MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #111
*******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.**********
...WEDNESDAY SEPTEMBER 15 2021 1:30 AM EDT...
Although Nicholas has weakened to a tropical depression transitioning into a remnant low as it pushes into Louisiana… the Atlantic tropics remain active with three areas of interest to watch. Each area of interest is detailed in its own section below.
TROPICAL STORM NICHOLAS (RECENTLY DOWNGRADED TO TROPICAL DEPRESSION)…Due to ongoing westerly shear on the north side of the Gulf of Mexico upper ridge... and land interaction with the center being over the southeast corner of Texas...Nicholas has weakened from a minimal category 1 hurricane to a low-end 40 mph maximum sustained wind tropical storm in the last 24 hours. As of 8 PM EDT… Nicholas weakened further to a 35 mph max sustained wind tropical depression. Nicholas has undergone a sluggish eastward drift over the last several hours as the storm has been tall enough to be dragged by upper westerly flow against the southeastern US surface ridge instead of being free to flow northward around the ridge. One would think as Nicholas weakens that a northward turn would begin as the circulation becomes more shallow. However as the upper trough from central Canada approaches...some of the upper westerly flow is deflecting north ahead of the trough and some of the flow is deflecting south around the Gulf of Mexico upper ridge... resutling in an upper divergence zone toward the Mississippi/Louisiana border. This upper divergence zone is illustrated in the upper winds panel of the above birdseye view chart. Nicholas appears to be in the process of transitioning into a less tropical system supported by this upper divergence zone and therefore will continue to drift east into the upper divergence zone over the next 24 hours. The updated forecast track below is nudged south due to the current positon of Nicholas with respect to the previous forecast... and is also nudged west as Nicholas's eastward forward speed has remained rather slow.
Despite the center being near the Gulf of Mexico... the westerly shear has been too much for Nicholas such that no new thunderstorm activity has developed near the center. My intensity forecast calls for Nicholas to weaken to a remnant low in the next 24 hours. Update as of 11 PM EDT… the NHC had issued their final advisory on Tropical Depression Nicholas as it is transitioning to a remnant low… as such this will also be my final statement on Nicholas on this blog. Impacts from the remnants will be noted on the home page bulletins on this site going forward.
**Whatever breezy winds that were just east of center… in the sheared off heavy rains and thunderstorms … have diminished as shown by wind observations in Beaumont Texas and Lake Charles Louisiana. Although Alexandria in central Louisiana is showing gusts in the 20s of mph in recent observations… the threat of wind damage has greatly diminished with Nicholas now a tropical depression transitioning into a remnant low.
**Heavy rainfall with flash flooding potential is an ongoing concern across southern Louisiana… and based on latest radar has spread north and east across the remainder of Louisiana… southern Mississippi… southern Alabama… and the western Florida panhandle. Be mindful of things that will keep you safe from floodwater... such as avoiding driving into a water-covered roadway to prevent your vehicle from getting stuck which could result in drowning.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 14)... 40 mph maximum sustained wind tropical storm centered just east of Galveston Bay Texas at 29.6N-94.6W
IOH 24 Hr Forecast (1800Z Sep 15)... Remnant low centered just west of the Mississippi/Louisiana border at 31N-92.5W
AREA OF INTEREST #1... The shower and thunderstorm activity that was previously north of the northeastern Caribbean Islands has migrated west to the waters north of the eastern Bahamas. This activity continues to be driven by the eastern divergence zone of the upper vortex in the region… and has become better organized while a surface trough of low pressure has become re-established with this activity. The upper vortex is expected to drift further west due to upper ridging to the north to be associated with the warm sector of the frontal system approaching from the central Canada and Great Lakes region of North America...which will allow the thunderstorms to shift west toward the waters adjacent to the central Bahamas in the next 24 hours. As the upper vortex fades while remaining cut-off from high-latitude cold air... the upper air pattern will become more anticyclonic with lower shear and upper outflow... thus more conducive for tropical development.
The forecast positions in the outlook below are initially based on the location of the eastern divergence zone of the upper vortex… and in the short-term are adjusted north and west due to the current position of the surface
trough. By days 2 and 3 the frontal system from central Canada/Great Lakes region will leave behind upper vorticity over the southeast US whose eastern divergence zone will create a narrow surface ridge weakness that this system will track northwest and northward into. By day 4… the next frontal system after that will push the southeast US upper vorticity toward this system and also create a surface ridge weakness in the northwest Atlantic which should attract this system on a northeast track out to sea. On day 5… the upper trough of this next frontal system is forecast to merge with the upper vorticity ejecting from the southeast US… resulting in an amplified upper trough that could transition this system into a strengthening non-tropical or subtropical system… more on that in the next paragraph below.
I have raised odds of development to 40% due to the re-formation of a surface trough and improved organization. At this time I have not established higher odds like the NHC has as there is not a well-defined surface spin at this time… the same story is repeated in the mid-levels as the CIMSS 850 mb vorticity product shows an elongated system without a defined center of spin (http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=atlantic&sat=wg8&prod=vor&zoom=&time=). Plus the window of time for favorable conditions is narrow as the approach of southeast US upper vorticity by day 3… and an amplified upper trough by day 5… will increase the shear and potentially make this system less tropical… which is why I taper down development odds after day 2. The GFS has a less amplified upper trough at day 5 that pulls this system north over cooler waters and transitions this system to non-tropical… whereas the upper trough presentation in the ECMWF… NAVGEM… and CMC is more amplified and almost an upper vortex which captures this system at a further south location. In this alternate scenario… lower shear and higher upper divergence on the east side of the upper vortex could help strengthen this system into a subtropical storm… essentially a hybrid system supported non-tropically by the upper vortex’s upper divergence while located south enough over warm water to still have thunderstorms and warm core outflow in the environment just east of the upper vortex. Out of respect for the consensus in the ECMWF… NAVGEM… and CMC… the day 5 outlook below has slightly elevated odds over day 4 to reflect possible subtropical cyclone formation.
Coastal sea swells could reach the Bahamas and eastern US coast in the coming days if this system goes on to develop. Gusty winds and heavy rains may reach the North Carolina Outer Banks in about 2 to 3 days (late Thursday or Friday).
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 15)... 10% chance of tropical cyclone formation (north of the central Bahamas near 26N-74W)
IOH 48 Hr Outlook (1800Z Sep 16)... 40% chance of tropical cyclone formation (offshore of the southeastern US near 31N-76W)
IOH 72 Hr Outlook (1800Z Sep 17)... 30% chance of tropical cyclone formation (just offshore of Cape Hatteras North Carolina near 35N-74W)
IOH 96 Hr Outlook (1800Z Sep 18)... 10% chance of tropical cyclone formation (offshore of the northeastern US near 37.5N-70W)
IOH 120 Hr Outlook (1800Z Sep 19)… 30% chance of subtropical cyclone formation (northwestern Atlantic near 38N-61W)
AREA OF INTEREST #2... The tropical wave of low pressure in the far eastern Atlantic has shown an improved structure with defined cloud bands rotating around a low pressure center that passed near 12N-22.5W around 1800Z earlier tonight. However the thunderstorm activity has weakened overall in the last 24 hours due to dry Saharan air. Due to the ongoing model support… defined low pressure spin… and favorable upper outflow and low shear of the stout tropical upper ridge in the region… I maintain high 80% odds of development by day 5 assuming this system will be able to better develop as it moves into the central tropical Atlantic where dry Saharan air concentrations tend to be less. However my short-term odds are set at a lower 60% due to the current dry Saharan air situation.
The current shortwave upper trough entering the northwest Atlantic from eastern Canada is expected to leave behind upper vorticity near the northern Lesser Antilles. Even though the upper vorticity may weaken by day 5 as it remains cut-off from high-latitude cold air… there may be enough upper southwesterly flow on its east side to deflect the track of this system north… especially if it goes on to develop a stronger/taller structure that can be steered by upper flow. Even at the surface… area of interest #1 may stay at a more south position while becoming a large subtropical system… creating a surface ridge weakness. Therefore by day 5 I begin to show some increase in the north angle of this system’s forecast track.
IOH 24 Hr Outlook (1800Z Sep 15)... 60% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-27.5W)
IOH 48 Hr Outlook (1800Z Sep 16)... 80% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-32.5W)
IOH 72 Hr Outlook (1800Z Sep 17)... 80% chance of tropical cyclone formation (eastern tropical Atlantic near 13.5N-37.5W)
IOH 96 Hr Outlook (1800Z Sep 18)... 80% chance of tropical cyclone formation (central tropical Atlantic near 14N-42.5W)
IOH 120 Hr Outlook (1800Z Sep 19)… 80% chance of tropical cyclone formation (central tropical Atlantic near 16N-47.5W)
AREA OF INTEREST #3... Another tropical wave of low pressure featuring widespread thunderstorms on its southwest and southeast sides… currently over western Africa near 10N-2W… is Forecast to enter the eastern tropical Atlantic by day 4. The ECMWF model has continued to suggest the wave could develop with the stout presence of low shear and upper outflow supplied by the eastern tropical Atlantic upper ridge… while the GFS has abdicated this solution at this time. The NHC has only recently introduced this wave into their outlook product… which is why in the above charts it is marked as not being in the NHC outlook when it now is. I agree with the NHC’s low 20% odds of development by day 5… as the wave only has model support from the ECMWF… and the recent waves that have emerged from Africa have struggled with dry Saharan air. The ECMWF is also a northern outlier in track while assuming this wave will quickly develop offshore of Africa to become tall enough to be tugged by upper vorticity that is currently being left behind by the northeast Atlantic upper trough. As a compromise between the ECMWF and the rest of the models… I assume slower development and a smaller increase in the north angle in the track by day 4. I project this system to then escape the upper vorticity and continue more west by day 5.
IOH 24 Hr Outlook (1800Z Sep 15)... 0% chance of tropical cyclone formation (western Africa near 10N-7W)
IOH 48 Hr Outlook (1800Z Sep 16)... 0% chance of tropical cyclone formation (western Africa near 10N-12W)
IOH 72 Hr Outlook (1800Z Sep 17)... 0% chance of tropical cyclone formation (just offshore of Western Africa near 10.5N-17W)
IOH 96 Hr Outlook (1800Z Sep 18)... 10% chance of tropical cyclone formation (southeast of the Republic of Cabo Verde Islands near 12N-22W)
IOH 120 Hr Outlook (1800Z Sep 18)... 20% chance of tropical cyclone formation (southwest of the Republic of Cabo Verde Islands near 13N-27W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...
**For Tropical Storm Nicholas... weakens to a surface trough on the southeast Texas coast at 30 hours
**For area of interest #1... surface low develops near 29N-76W at 36 hours... transitions to large and strong subtropical storm centered at 40N-65W by 126 hours.
**For area of interest #2... tropical wave amplifies into a tropical low pressure spin just north of the northeastern Caribbean Islands at 168 hours.
**For area of interest #3... no development shown
1200Z ECMWF Model Run...
**For Tropical Storm Nicholas... remnant low dissipates between 48 and 72 hours near the southern Mississippi/Louisiana border
**For area of interest #1...transitions to large and strong subtropical storm centered at 37.8N-64W by 144 hours.
**For area of interest #2... located at 17.8N-54.5W as a compact tropical storm by 120 hours
**For area of interest #3... tropical wave departs Africa at 72 hours... passes over the Republic of Cabo Verde Islands between 96 and 120 hours as a tropical storm
1200Z GFS Model Run...
**For Tropical Storm Nicholas... remnant low weakens to a trough on the southwestern Louisiana coast at 45 hours.
**For area of interest #1... by 120 hours is located just offshore of Nova Scotia after having transitioned into a non-tropical frontal low.
**For area of interest #2... located at 13N-52W a compact hurricane by 120 hours.
**For area of interest #3... tropical wave departs Africa at 78 hours... no development shown afterwards
1200Z NAVGEM Model Run...
**For Tropical Storm Nicholas... remnant low weakens to a trough over the southeast corner of Texas at 60 hours
**For area of interest #1...transitions to large and strong subtropical storm centered at 39.5N-59.8W by 144 hours.
**For area of interest #2... becomes a tropical depression near 11.5N-39.5W at 138 hours
**For area of interest #3... tropical wave departs Africa at 90 hours... organizes into broad tropical low due south-southwest of the Republic of Cabo Verde Islands by 138 hours.