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 #97

*******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 SEPTEMBER 1 2021 1:22 AM EDT...


See Tropical Depression Kate and Tropical Depression Twelve (Area of Interest #1) sections below for an update on all currently active Atlantic tropical cyclones. See area of interest #2 to #4 sections below for all areas being monitored for tropical cyclone formation in the Atlantic


Elsewhere… another tropical wave of low pressure… located east of Tropical Depression Twelve and over western Africa near the Atlantic coast… has lost thunderstorm activity except in a western squall that is becoming entangled with the depression. This suggests the depression is dominating the surface inflow in the region which will make this tropical wave struggle to have its own inflow to collect moisture and make thunderstorms. In addition the depression will likely push this wave on a more north angle in track toward cooler water and dry Saharan air as the wave moves offshore… therefore development of this wave is not expected.


TROPICAL DEPRESSION KATE... Kate has weakened to a tropical depression over the last 24 hours while largely absent of thunderstorm activity thanks to westerly shear and upper outflow suppression induced beneath the string of upper vorticity in the region. By late afternoon and into tonight… the upper vorticity string has undergone some consolidation off to the west as the GFS has been forecasting… allowing for Kate’s eastern side to pop up some scattered thunderstorms where the upper outflow suppression has lessened. This is how I theorized Kate would survive it’s interaction with the upper vorticity and go on to take advantage of a better low shear and upper outflow environment while moving north into mid-latitude upper ridging. However… the models still agree on dissipating Kate quickly. A closer look at the GFS upper wind forecast suggests a zone of possible suppressive upper convergence between northerlies on the south side of the mid-latitude upper ridging and southerlies on the northeast side of the upper vorticity string that Kate may have to deal with in the next 24 hours… and the 11 PM EDT NHC advisory discussion mentioned upper convergence being a factor in the NHC forecasting Kate to now not survive. My updated forecast has a lower short-term intensity due to the potential upper convergence… then assumes Kate survives and regains tropical storm status in the better upper wind environment further north. I still forecast only slow strengthening to 50 mph max sustained winds as there is no guarantee Kate will survive in the shorter term… which could result in larger error if forecasting bullish strengthening. And If Kate does dissipate in the next 24 hours… will have to evaluate whether or not to keep Kate as an area of interest for tropical re-development.


Forecast track in adjusted south and east due to Kate’s current position. The track bends west thru 48 hours while the Atlantic surface ridge centered to the east intensifies while aided by the western convergence zone of the upper trough in the northwest Atlantic as that trough slides into the northeast Atlantic. By 72+ hours the track bends northeast in the flow between the Atlantic surface ridge and approaching remnants of Ida to the west. Because Ida is forecast to become a large strong remnant cyclone near Atlantic Canada… my forecast assumes Kate will be absorbed by Ida’s remnant cyclone by day 4.

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

0 Hr Position (0000Z Sep 1)… 35 mph maximum sustained wind tropical depression centered at 25N-50.9W

IOH 24 Hr Forecast (0000Z Sep 2)… 35 mph maximum sustained wind tropical depression centered at 28N-52.5W

IOH 48 Hr Forecast (0000Z Sep 3)… 45 mph maximum sustained wind tropical storm centered at 31N-55W

IOH 72 Hr Forecast (0000Z Sep 4)… 50 mph maximum sustained wind tropical storm centered at 35N-54.5W

IOH 96 Hr Forecast (0000Z Sep 5)…Absorbed by Ida’s remnant cyclone while located near 41N-51W


AREA OF INTEREST #1 (RECENTLY UPGRADED TO TROPICAL DEPRESSION TWELVE)… The vigorous tropical wave of low pressure in the far eastern tropical Atlantic has intensified into the twelfth tropical depression of the 2021 Atlantic Hurricane Season. The center of the depression in the afternoon was fixed on the southeast edge of its organizing thunderstorm mass… however I estimated at 0000Z (8 PM EDT) that the center migrated closer to the center of the mass near 11.5N-22W. As of 0300Z (11 PM EDT) the NHC center fix is even closer to the center of the thunderstorm mass… near 12.1N-23.2W. This system will be aided by the low shear and upper outflow of the tropical Atlantic upper ridge… and due to the slightly higher initial strength when compared to my previous intensity forecast I have slightly raised the intensity in the new forecast below. I call for a category 3 hurricane by days 4 and 5… and prefer to wait and see how this system actually starts developing before suggesting any strength higher than this.


For the first 72 hours… I assume a slightly slower track with some north angle from a surface ridge weakness to be induced to the northeast by the divergence zone of a northeast Atlantic upper vortex to be deposited by the upper trough currently in the northwest Altantic as that trough slides east… and to be enforced by the presence of an adjacent tropical wave of low pressure currently located just to the east which will likely pivot northwest on the northeast side of this system. Although I speed up the track by days 4 and 5 as this system escapes the surface ridge weakness… some north angle in track remains possible due to the presence of the upper vorticity currently near Kate which will become re-enforced by the approach of the amplifying upper trough to be associated with Ida’s remnants. The re-enforced upper vorticity will likely play a role in the steering as this system is expected to become a strong/tall hurricane that can be steered by upper flow. On a final note… my Forecast track points are nudged north due to the current center fix of the depression relative to my prior forecast.

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

0 Hr Position (0000Z Sep 1)… 35 mph maximum sustained wind tropical depression centered due south of the Republic of Cabo Verde Islands at 11.5N-22W

IOH 24 Hr Forecast (0000Z Sep 2)… 60 mph maximum sustained wind tropical storm centered southwest of the Republic of Cabo Verde Islands at 12.5N-26W

IOH 48 Hr Forecast (0000Z Sep 3)… 80 mph maximum sustained wind hurricane centered at 13.5N-30.5W

IOH 72 Hr Forecast (0000Z Sep 4)… 95 mph maximum sustained wind hurricane centered at 14.5N-35W

IOH 96 Hr Forecast (0000Z Sep 5)… 115 mph maximum sustained wind major hurricane centered at 15.5N-40W

IOH 120 Hr Forecast (0000Z Sep 6)…125 mph maximum sustained wind major hurricane centered at 16.5N-45W


AREA OF INTEREST #2… The area of low surface pressure offshore of the US northeast coast has lost its identity to the much larger low pressure field of the approaching east Canada frontal low… this is my final statement on this area of interest.


AREA OF INTEREST #3…The tropical wave of

low pressure that a few days ago spawned what is now Tropical Depression Kate is currently inactive while in the eastern Caribbean Sea. The current cool core upper vorticity string in the west Atlantic is forecast to potentially enhance the poleward outflow of the tropical wave as it moves across the Caribbean as the upper vorticity avoids shearing the wave while it weakens from being cut-off from high-latitude cold air… a pattern similar to the genesis of Ida when its seedling wave was in the Caribbean. The only global models that shows this system developing are the CMC and NAVGEM… however they show development in the western Gulf of Mexico and in the longer range beyond day 5. I agree with the NHC in not raising development odds above 20% for the following reasons: (1) the CMC and NAVGEM are usually not the most reliable… (2) the GFS has trended with a slower breakup of the west Atlantic upper vorticity string and keeps some upper vorticity over the west side of this system as it moves across the western Caribbean… (3) the Central

Caribbean activity that was downstream of the tropical wave and previously enhanced by poleward outflow streaming into the west Atlantic upper vorticity string has fizzled out tonight… a factor that has also caused me to lower shorter-term development odds as well. By day 5 when this system reaches the southwest Gulf… I only have 10% odds of development as the GFS shows more Gulf upper vorticity being left behind by the upper trough interacting with Ida’s remnants… more than it did in prior runs.


I forecast this system to stay north of the Central America land mass due to the ridge weakness to be created by Ida’s remnants… but a sharp north turn is not anticipated either due to the eastern US surface ridge forecast to be west of Ida’s remnants and induced by the western convergence zone of the amplified upper trough forecast to support Ida’s remnants. The northwest convergence zone of the upper vorticity to be left behind by this upper trough will likely keep the surface ridge going over the southeast US by day 5… helping to push this system into the western Gulf after crossing the Yucatan peninsula. I have also adjusted the forecast positions in the Outlook below to the west due to the current position of the wave in the NHC TAFB surface analysis relative to the previous forecast. Based on the latest forecast track… interests in Belize and the Mexican Yucatan provinces should be aware of this system as it is expected to approach by this weekend.

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

IOH 24 Hr Outlook (0000Z Sep 2)…0% chance of tropical cyclone formation (central Caribbean Sea near 13.5N-75W)

IOH 48 Hr Outlook (0000Z Sep 3)…0% chance of tropical cyclone formation (central Caribbean Sea near 14.5N-80W)

IOH 72 Hr Outlook (0000Z Sep 4)…20% chance of tropical cyclone formation (western Caribbean Sea near 16.2-85W)

IOH 96 Hr Outlook (0000Z Sep 5)… 0% chance of tropical cyclone formation (Yucatan peninsula near 19N-89.5W)

IOH 120 Hr Outlook (0000Z Sep 6)… 10% chance of tropical cyclone formation (southwestern Gulf of Mexico near 21N-92.5W)


AREA OF INTEREST #4… Ida’s remnant low and it’s amplifying supporting upper trough is forecast to drive a cold front into the west Atlantic in the coming days. Will watch to see if the tail end of the front evolves into a subtropical or tropical disturbance to be enhanced by reducing shear and increased upper divergence on the east side of amplifying upper vorticity to be cut-off from the remainder of the upper trough. This cut-off event is expected due to adjacent amplification of warm upper ridging over the US to be caused by the warm sector of the next frontal system to swing across North America. Models runs over the last couple of days have varied on how to handle this situation… with the GFS showing in some runs a compact tropical system developing along the decaying front which moves quickly northeast out to sea while chasing the surface ridge weakness to be induced by Ida’s presence… while other model runs showed a stalled system further southwest that does not move immediately out to sea. My forecast below is a compromise between these two ideas and uses 32.5N-72.5W as a forecast point for days 3 to 5. I also raise development odds above 0% by days 4 and 5… which is the most likely timeframe for the cut-off amplified upper vorticity as suggested in the modeling. However my peak odds are only 10% as the models are not consistent in showing development at this time.

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

IOH 24 Hr Outlook (0000Z Sep 2)…0% chance of tropical cyclone formation (southern tip of South Carolina near 32.5N-81W)

IOH 48 Hr Outlook (0000Z Sep 3)…0% chance of tropical cyclone formation (offshore of the southeastern US near 32.5N-77.5W)

IOH 72 Hr Outlook (0000Z Sep 4)…0% chance of tropical cyclone formation (western Atlantic near 32.5N-72.5W)

IOH 96 Hr Outlook (0000Z Sep 5)…10% chance of tropical cyclone formation (western Atlantic near 32.5N-72.5W)

IOH 120 Hr Outlook (0000Z Sep 6)… 10% chance of tropical cyclone formation (western Atlantic near 32.5N-72.5W)


...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 Tropical Depression Kate… weakens to a remnant trough near 30N-54W by 36 hours

**For area of interest #1… located at 17.5N-45W at 120 hours as a hurricane

**For area of interest #2… no development shown

**For area of interest #3… consolidates into a tropical low in the Bay of Campeche near 21.5N-94.5W at 126 hours

**For area of interest #4… no development shown


1200Z ECMWF Model Run…

**For Tropical Depression Kate… entire circulation dissipates in 24 hours

**For area of interest #1… located at 17.5N-46.5W at 120 hours as a hurricane

**For area of interest #2… no development shown

**For area of interest #3… no development shown

**For area of interest #4… no development shown


1200Z GFS Model Run...

**For Tropical Depression Kate… weakens to a remnant trough near 27.5N-52.5W at 27 hours

**For area of interest #1… located at 19N-45W at 120 hours as a hurricane

**For area of interest #2… no development shown

**For area of interest #3… no development shown

**For area of interest #4… compact tropical circulation develops along front left behind by Ida’s remnant near 35N-70W at 81 hours… reaches 35.5N-66W at 120 hours


1200Z NAVGEM Model Run...

**For Tropical Depression Kate… weakens to a remnant trough near 26.5N-54.5W at 30 hours

**For area of interest #1… located at 17.5N-46W at 120 hours as a hurricane

**For area of interest #2… no development shown

**For area of interest #3… consolidates into a tropical low in the just northwest of the Yucatan peninsula at 138 hours

**For area of interest #4… no development shown

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