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 #122 (Weekend Edition)

*******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 SEPTEMBER 27 2021 1:45 AM EDT...

Satellite image from 0850Z on Sunday. 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 Saturday September 25. For features that have moved over longer distances in the last day or so… arrows show the direction of travel and an X enclosed by a circle indicates the system’s current position:

GFS Model Upper-Level Winds 0000Z Sunday September 26. For features that have moved over longer distances in the last day or so… arrows show the direction of travel and an X enclosed by a circle indicates the system’s current position:

The Atlantic tropics remain very active with multiple systems to watch:

**See Hurricane Sam section below for an update on the only currently active storm in the Atlantic basin.

**See area of interest #1… #4… and #5 sections below for info on a train of tropical waves of low pressure spanning from the eastern Atlantic and into Africa being monitored for possible development in the days ahead.

**See area of interest #2 section below for an update on the decaying cold front in the Gulf of Mexico.

**See area of Interest #3 section below for an update on the remnants of Peter.


Elsewhere…the upper trough over eastern North America may amplify enough to leave behind an upper vortex that induces potential subtropical cyclone development just offshore of the US east coast over the next few days. The models however have not been consistent on exactly how amplified this upper trough will get… therefore no new area of Interest offshore of the eastern US is being added at the present time.


MAJOR HURRICANE SAM…Satellite image of Hurricane Sam taken at 1810Z on Sunday while reaching a peak of 150 mph maximum sustained winds:

As Sam continues to traverse west-northwest across the central tropical Atlantic under a vast area of tropical upper ridging with low shear and upper outflow… and also over warm 28 deg C waters… the hurricane for much of Sunday maintained a small but very clear eye and the core of thunderstorms became less ragged and almost a perfect circle. The improvement in the structure indicated Sam reached powerful category 4 strength with 150 mph max sustained winds as of 5 PM EDT. As of 11 PM EDT… aircraft recon found Sam started to decline to 145 mph max sustained winds while the hurricane has started an eye wall replacement cycle… and the loss of the eye in the most recent satellite photos confirms this. The intensity forecast below therefore shows a low-end but still powerful category 4 at 24 hours. With the favorable conditions remaining… and the outer bands around the core no longer struggling with dry Saharan air… I now assume Sam will quickly recover from the eye wall replacement and vault to high-end category 4… knocking on the door of category 5… from 48 to 72 hours. Models still agree that the current central eastern North America upper trough will enter the western Atlantic in a more amplified state and thus have a harder time lifting out… and the southeast side of the large upper trough may start to shear or block the western outflow of the hurricane by 96+ hours. As a result… I forecast Sam to weaken at that time. Just after 120 hours… Sam will be tracking into waters in Bermuda’s proximity that are currently marginally warm for tropical development… and this could be another factor that weakens Sam during that time.


Regarding track… Sam’s west-northwest speed has slowed down with the eye undergoing wobbles in various directions due to the expansive ridge weakness to the northwest caused by Peter’s and Teresa’s remnants. The slow down is enough for me to adjust the track by taking the 0000Z forecast points from previous post #121 and assuming Sam will arrive to those points 12 to 18 later than I previously thought. Although the current eastern Canada frontal low is expected to sweep up ex-Teresa to the north…the ridge weakness is expected to linger as the current upper trough over eastern North America amplifies on its way into the western Atlantic… with the eastern divergence zone of the upper trough maintaining the weakness while generating another frontal low offshore of the northeast US . Thus the slow northwest track into the weakness is likely to continue through day 4. As the aforementioned frontal low offshore of the northeastern US begins lifting to the north… this will allow the steering Atlantic surface ridging to recover which should allow Sam to move faster by day 5 as it bends north in track while chasing the frontal low.


Given these forecast updates:

**The Lesser Antilles are quiet likely to see coastal sea swells generated by Sam this week.

**Bermuda should monitor the progress of Sam for possible impacts after day 5. Although the forecast projects longer-range weakening… Sam could still pass over or near Bermuda as a still dangerous category 2 or 3 hurricane.

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

0 Hr Position (1800Z Sep 26)… 150 mph maximum sustained wind major hurricane centered at 14.2N-50.5W

IOH 24 Hr Forecast (1800Z Sep 27)… 130 mph maximum sustained wind major hurricane centered at 16.5N-52.5W

IOH 48 Hr Forecast (1800Z Sep 28)… 155 mph maximum sustained wind major hurricane centered at 18.5N-55.2W

IOH 72 Hr Forecast (1800Z Sep 29)… 155 mph maximum sustained wind major hurricane centered northeast of the Lesser Antilles at 20.5N-57.7W

IOH 96 Hr Forecast (1800Z Sep 30)… 130 mph maximum sustained wind major hurricane centered at 22.5N-61.5W

IOH 120 Hr Forecast (1800Z Oct 1)… 125 mph maximum sustained wind major hurricane centered south-southeast of Bermuda at 28.5N-63.5N


AREA OF INTEREST #1… The tropical wave of low pressure over Western Africa has seen an increase in organizing thunderstorms as it makes its way into the eastern tropical Atlantic from its current position at 8N-12W. Therefore I agree with the raised 70% odds of development by day 5 shown in the NHC outlook product as of this writing (update as of 2 AM EDT… the NHC now projects 80% odds of development by day 5 for this wave). Note that as this wave exits Africa the eastern Atlantic surface ridge is forecast to strengthen due to the western convergence zone of the current northeast Atlantic upper trough… which should keep this wave’s track toward the south. Models also agree on some kind of development offshore of Africa… but a few days from now. This seems puzzling considering this wave is already on its way into the eastern tropical Atlantic and should already be well offshore of Africa in a few days… especially while steered by the strong eastern Atlantic surface ridge. This means either one of the two scenarios followed scenarios are possible (1) this wave becomes entangled in a fujiwhara-type interaction with the adjacent wave to the east and as a result slows down its west speed while offshore of Africa… (2) the adjacent wave to the east is the one that develops just offshore of Africa while this wave doesn’t. Given that this wave is already increasing in organization and the other wave to the east is not… the forecast below is based on the first scenario. The forward speed in the forecast below is slowed down after 48 hours and position kept toward the south… consistent with a fujiwhara-type interaction with the adjacent wave to the east. For more info on the wave to the east… see area of Interest #4 section.

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

IOH 24 Hr Outlook (1800Z Sep 27)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 8N-17W)

IOH 48 Hr Outlook (1800Z Sep 28)… 50% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-22W)

IOH 72 Hr Outlook (1800Z Sep 29)… 70% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-25W)

IOH 96 Hr Outlook (1800Z Sep 30)… 70% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-28W)

IOH 120 Hr Outlook (1800Z Oct 1)… 70% chance of tropical cyclone formation (eastern tropical Atlantic near 8.5N-31W)


AREA OF INTEREST #2… The tail end of the surface frontal boundary in the southwest Gulf of Mexico has not developed any concentrated shower or thunderstorm activity… this is my final statement on this area of Interest on this blog as the surface trough left behind by the decaying frontal boundary will be pushed into Mexico by the southeast US surface ridge and will not have time to develop even if it starts producing thunderstorms later on.


AREA OF INTEREST #3 (REMNANTS OF PETER)… The upper vortex that was north of Tropical Storm Peter’s remnants… the same vortex that generated Teresa a couple of days ago… has dissipated which has caused the shear over Peter’s remnants to relax. Ex-Peter has been moving northeast into the central Atlantic in the flow ahead of ex-Teresa… and will continue to move northeast in the flow ahead of the current east Canada frontal low and another frontal low to form offshore of the northeast US as both lows are supported by the large eastern North America upper trough. Although this trough is shearing Ex-Peter… the shear is not as strong as before as the upper trough is a good distance away… and Ex-Peter has gotten better organized while showing a spin near 30N-60W with thunderstorm over and just east of the spin. And given that the usually more reliable ECMWF and GFS suggesting development… I already forecast a high 70% chance that Peter regains tropical cyclone status. By 72 hours the track takes Peter to cooler waters below 26 deg C…. and this is when I drop developments odds to 0%.

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

IOH 24 Hr Outlook (1800Z Sep 27)…70% chance of tropical cyclone formation (central Atlantic near near 32.5N-58W)

IOH 48 Hr Outlook (1800Z Sep 28)… 70% chance of tropical cyclone formation (central Atlantic near near 34N-56W)

IOH 72 Hr Outlook (1800Z Sep 29)… 0% chance of tropical cyclone formation (northwest Atlantic near near 38N-54.5W)


AREA OF INTEREST #4… A tropical wave of low pressure over central Africa appears to be in progress near 8N-0W where a cluster of thunderstorms is present. The wave was also defined on Saturday morning with cyclonic curvature in the low clouds. Based on the latest model runs… this wave is likely to undergo an interaction with area of interest #1 to the west. I have added this wave as an area of interest until it is definite which wave will become the dominant. The forecast below assumes this wave will have a slightly more north angle in track than area of interest #1 due to the interaction… however given that area of interest #1 is more organized I assume that it will become the dominant feature and therefore assign low 20% odds of development to this wave.

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

IOH 24 Hr Outlook (1800Z Sep 27)…0% chance of tropical cyclone formation (Western Africa near 8.5N-5W)

IOH 48 Hr Outlook (1800Z Sep 28)…0% chance of tropical cyclone formation (Western Africa near 8.5N-10W)

IOH 72 Hr Outlook (1800Z Sep 29)…0% chance of tropical cyclone formation (west coast of Africa near 9N-15W)

IOH 96 Hr Outlook (1800Z Sep 30)…10% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-20W)

IOH 120 Hr Outlook (1800Z Oct 1)…20% chance of tropical cyclone formation (south-southwest of the Republic of Cabo Verde Islands near 12N-25W)


AREA OF INTEREST #5… Between area of interest #1 and Hurricane Sam… a broad area of showers and thunderstorms is occurring with the support of two surface tropical waves of low pressure and upper outflow supplied by tropical upper ridging in the region. This activity is in the vicinity of 9N-28W and points west… albeit this activity appears to be organizing toward the west in the most recent satellite frames. Because computer models have recently been picking up on this area developing… the NHC has also called an area of interest here. This system will be steered west initially by the strengthening east Atlantic surface ridge mentioned in the area of interest #1 section. By 72 and 96 hours… the upper vortex currently in the eastern Atlantic is forecast to be pushed toward this disturbance by the current northwest Atlantic upper ridge. Assuming this disturbance becomes strong/tall enough to couple with upper winds… my forecast track below shows some north bend due to the upper vortex in the 72 to 96 hour timeframe. The approach of the upper vortex likely means wind shear is in the future for this disturbance… thus my peak odds of development are kept at 50% which is lower than the NHC’s outlook as of this writing. I lower odds of development to 20% by day 5 as the track of this system takes it into the southwest side of the vortex where shear is more hostile. The forecast track assumes the shear will weaken this system to a shallow state that decouples from the upper flow… which is why the forecast track angles more west in the surface flow on the south side of the eastern Atlantic surface ridge by 120 hours as well.

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

IOH 24 Hr Outlook (1800Z Sep 27)…0% chance of tropical cyclone formation (eastern tropical Atlantic near near 9N-33W)

IOH 48 Hr Outlook (1800Z Sep 28)…20% chance of tropical cyclone formation (eastern tropical Atlantic near near 9N-38W)

IOH 72 Hr Outlook (1800Z Sep 29)…50% chance of tropical cyclone formation (central tropical Atlantic near near 9.5N-42W)

IOH 96 Hr Outlook (1800Z Sep 30)…50% chance of tropical cyclone formation (central tropical Atlantic near near 10.5N-45W)

IOH 120 Hr Outlook (1800Z Oct 1)…20% chance of tropical cyclone formation (central tropical Atlantic near near 11N-48W)

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