MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #103 (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.**********
...MONDAY SEPTEMBER 6 2021 9:45 PM EDT…
Satellite image as of 2020Z. 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:
The weekend edition format… with the above style charts created on mobile phone and no published computer model summary… has been continued for one more day due to the Labor Day holiday. I plan to resume the regular format tomorrow.
See Larry section below for more info on the only currently active tropical cyclone in the Atlantic basin. See area of interest section below for an update on the tropical disturbance that has entered the southwestern Gulf of Mexico from the Yucatan peninsula.
Elsewhere… some model runs have converged on two possible areas that could also become a concern for tropical development in the Atlantic in the coming days: (1) A vigorous tropical wave of low pressure forecast to emerge from western Africa in about four days after the upper ridge over Africa expands into the eastern tropical Atlantic to provide a supply of low shear and upper outflow… (2) a tropical low pressure currently forecast to develop in the Bay of Campeche in about six days with the support of outflow from upper ridging to expand in the wake of the western Gulf cold core upper vortex as the vortex weakens while remaining cut-off from high-latitude cold air.
MAJOR HURRICANE LARRY… Larry has spent the last 24 hours recovering from upper outflow disruption to the west caused by upper vorticity… as the latent heat release of the hurricane helps to weaken the cool core upper vorticity. The outflow on the west side of the hurricane is now healthy with plenty of cirrus outflow clouds… and remains healthy in all other quadrants of the hurricane. Also the the thunderstorm bands west of the eye have re-solidified… and the eye wall replacement cycle that began last night is complete with Larry displaying a singular large eye. During this recovery process Larry has maintained category 3 strength with 120 to 125 mph max sustained winds. Because Larry has not strengthened yet… I have lowered my updated intensity forecast a touch… but I still project Larry to reach the bottom-end of category 4 status. By day 3 Larry is forecast to move into waters with less heat content… below 28 deg C… and so I taper down the forecast intensity back to category 3 by then.
Regarding track… in the short-term my updated track forecast is nudged east due to the current position of the hurricane. The track is expected to make a full turn to the north after 24 hours due a large surface ridge weakness to be induced by a pair of frontal systems to eject from North America… the first of which is currently over eastern Canada. The north turn will also be aided by what will be left of the aforementioned upper vorticity…to be located east of the Bahamas. It should be noted the western convergence zone of the upper trough tied to the first of the ejecting frontal systems (the system currently over eastern Canada) will produce a passing surface ridge in the northwest Atlantic which will keep Larry’s track on a slight west lean thru day 2… and prevent an immediate east turn at day 3. As most models bring Larry just past 60W longitude… I have not nudged the longer term forecast track to the east despite the shorter-term eastward adjustment. On day 4… Larry is expected to accelerate north-northeast in the flow ahead of the second of the two frontal systems. The acceleration will give little time for Larry to weaken over cooler waters before it aligns with the supportive eastern divergence zone of the upper trough tied to the second frontal system… with the divergence zone then helping to make the weakening rate of Larry slow after it loses tropical character. This is why by day 4 I have Larry at strong category 1 hurricane force after transitioning to a non-tropical frontal cyclone supported by the upper trough. This could result in a significant frontal cyclone that could affect Newfoundland… more on that in the bulletins below:
**Larry will bring coastal sea swells to the northern Lesser Antilles over the next day or so.
**Coastal sea swells will reach Bermuda by 24 to 72 hours… and become vigorous as the hurricane passes just to the east. The chances for the west side of the hurricane to pass over the island and bring tropical storm force winds continues to drop as the forecast track has remained essentially unchanged
** Coastal sea swells will reach the northeastern US and Atlantic Canada coasts in 48 to 96 hours. As mentioned above… dynamics could converge in a way that helps Larry maintain strength after losing tropical characteristics while heading toward Newfoundland just after day 4… which could result in coastal storm surge and gusty winds with damage potential. Interests in Newfoundland should be continue to monitor the progress of Larry.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 6)… 125 mph maximum sustained wind major hurricane centered at 22.5N-53.9W
IOH 24 Hr Forecast (1800Z Sep 7)… 130 mph maximum sustained wind major hurricane centered at 25N-56W
IOH 48 Hr Forecast (1800Z Sep 8)… 130 mph maximum sustained wind major hurricane centered east-southeast of Bermuda at 29N-59W
IOH 72 Hr Forecast (1800Z Sep 9)… 120 mph maximum sustained wind major hurricane centered east-northeast of Bermuda at 34N-60.5W
IOH 96 Hr Forecast (1800Z Sep 10)… 95 mph maximum sustained wind non-tropical frontal cyclone centered south-southwest of Newfoundland at 42.5N-58W
AREA OF INTEREST #1… Despite an increase in showers and thunderstorms in the Gulf of Mexico to the north of the Yucatan peninsula this past afternoon… in the eastern divergence zone of the upper vortex currently in the western Gulf… the surface trough of low pressure that was over the Peninsula has not reformed to the north into this activity and instead is located further southwest in southwestern Gulf and Bay of Campeche. This is confirmed by the most recent ASCAT-A pass of surface winds which sampled the region and did not show a circulation to the north… and the CIMSS 850 mb vorticity product which shows mid-level rotation also in the Bay of Campeche (http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=atlantic&sat=wg8&prod=vor&zoom=&time=). In addition the most recent Satellite frames show the thunderstorm activity re-grouping southwestward over and near the Yucatan peninsula and closer to the surface trough. The modeling is adamant on reforming this system north into the divergence zone of the western Gulf upper vortex… followed by shifting this system east-northeast toward the Florida panhandle in the flow ahead of a pair of frontal systems to cross North America (the first system of the pair is already currently crossing eastern Canada). Due to a lack of a well-defined surface spin and thunderstorms being disorganized while also tending to be northeast of the surface trough due to the light southerly shear on the east side of the upper vortex… I have dropped development odds to 15% in the Gulf of Mexico. Odds are dropped to 0% by day 3 due to the inland location of my forecast track during that timeframe. It should be noted that my updated forecast track is also shifted south and west due to the current position of the surface trough.
Models have continued to suggest that this system will re-emerge into water at a location offshore of the southeast US by days 4 and 5 and make a second attempt at developing… therefore my outlook below remains extended through 5 days. This development would be facilitated by the eastern divergence zone of the upper trough associated with the second of the two frontal systems to traverse North America within the pair mentioned in the prior paragraph. The GFS has trended with an upper trough that is not very amplified… resulting in southwesterly wind shear. In addition the southwest adjusted forecast track causes this system to miss the surface ridge weakness to be caused by the second frontal low and Hurricane Larry… which will result in this system becoming nearly stalled by day 5 while trapped between the Atlantic surface ridge to the east and an eastern US surface ridge to the north to be supported by the northwestern convergence zone of the second frontal low’s upper trough. The stalled track below the southwesterly upper flow would make the shear worse. With the negative shear forecast… I have dropped development odds offshore of the southeast US (days 4 and 5) to 10%.
**While it is concerning that this system in 24
to 48 hours will be near southeast Louisiana where Hurricane Ida recovery efforts are ongoing… it appears this system has lower development potential than Ida did as it will have to negotiate Gulf of Mexico upper vorticity as discussed above… followed by possible westerly shear on the South side of the North American frontal systems’ upper troughs while nearing the US Gulf coast. Also the steering flow suggests a track that turns this system east away from the area such that landfall is not expected here
**Odds are decreasing that tropical storm conditions (gusty winds… some coastal sea swells… and heavy rains with flash flooding potential) will occur for the Alabama and Florida panhandle coastal regions. However this system could still produce flash flooding heavy rainfall in this area and also for southern Georgia… northeast Florida… and southern South Carolina on Thursday.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 7)… 5% chance of tropical cyclone formation (Gulf of Mexico near 26N-91W)
IOH 48 Hr Outlook (1800Z Sep 8)… 15% chance of tropical cyclone formation (northeastern Gulf of Mexico near 28N-87.5W)
IOH 72 Hr Outlook (1800Z Sep 9)… 0% chance of tropical cyclone formation (southeast Georgia near 31N-81.5W)
IOH 96 Hr Outlook (1800Z Sep 10)… 10% chance of tropical cyclone formation (offshore of the southeastern US near 32.5N-72.5W)
IOH 120 Hr Outlook (1800Z Sep 11)… 10% chance of tropical cyclone formation (offshore of the southeastern US near 33N-72W)