MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #84
*******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.**********
...THURSDAY AUGUST 19 2021 12:12 AM EDT...
See Grace and Henri sections below for an update on all active tropical cyclones in the Atlantic basin. Both are currently or are forecast to bring impacts to land areas in the coming days.
Elsewhere...satellite imagery has shown the strong tropical wave of low pressure that was over Western Africa 24 hours ago has moved offshore with signs of rotation and thunderstorm banding. See area of interest #1
section below for more details.
HURRICANE GRACE...After solidifying a shield of thunderstorms… Grace in the western Caribbean Sea earlier today became the second hurricane of the 2021 Atlantic season shortly after striking the Cayman Islands with tropical storm conditions. The solid shield has since degraded with outer bands of thunderstorms gone or weakened… thus the hurricane has not strengthened past 80 mph max sustained winds. This and the overall east-west elongation of Grace’s outer cloud pattern might be signs of disruption from the nearby upper vorticity string that has been lurking to the north. However by no means is Grace weakening either as the center has developed a small thunderstorm core which has recently shown an eye-type feature.
Due to the hurricane’s current position… I have shifted the short-term forecast track points a little to the north. 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 upper ridging is forecast to expand in the warm sector of the frontal systems… resulting in upper-level steering wanting to push a strong/tall Grace west-southwest. Even if Grace weakens from a hurricane to a tropical storm while crossing the Yucatan… it could still be strong/tall enough to be coupled to the upper-level steering once it moves out over the Gulf of Mexico waters.
Also all of the global models now agree with a more south track…. and so my long-term track forecast is left the same with no north adjustment.
Regarding intensity… the less than ideal upper air environment will improve for Grace once the storm moves away from the nearby upper vorticity string and into a vast area of upper ridging in the Gulf of Mexico. Even with the current upper air environment Grace has developed a core with an eye type feature… so interests in the northeast Yucatan who will get the brunt of Grace in the coming hours should not let their guard down as some additional strengthening is still on the table. I forecast Grace to lose hurricane strength over the northern Yucatan… and my longer term intensity forecast remains lower than the NHC as of this writing as I forecast recovery to a minimal hurricane for the Veracruz landfall late Friday whereas the NHC shows a stronger category 1 hurricane. This is because I see that Grace has a tiny core and will move well inland across the northern Yucatan which will cause this core to collapse as it’s too small to stay in touch with offshore water… and I currently predict Grace will have to spend some time re-organizing a new core before it re-strengthens over the southwest Gulf.
With these forecast updates:
** Coastal sea swells will remain possible for the Cayman Islands and western Cuba tonight until Grace makes landfall in the northern Yucatan.
**Interests across the northern Yucatan peninsula should have finished preparing for Grace. Expect tropical storm to hurricane force conditions in the northeast Yucatan by morning… and tropical storm conditions in the northwest Yucatan later in the day.
** Per the home page bulletins of this site… interests in Veracruz should continue preparing for Grace as the storm has potential to make landfall Friday night or Saturday 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.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Aug 19)… 80 mph maximum sustained wind hurricane in the northwest Caribbean centered at 19.8N-84.9W
IOH 24 Hr Forecast (0000Z Aug 20)… 50 mph maximum sustained wind tropical storm centered just offshore of the northwest Yucatan peninsula coast at 20.5N-91W
IOH 48 Hr Forecast (0000Z Aug 21)… 75 mph maximum sustained wind hurricane centered just offshore of central Veracruz at 20.5N-96.5W
IOH 72 Hr Forecast (0000Z Aug 22)… Remnant low over central Mexico centered at 20.5N-100.5W
TROPICAL STORM HENRI… Henri has not been able to strengthen to a hurricane just yet… but is on the cusp of 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 again nudge the shorter-term forecast track points southward with these observations. By 48 hours and while rounding the west side of the northwest Atlantic surface ridge… and also while tracking toward the surface ridge weakness of Fred’s remnants… I expect the track to turn north. The 1200Z GFS puzzled me and was also a western outlier while having Henri still drift west for a bit longer during that timeframe… as even in the upper-levels the south side of the western Atlantic upper ridge won’t be able to push Henri west by that time while shift east and aligning its center directly over Henri.
After 48 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. With each day that passes the models are showing the warm upper ridging over the Gulf of Mexico and US becoming more amplified than before in the warm sector of a pair of western North America frontal systems. This appears to be due to the upper trough of the leading frontal system which has been shown to be more amplified/stronger after now making landfall on the North American continent… resulting in a stronger surface frontal system that pumps up the Gulf/US warm upper ridging with stronger northbound warm air transport on its east side (perhaps the models have gotten better data from land based weather stations about the state of this upper trough than when it was over the Pacific waters). The implication is the stronger Gulf/US upper ridging caused the upper vorticity nearing Henri to also be more amplified… which requires adjusting the longer term track more west and even closer to the northeast US coast. This is because the more 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 previously shown. The 1200Z GFS was more extreme by having the surface northerly flow on the west side of Henri push the cool air associated with the upper vorticity more south… which causes the upper vorticity to be even more amplified to the degree Henri whirls west into New York’s coast. However given that I think the GFS around 48 hours moved Henri too much to the west and closer to the upper vorticity… I am not forecasting that scenario at this time.
Regarding intensity… I have lowered the short-term expectations as Henri didn’t make it to a hurricane yet… but by 48 hours I still ramp Henri to a 95 mph max sustained wind hurricane. This is because by that point the west Atlantic upper ridging will have shifted east over Henri which will help end the shear and produce an excellent upper outflow environment. A little weakening is shown by day 3 due to cooler waters. However even as Henri loses tropical character over cooler water by day 4… I still forecast hurricane force intensity from tremendous divergence expected on the east side of the upper vorticity.
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 and Nova Scotia/New Brunswick region is increasing for this weekend… interests here should be aware of Henri and think about preparations for tropical storm to hurricane force conditions in case such actions are warranted by tomorrow or Friday.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Aug 19)… 70 mph maximum sustained wind tropical storm centered southwest of Bermuda at 29.9N-67.8W
IOH 24 Hr Forecast (0000Z Aug 20)… 85 mph maximum sustained wind hurricane centered in the western Atlantic at 30.8N-71W
IOH 48 Hr Forecast (0000Z Aug 21)… 95 mph maximum sustained wind hurricane centered in the western Atlantic at 34N-71.5W
IOH 72 Hr Forecast (0000Z Aug 22)… 85 mph maximum sustained wind hurricane centered offshore of the northeast US at 38N-71W
IOH 96 Hr Forecast (0000Z Aug 23)… Hurricane-Force frontal cyclone centered between Cape Cod Massachusetts and Maine at 42.5N-69.5W
AREA OF INTEREST #1… Satellite imagery has shown the strong tropical wave of low pressure that was over Western Africa 24 hours ago has moved offshore with signs of rotation and thunderstorm banding. Based on the banding… the center of lowest pressure appears to be near 12.5N-23W. I have marked the wave as an area of interest as the dry Saharan air layer although stronger 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. However I assign low 10% odds of development as the thunderstorm banding has weakened in intensity… a sign that the wave may still struggle with the dry air. Also none of the models develop this wave at the present time.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Aug 20)… 0% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-29W)
IOH 48 Hr Outlook (0000Z Aug 21)… 10% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-35W)
IOH 72 Hr Outlook (0000Z Aug 22)… 10% chance of tropical cyclone formation (central tropical Atlantic near 12.5N-41W)
IOH 96 Hr Outlook (0000Z Aug 23)… 10% chance of tropical cyclone formation (central tropical Atlantic near 12.5N-47W)
IOH 120 Hr Outlook (0000Z Aug 24)… 10% chance of tropical cyclone formation (central tropical Atlantic near 13N-53W)
...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 Grace... crosses the northern Yucatan peninsula at 18 to 24 hours which causes it to weaken to a tropical storm… makes landfall in east-central Mexico near 21N-97W at 66 hours while regaining hurricane strength… weakens very rapidly after landfall.
**For Tropical Storm Henri... reaches 31N-72.5W at 36 hours after which time it turns north in track… despite cooler waters Henri strengthens offshore of Massachusetts and Nova Scotia near 40.5N-67.5W at 120 hours due to the support of the incoming amplified upper vorticity to the west.
**For area of interest #1… no development shown as a wave to the east overtakes this wave by 54 hours… the overtaking wave loses its spin while passing 40W longitude at 120 hours.
**Strong tropical wave forecast to emerge from Africa at 108 hours… spin of wave located at 10N-24W at 138 hours
1200Z ECMWF Model Run...
**For Hurricane Grace... crosses the northeast Yucatan at 24 hours… makes landfall over east-central Mexico near 20N-96W between 48 and 72 hours after which time it weakens very rapidly.
**For Tropical Storm Henri... reaches 31.5N-72.5W at 48 hours after which time it turns north in track… weakens to a remnant low near 36N-71W at 72 hours… remnant low strengthens offshore of Massachusetts and Nova Scotia near 41.5N-67.5W at 120 hours due to the support of the incoming amplified upper vorticity to the west.
**For area of interest #1… no development shown as a wave to the east overtakes this wave by 48 hours… the overtaking wave loses its spin while passing 41W longitude at 120 hours.
1200Z GFS Model Run...
**For Hurricane Grace... crosses the northeastern Yucatan peninsula at 24 hours… makes landfall in east-central Mexico near 20N-96W at 69 hours after which time it weakens very rapidly.
**For Tropical Storm Henri… attains hurricane strength in the next 3 hours… reaches 31.2N-73.5W at 51 hours after which time it turns north… by 120 makes landfall in southeastern New York while maintaining strength over cooler water due to the support of the incoming amplified upper vorticity to the west.
**For area of interest #1…no development shown as a wave to the east overtakes this wave by 48 hours… the overtaking wave loses its spin while passing 35W longitude at 102 hours.
1200Z NAVGEM Model Run...
****For Hurricane Grace... crosses the northern Yucatan peninsula from 24 to 36 hours… makes landfall in east-central Mexico near 20N-96W at 66 hours after which time it weakens very rapidly.
**For Tropical Storm Henri... reaches 30.5N-72W at 36 hours after which time it turns north… over cooler waters Henri transitions to a strengthening non-tropical low between Massachusetts and Nova Scotia near 43N-67.5W at 120 hours due to the support of the incoming amplified upper vorticity to the west.
**For area of interest #1…no development shown as a wave to the east overtakes this wave by 54 hours… the overtaking wave loses its spin while passing 36W longitude at 126 hours.