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 #131

*******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.**********

...THURSDAY OCTOBER 7 2021 8:30 AM EDT...

Satellite image as of 0200Z. 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 (Wednesday Oct 6):

GFS Model Upper-Level Winds 1800Z (Wednesday Oct 6):

See pair of area of interest sections below for two areas being watched for development in the Atlantic tropics. Elsewhere… a western Caribbean Sea tropical wave of low pressure is producing bands of thunderstorm activity with the aid of upper outflow generated by an upper ridge in the region. However based on the arc of the bands.. the lowest pressure of the wave is over Honduras and now heading toward Belize instead of over water… and the offshore thunderstorm bands have weakened as of this morning. Tropical development here is not anticipated.


AREA OF INTEREST #1 ... The tropical wave of low pressure in the central tropical Atlantic remains well-defined with a large area of curved thunderstorm bands that suggest the lowest pressure of the wave is near 10N-49W as of 0000Z. My peak 5-day odds of development have been slightly lowered to 10% 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 central Atlantic upper vortex. The central Atlantic upper vortex 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 east in the mid-latitude westerlies. The central Atlantic upper vortex will also continuously weaken while remaining cut-off from high-latitude cold air. The west drift and continuous weakening of the upper vortex suggests a possible relaxation of the shear in the long range… however the GFS is too slow to weaken the upper vortex such that I have 0% odds of development for this wave by 48 hours.


The GFS has stopped showing the formation of another disturbance to the east of this wave in the eastern divergence zone of the weakening central Atlantic upper vortex… and the GFS and ECMWF do not develop this wave. However a new alternate scenario is proposed by recent runs of the CMC and NAVGEM which suggests this wave could make a comeback either in the central Caribbean or over/near the northern Caribbean Islands. This alternate scenario is possible as the central Atlantic upper vortex will dissipate… becoming replaced by a more favorable regime of upper divergence in the central Caribbean and northern Caribbean Islands region to be induced by the southeast side of the amplified upper vorticity currently over the southeast US… once the south part of that vorticity settles over the Bahamas in the days ahead. If this scenario continues to show in various model runs… will consider extending the outlook for this tropical wave in future updates.


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. Regardless of whether or not this wave develops into a tropical cyclone in the short-term… heavy rains maybe possible in the Lesser Antilles by late Friday… especially if the eastern divergence zone of the weakening central

Atlantic upper vortex 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 8)… 10% chance of tropical cyclone formation (central tropical Atlantic near 11N-54W)

IOH 48 Hr Outlook (0000Z Oct 9)… 0% chance of tropical cyclone formation (just east of the southern Lesser Antilles near 12.5N-59W)


AREA OF INTEREST #2...More recent satellite image of the surface low pressure disturbance offshore of the southeastern US as of 1216Z showing an increase in its thunderstorms. Yellow plus marks the center of lowest pressure… which is southwest of the thunderstorms as southwesterly shear is being generated by an upper vortex over the southeast US:

The surface trough of low pressure north of the Bahamas has shifted northwest and evolved into a surface low pressure center offshore of the southeast US coast located near 30N-78W (as of 0000Z) while transitioning into a feature supported by the eastern divergence zone of the current southeast US upper vortex. The forecast track below is based on the location of the divergence maximum being generated by the upper vortex as presented in the GFS model which results in a slow northeast drift through 72 hours… especially as the upper vortex pushes east into the western Atlantic. The GFS previously had the vortex stay as a consolidated feature further south toward the the Bahamas… which resulted in my previous forecast swinging south more quickly in the long range while the surface low whirls toward that upper vortex location. My updated forecast track in the long range now has a cyclonic whirl from 72 to 96 hours as the upper vortex is now shown to become elongated and be located near the surface low instead of toward the south. By 120 hours the track drifts south while the surface low reaches the west side of the elongating upper vortex. The NAVGEM model is also in agreement with the GFS.


I have slightly raised my peak 5-day development odds to 40% for this disturbance as most models still agree on keeping a well-defined disturbance in the days ahead with an amplified surface trough signature… and the GFS and NAVGEM are now explicit in showing the formation of a subtropical to tropical cyclone. In addition thunderstorm activity has markedly increased with this disturbance as of this morning. I drop odds of development after day 4 as this system will have whirled to a position beneath the center of the upper vortex by then… where supportive upper divergence is lacking. Of note… the idea of an additional nearby disturbance forming to the southeast (with the support of the eastern divergence zone of the southeast US upper vortex once the vortex moves offshore) has been dropped in the models. This simplifies the forecasting of this area of interest for now as we would be dealing with one disturbance instead of multiple ones if the current model trends hold.


With these forecast updates:

**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 southeastern US upper vortex. In addition surface pressures over the northeast US coast will be elevated due to the western convergence zone of the current upper trough in 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 8)… 0% chance of tropical cyclone formation (offshore of southeastern US near 30N-78W)

IOH 48 Hr Outlook (0000Z Oct 9)… 20% chance of tropical cyclone formation (offshore of southeastern US near 30.5N-77.5W)

IOH 72 Hr Outlook (0000Z Oct 10)… 30% chance of tropical cyclone formation (offshore of southeastern US near 32.5N-75W)

IOH 96 Hr Outlook (0000Z Oct 11)… 40% chance of tropical cyclone formation (offshore of southeastern US near 32.5N-76W)

IOH 120 Hr Outlook (0000Z Oct 12)… 25% 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 tropical low southeast of Jamaica by 156 hours

**For area of interest #2...surface low weakens to a trough that settles on Carolina coast by 48 hours… subsequently stalls just offshore through 120 hours


1200Z ECMWF Model Run...

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

**For area of interest #2... surface low weakens to a trough that settles on Carolina coast by 48 hours… shifts offshore while strengthening into an elongated low pressure by 96 hours… northern part of elongated low shifts north to waters offshore of New Jersey by 120 hours


1800Z GFS Model Run...

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

**For area of interest #2... drifts slowly northeast parallel to southeast US coast thru 75 hours… north part of circulation evolves into a subtropical to tropical cyclone at 90 hours just offshore of the North Carolina Outer Banks… drifts southwest while weakening to a remnant low by 120 hours

**South part of circulation associated with area of interest #2 swings east-northeast after 90 hours initially in a fujiwhara interaction with the remainder of area of interest #2… becomes possible subtropical to tropical cyclone near 38N-65.5W by 168 hours

**Tropical wave currently at 27W longitude develops into tropical cyclone near 25N-65W by 168 hours


1800Z NAVGEM Model Run...

**For area of interest #1...Development suggested east of the Bahamas near 21.2N-69W at 150 hours

**For Area of Interest #2... drifts slowly northeast parallel to southeast US coast thru 48 hours… subtropical to tropical cyclone formation suggested near 33N-74W at 78 hours… drifts southwest through 120 hours

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