offshore of the Guajira peninsula of Colombia shortly thereafter (42 hours)
0600Z (Nov 20) GFS Model Run...
**For area of interest #54... frontal low currently positioned just south of Bermuda moves east-southeast across the open central Atlantic and evolves into a possible subtropical cyclone near 25N-50W at 60 hours... subtropical cyclone then continues further southeast to 22N-48W through 90 hours... subtropical cyclone then moves east-northeast to 24N-43W through 120 hours
**For area of interest #55... under influence of deep-layer ridge passing to the north the frontal low currently over the eastern Azores accelerates west-southwest while weakening and dissipates near 35N-35W at 48 hours
**For area of interest #56... current central Caribbean tropical low quasi-stationary until 36 hours when current eastern North America surface ridge moves offshore and begins to push this system west... dissipates offshore of Nicaragua just after 102 hours
0600Z (Nov 20) NAVGEM Model Run...
**For area of interest #54... frontal low currently positioned just south of Bermuda moves east-southeast across the open central Atlantic and evolves into a possible subtropical cyclone near 29.5N-44W at 66 hours... as a strengthening subtropical cyclone moves northeast into the waters just southwest of the Azores through 120 hours
**For area of interest #55... under influence of deep-layer ridge passing to the north the frontal low currently over the eastern Azores accelerates west-southwest while weakening and dissipates near 33N-35W at 48 hours
**For area of interest #56... no development shown