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 #125

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


...FRIDAY OCTOBER 1 2021 1:20 AM EDT...

Satellite image as of 0110Z. 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 (Sept 30):

GFS Model Upper-Level Winds 1800Z (Sept 30):

As we roll into the month of October…. the Atlantic tropics remain busy with multiple systems to watch:

**See Hurricane Sam and Tropical Storm Victor sections below for an update on the two currently active tropical cyclones in the Atlantic basin.

**See area of interest #1 and #2 sections below for an update on low pressure areas west and east of Victor.

**See area of Interest #3 section below for the potential for heavy rains and possible formation of a subtropical to tropical disturbance over the Dominican Republic and the Bahamas by early next week.


MAJOR HURRICANE SAM...Sam has remarkably kept intense category 4 strength strength with 145 mph maximum sustained winds in the last 24 hours despite nearing the large upper trough to the west. The intensity of the thunderstorms around the eye on colorized infrared satellite has not been very intense as typically seen in such a strong hurricane… and southerly shear generated by the east side of the upper trough is keeping the ring of thunderstorms around the eye lopsided toward the north. It is possible the strength of the hurricane is being maintained by the jet of stronger upper southerly winds to the north which could very well be enhancing the northern outflow of the hurricane. In addition no new eye wall replacement cycles have occurred which would have otherwise disrupted the hurricane.


In lieu of the above notes… my intensity forecast for Sam is nudged upward but still calls for weakening as Sam moves directly into the stronger jet of southerly upper winds which would allow these winds to more strongly shear the hurricane instead of aiding it. However the weakening rate will tend to be gradual as the east side of the aforementioned upper trough is divergent as we always see with a mid-latitude upper trough which will aid Sam. Sam is also likely to grow in size during this weakening phase due to the size of the upper divergence region which will lower surface pressures over a wide area. Weakening after 24 hours will also be facilitated by the marginally warm 26 deg C waters east of Bermuda...which are cooler than the 28 deg C water Sam is currently over.


Regarding track... a classic north then northeast turn into the surface ridge weakness induced by the eastern divergence zone of the upper trough is expected. Sam has remained on track and so my updated forecast track is the same through 72 hours. The track by 96 hours depicts a faster forward speed to the northeast as all models are now in agreement on the upper trough being amplified enough to scoop Sam into the high latitudes instead of leaving Sam behind. I expect transition into a hurricane-force non-tropical frontal cyclone supported by the divergence zone of the upper trough by 96 hours as Sam will be over waters below 26 deg C by that time on the current forecast track.


Given these forecast updates:

**The Lesser Antilles… Bermuda… the mid-Atlantic US coast...northeast US coast...and Atlantic Canada coasts will see coastal sea swells generated by Hurricane Sam

**Sea swells will be particularly vigorous for Bermuda by 24 hours… and be vigorous for coastal Newfoundland by early next week… as Sam passes nearby.

**A tropical storm watch then warning has been raised for Bermuda in the last 24 hours in case Sam grows large enough in size to produce gusty winds over the island with its west edge by 24 hours.

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

0 Hr Position (0000Z Oct 1)... 145 mph maximum sustained wind major hurricane centered at 24.3N-61W

IOH 24 Hr Forecast (0000Z Oct 2)… 115 mph maximum sustained wind major hurricane centered east-southeast of Bermuda at 30N-61.5W

IOH 48 Hr Forecast (0000Z Oct 3)… 100 mph maximum sustained wind hurricane centered at 35N-59W

IOH 72 Hr Forecast (0000Z Oct 4)… 85 mph maximum sustained wind hurricane centered at 37.5N-55W

IOH 96 Hr Forecast (0000Z Oct 5)… 80 mph maximum sustained wind remnant frontal cyclone centered at 40N-45W


TROPICAL STORM VICTOR... Although Victor has maintained a large field of cloud bands… for a time on Thursday the intensity of thunderstorms tended to be on the low side on colorized infrared satellite… perhaps the negative effects of the nearby dry Saharan air layer to the north. This kept Victor’s strengthening rate slow. Tonight… a core of stronger thunderstorms has developed which has allowed Victor to strengthen more quickly to 60 mph max sustained winds.


The tropical low pressure to the west… area of interest #2… has dissipated. Despite this and the ongoing presence of a strong eastern Atlantic surface ridge… Victor has maintained a north angle in track due to the southerly upper flow ahead of the central Atlantic upper vortex which the tropical storm has been able to couple with as it has developed into a stronger/taller system. In fact the north angle in track is slightly larger than my previous forecast… and so my updated one is adjusted accordingly. By day 4… the surface ridge weakness induced by Hurricane Sam will also contribute to the north angle of Victor’s track…and should result in a complete turn to the north by day 5.


Regarding intensity… due to the delay Victor has had in strengthening my peak intensity forecast is lowered a notch but still calls for Victor to become a category 1 hurricane. Weakening is shown at 48 and 72 hours as Victor moves into southerly upper winds on the east side of the central Atlantic upper vortex which will shear this system. The GFS has trended slower with the northeastward retreat of the upper vortex in the flow ahead of the upper trough to approach from the western Atlantic… which could result in continued shear by 96 hours and therefore I no longer show re-strengthening in that timeframe. As Victor moves into the south side of the approaching upper trough the shear will only worsen by 120 hours and so this is when I project Victor to weaken to a remnant low.

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

0 Hr Position (0000Z Oct 1)… 60 mph maximum sustained wind tropical storm centered at 10.6N-30.7W

IOH 24 Hr Forecast (0000Z Oct 2)… 85 mph maximum sustained wind hurricane centered at 12N-34W

IOH 48 Hr Forecast (0000Z Oct 3)… 75 mph maximum sustained wind hurricane centered at 15N-37.5N

IOH 72 Hr Forecast (0000Z Oct 4)… 50 mph maximum sustained wind tropical storm centered at 19.5N-40W

IOH 96 Hr Forecast (0000Z Oct 5)… 45 mph maximum sustained wind tropical storm centered at 24N-43W

IOH 120 Hr Forecast (0000Z Oct 6)… Remnant low centered at 29N-43W


AREA OF INTEREST #1… The tropical wave of low pressure currently over western Africa appears to be nearing the coast while located near 7.5N-12.5W based on a recent increase in thunderstorms after it lost thunderstorm activity for much of Thursday. Although none of the models develop this tropical wave as it moves offshore... the tropical upper ridging with low shear and upper outflow that has aided in the development of Victor will remain in place in the days ahead. Therefore I am maintaining this tropical wave as an area of interest for tropical development. Because the wave has not made much progress in developing… I have lowered short-term development odds but keep 15% odds of development by day 5 as the wave has been maintaining thunderstorms tonight.


What will be the warm sector of Sam’s remnant frontal cyclone will amplify upper ridging in the northeast Atlantic… which will cause the upper vorticity currently in the east Atlantic to shift south by days 4 and 5. This upper vorticity should prevent any waves behind this wave (to the east) from developing. Meanwhile this wave should just escape the influence of this upper vorticity… and it will be interesting to see if split flow upper divergence between the west side of the upper vorticity and south side of the tropical upper ridging manifests itself over this wave. Such a split flow upper divergence regime could aid in this wave’s development.

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

IOH 24 Hr Outlook (0000Z Oct 2)… 0% chance of tropical cyclone formation (offshore of Africa near 8N-17.5W)

IOH 48 Hr Outlook (0000Z Oct 3)… 5% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-22W)

IOH 72 Hr Outlook (0000Z Oct 4)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 10.5N-26W)

IOH 96 Hr Outlook (0000Z Oct 5)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-30W)

IOH 120 Hr Outlook (0000Z Oct 6)… 15% chance of tropical cyclone formation (eastern tropical Atlantic near 11.5N-35W)


AREA OF INTEREST #2… The tropical low pressure area just west of Tropical Storm Victor has dissipated as Victor has dominated the surface inflow in the region… which prevented the tropical low from having its own surface inflow needed to build thunderstorms and survive. This is my final statement on this area of interest on this blog.


AREA OF INTEREST #3…The north end of a currently inactive tropical wave of low pressure currently passing 50W longitude has potential to interact with amplified upper vorticity to be left behind by the current west Atlantic upper trough and also the upper trough currently approaching from western North America by early next week. This could result in the development of a subtropical to tropical disturbance in the vicinity of the Dominican Republic and Bahamas by early next week… especially if the upper vorticity is amplified enough to keep shear levels lower and upper divergence elevated. The GFS has been suggesting the formation of this disturbance over the last few days… and the NAVGEM has recently joined. As a result I have added this tropical wave as an area of interest to watch in the days ahead.

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

IOH 24 Hr Outlook (0000Z Oct 2)… 0% chance of tropical cyclone formation (east of the Lesser Antilles near 13N-55W)

IOH 48 Hr Outlook (0000Z Oct 3)…0% chance of tropical cyclone formation (just east of the Lesser Antilles near 15N-60W)

IOH 72 Hr Outlook (0000Z Oct 4)…0% chance of tropical cyclone formation (just southeast of Puerto Rico near 16.5N-65W)

IOH 96 Hr Outlook (0000Z Oct 5)…0% chance of tropical cyclone formation (Dominican Republic near 19N-70.5W)

IOH 120 Hr Outlook (0000Z Oct 6)…10% chance of tropical cyclone formation (southeastern Bahamas near 22N-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 Hurricane Sam...passes east of Bermuda and located east of Newfoundland as an intense remnant frontal cyclone by 120 hours

**For Tropical Storm Victor... located at 29N-41.5W at 120 hours

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

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

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


1200Z ECMWF Model Run...Not available at above-mentioned source


1800Z GFS Model Run...

**For Hurricane Sam... passes east of Bermuda and located east of Newfoundland as an intense remnant frontal cyclone by 120 hours

**For Tropical Storm Victor... located at 29.5N-40.5W at 120 hours as a remnant low

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

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

**For area of interest #3… north end of wave currently at 50W longitude evolves into amplified surface trough over western Bahamas by 120 hours


1800Z NAVGEM Model Run...

**For Hurricane Sam... passes east of Bermuda and located east of Newfoundland as an intense remnant frontal cyclone by 120 hours

**For Tropical Storm Victor... located at 26N-45W at 120 hours

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

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

**For Area of Interest #3…north end of wave currently at 50W longitude evolves into amplified surface trough north of the Bahamas by 120 hours

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