*******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.**********
...TUESDAY SEPTEMBER 5 2023 3:49 AM EDT...
The Atlantic tropics are expected to remain hyperactive over the next several days… see the sections below all current areas of interest.
REMNANTS OF KATIA... As of 5 PM EDT Katia was located at 28N-34.4W. Despite the nearby upper vorticity shrinking to a smaller central Atlantic vortex off to the west… resulting in less southerly shear… the cloud swirl of Katia has only been able to produce occasional shower and thunderstorm bursts and thus it weakened to a tropical depression. More recently the NHC has downgraded Katia to a remnant low as it has lacked sufficient activity over the last several hours to be classified as a tropical cyclone. The lack of activity in the lower shear environment is due to dry air per recent NHC advisories. I previously speculated Katia could make a comeback while moving north of the beige tint of dry Saharan air in visible satellite pictures… however it is likely the air is still dry as Katia is probably close enough to Africa to experience low humidity values wafted in from the Saharan desert by the Atlantic surface ridge. Going forward… the remnant low of Katia should remain quasi-stationary for a day or so while trapped in conflicting steering between the Atlantic surface ridge trying to pull Katia west and a duplex of low pressures to the northeast (ex-Franklin/AOI #33 and a frontal low offshore of Portugal) trying to pull Katia east. The models resolve the conflicting steering eventually by agreeing on Katia by day 3 drifting east when ex-Franklin dives south and moves in closer… followed by a southwest acceleration by days 4 and 5 as the Atlantic surface ridge rebuilds under the western convergence zone of the large upper vortex over ex-Franklin. The southwest acceleration will bring ex-Katia into where the higher concentrations of dry Saharan tend to be and possibly into southerly shear on the east side of the central Atlantic upper vortex… and the models agree on ex-Katia not making a comeback and instead dissipating. Therefore this is my planned final statement on Katia on this blog.
REMNANTS OF GERT... At this point not much to say about a tropical system that has been around for several days seemingly invincible. Gert did not achieve a peak strength above 60 mph max sustained winds while accelerating northward in the flow on the east side of the remnant frontal cyclone of Idalia. Gert has recently weakened to a remnant low while becoming absorbed by the east side of ex-Idalia… and this is my final statement on Gert on this blog as this long-lasting system finally comes to an end.
AREA OF INTEREST #30... The western of the two eastern Atlantic tropical waves of low pressure has continued to become better organized with increased thunderstorm banding appearing around its center of rotation at times (for the eastern of the two waves… refer to Area of Interest #32 section below). This system is expected to continue west-northwest across the Atlantic tropics as the Atlantic surface ridge has begun to recover under the western convergence zone of the current northeast Atlantic upper vorticity… with the north angle in the track being a result of the gradual rate of recovery of the ridge followed by the lingering surface ridge weakness of ex-Idalia by later in the forecast period. This system has been moving faster than the typical 5W longitude per day clip and my short-term forecast track is adjusted accordingly… however the models do slow the forward speed to this typical clip perhaps due to the ridge weakness of ex-Idalia and so my forecast track in the longer range shows this. By day 5 I am south of the model consensus for two reasons… (1) ex-Idalia dissipates while completing its post-mature phase as a frontal cyclone decaying below the non-divergent core of its upper trough… (2) the surface and upper level southerly flow associated with the next frontal system and its upper trough in the mid-latitudes will be over the eastern US by day 5 and well off to the west. Providing an intensity forecast below instead of probabilities of cyclone formation as tropical cyclone formation appears imminent with vast tropical upper ridging (low shear and outflow) that this system will be under… warm waters… and strong computer model support. My intensity forecast is similar to the previous idea in full update #68 of bringing this system to hurricane status by the time it crosses 50W longitude… and thus by day 5 I already have this system as a major hurricane. Interests across the northern Lesser Antilles… Virgin Islands… and Puerto Rico need to monitor this system as by day 5 it could be over or very near as a major hurricane.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Sep 5)… Tropical low centered in the eastern tropical Atlantic at 11.5N-36W
IOH 24 Hr Forecast (0000Z Sep 6)… 45 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 12.5N-42W
IOH 48 Hr Forecast (0000Z Sep 7)… 70 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 13.5N-47W
IOH 72 Hr Forecast (0000Z Sep 8)… 95 mph maximum sustained wind hurricane centered in the central tropical Atlantic at 15N-52W
IOH 96 Hr Forecast (0000Z Sep 9)… 110 mph maximum sustained wind hurricane centered east of the northern Lesser Antilles at 16N-57W
IOH 120 Hr Forecast (0000Z Sep 10)… 130 mph maximum sustained wind major hurricane centered over the northern Lesser Antilles at 17.5N-62W
******National Hurricane Center (hurricanes.gov) official outlook as of 2 AM EDT*****************************
Formation chance through 48 hours…90%
Formation chance through 7 days (168 hours)... near 100%
AREA OF INTEREST #32… A tropical wave of low pressure is now entering the eastern tropical Atlantic from the west coast of Africa with rotating cloudiness… and has on occasion featured eruptions of widespread thunderstorm activity. The wave is expected to move slowly over/near the Republic of Cabo Verde Islands thru 72 hours as the east end of the steering Atlantic surface ridge is kept weak by ex-Franklin. Afterwards 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 favorable regional tropical upper ridging with low shear and outflow… occasional robust nature of the wave… and ongoing model support at least showing the wave evolving into a tropical low… I have increased odds of development to above the 50% mark by day 5… but not quiet as high as the NHC as the models have been trending toward showing a broad tropical low rather than outright tropical cyclone formation. 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 likely for the islands over the next few days.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Sep 6)… 0% chance of tropical cyclone formation (offshore of Africa near 12.5N-19W)
IOH 48 Hr Outlook (0000Z Sep 7)… 15% chance of tropical cyclone formation (just southeast of the Republic of Cabo Verde Islands near 13.5N-22W)
IOH 72 Hr Outlook (0000Z Sep 8)… 30% chance of tropical cyclone formation (just southwest of the Republic of Cabo Verde Islands near 14.5N-26W)
IOH 96 Hr Outlook (0000Z Sep 9)… 50% chance of tropical cyclone formation (eastern tropical Atlantic near 15N-31W)
IOH 120 Hr Outlook (0000Z Sep 10)… 60% chance of tropical cyclone formation (eastern tropical Atlantic near 15.5N-36W)
******National Hurricane Center (hurricanes.gov) official outlook as of 2 AM EDT*****************************
Formation chance through 48 hours…20%
Formation chance through 7 days (168 hours)...70%
AREA OF INTEREST #33 (REMNANTS OF FRANKLIN)… As marked in the above birdseye view chart… Franklin remains a North Atlantic remnant frontal cyclone. Ex-Franklin and its upper trough fragment will be merging with the cut-off vortex west of Portugal soon… resulting in a large cut-off upper vortex midway between Portugal and the Azores that ex-Franklin will be cyclonically whirling beneath over the next few days. Upper air temps will be cold with the upper vortex forecast to have a 200 mb height of about 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 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 2 AM EDT*************************
Formation chance through 48 hours…0%
Formation chance through 7 days (168 hours)...20%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/).
0000Z (Sep 4) CMC Model Run...
**For Tropical Depression Katia… reaches 29N-35W at 42 hours… subsequently weakens to a remnant low that begins an anticyclonic turn while first going east toward ex-Franklin and then rapidly accelerating southwest under the influence of the Atlantic surface ridge with remnant low arriving west of AOI #32 and near 16.2N-40W at 120 hours
**For Tropical Storm Gert… through 12 hours is pulled north and absorbed into the east side of ex-Idalia.
**For Area of Interest #30… tropical cyclone formation suggested near 15N-45W at 66 hours and intensifies into a hurricane that reaches the waters east-northeast of the Lesser Antilles and near 17.5N-56W by 120 hours
**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 42 hours and passes over the Republic of Cabo Verde Islands at 90 hours as a tropical low… tropical low reaches 17.5N-32.5W at 120 hours
**For Area of Interest #33… ex-Franklin midway between Spain and Portugal at 120 hours
0000Z (Sep 4) ECMWF Model Run...
**For Tropical Depression Katia… weakens to a remnant low while becoming stationary near 28.5N-34W at 30 hours…remnant low begins an anticyclonic turn while first going east toward ex-Franklin and then turning southwest under the influence of the Atlantic surface ridge and becomes absorbed into the northwest side of AOI #32 by 120 hours.
**For Tropical Storm Gert… through 12 hours is pulled north and absorbed into the east side of ex-Idalia.
**For Area of Interest #30… tropical cyclone formation suggested near 14.5N-43W at 54 hours and intensifies into a compact hurricane that reaches the waters east-northeast of the Lesser Antilles and near 19N-57W by 120 hours
**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 30 hours and passes over the Republic of Cabo Verde Islands at 78 hours as a tropical low… tropical low reaches 15N-33W as a rather broad feature by 120 hours
**For Area of Interest #33… ex-Franklin east-northeast of the Azores at 120 hours while centered near 39N-19W
1200Z (Sep 4) GFS Model Run...
**For Tropical Depression Katia… stalls at its current location near 28N-34W thru 45 hours while weakening to a remnant low… remnant low begins an anticyclonic turn while first going east toward ex-Franklin and then turning southwest and west under the influence of the Atlantic surface ridge and weakens further to a trough that reaches 22.5N-41W by 120 hours
**For Tropical Storm Gert… absorbed into northeast side of ex-Idalia over the next 9 hours
**For Area of Interest #30… tropical cyclone formation suggested near 12.5N-40W at 27 hours and intensifies into a hurricane that reaches the waters east-northeast of the Lesser Antilles and near 19.5N-57W by 120 hours
**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 15 hours which evolves into a tropical low that moves across the southern Republic of Cabo Verde Islands thru 75 hours… compact tropical cyclone formation suggested near 19N-32.5W at 120 hours
**For Area of Interest #33… ex-Franklin located east of the Azores at 120 hours while centered near 37.5N-19.5W
0600Z (Sep 4) NAVGEM Model Run...
**For Tropical Depression Katia… stalls at its current location near 28N-34W thru 24 hours while weakening to a remnant low… remnant low begins an anticyclonic turn while first going east toward ex-Franklin and then turning southwest under the influence of the Atlantic surface ridge and weakens further to a trough that reaches 21N-41.5W by 120 hours
**For Tropical Storm Gert… absorbed into northeast side of ex-Idalia over the next 18 hours
**For Area of Interest #30… tropical cyclone formation suggested near 15N-46W at 60 hours followed by weakening to a remnant trough that reaches the waters northeast of the Lesser Antilles thru 120 hours
**For Area of Interest #32… Additional tropical wave emerges from Western Africa at 24 hours and evolves into a broad tropical low centered just south of the Republic of Cabo Verde Islands thru 72 hours… very broad tropical low centered at 15N-33W at 120 hours
**For Area of Interest #33… ex-Franklin located east of the Azores while centered near 38.5N-20.5W at 120 hours
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