MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #144
*******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.**********
…MONDAY OCTOBER 25 2021 4:50 AM EDT...
Satellite image as of 0040Z. 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 (Sunday Oct 24):
GFS Model Upper-Level Winds 1800Z (Sunday Oct 24):
Models have recently trended with a more amplified version of the current wave of upper vorticity across the central US as that wave moves into the northwest Atlantic… and the potential for subtropical development in the northwest Atlantic in a few days has increased. See area of interest #1 section below for more details.
In addition… some recent model runs suggest a strong tropical wave of low pressure will emerge from Africa and enter the eastern tropical Atlantic in about three days. However upper vorticity in the eastern Atlantic is expected to persist while re-enforced by the pocket of upper vorticity recently deposited by the current Northeast Atlantic upper trough… with another pocket soon to be deposited by the current western Atlantic upper trough. This upper vorticity is likely to make conditions for this wave’s development hostile while inducing westerly wind shear.
And finally… the current wave of cold core upper vorticity in the northeast Pacific is expected to cross North America and drive a surface cold front as far south as the southern Caribbean Sea in a few days. The tail end of this front may evolve into a tropical disturbance supported by the outflow of the ongoing upper ridge in the Caribbean.
AREA OF INTEREST #1... Since Saturday… shower and thunderstorm activity has been present across the southern Gulf of Mexico due to split flow upper divergence between the north side of Caribbean upper ridging and the south side of the upper trough that has recently moved across the eastern US and into the western Atlantic. This activity has continued and strengthened in the eastern Gulf of Mexico on Sunday as a fast-moving upper trough is approaching from the central US… with this trough originating from the large area of northeastern Pacific upper vorticity. A surface trough of low pressure has also formed just west of the Florida peninsula in this activity.
Models have been varying on the amplitude of the approaching central US upper trough as that trough moves into the northwestern Atlantic. In the last 24 hours… they have trended back to a more amplified upper trough while once again showing more separation between this trough and the remainder of the northeast Pacific upper vorticity as that vorticity swings into western North America. The increased separation between the two leaves a more intact warm core upper ridge over Canada… with the upper ridge remaining amplified in the warm sector of surface frontal systems to be generated by the remainder of the upper vorticity from the northeast Pacific. The more amplified adjacent upper ridge results in a more amplified upper trough that moves into the northwest Atlantic. The surface trough west of Florida is forecast in the next 48 hours to quickly cross the Florida peninsula while rapidly intensifying into a surface frontal cyclone offshore of the eastern US… supported by the tremendous eastern divergence zone of the amplified upper trough that moves into the northwest Atlantic. The upper trough is now likely to amplify into an upper vortex based on the recent model trends… which will support reduced wind shear needed for possible subtropical development of the surface cyclone. The NHC as a result has introduced an area of interest in their 5-day outlook for this developing situation.
Forecast track in my updated outlook below is initially northeast as the aforementioned surface trough near Florida intensifies into a surface cyclone in the divergence maximum of the approaching central US upper trough. Once the upper trough amplifies into a vortex… a cyclonic whirl of the surface cyclone is likely offshore of Massachusetts in approximately 48 hours… at a location below the vortex. Ad the upper vortex swings eastward out to sea… the surface cyclone is expected to follow suit and also swing east. Factors aiding in potential subtropical development of the surface cyclone include the rather cold temps Forecast with the upper vortex which could aid in thunderstorm generation even at water temps below 26 deg C… and forecast passage of the surface cyclone near the north wall of the warm Gulf Stream waters at 72 to 96 hours. The NHC has recently increased odds of subtropical cyclone formation to 40% by day 5. I am more cautious while assigning 30% odds as the GFS model is not yet on board with as amplified of an upper vortex… instead presenting an elongated upper vortex/trough with potential wind shear and also a broad eastern divergence zone that would result in multiple surface cyclone centers instead of a single surface cyclone center needed for subtropical development.
Regardless of subtropical cyclone development or not… a strong surface cyclone is imminent offshore of the eastern US in the next 48 hours. This will result in an increase in coastal surf for the mid-Atlantic and northeast US coasts… as well as the Atlantic Canada coast… through Tuesday. The northwest side of the surface cyclone is expected to bring gusty winds to the mid-Atlantic and northeast US coasts by Tuesday as well.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Oct 26)… 0% chance of subtropical cyclone formation (offshore of the eastern US coast near 33N-74W)
IOH 48 Hr Outlook (0000Z Oct 27)…0% chance of subtropical cyclone formation (southeast of Cape Cod Massachusetts near 40N-70W)
IOH 72 Hr Outlook (0000Z Oct 28)… 30% chance of subtropical cyclone formation (northwest Atlantic near 40N-65W)
IOH 96 Hr Outlook (0000Z Oct 29)…30% chance of subtropical cyclone formation (northwest Atlantic near 40N-60W)
IOH 120 Hr Outlook (0000Z Oct 30)…30% chance of subtropical cyclone formation (north Atlantic near 40N-54W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (Oct 24) CMC Model Run…
**For area of interest #1… surface low forms offshore of Carolinas at 36 hours… becomes a rapidly intensifying frontal cyclone that makes a cyclonic loop just offshore of Massachusetts from 54 to 72 hours after which time it accelerates east… located at 39.5N-55W at 120 hours as a possible subtropical cyclone.
**Tropical wave emerges from Africa at 72 hours… located near 8.5N-25W as a broad tropical low at 120 hours
**Tail end of cold front evolves into a southern Caribbean tropical low pressure near 12.5N-80W at 168 hours
1200Z (Oct 24) ECMWF Model Run...
**For area of interest #1… surface low forms offshore of South Carolina at 24 hours… becomes a rapidly intensifying frontal cyclone that makes a cyclonic loop offshore of the northeast US from 48 to 72 hours after which time it accelerates east… located at 38.5N-58.5W at 120 hours as a possible subtropical cyclone.
**Tail end of cold front evolves into a broad southern Caribbean tropical low pressure near 12.5N-78W at 168 hours
1800Z (Oct 24) GFS Model Run...
**For area of interest #1… surface low forms offshore of South Carolina at 18 hours… becomes a rapidly intensifying frontal cyclone that makes a cyclonic loop just offshore of Massachusetts from 42 to 60 hours after which time it accelerates east… located at 39.5N-52.5W at 120 hours as a non-tropical frontal low
**Tail end of cold front evolves into a southern Caribbean tropical low pressure near 13.5N-81.5W at 120 hours
1800Z (Oct 24) NAVGEM Model Run...
**For area of interest #1… surface low forms near 31.5N-76W at 24 hours… becomes a rapidly intensifying frontal cyclone that makes a cyclonic loop offshore of the northeast US from 54 to 72 hours after which time it accelerates east… located at 40.5N-57.8W at 120 hours… in long range continues east along 40N latitude while developing a circular core suggesting tropical characteristics
**Tropical wave emerges from Africa at 72 hours…located at 11N-22.5W as a broad tropical low at 120 hours