MY 2023 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #68

*******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.**********

...SUNDAY SEPTEMBER 3 2023 9:30 AM EDT...

The Atlantic tropics are expected to remain hyperactive over the next several days… see the sections below for all current active tropical cyclones and areas of interest.

Due to a busy day personally and the number of systems being tracked… only providing general outlooks for each system instead of the usual detailed track… intensity… and probability of tropical cyclone formation callouts.

TROPICAL STORM KATIA... Satellite view of Katia acquiring peak strength on Saturday as of 1640Z:

A tropical low pressure northwest of the Republic of Cabo Verde Islands became Tropical Depression Twelve at 11 AM EDT Friday and then Tropical Storm Katia at 11 AM EDT Saturday while pulled north-northwestward by a combination of the Atlantic surface ridge trying to push the storm west and southerly flow on the east side of the east Atlantic upper vorticity trying to pull the storm north. Despite southerly shear Katia strengthened to a peak of 60 mph maximum sustained winds as of 5 PM EDT Saturday. The thunderstorm canopy has collapsed tonight as Katia has ingested a renewed surge of dry Saharan air to the southeast brought in by the north apex of the tropical wave associated with Area of Interest #30… and while entering the mid-latitudes of the eastern Atlantic it has weakened to 45 mph maximum sustained winds as of 11 PM EDT while centered at 26.5N-31W…. and as of 5 AM EDT it has weakened further to 40 mph max sustained winds. By 48 hours I expect Katia to continue northwest to the vicinity of 30N-35W where it will likely become quasi-stationary thru 120 hours while in conflicting steering between the Atlantic surface ridge trying to pull Katia west and ex-Franklin (AOI #33) dropping in to the northeast. While further away from dry Saharan air to the south at this position… and as southerly shear relaxes with the eastern Atlantic upper vorticity weakening to a small upper vortex… I forecast Katia could re-strength to a peak of 60 mph max sustained winds within the next 72 hours. After that time westerly shear is forecast to increase as the southwest side of the large upper vortex over ex-Franklin (AOI #33) approaches… and Katia is likely to weaken from 72 to 120 hours.

******National Hurricane Center (hurricanes.gov) official forecast as of 11 PM EDT*****************************

Loss of Tropical Cyclone Status (0000Z Sep 5)… 35 mph maximum sustained wind remnant low centered at 29.4N-34.9W

TROPICAL STORM GERT... Franklin lifted further north of Gert more than previously anticipated… reducing the northerly shear from Franklin’s outflow enough such that Gert came back alive as a tropical cyclone on 5 AM EDT Friday. Franklin has also not pulled Gert as far to the east as previously anticipated either due to the increased distance between the two… and as of 11 PM EDT Saturday Gert is is hanging out near 29.1N-53.4W instead of 50W longitude. Some northerly shear has remained over Gert as the storm is near the east end of the west Atlantic upper ridge instead of more underneath it… thus Gert has only been gradually strengthening to 60 mph max sustained winds thru 11 PM EDT Saturday. Idalia (AOI #31) has moved east of previous forecasts and closer to Gert… and so now the east side of Idalia is expected to lift Gert northward and absorb it over the next 48 hours. Warm southerly flow on Idalia’s east side will amplify upper ridging over Gert… which could lower the shear… thus I forecast Gert could reach a peak of 70 mph max sustained winds while lifting north to 35N latitude in the next 24 hours before it becomes absorbed by Idalia at 48 hours.

******National Hurricane Center (hurricanes.gov) official forecast as of 11 PM EDT*****************************

Loss of tropical cyclone status (1200Z Sep 4)… Absorbed by Idalia

AREA OF INTEREST #30... A tropical wave of low pressure that recently emerged from Western Africa has produced a better defined area of rotating showers and thunderstorms that have passed south of the Republic of Cabo Verde Islands so far this weekend. This system is expected to continue west-northwest across the Atlantic tropics as the Atlantic surface ridge recovers with the northward lifting of ex-Idalia (AOI #31)… Gert… and Katia… the northward angle in the track being a result of the surface ridge only gradually recovering. Conditions are expected to be favorable for development for this troipcal wave due to the low shear and outflow of tropical upper ridging persisting... and combined with a model consensus suggesting development overall I expect tropical cyclone formation from this wave… currently forecast if it to be a 75 mph maximum sustained wind hurricane when it reaches the central tropical Atlantic near 16N-50W by 120 hours.

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT*****************************

Formation chance through 48 hours…40%

Formation chance through 7 days (168 hours)...80%

AREA OF INTEREST #31 (REMNANTS OF IDALIA)… The remnant frontal cyclone of Idalia was in a hurry to cut eastward through Bermuda and is already northeast of the island as of Sunday morning. Southward cool air transport on Idalia’s northwest side has allowed it to carve its own upper trough in the mid-latitude westerlies… with the eastern divergence zone of said trough helping the frontal cyclone maintain strength. Although it did redevelop thunderstorms near the center for a time… the center of circulation has been oblong instead of circular and the thunderstorm activity has since become more removed from the center thru Sunday morning. Going forward… the nearby upper trough will merge with the upper trough incoming from eastern Canada… and ongoing southward cold air transport on the west side of ex-Idalia will help the merged upper trough amplify with increased divergence on the east side of the amplifying upper trough helping ex-Idalia hang on for a few more days. As seen classically with frontal cyclones… ex-Idalia will be taking a cyclonically arcing northbound track into the core of the amplifying upper trough where then by day 5 ex-Idalia is a post-mature weakening frontal cyclone just offshore of Atlantic Canada as there is a lack of divergence at the upper trough’s core. Models generally agree that ex-Idalia will remain more oblong instead of circular through this process… probably as the amplifying trough has an elongated divergence zone… and thus reformation into a tropical cyclone is not expected (I assign 0% chance of this happening in this update).  Ex-Idalia has replaced ex-Franklin as a coastal surf generator Bermuda… the northeastern US coast… and Atlantic Canada coast and the surf will linger till ex-Idalia’s weakens as disused above. Heavy rainfall across Atlantic Canada is likely within the next few days as well. This is my planned final statement on ex-Idalia on this blog… refer to the home page bulletins on this site for impacts to land areas going forward.

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT*************************

Not in the official outlook

AREA OF INTEREST #32… Satellite imagery shows a large and vigorous tropical wave of low pressure over Western Africa that models have latched on in showing development when it later moves offshore and I was in the process of adding it in as another area of interest… and thru 8 AM this morning the NHC has joined in and also added it as an area of interest in their outlook. That is why it’s marked as not in the NHC outlook in the above birdseye view graphic but is in fact now in the outlook. The wave is expected to move offshore thru 48 hours… and then meander over/near the Republic of Cabo Verde Islands thru 96 hours as the east end of the steering Atlantic surface ridge is kept weak by ex-Franklin. Then by 120 hours a westward motion pulling away from the islands is expected as it reaches the same recovering Atlantic surface ridge that AOI #30 will be moving westward with. Given the model support… favorable regional tropical upper ridging with low shear and outflow… and already robust nature of the wave… I am already assigning 40% odds of tropical cyclone formation thru day 5. Interests across the Republic of Cabo Verde Islands should monitor the progress of this system… and regardless of tropical cyclone formation or not heavy rainfall and gusty winds are increasingly likely for the islands by Tuesday and Wednesday.

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT*****************************

Formation chance through 48 hours…0%

Formation chance through 7 days (168 hours)...20%

AREA OF INTEREST #33 (REMNANTS OF FRANKLIN)… As marked in the above birdseye view chart… Franklin is now a North Atlantic remnant frontal cyclone supported by the divergence zone of a southern fragment of the upper trough now over southern Greenland. Franklin was able to make the southern part of the trough fracture while pulling in cold air associated with the passing trough toward itself. The models thru day 5 now agree that ex-Franklin and its upper trough fragment will merge with the cut-off vortex over southern Portugal… resulting in a larger cut-off upper vortex midway between Portugal and the Azores that ex-Franklin will be cyclonically whirling beneath. Upper air temps will be cold with the upper vortex forecast to have a 200 mb height of 1205 dekameters and sea surface temps between Portugal and the Azores are in the 22 to 23 deg C range… this combination may be enough for ex-Franklin to regain a thunderstorm core and tropical characteristics by day 5. I forecast a 40% chance that ex-Franklin becomes a subtropical cyclone thru day 5… preferring to keep odds below the 50% mark as I would have liked to see colder upper air temps (200 mb heights at or below 1200 dekameters) to have more confidence in subtropical development. Regardless of subtropical cyclone formation or not… expect an increase in coastal surf for the Azores… Portugal… northwest Spain… and the Canary Islands over the next few days.

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT*************************

Not in the official outlook

...COMPUTER MODEL SUMMARY...

Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/).

0000Z (Sep 2) CMC Model Run...

**For Tropical Storm Katia… weakens to a remnant low near 27.5N-34W at 66 hours… remnant low becomes stationary through 120 hours while in a steering saddle point between Atlantic surface ridge to northwest and ex-Franklin to the northeast

**For Tropical Storm Gert… through 66 hours is pulled north and absorbed into the northeast side of AOI #31 (ex-Idalia).

**For Area of Interest #30… tropical cyclone formation suggested near 13.5N-45W at 120 hours

**For Area of Interest #31… remnant frontal cyclone of Idalia does not regenerate into a tropical cyclone while taking a large cyclonic curve across the western Atlantic that takes it just offshore of Nova Scotia through 120 hours

**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 96 hours and passes over the Republic of Cabo Verde Islands at 132 hours as a tropical low… tropical low reaches 19N-34W at 168 hours

**For Area of Interest #33… ex-Franklin midway between the Azores and northwest Spain at 120 hours

**Additional strong tropical wave emerging from west coast of Africa at 168 hours

0000Z (Sep 2) ECMWF Model Run...

**For Tropical Storm Katia… weakens to a remnant low near 27.5N-35W at 72 hours… while dragged east by ex-Franklin dissipates at 108 hours

**For Tropical Storm Gert… through 54 hours is pulled north and absorbed into the east side of AOI #31 (ex-Idalia).

**For Area of Interest #30… tropical cyclone formation suggested near 14N-45W at 102 hours… strengthening tropical cyclone suggested near 15N-46.5W at 120 hours

**For Area of Interest #31… remnant frontal cyclone of Idalia does not regenerate into a tropical cyclone while taking a large cyclonic curve across the western Atlantic that takes it just offshore of Nova Scotia through 120 hours

**For Area of Interest #32… no development shown

**For Area of Interest #33… ex-Franklin midway between the Azores and Portugal at 120 hours

1800Z (Sep 2) GFS Model Run...

**For Tropical Storm Katia… reaches 30N-35W at 78 hours while weakening… slowly drifts east toward ex-Franklin through 120 hours while weakening further to a remnant low

**For Tropical Storm Gert… through 45 hours is pulled north and absorbed into the northeast      side of AOI #31 (ex-Idalia).

**For Area of Interest #30… compact tropical cyclone formation suggested near 15.5N-55W at 132 hours

**For Area of Interest #31… remnant frontal cyclone of Idalia does not regenerate into a tropical cyclone while taking a large cyclonic curve across the western Atlantic that takes it to 40.5N-55W by 78 hours… subsequently weakens to a frontal low that then dissipates while moving into southwestern Newfoundland through 114 hours

**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 66 hours which evolves into a tropical low that moves across the southern Republic of Cabo Verde Islands at 105 hours… tropical cyclone formation suggested just west of the southern islands at 114 hours which reaches 15.5N-27.5W at 120 hours

**For Area of Interest #33… ex-Franklin midway between the Azores and Portugal at 120 hours

1800Z (Sep 2) NAVGEM Model Run...

** For Tropical Storm Katia… weakens to a remnant low near 27.5N-35W at 54 hours… remnant low drifts west under influence of surface ridge to 29N-41W through 120 hours

**For Tropical Storm Gert… through 30 hours is pulled north and absorbed into the east side of AOI #31 (ex-Idalia).

**For Area of Interest #30… no development shown

**For Area of Interest #31… remnant frontal cyclone of Idalia does not regenerate into a tropical cyclone while taking a large cyclonic curve across the western Atlantic that takes it just offshore of Nova Scotia through 120 hours

**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 66 hours which evolves into a tropical low that moves across the southern Republic of Cabo Verde Islands at 105 hours… tropical cyclone formation suggested just west of the southern islands at 114 hours which reaches 15.5N-27.5W at 120 hours

**For Area of Interest #33… ex-Franklin west of Portugal near 39.5N-15W at 120 hours