*******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 16 2022 3:54 AM EDT...
Tropical Storm Fiona is hanging on in an environment of wind shear and is expected to bring impacts to the northern Caribbean Islands and possibly the Bahamas over the next few days. See Fiona section below for more information. See area of interest #29 and #30 sections below for an update on the pair of tropical waves of low pressure located well east of Tropical Storm Fiona and in the eastern tropical Atlantic.
Elsewhere... the broad area of low surface pressure materializing underneath the upper ridge axis over southeastern Mexico and western Central America has already consolidated into eastern Pacific Tropical Depression Thirteen-E. Although this upper ridge and its supporting upper outflow could expand as the current Caribbean upper vorticity dissipates from its ongoing isolation from high-latitude cold air... the potential for another disturbance developing in the western Caribbean or Bay of Campeche is diminishing as a cut-off upper trough over the southeastern United States is expected to soon after amplify southward from an amplifying United States upper ridge. The amplifying cut-off upper trough will displace the upper ridge southward away from the Bay of Campeche and western Caribbean. Also noting a pair of tropical waves of low pressure over central and western Africa as follows:
(1) A tropical wave of low pressure over western Africa... in the vicinity of 5W longitude... has become better defined with a notable increase in thunderstorms on Thursday afternoon followed by an area of remaining clouds tonight featuring curved bands toward the west. There is a current lack of computer model support showing this wave developing as it later moves into the tropical Atlantic... however should this change or should the wave show signs of organization in the days ahead will consider adding it as an area of interest in future updates.
(2) A tropical wave of low pressure over central Africa... in the vicinity of 12E longitude... has also become better defined. The current global model consensus suggests some development of this wave as it later moves into the eastern tropical Atlantic in about six days. This tropical wave could emerge as another area of interest in future updates if these trends continue.
TROPICAL STORM FIONA...The sixth tropical storm of the 2022 Atlantic Hurricane Season continues to retain a classic sheared appearance... with a swirl of low clouds featuring an ongoing thunderstorm complex displaced to the east side of the swirl by the shear. The source of shear is upper vorticity that has been cut-off from the central Atlantic upper trough associated with ex-Earl and being sent southward toward Fiona due to the building western Atlantic upper ridge... with the upper westerlies on the south side of the upper vorticity opposing the forward motion of the storm being the shearing mechanism. The 200 mb layer of the upper atmosphere... which is always plotted in the above birdseye view charts and normally what I use to assess wind shear... shows Fiona being tucked under the tropical upper ridge in the region which would normally suggest low shear. However their must be upper westerly shearing flow at a lower layer of the atmosphere undercutting the 200 mb layer of the tropical upper ridge. Another puzzling thing about Fiona is the slight south angle in its westward track despite the steering Atlantic surface ridge not being strong enough to induce such an angle. Perhaps the sheared-off thunderstorm complex is producing a mid-level low pressure circulation just east of Fiona's surface swirl which is helping to ever-so-slightly bend the track of the surface swirl on a southward angle. I have no choice but to shift the forecast track southward in this update. Downstream and well to the west... the current upper trough over eastern North America is in the process of leaving behind a cut-off upper trough over the southeastern United States whose eastern divergence zone will support an offshore cold front/inverted surface trough... therefore an eventual northward angle in the track is expected in the next 72 hours as Fiona is funneled in the flow around the south side of the Atlantic surface ridge and east side of the inverted surface trough. At 96 to 120 hours... an upper trough sliding eastward across the US/Canada border region is expected to team up with the cut-off upper trough over the southeastern United States to expand on the ridge weakness being created by the inverted surface trough... therefore an increase in the north angle of Fiona's westward track is anticipated during that timeframe. Where Fiona tracks just beyond 120 hours will depend on the storm's strength as the surface ridge weakness closes when the upper trough from the US/Canada border region moves away... with a weaker/shallower system moving more west in the surface flow caused by the recovering surface ridging. The weaker/shallower scenario cannot be ruled out as the southward adjusted forecast track increases land interaction prospects with Hispaniola (Haiti and the Dominican Republic) and eastern Cuba... with that scenario then exacerbated as the weaker/shallower system would also walk more westward into the shearing winds on the east side of the southeastern US cut-off upper trough that would re-enforce a weaker strength. However as discussed in the next paragraph... the upper wind pattern over Fiona could allow it to recover in the 96 to 120 hour timeframe... resulting in a stronger/taller system that bends north into the Bahamas while feeling the steering influence on the east side of the southeastern US cut-off upper trough. Interesting to note that the current global model consensus favors the stronger/taller scenario.
Regarding intensity... an aircraft recon mission on Thursday found Fiona has strengthened to 60 mph maximum sustained winds despite its sheared nature. Since Fiona has maintained the same appearance since then the NHC has maintained this intensity estimate. Noting that the cut-off upper trough over the southeastern United States is now expected to be more amplified and positioned further west... perhaps later settling into the Gulf of Mexico... while models now depict a more amplified adjacnet upper ridge building over the United States. This in turn allows the western Atlantic upper ridge to remain more intact such that it may now be able to continue pushing the shearing upper vorticity just north of Fiona southwestward toward the storm. Therefore I do not forecast additional strengthening for the next 48 hours... and forecast some weakening by 72 hours around the time the 1800Z GFS model run showed this upper voritcity getting very close to Fiona from the east. This upper vorticity could be why mutliple global model runs... cited in the computer model summary section below... also want to weaken Fiona during the next 72 hours. At 96 to 120 hours... the upper air pattern improves as the cool core upper vorticity near Fiona dissipates while remaining cut-off from high-latitude cold air thanks to the warm western Atlantic upper ridge. And with the above observation that the southeast US cut-off upper trough could be positioned further west and away from Fiona while also being more amplified... this would mean favorable low wind shear over Fiona during this timeframe. However I show more weakening between 72 and 96 hours due to potential land interaction with Hispaniola (Haiti and the Dominican Republic)... followed by recovery in the low shear environment between 96 and 120 hours after Fiona departs Haiti. However I only show a little re-strengthening as the forecast track then brings the circulation of Fiona into eastern Cuba.
Regarding impact to land areas:
(1) I recommend interests in the northern Lesser Antilles... Virgin Islands... and Puerto Rico begin to finish preparations for tropical storm conditions (gusty winds... heavy rains with flash flooding potential... and some coastal sea swells). The timeframe for impacts in this region is expected to be tonight through Saturday.
(2) Potential for tropical storm conditions in the eastern Bahamas has reduced with the southward adjustment in the forecast track which raises prospects for Fiona weakening to the south of the islands while passing over Haiti and the Dominican Republic. However interests here should remain aware of the storm for possible impacts early next week in case the forecast shifts again.
(3) For Haiti and the Dominican Republic... heavy rainfall with flash flooding potential appears to be the primary threat on Sunday and Monday. The eastern Dominican Republic may see some gusty winds and coastal surf before Fiona weakens from wind shear and increased land interaction with the Haiti/Dominican Republic landmass.
(4) Interests in the central and western Bahamas... as well as eastern Cuba... should be aware of Fiona for possible impacts by Tuesday as the long-range intensity forecast remains uncertain.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Sep 16)...60 mph maximum sustained wind tropical storm centered east of the northern Lesser Antilles at 16N-56.9W
IOH 24 Hr Forecast (0000Z Sep 17)...60 mph maximum sustained wind tropical storm centered just west of the northern Lesser Antilles at 16.2N-62W
IOH 48 Hr Forecast (0000Z Sep 18)...60 mph maximum sustained wind tropical storm centered south of Puerto Rico at 16.8N-66W
IOH 72 Hr Forecast (0000Z Sep 19)...50 mph maximum sustained wind tropical storm centered just south of the southern Dominican Republic coast at 17.5N-70W
IOH 96 Hr Forecast (0000Z Sep 20)...40 mph maximum sustained wind tropical storm centered just west of Haiti at 19.5N-74W
IOH 120 Hr Forecast (0000Z Sep 21)...50 mph maximum sustained wind tropical storm centered over eastern Cuba at 20.8N-77W
AREA OF INTEREST #29...The western of the pair of eastern Atlantic tropical waves of low pressure remains generally calm with an absence of thunderstorms except in an ongoing small area on the north side of the wave. The GFS model suggests that some of the upper vorticity to the northwest of the tropical wave... being separated and cut-off from the central Atlantic upper trough associated with ex-Earl by the building western Atlantic upper ridge... could organize into a cut-off upper vortex. In turn an upper divergence and low shear environment on the east side of the upper vortex could allow the north side of the tropical wave to evolve into a tropical or subtropical disturbance in the open central Atlantic... and the NHC has brought back this tropical wave of low pressure into its tropical weather outlook. However also noting that the GFS is the only major global model suggesting some development on the north side of the tropical wave... therefore at this time retaining 0% odds of tropical cyclone formation unless other models join or observations during later updates show a tropical or subtropical disturbance materializing on the north side of the wave.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Sep 17)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 17N-41W)
AREA OF INTEREST #30...The eastern of the pair of east Atlantic tropical waves of low pressure is now passing south of the Republic of Cabo Verde Islands. Although the wave has seen some increase in thunderstorm activity over the last 24 hours... the wave has not become better organized while lacking a defined center of rotation. My current best guess at an area of maximum spin... using satellite animation... is 11N-22.5W which it a tad west of my previous forecast track... and so my updated one is nudged accordingly. Favoring this tropical wave as it moves across the eastern and central tropical Atlantic over the next five days is the persistent tropical upper ridge in the region featuring low shear and upper outflow. The longer range global model consensus wants to strengthen Fiona and its upper anticyclonic outflow... with the outflow helping to push the upper vortex to interact with area of interest #29 southward toward the path of this tropical wave. If this in fact occurs... upper-level winds over this tropical wave would become less favorable for development just after day 5. I continue to cap peak odds of tropical cyclone formation at 20% due to the current lack of computer model support showing development. Also noting that I have lowered short term odds of tropical cyclone formation as the wave has not made progress in organizing further.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Sep 17)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 11.5N-27.5W)
IOH 48 Hr Outlook (0000Z Sep 18)...0% chance of tropical cyclone formation (eastern tropical Atlantic near 12N-32.5W)
IOH 72 Hr Outlook (0000Z Sep 19)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-37.5W)
IOH 96 Hr Outlook (0000Z Sep 20)...20% chance of tropical cyclone formation (central tropical Atlantic near 13N-42.5W)
IOH 120 Hr Outlook (0000Z Sep 21)...20% chance of tropical cyclone formation (central tropical Atlantic near 13.5N-47.5W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (Sep 15) CMC Model Run...
**For Tropical Storm Fiona... weakens to a tropical depression while moving across the northern Lesser Antilles... Virgin Islands... and Puerto Rico from 36 to 54 hours... weakens to a remnant low while moving across the south coasts of the Dominican Republic and Haiti through 84 hours... remnant low moves northwest across eastern Cuba at 108 hours and regains tropical storm status between eastern Cuba and the central Bahamas at 120 hours... strengthens into a strong tropical storm while moving slowly northwest into the western Bahamas through 168 hours.
**Tropical wave emerges from west coast of Africa at 150 hours... organizes into a broad tropical low midway between the west coast of Africa and Republic of Cabo Verde Islands through 168 hours.
1200Z (Sep 15) ECMWF Model Run...
**For Tropical Storm Fiona... oscillates between tropical storm and tropical depression strength through 72 hours while passing through the northern Lesser Antilles and then just south of the Virgin Islands and Puerto Rico... weakens to a remnant low while moving northwest across the Dominican Republic through 120 hours... remnant low moves west-northwest along northeast coast of Cuba where it re-organizes into a tropical depression through 168 hours.
**Tropical wave emerges from west coast of Africa at 144 hours... organizes into a broad tropical low that moves into the Republic of Cabo Verde Islands through 168 hours.
1800Z (Sep 15) GFS Model Run...
**For Tropical Storm Fiona... gradually weakens to a tropical depression and then remnant low while passing through the northern Lesser Antilles and then just south of the Virgin Islands and Puerto Rico from 33 to 60 hours... remnant low moves west-northwest across southern Haiti through 102 hours and becomes a broad feature while moving northwest into eastern Cuba through 123 hours.. northeast side of remmant low re-organizes into a tropical storm over the central Bahamas through 147 hours and becomes a strong tropical storm while drifting north into the western Bahamas through 168 hours.
**For area of interest #29... through 63 hours north side of tropical wave fractures into a surface trough supported by the eastern divergence zone of an upper vortex that gets left behind by the current central Atlantic upper trough interacting with ex-Earl... surface trough strengthens into a surface low near 26N-46.2W at 87 hours... surface low moves northwest around northeast side of upper vortex and reaches 28.5N-49W at 120 hours
1800Z (Sep 15) NAVGEM Model Run...
**For Tropical Storm Fiona... passes over the northern Lesser Antilles and then just south of the Virgin Islands and Puerto Rico from 30 to 60 hours while gradually reaching strong tropical storm strength... turns northwest between Puerto Rico and the Dominican Republic through 72 hours and reaches hurricane strength just east of the eastern Bahamas at 96 hours... turns north and reaches 22.5N-70.5W at 120 hours