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 #80 (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.**********


...SUNDAY AUGUST 15 2021 12:09 AM EDT...

Satellite image as of 2350Z. 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 1200Z:



GFS Model Upper-Level Winds 1200Z:



See Grace section below for an update on the tropical storm as it marches into the northern Caribbean Islands. For the remnants of Fred… the above surface and upper air charts marked this system as not in the NHC outlook product as they were created before the 8 PM EDT introduction of this system into the outlook. See Fred (area of interest #1) section below for an update on this system.


Elsewhere… a new tropical disturbance has developed north of Bermuda and could affect the island over the next 72 hours… see area of interest #2 section for details. Note the above surface and upper air charts marked this system as not in the NHC outlook product as they were created before the 8 PM EDT introduction of this system into the outlook.


In addition...satellite imagery suggests the recent round of tropical waves emerging from Africa are being suppressed by dry Saharan air as they enter the Atlantic. None of the models forecast development from these waves at this time. However note that one of these waves has produced a small rotating thunderstorm cluster southeast of the Republic of Cabo Verde Islands… but has a chance to succumb soon to the large amount of dry Saharan air to the north and west.


TROPICAL STORM GRACE… The NHC has recently been adjusting the north-south position of Grace with each advisory… probably indicating that the circulation of Grace is more oblong rather than circular. Aircraft recon data also found Grace was not quiet as strong as previously estimated… with winds being downgraded to 40 mph max sustained based on the data. The tropical storm continues its brisk west-northwest march… suggesting Grace is being channeled into strong east-southeasterly flow on the east side of the large tropical wave to the west that used to be tagged as invest 92-L. The forecast track is west-northwest for the next 96 hours while aiming for a surface ridge weakness created by Fred in the eastern Gulf of Mexico and also the lingering tail end of a front over the interior southeast US. By 120 hours once Fred and the front dissipate I bend the track more west. The NHC has adjusted their forecast track more south as the 5 PM center fix was further south… but with the 8 PM fix being again further north I have not made any adjustment.


I have slightly lowered the intensity forecast due to Grace’s recent intensity downgrade… which now aligns my peak intensity forecast (when Grace moves over the Virgin Islands and Puerto Rico) with the NHC’s intensity forecast. After Grace passes through Puerto Rico… I still forecast weakening as the storm begins battle with westerly shear to be generated by the patch of upper vorticity lingering northeast of the Bahamas. By 72+ hours the forecast track will take the center of Grace directly below the axis of upper vorticity…and so I forecast Grace to struggle as a tropical depression whose thunderstorms become displaced east of the upper vorticity axis. Of note some recent model runs have a different idea… keeping Grace as a bonafide tropical cyclone as it moves across the Bahamas and toward south Florida while having Grace’s center reform or stay a little further east while more coupled with the thunderstorms…. resulting in a compact tropical cyclone with a small area of warm core upper anticyclonic outflow (caused by thunderstorm latent heat release) that battles the axis of upper vorticity. Because of this solution… I have now kept Grace as a tropical depression instead of a remnant low by days 3 to 5… but the longer term intensity of Grace is now in question… does it slide under the axis of upper vorticity and weaken or does it stay coupled with the thunderstorms and fight off the upper vorticity? Also if indeed Grace tracks more south…it could end up over the Dominican Republic mountainous terrain which would negatively affect Grace’s strength.


** Interests in the northern Lesser Antilles should have finished preparing for Grace… heavy rains and the potential for gusty winds have already overspread the area.

**Interests in the Virgin Islands and Puerto Rico should have also finished preparing for Grace… weather conditions will deteriorate by morning. Wind damage and coastal

sea swell potential is higher here as Grace could strengthen to a more high-end tropical storm.

** Interests in the Dominican Republic are more likely to see tropical storm conditions (some gusty winds and coastal sea swells) by Monday on the north coast… now is the time to prepare. Heavy rainfall with flash flooding could become a concern for all of the Dominican Republic.

**Interests across the Bahamas and south Florida and the Florida Keys should be aware of Grace as the intensity forecast is uncertain by days 4 and 5.

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

0 Hr Position (0000Z Aug 15)…40 mph maximum sustained wind tropical storm centered over the northern Lesser Antilles at 16.6N-61.5W

IOH 24 Hr Forecast (0000Z Aug 16)…60 mph maximum sustained wind tropical storm centered over Puerto Rico at 18.2N-66.2W

IOH 48 Hr Forecast (0000Z Aug 17)… 45 mph maximum sustained wind tropical storm centered over the north coast of the Dominican Republic at 19.8N-70.2W

IOH 72 Hr Forecast (0000Z Aug 18)… 30 mph maximum sustained wind tropical depression centered over the eastern Bahamas at 21.6N-73.6W

IOH 96 Hr Forecast (0000Z Aug 19)… 30 mph maximum sustained wind tropical depression centered over the western Bahamas at 23.2N-76.7W

IOH 120 Hr Forecast (0000Z Aug 20)… 30 mph maximum sustained wind tropical depression centered south of Florida and southeast of the Keys at 24N-80.2W


AREA OF INTEREST #1 (REMNANTS OF FRED)... The remnant low pressure trough of Fred has established an east-west oblong circulation in the southeastern Gulf of Mexico. However the circulation is too elongated to re-declare Fred a tropical depression. I currently fix the center of Fred’s remnants to be on the east extent of the circulation… toward 24N-83W where the thunderstorms have been gradually re-organizing into rotating banding features. This is also where the upper divergence zone of the lingering eastern Gulf of Mexico upper vorticity is located… with the divergence helping to keep surface pressures lower… another reason why I forecast that the east side of the oblong circulation will become the dominant and re-generate into a tropical cyclone. With this forecast philosophy… Fred remains aligned with my previous track and so my updated one remains the same. Fred is expected to track northwest toward the surface ridge weakness associated with the lingering front over the eastern US being supported by the divergence zone of cut-off upper vorticity over the north-Central US.


Due to Fred remaining a remnant low for as long as it has… I agree with the downward adjusted NHC intensity forecast which has Fred making landfall on the northeast US Gulf coast region as a 50 mph maximum sustained wind tropical storm. Fred is expected to make a comeback and regain Storm strength due to an expected reduction in wind shear as the lingering upper vorticity in the eastern Gulf dissipates. This seems plausible as Fred despite its struggles has had a history of strong thunderstorm activity whose latent heat release would help weaken the cool core upper vorticity. Of note… rapid re-strengthening before landfall is not anticipated as the aforementioned north-Central US upper vorticity may produce light shear with some upper southwesterly flow over the forecast landfall region.


** For western Cuba… additional heavy rainfall is possible overnight.

**Interests across the west Florida peninsula coast are not likely to see tropical storm conditions if the current forecast track holds… however coastal sea swells and rip currents are still possible this weekend.

**Interests across the eastern Florida panhandle and southwest Georgia are less likely to see tropical storm force winds if the latest forecast holds. However heavy rains and coastal sea swells are likely by late this weekend and into Monday.

**I recommend interests in the western Florida panhandle and southern Alabama gather storm prep supplies… and begin preparing should a tropical storm watch or warning be issued over the next several hours. Fred is forecast to be a 50 mph max sustained wind tropical storm in the region late this weekend and into Monday.

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

0 Hr Position (0000Z Aug 15)… Low pressure centered offshore of northwestern Cuba at 24N-83W

IOH 24 Hr Forecast (0000Z Aug 16)… 35 mph maximum sustained wind tropical depression centered offshore of western Florida at 27N-85W

IOH 48 Hr Forecast (0000Z Aug 17)… 50 mph maximum sustained wind tropical storm centered over the northeastern Gulf of Mexico at 28.5N-87.5W

IOH 72 Hr Forecast (0000Z Aug 18)… Remnant low centered over the southern Mississippi/Alabama border at 32N-88.5W


AREA OF INTEREST #2…Hovmoller diagrams of the mid-latitudes of the Atlantic (https://www.nhc.noaa.gov/tafb_latest/gehov3latest.gif ) showed that on 1140Z Friday August 13 a thunderstorm complex developed north of Bermuda. A look at the upper air pattern in this region has shown split flow upper divergence between the current western Atlantic upper ridging to the west and north and upper vorticity to the south and east in the western and central Atlantic. This upper air pattern has resulted in a new tropical disturbance with intermittent rotating showers and thunderstorms. The center of low pressure spin is located north-northeast of Bermuda near 35N-64W with the thunderstorm activity a bit displaced to the southwest of the center due to light northerly shear induced by the east edge of the upper ridging.


The forecast track of this system is expected to be a slow anti-cyclonic loop around the island of Bermuda while pushed around by the eastern US surface ridge as that ridge moves offshore into the northwest Atlantic. The slow speed in the track is due to the drag of the Atlantic surface ridge to the east which will try to oppose the force applied by the nearing eastern US surface ridge. Due to the healthy surface spin this disturbance has had… I assign a peak of 50% development odds… higher than the NHC outlook as of this writing. My short-term odds are a bit lower due to the current northerly shear… and are raised to the 50% once the upper ridging is forecast to shift east over this disturbance which may help to lower the shear.


Interests in Bermuda should be aware that this system will move slowly over or near the island over the next 72 hours… with a possible prolonged period of heavy rains and maybe gusty winds should further development in fact occur.

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

IOH 24 Hr Outlook (0000Z Aug 16)…40% chance of tropical cyclone formation (just northeast of Bermuda near 33N-63W)

IOH 48 Hr Outlook (0000Z Aug 17)…40% chance of tropical cyclone formation (just southeast of Bermuda near 32N-64.5W)

IOH 72 Hr Outlook (0000Z Aug 18)…50% chance of tropical cyclone formation (just southwest of Bermuda near 32N-66W)

IOH 96 Hr Outlook (0000Z Aug 19)…50% chance of tropical cyclone formation (west-southwest of Bermuda near 32N-67.5W)

IOH 120 Hr Outlook (0000Z Aug 19)…50% chance of tropical cyclone formation (western Atlantic near 34N-69W)

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