*******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 OCTOBER 8 2021 8:23 AM EDT...
Satellite image as of 0400Z. Areas of interest circled in yellow are not mentioned in the NHC tropical weather outlook. Areas circled in a green dashed line are in the NHC 5-day tropical weather outlook. Areas circled in a solid green line are in the NHC 2-day tropical weather outlook:
NHC TAFB Surface Analysis 1800Z (Thursday Oct 7):
GFS Model Upper-Level Winds 1800Z (Thursday Oct 7):
See pair of area of interest sections below for two areas being watched for development in the Atlantic tropics. Elsewhere… the current central Atlantic upper vorticity is forecast to diminish and become replaced with a more favorable upper ridge with low shear and upper outflow. Will monitor the current east Atlantic tropical wave of low pressure and also a new surface trough of low pressure that has developed east of area of Interest #1 for any signs of development in the days ahead as both features head toward the forecast and more favorable upper ridge.
AREA OF INTEREST #1... The tropical wave of low pressure in the central tropical Atlantic has become disorganized while encountering southerly shear induced by upper vorticity in the region. The potential for this wave to develop in the short-term has therefore ended. However I have extended my updated outlook for this tropical wave through day 5 as upper-level winds will potentially become more conducive for development as (1) the suppressing and shearing upper vorticity in the region dissipates… (2) upper divergence increases across the southeastern Bahamas and central Caribbean without much wind shear due to the amplified nature of upper vorticity to approach from the eastern US and settle over the western Bahamas. The forecast track in the outlook below is west-northwest toward the ridge weakness associated with area of interest #2… taking the wave into the upper divergence maximum of the aforementioned Bahamas upper vorticity by day 4. The position at day 5 is based on the upper divergence maximum of the upper vorticity. My odds of development by day 5 are currently at a low 10% as the CMC and NAVGEM which suggest some development of this wave do so after day 5.
With these forecast updates:
**Although the tropical wave has become less organized while moving into the Lesser Antilles islands… some squalls of thunderstorms have developed west and east of the islands due to upper divergence on the east side of the upper vorticity currently in the region. Therefore heavy rainfall cannot be ruled out in the short-term.
**Will watch for possible heavy rains by early next week for the Dominican Republic... Haiti…and southeastern Bahamas as upper divergence over the wave increases markedly from the arrival of amplified upper vorticity to settle into the western Bahamas.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Oct 9)… 0% chance of tropical cyclone formation (eastern Caribbean Sea near 13N-62W)
IOH 48 Hr Outlook (0000Z Oct 10)… 0% chance of tropical cyclone formation (eastern Caribbean Sea near 15N-66W)
IOH 72 Hr Outlook (0000Z Oct 11)… 0% chance of tropical cyclone formation (just south of the Dominican Republic near 17.5N-70W)
IOH 96 Hr Outlook (0000Z Oct 12)… 0% chance of tropical cyclone formation (between Haiti and the southeastern Bahamas near 20.2N-73.8W)
IOH 120 Hr Outlook (0000Z Oct 13)… 10% chance of tropical cyclone formation (southeastern Bahamas near 23N-73.8W)
AREA OF INTEREST #2...The surface low pressure area offshore of the southeastern US as of 0000Z was located near 32.2N-77.6W while supported by upper divergence ahead of the current eastern US upper vortex. The increased thunderstorm activity that this system developed yesterday has become sheared off to the east… and another thunderstorm complex has developed to the west over eastern Georgia and South Carolina… which leaves this surface low pressure area thunderstorm-free at the moment. However with the upper divergence regime expected to last in the region… the surface low pressure could redevelop thunderstorms and tropical characteristics in the days ahead. The forecast track below is based on the location of the divergence maximum being generated by the eastern US upper vortex as presented in the GFS model which results in no forward motion in the next 24 hours… then an east shift in position by 48 hours as the upper vortex pushes east into the western Atlantic. The model solutions have continued to change regarding the shape and position of the upper vortex as it enters the west Atlantic… with the most recent solutions showing the upper vortex becoming quiet elongated north to south… with the south part settling over the western Bahamas and the north part becoming a shortwave upper trough that accelerates east toward the current North Atlantic upper vortex. This has resulted in a bifurcation of the long-range track solutions… with some showing an east acceleration in track with the shortwave upper trough and others keeping the surface low on a long-term southwest drift under the northwest side of the Bahamas upper vorticity while a separate surface low forms with the shortwave upper trough. For the sake of continuity with my previous outlook …. I am keeping the latter scenario which shows a southwest drift in the long range.
I am maintaining peak development odds of 40% for this disturbance as most models still agree on keeping a well-defined disturbance in the days ahead while presenting a gradually strengthening surface low through 72 hours due to the supportive upper divergence regime associated with the approaching eastern US upper vortex. I sharply drop development odds to 0% by day 4 as models show the surface low accelerate east with the shortwave upper trough as discussed above and lose tropical character while becoming a frontal low supported by the shortwave… or show the surface low weaken while settling beneath the northwest side of the remainder of the upper vortex to settle over the western Bahamas where upper divergence will be lacking.
With these forecast updates:
**Even without the formation of a subtropical or tropical cyclone… surface pressures are expected to be rather low offshore of the southeast US in the days ahead due to the upper divergence pattern to be induced by the eastern US upper vortex. In addition surface pressures over the northeast US coast will be elevated due to the western convergence zone of the current North Atlantic upper vortex. The pressure difference between the northeast US coastal high pressure and surface low pressure offshore of the southeast US will drive strong onshore winds that will kick up the surf across the mid-Atlantic and northeast US coast in the days ahead. This pattern could last for multiple days… with beach erosion possible.
**The gusty onshore winds could reach land areas from Cape Hatteras North Carolina to New Jersey through 72 hours. After that time the surface low is forecast to weaken or shift east and away as discussed above… which will relax the pressure difference with respect to the northeast US coastal high pressure and thus diminish the gusty winds.
**Heavy rainfall for the mid-Atlantic and northeast US is also possible over the next few days from this disturbance.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Oct 9)… 10% chance of tropical cyclone formation (offshore of southeastern US near 32.2N-77.6W)
IOH 48 Hr Outlook (0000Z Oct 10)… 30% chance of tropical cyclone formation (offshore of southeastern US near 32.5N-75W)
IOH 72 Hr Outlook (0000Z Oct 11)… 40% chance of tropical cyclone formation (offshore of southeastern US near 32.5N-76W)
IOH 96 Hr Outlook (0000Z Oct 12)… 0% chance of tropical cyclone formation (offshore of southeastern US near 31N-77.5W)
...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 area of interest #1... tropical wave evolves into broad low pressure area spanning from the central Caribbean to the central Bahamas by 168 hours
**For area of interest #2...while drifting north makes landfall on North Carolina Outer Banks at 66 hours… weakens to a remnant surface trough soon after
1200Z ECMWF Model Run...
**For area of interest #1... no development shown
**For area of interest #2... surface low continuously strengthens offshore of Carolinas through 48 hours… makes landfall on North Carolina Outer Banks at 72 hours… shifts northeast and is located offshore of New Jersey at 96 hours… transitions into a strengthening non-tropical frontal low that accelerates east by 120 hours while north part of eastern US upper vortex becomes an eastward-accelerating shortwave upper trough that heads toward North Atlantic upper vortex.
1800Z GFS Model Run...
**For area of interest #1... no development shown
**For area of interest #2... surface low continuously strengthens offshore of Carolinas through 66 hours… continuously weakens just offshore of southeastern North Carolina through 90 hours… drifts west into central North Carolina as a weakening surface trough through 105 hours
**Tropical wave currently at 32.5W longitude develops into tropical low near 22.5N-66W by 168 hours
1800Z NAVGEM Model Run...
**For area of interest #1... north end of tropical wave evolves into amplified surface trough just east of the eastern Bahamas by 120 hours
**For Area of Interest #2... moves northeast parallel to mid-Atlantic US coast through 96 hours… transitions into a strengthening non-tropical frontal low that accelerates east after 96 hours while north part of eastern US upper vortex becomes an eastward-accelerating shortwave upper trough that heads toward North Atlantic upper vortex.
**Tropical wave currently at 32.5W longitude develops into tropical low near 16.5N-56W by 120 hours