*******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 15 2024 6:11 PM EDT...
Regarding current Atlantic tropical systems... Tropical Storm Gordon is moving slowly in the central Atlantic… and it has recently weakened to a tropical depression due to wind shear… see Gordon section below for more info. See area of interest #28 section below for more info on the tropical disturbance expected to move into the Carolinas in the next 24 hours while potentially becoming a tropical storm.
Elsewhere... watching for the possible emergence of additional areas of interest in the days ahead as follows:
(1) For the eastern tropical Atlantic... a tropical wave of low pressure is currently in the eastern tropical Atlantic and is inactive due to dry Saharan air… while a pair of tropical waves that were over central and Western Africa on Friday appear to have merged and will soon merge with the aforementioned offshore wave. The merger between the multiple waves has high potential to result in a broad eastern tropical Atlantic surface trough. Steering current are expected to weaken in the eastern tropical Atlantic... due to the sprawling surface ridge weakness that will be associated with tropical cyclone Gordon and a central Atlantic surface front... and due to another surface ridge weakness toward northwestern Africa/southwestern Europe to be driven by the eastern divergence zone of the current eastward-shifting mid-latitude eastern Atlantic upper vorticity. Therefor the potential surface trough is expected to be slow-moving at a position southeast of tropical cyclone Gordon. And with the regional tropical upper ridge axis forecast to persist... featuring low shear and outflow... it is not out of the question that within the next week a meandering tropical disturbance emerges within the possible eastern tropical Atlantic surface trough.
(2) For the mid-latitudes of the central Atlantic... including the waters toward the west between Bermuda and the northeastern Caribbean Islands... a surface front continues in the region linking area of interest #28 to the current north-central Atlantic frontal cyclone. This segment of surface front may decay into one or more low pressure spins. If tropical cyclone Gordon incoming from the southeast does not dominate… which now looks more likely given the current poor state of Gordon… this may allow for one of the potential mid-latitude central Atlantic spins to become a new tropical area of interest in future updates... especially if one of the spins ends up in an area of lower shear to the south or west of the currently forecast central Atlantic complex upper trough (the details of the complex upper trough are in the Tropical Storm Gordon section below).
TROPICAL STORM GORDON… The tropical storm that was in the eastern tropical Atlantic has continued west-northwest into the central Atlantic. Although the forward speed of the storm has slowed due to the ridge weakness of the current north-central Atlantic frontal cyclone… the track has actually changed to a west-southwest angle tilted away from the weakness due to a remainder southwest extension of the steering east Atlantic surface ridge.. and the updated track shown below was nudged south and west accordingly. The current northwest Atlantic and northeast Canada upper troughs will soon merge and dive into the central Atlantic… and the latest model runs show the southwest tail of the central Atlantic upper trough will be re-enforced by a piece of energy that leaves the upper trough tied to area of interest #28 (previously thought some energy from the current western Canada upper trough would re-enforce the southwest tail of the central Atlantic upper trough… either way the southwest tail of the upper trough looks to be around for a few days). The divergence zone of the lingering southwest tail of the upper trough will produce one or more frontal lows to the north and northwest of tropical storm Gordon that will eventually pull the tropical storm north… with a northeast turn shown by day 5 as the southwest tail of the upper trough pushes toward Gordon. Regarding intensity… some of the mid-latitude eastern Atlantic upper vorticity continues to hang around Gordon’s environment… inducing westerly shear that has kept Gordon weak (the tropical storm reached a peak of 45 mph maximum sustained winds on Saturday… however it has weakened back to 40 mph maximum sustained winds as shear has kept the thunderstorms more separated from the storm’s swirl center). The latest model runs show the southwestern part of the eastern Atlantic upper vorticity getting pushed southwest into Gordon due to western Atlantic warm core upper ridging that amplifies due to warm southerly flow on the east side of area of interest #28… if so this will result in the upper outflow of the storm becoming suppressed through 48 hours. This could be why some of the models have wanted to dissipate Gordon into a remnant low all along… and although this could actually happen I currently agree with the NHC strategy of hanging on to Gordon as a minimal tropical depression as Gordon’s forecast track also keeps it on the cusp of taking advantage of the upper ridging (low shear/outflow region) that amplifies from the western Atlantic and just north of the storm. By 72 hours I show a conservatively slow re-strengthening rate when Gordon turns north into the more favorable upper ridging… as it is possible Gordon does not survive the short-term unfavorability to begin with. Westerly wind shear is then expected to increase by 96+ hours when the southwest tail of the central Atlantic upper trough swings toward Gordon… and I stop the forecast intensification and show weakening once again through 120 hours.
Update as of 5 PM EDT… the NHC has downgraded Gordon to a tropical depression
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0600Z Sep 15)… 40 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 19.6N-44.8W
IOH 24 Hr Forecast (0600Z Sep 16)... 30 mph maximum sustained wind tropical depression centered in the central tropical Atlantic at 19.2N-47.5W
IOH 48 Hr Forecast (0600Z Sep 17)… 30 mph maximum sustained wind tropical depression centered in the central Atlantic at 20N-50W
IOH 72 Hr Forecast (0600Z Sep 18)… 40 mph maximum sustained wind tropical depression centered in the central Atlantic at 22N-50W
IOH 96 Hr Forecast (0600Z Sep 19)… 45 mph maximum sustained wind tropical storm centered in the central Atlantic at 26N-49W
IOH 120 Hr Forecast (0600Z Sep 20)… 30 mph maximum sustained wind tropical depression centered in the central Atlantic at 30N-45W
*****National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT*************************
5-Day Position (1200Z Sep 20)… 40 mph maximum sustained wind tropical storm centered in the central Atlantic at 23.5N-48.5W
AREA OF INTEREST #28 (POTENTIAL TROPICAL CYCLONE EIGHT)… More recent satellite image of frontal low offshore of the Carolinas as it potentially begins to take on tropical characteristics… image taken as of 2031Z
The divergence zone of upper vorticity parked over the southeast US has led to the gradual formation of a non-tropical frontal low over offshore waters over the weekend. Since 0300Z this morning… the frontal low had a clump of thunderstorms northeast of its center… which as of this afternoon has become replaced by an even larger clump northwest of the center that is beginning to resemble the northwest quadrant of a tropical system. As such the NHC has upgraded this system to potential tropical cyclone eight… in order to issue tropical storm advisories for parts of the Carolina coast ahead of time in case it becomes a tropical storm at the last minute before landfall. I think the strategy of potential tropical cyclone advisories also makes sense because the pressure gradient between the north side of the frontal low and south side of the northeastern US surface ridge is already producing storm force conditions… therefore tropical storm conditions are imminent for parts of the Carolinas in the next 24 hours regardless of whether or not this system will technically qualify as a tropical cyclone during its lifecycle. The only reason that may not happen is the broad east-west elongated circulation of the frontal low never consolidates into a well-defined circular center necessary for cyclone status. As of this afternoon I position the centroid of the oblong spin toward 75W longitude where the NHC TAFB surface analyses had it as of late Saturday and early today… and recent visible satellite animation seems to still suggest this. This is east of where the NHC positioned the center of the potential tropical cyclone as of 5 PM EDT… which is why my forecast track in my outlook below is aligned northeast of the NHC. The materializing steering flow between the northeast US surface ridge and inland eastern US upper vorticity is more easterly and less southerly then conveyed in my previous Friday update… therefore the latest track is more west and less north and does not call for impacts further north than the North Carolina coast. In the next 24 hours the updated track moves this system northwest into the Carolinas… then beyond that time this system whirls northwest across the Appalachian mountains and into the Ohio Valley while pulled into the core of the inland upper vorticity where ultimately the inland remnant low dissipates due to the lack of divergence beneath the core of the upper vorticity. Regarding odds of tropical cyclone formation… I have increased them to above 50% given the recent increase in organized thunderstorms and as models have trended toward showing tropical cyclone formation around landfall time.
Regarding impact to land areas for the next 24 hours:
(1) Coastal surf is expected across southeastern North Carolina and northeastern South Carolina… along with tropical storm force winds with some power outages and/or damage potential both on the coast and inland.
(2) Heavy rainfall with flash flood potential is expected across northeastern South Carolina… southeastern North Carolina… central and western North Carolina… and perhaps as far north as south-central and southwestern Virginia.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 12 Hr Outlook (0000Z Sep 16)… 60% chance of tropical cyclone formation (just east of Cape Fear North Carolina near 34N-77.5W)
IOH 24 Hr Outlook (1200Z Sep 16)… 0% chance of tropical cyclone formation (central North Carolina near 35.2N-79.2W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 5 PM EDT***************************
Formation chance through 48 hours... 70%
Formation chance through 7 days... 70%
Was recently increased from 50% to 70% from 2 PM to 5 PM EDT
****National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT***************************
Landfall (1800Z Sep 16)… 50 mph maximum sustained wind tropical storm centered on the northeastern South Carolina coast at 33.1N-79.4W
3-Day Position (1800Z Sep 18)… 25 mph maximum sustained wind dissipating remnant low centered over southern West Virginia at 37.5N-81.5W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/).
1200Z (Sep 14) CMC Model Run...
**For Tropical Storm Gordon... weakens to a tropical depression near 19N-44W at 12 hours then a remnant low near 18.8N-52.5W at 72 hours… remnant low becomes absorbed by a large frontal low to the northwest at 132 hours
**For area of interest #28... surface frontal low moves west-northwest toward Carolina coast through 48 hours… then makes landfall between Cape Fear and Cape Lookout on the North Carolina coast at 60 hours while potentially becoming a subtropical or tropical cyclone… the weakening inland remnant low continues north-northwest to Ohio through 114 hours then dissipates shortly thereafter while drifting east
1200Z (Sep 14) ECMWF Model Run...
**For Tropical Storm Gordon… weakens to a tropical depression near 19N-46W at 30 hours then remnant low near 19N-47.5W by 48 hours… remnant low becomes absorbed by a large frontal low to the northwest at 126 hours
**For area of interest #28... surface frontal low moves west toward waters offshore of the GA/SC border through 48 hours…then turns north and makes landfall at the NC/SC border at 72 hours while potentially becoming a subtropical or tropical cyclone… inland weakening remnant low then continues north into Virginia where it then dissipates just after 120 hours
1800Z (Sep 14) GFS Model Run...
**For Tropical Storm Gordon… weakens to a tropical depression near 19.8N-46W at 27 hours then remnant low near 19N-50W by 54 hours… while reaching 20.5N-51.5W remnant low becomes absorbed by a frontal low to the north at 108 hours
** For area of interest #28... surface frontal low makes landfall on the NE South Carolina coast at 33 hours while potentially becoming a subtropical or tropical cyclone… while continuing northwest the inland weakening remnant low decays to a surface trough while near the OH/KY/WV border at 69 hours with the trough dissipating over Indiana by 93 hours
1800Z (Sep 14) NAVGEM Model Run...
**Not available at above-mentioned source
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