MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #85
*******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 AUGUST 20 2021 12:27 AM EDT...
See Grace and Henri sections below for an update on all active tropical cyclones in the Atlantic basin. Both are forecast to bring impacts to land areas in the coming days. See area of interest #1 section below for an update on the tropical wave in the east Atlantic. Note just after the above chart was created that the tropical wave was introduced into the NHC Outlook… which is why it’s marked as not being in the outlook even though it now is.
Elsewhere...models have converged on forecasting another vigorous wave emerging from Africa in 3 to 4 days… this wave may become another area of Interest.
TROPICAL STORM GRACE...As expected Grace has weakened to a tropical storm due to its passage over the northern Yucatan. However the tropical storm is doing a good job re-organizing over warm Gulf of Mexico water (29 to 30 deg C) while also under vast Gulf of Mexico upper ridging featuring low shear and upper outflow. After dipping to 50 mph max sustained winds… the tropical storm is back up to 60 mph. As of 11 PM EDT… the winds have climbed up another notch to 65 mph.
The previous forecast track has done excellent… so I have made no changes to it tonight. During the forecast period there will be a pair of frontal systems moving through western North America… with the surface steering layer wanting to pull Grace more west-northwest into the surface ridge weakness of the fronts. In the upper layers the vast area of warm Gulf of Mexico upper ridging is forecast to expand in the warm sector of the frontal systems… resulting in upper-level steering wanting to push the strong/tall structure of Grace west-southwest. The net result will be a straight west track into Veracruz and inland provinces with some south angle in the track possible from the upper-level steering as the models keep hinting at.
Regarding intensity… I have slightly bumped up my intensity forecast and show an 80 mph hurricane for the Veracruz landfall due to Grace’s current intensity. This is slightly lower than the NHC Forecast as infrared Satellite does not show Grace has redeveloped a core just yet… but rather a small area of thunderstorms just east of the center.
With these forecast updates:
** Interests in Veracruz should finish preparing for Grace by morning as the storm is expected to make landfall tomorrow night as a hurricane. There is potential for a strong post-landfall rain event with flood potential inland across the provinces of Tlaxcala… Hidalgo… northern Puebla… Mexico… and the Federal District on Saturday. Gusty winds may spread inland across northern Puebla as Grace will take some time to lose its winds after landfall.
**As alluded to in the above discussion… upper-level winds may try to push Grace or it’s moisture more south around or just after landfall time. Heavy rains and flooding potential may therefore also spread into northern Oaxaca… southern Puebla… and Morelos.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Aug 20)… 60 mph maximum sustained wind tropical storm centered just offshore of the northwest Yucatan peninsula coast at 20.7N-91W
IOH 24 Hr Forecast (0000Z Aug 21)… 80 mph maximum sustained wind hurricane centered just offshore of central Veracruz at 20.5N-96.5W
IOH 48 Hr Forecast (0000Z Aug 22)… Remnant low over central Mexico centered at 20.5N-100.5W
TROPICAL STORM HENRI… Henri has still not been able to strengthen to a hurricane just yet… and remains just below hurricane strength. This appears to be due to the fact that Henri has continued to track a little more south than previously forecast… keeping the storm on the southeast edge of western Atlantic upper ridging where some northerly shear has affected Henri. It may be these shearing upper winds are also why Henri is dragged more south… as the strong/tall structure of the storm is susceptible to being influenced by the upper winds. In the end… I have had to yet again nudge the shorter-term forecast track points south and west based on the storm’s current position. In the next 24 hours I expect Henri will be rounding the west side of the Atlantic surface ridge… and also track toward the surface ridge weakness of Fred’s remnants… thus I forecast a northward turn around 72.5W which most models have been in agreement upon. I do not expect the northward turn to be delayed by the upper winds on the south side of the upper ridge as the upper ridge center is in the process of shifting east toward Henri… and so Henri will no longer be aligned with the south side of the upper ridge by 24 hours.
After 24 hours the upper vorticity over the Central US will be heading into the northwest Atlantic... creating a surface ridge weakness with its eastern divergence zone that should continue to carry Henri north. The upper vorticity is Forecast to remain highly amplified which will keep the track toward the northeast US coast… because the amplified upper vorticity will provide a more southerly steering flow in the upper levels (which Henri will be coupled to as a strong/tall tropical cyclone) and cause the surface ridge weakness to not be as Far East as shown in model runs from days past. Given the short-term positions of Henri have been more toward the west…. chances are increasing that Henri will interact with the incoming upper vorticity as follows: (1) the surface northerly flow on the west side of Henri pushes the cool air associated with the upper vorticity more south… which causes the upper vorticity to be even more amplified (2) the more amplified upper vorticity could whirl Henri west into the northeast US coast… and also will have a stronger Eastern divergence zone that will help Henri maintain strength even as it loses tropical character over cooler water. As such the latest models have agreed on whirling Henri inland into Massachusetts instead of allowing Henri to only skirt Cape cod… and my updated forecast track agrees. Because the NAVGEM had Henri initialized too big in size… I think it overdoes the amount of southward cool air transport on Henri’s west side and resulting upper vorticity amplification… which caused Henri in its solution to whirl more west in track into New York.
Regarding intensity… due to Henri still not being a hurricane… I have lowered the peak intensity forecast and now match the NHC… which still forecasts Henri to become a strong 90 mph category 1 hurricane. This is because the west Atlantic upper ridging will shift east over Henri which will help end the shear and produce an excellent upper outflow environment. A little weakening is shown by day 2 due to cooler waters. However even as Henri loses tropical character over cooler water by day 3… I still forecast hurricane force intensity from tremendous divergence expected on the east side of the upper vorticity.
With these Forecast updates:
**Coastal sea swells and rip currents are expected for Bermuda… the US east coast… and Atlantic Canada coast in particular toward Nova Scotia.
**The potential for gusty winds on the northeast US coast is increasing for this weekend… and reducing for the Nova Scotia/New Brunswick region due to the latest track Forecast.
**As stated on the home page bulletins of this site… I recommend interests in eastern Massachusetts and Rhode Island begin preparing for tropical storm to hurricane force conditions as there is growing model consensus of Henri making a direct strike in this area
**In addition with the current Forecast… Henri would bring tropical storm to hurricane force conditions to the rest of Massachusetts… southern Vermont.. southern New Hampshire… Connecticut… and Long Island New York… I also recommend preparing now. In addition to the wind… coastal storm surge is a concern. Heavy rains with flooding potential are possible in this region and also northern Vermont.. northern New Hampshire… and Maine.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Aug 20)… 65 mph maximum sustained wind tropical storm centered in the western Atlantic at 29.7N-71.8W
IOH 24 Hr Forecast (0000Z Aug 21)… 90 mph maximum sustained wind hurricane centered in the western Atlantic at 32N-72.5W
IOH 48 Hr Forecast (0000Z Aug 22)… 85 mph maximum sustained wind hurricane centered offshore of the northeast US at 37.5N-72.5W
IOH 72 Hr Forecast (0000Z Aug 23)… Hurricane-Force frontal cyclone centered over Massachusetts at 42.2N-72.5W
AREA OF INTEREST #1… Based on thunderstorm banding shown on satellite pictures … the lowest pressure of the vigorous east Atlantic tropical wave appears to be near 12.5N-30W. Although the dry Saharan air layer has become stronger… it is also more north than recent days… perhaps allowing for an opportunity for this wave to develop South of the dry air in a low shear and upper outflow environment beneath the tropical upper ridge axis in the region. I have slightly raised my peak odds of development to 15% as the thunderstorm banding has slightly increase. However this is lower than the NHC outlook of 20% as of this writing as there is still the possibility of ingestion of the dry Saharan air and also as none of the models develop this wave at the present time.
Of note… the NHC forecast track is notably north of my track. The models show this wave being overtaken by and absorbed into the adjacent wave to the east currently located southeast of the Republic of Cabo Verde Islands. The merged wave is shown to have a large circulation whose north end becomes trapped into a surface ridge weakness to be induced by some of the current west Atlantic upper vorticity string and also the current Canadian shortwave upper trough as that trough amplifies while entering the Atlantic. However given that this wave is maintaining independence from the adjacent wave to the east… I do not forecast this scenario and thus forecast this wave to escape the surface ridge weakness while the adjacent wave to the east potentially becomes the one who interacts with the weakness… and so my forecast track at this time is more to the south.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Aug 21)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-35W)
IOH 48 Hr Outlook (0000Z Aug 22)… 15% chance of tropical cyclone formation (central tropical Atlantic near 12.5N-41W)
IOH 72 Hr Outlook (0000Z Aug 23)… 15% chance of tropical cyclone formation (central tropical Atlantic near 12.5N-47W)
IOH 96 Hr Outlook (0000Z Aug 24)… 15% chance of tropical cyclone formation (central tropical Atlantic near 13N-53W)
IOH 120 Hr Outlook (0000Z Aug 25)… 15% chance of tropical cyclone formation (just east of the Lesser Antilles near 13.5N-59W)
...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 Storm Grace… makes landfall in east-central Mexico near 21N-97W at 42 hours while regaining hurricane strength… weakens very rapidly after landfall.
**For Tropical Storm Henri... reaches 30.2N-72.5W at 18 hours after which time it turns north in track… reaches hurricane strength near 38N-70W… despite cooler waters Henri maintains strength while moving into Cape Cod Massachusetts at 78 hours due to the support of the incoming amplified upper vorticity to the west… whirls west into Massachusetts and weakens into a remnant low at 96 hours… remnant low slides northeast into the waters offshore of Maine by 120 hours.
**For area of interest #1… no development shown as a wave to the east overtakes this wave by 30 hours… the overtaking wave loses its spin while passing 37.5W longitude at 84 hours.
**Strong tropical wave forecast to emerge from Africa at 84 hours… develops a spin near 10.8N-33.5W at 132 hours
**Tropical disturbance initiated offshore of the eastern US by the divergence zone of upper vorticity interacting with Henri near 35N-72W by 120 hours
1200Z ECMWF Model Run...
**For Tropical Storm Grace… makes landfall over east-central Mexico near 20N-96W between 24 and 48 hours after regaining hurricane strength…after landfall it weakens very rapidly.
**For Tropical Storm Henri... attains Hurricane strength within 24 hours… reaches 31.2N-72.5W at 24 hours after which time it turns north in track… weakens to a tropical storm near 35N-71.5W at 48 hours… whirls west into Massachusetts between 72 and 96 hours due to approaching upper vorticity to the west… weakens to a remnant low while stalled over Massachusetts and under the upper vorticity thru 120 hours
**For area of interest #1… no development shown as a wave to the east overtakes this wave by 24 hours… the overtaking wave loses its spin while passing 42W longitude at 96 hours.
** Strong tropical wave forecast to emerge from Africa between 72 and 96 hours… spin of wave reaches 14N-32.5W at 144 hours
1800Z GFS Model Run...
**For Tropical Storm Grace… makes landfall in east-central Mexico near 20.5N-96.5W at 36 hours after regaining hurricane strength…after landfall it weakens very rapidly.
**For Tropical Storm Henri… attains hurricane strength in the next 3 hours… reaches 30.2N-73.5W at 18 hours after which time it turns north… by 72 hours makes landfall at Cape Cod Massachusetts while maintaining strength over cooler water due to the support of the incoming amplified upper vorticity to the west… thru 90 hours whirls west beneath the upper vorticity and further into eastern Massachusetts while weakening… weakened remnant low turns northeast and offshore and reaches southwest Nova Scotia at 120 hours.
**For area of interest #1…no development shown as a wave to the east overtakes this wave by 24 hours… the overtaking wave loses its spin while passing 37.5W longitude at 84 hours.
** Strong tropical wave forecast to emerge from Africa at 96 hours… wave develops spin southeast of the Republic of Cabo Verde Islands near 12.5N-20.5W at 120 hours.
1200Z NAVGEM Model Run...
****For Tropical Storm Grace... regains hurricane strength near 21N-95W at 24 hours… loses hurricane strength just before landfall in east-central Mexico near 20.5N-96.5W at 42 hours… weakens very rapidly after landfall
**For Tropical Storm Henri... reaches 31N-72.5W at 18 hours after which time it turns north… over cooler waters Henri transitions to a strengthening non-tropical low that makes landfall over southeastern New York at 78 hours due to the support of the incoming amplified upper vorticity to the west… remains stationary over southeast New York under the upper vorticity while weakening through 102 hours… weakened remnant low moves east-northeast across Massachusetts and into offshore waters thru 126 hours.
**For area of interest #1…no development shown as a wave to the east overtakes this wave by 30 hours… the large broad spin of the overtaking wave reaches 17.5N-42.5W at 120 hours.
**Strong tropical wave forecast to emerge from Africa at 102 hours… the large broad spin of the wave passes just south of the Republic of Cabo Verde Islands at 138 hours