BIRDSEYE VIEW POSTS

Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at IOHurricanes@outlook.com 

 
 
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MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #130

*******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 OCTOBER 6 2021 7:28 AM EDT...

Satellite image as of 0220Z. 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 (Tuesday Oct 5):

GFS Model Upper-Level Winds 1800Z (Tuesday Oct 5):

See pair of area of interest sections below for two areas being watched for development in the Atlantic tropics. Elsewhere… a central Caribbean Sea tropical wave of low pressure is producing bands of thunderstorm activity with the aid of poleward (northbound) upper outflow streaming into the current axis of western Atlantic upper vorticity. No computer models at present support this wave developing. However will continue to monitor this wave for any signs of development as it moves into the western Caribbean in the days ahead… adding another area of Interest in future updates if needed.


AREA OF INTEREST #1 ... The tropical wave of low pressure that was in the eastern tropical Atlantic has now in made its way into the central tropical Atlantic. The wave has continuously become better defined while the thunderstorm activity continues to increase around a center of rotation that appears to be near 7.5N-42.5W as of tonight. In addition the wave remains under tropical upper ridging with low shear and upper outflow… therefore I am maintaining this tropical wave as an area of interest for tropical development. However my peak 5-day odds of development have been slightly lowered to 15% as the wave has an increasingly narrower window of time to develop further before it encounters less favorable southerly shearing upper winds being generated by the east side of the current axis of central Atlantic upper vorticity. The central Atlantic upper vorticity will tend to drift west into the Lesser Antilles while trying to link up with the current west Atlantic axis of upper vorticity as the west Atlantic axis shifts northeast in the mid-latitude westerlies. The central Atlantic upper vorticity will also continuously weaken while remaining cut-off from high-latitude cold air. The west drift and continuous weakening of the upper vorticity suggests a possible relaxation of the shear in the long range… however the GFS is too slow to weaken the upper vorticity such that I have 0% odds of development for this wave by day 4. It is interesting to note the GFS develops another disturbance to the east of this wave in the eastern divergence zone of the weakening upper vorticity… so it will be interesting to see if in fact we end up with yet another disturbance to monitor in the days ahead. It is also interesting to see the NAVGEM try and develop this wave by 4+ days… perhaps as it sees the upper vorticity weakening fast enough to allow this wave to develop. Taking the NAVGEM idea with a grain of salt at this time as it tends to be a less reliable model when it comes to tropical development.


The forecast track is due west with a north angle toward the Lesser Antilles… with the north angle induced by the ridge weakness associated with area of interest #2… especially towards day 4. Regardless of whether or not this wave develops into a tropical cyclone… heavy rains maybe possible in the Lesser Antilles by day 4… especially if the eastern divergence zone of the weakening upper vorticity aids in generating thunderstorm activity during that timeframe.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (0000Z Oct 7)… 15% chance of tropical cyclone formation (central tropical Atlantic near 8.5N-47.5W)

IOH 48 Hr Outlook (0000Z Oct 8)… 15% chance of tropical cyclone formation (central tropical Atlantic near 9.5N-52.5W)

IOH 72 Hr Outlook (0000Z Oct 9)… 10% chance of tropical cyclone formation (east of the southern Lesser Antilles near 10.5N-57.5W)

IOH 96 Hr Outlook (0000Z Oct 10)… 0% chance of tropical cyclone formation (southern Lesser Antilles near 12N-61.5W)


AREA OF INTEREST #2...The eastern divergence zone of upper vorticity in the west Atlantic continues to produce an area of disturbed weather featuring a surface trough of low pressure north of the Bahamas with curved bands of showers and thunderstorms… albeit the activity is also being sheared eastward from the surface trough by the upper vorticity. The west Atlantic upper vorticity in the next 24 hours expected to shift northeast and away in the flow ahead of a stronger batch of upper vorticity that has recently been left behind over the eastern US by a mid-latitude trough now moving through the northeast US. This disturbance is expected to shift northwest through 72 hours while evolving into a feature supported by the eastern divergence zone of the eastern US upper vorticity. The forecast track below is based on the location of the divergence maximum to be generated by the eastern US upper vorticity. The track is likely to shift east or northeast by day 4 as the eastern US upper vorticity pushes into the western Atlantic. I forecast this disturbance to shift south by day 5 as the upper vorticity in the GFS settles over the northwest Bahamas as a vortex… creating an upper divergence zone in the northeast quadrant of the vortex by 120 hours located further south.


I maintain peak 5-day development odds of 30% for this disturbance as most models still agree on keeping a well-defined disturbance in the days ahead with an amplified surface trough signature. Of note… some model runs suggest another disturbance that forms southeast of this area of interest and north of the Bahamas with the support of the eastern divergence zone of the eastern US upper vorticity once that vorticity moves into the west Atlantic after day 4. It is possible the remnants of Victor become involved in the genesis of this second disturbance if the remnants survive long enough. It is also possible that instead of having a second disturbance… this area of interest instead reforms to the southeast after day 4 and takes another stab at trying to develop. The forecast presented in the outlook below assumes the latter scenario.


With these forecast updates:

**The heavy rain risk remains low for the Bahamas as the disturbance remains concentrated more toward the north.

**Coastal sea swells are possible for the Bahamas… southeast United States coast… and mid-Atlantic United States coast should tropical or subtropical cyclone development occur. Even without the formation of a 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 vorticity. In addition surface pressures over the northeast US coast will be elevated due to the western convergence zone of the current northeast Canada upper trough as that trough later shifts into the northwest Atlantic. 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 US coast in the days ahead. This pattern could last for several days… with beach erosion possible.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (0000Z Oct 7)… 0% chance of tropical cyclone formation (north of the central Bahamas near 26N-75W)

IOH 48 Hr Outlook (0000Z Oct 8)… 20% chance of tropical cyclone formation (offshore of southeastern US near 29.8N-76W)

IOH 72 Hr Outlook (0000Z Oct 9)… 30% chance of tropical cyclone formation (offshore of southeastern US near 30.2N-77.5W)

IOH 96 Hr Outlook (0000Z Oct 10)… 30% chance of tropical cyclone formation (offshore of southeastern US near 32.5N-75W)

IOH 120 Hr Outlook (0000Z Oct 11)… 30% chance of tropical cyclone formation (offshore of southeastern US near 30N-75W)


...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... no development shown

**For area of interest #2...surface trough reaches South Carolina and Georgia coast at 48 hours… drifts very slowly offshore and becomes better defined offshore of the Carolinas by 108 hours… becomes a surface low over North Carolina Outer Banks by 126 hours… moves inland into eastern Virginia in the long range

**Additional surface trough develops offshore of southeast US by 156 hours… subtropical to tropical development offshore of mid-Atlantic US suggested in long range.


1200Z ECMWF Model Run...

**For area of interest #1... no development shown

**For area of interest #2... surface trough stalls offshore of southeast US from 48 to 72 hours… develops into a surface low near 31.2N-77.5W by 96 hours which moves north into the North Carolina Outer Banks by 120 hours


1800Z GFS Model Run...

**For area of interest #1... no development shown

**For area of interest #2... surface trough reaches Georgia coast by 48 hours… drifts northeast back offshore by 75 hours… develops into a surface low near 32N-78W at 102 hours… drifts west-southwest into northeast Florida by 132 hours while weakening to a surface trough

**Additional surface trough becomes defined east of the Lesser Antilles at 150 hours… possible tropical cyclone formation suggested northeast of the islands in the long range


1800Z NAVGEM Model Run...

**For area of interest #1...Tropical wave develops into a tropical low just east of the northern Lesser Antilles by 96 hours… located just north of the Lesser Antilles by 120 hours

**For Area of Interest #2... surface trough reaches South Carolina and Georgia coast at 48 hours… becomes stationary and then dissipates just offshore by 120 hours

**Additional surface trough develops northeast of the Bahamas by 126 hours… while remaining well-defined drifts west toward southeast US coast in long range.

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