MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #174
*******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 NOVEMBER 8 2020 9:30 PM EDT...
See Tropical Storm Eta section below for the only active tropical cyclone in the Atlantic basin. See area of interest sections below for multiple areas being monitored for tropical development.
TROPICAL STORM ETA...A deep-layered ridge has built over the eastern United States...to the north of Eta and the cut-off upper trough to the northwest of Eta. The surface layer of the ridge is supported by the western convergence zone of the current northwest Atlantic upper trough. The upper layer of the ridge is supported by warm air advection ahead of a strong surface frontal system/upper trough over the western US. This deep-layered ridge is causing the cut-off upper trough next to Eta to amplify into an upper vortex soon to be centered southwest of Tropical Storm Eta. The tropical storm has already responded to the amplification of the cut-off upper trough by curving westward in track towards south Florida and the Florida Keys...and this bend in track is also caused by the deep-layered ridge to the north. As the cut-off upper trough amplifies...the shear over Eta decreases and upper divergence increases...and so despite Eta recently losing thunderstorm intensity due to dry air from the nearby western convergence zone of the cut-off upper trough...it has been able to retain strong tropical storm strength due to the support of the upper divergence. In essence Eta is somewhat of a hybrid subtropical system instead of a purely tropical one...getting help from an upper trough like we see with non-tropical systems as it’s thunderstorm latent heat release and warm core upper outflow has recently played less of a role in Eta’s current strength.
With the exception of the NAVGEM...the latest model consensus is that Eta will bend southward in track after crossing south Florida under the influence of the northwest quadrant of the upper vortex. Given that Eta has recently lost thunderstorm intensity...this seems plausible as there will be less latent heat to weaken the forecast upper vortex...thus the vortex will have more influence on the steering. Thus my short term (thru 48 hour) forecast points are adjusted south and west...and I have lowered the intensity forecast to be just below hurricane force during this time. After 48 hours...the current western US frontal system/upper trough will have reached the central US and have weakened the eastern US deep-layered ridge. In response to the ridge weakening...a northward turn away from the upper vortex and its dry air is anticipated during that time...thus I forecast hurricane strength by 72 hours as the southward-adjusted forecast track will keep Eta south of the shearing upper southwesterly flow ahead of the central US upper trough...and also another upper trough over central Canada to quickly follow behind. By 96 hours...a northeast drift closer to northern Florida is likely due to the upper southwesterly flow and surface cold front tied to the leading upper trough from the central US...with the track being slow due to the narrowness of ridge weakness associated with the surface front. The upper southwesterly flow would likely be shearing Eta during this time...thus I drop forecast intensity below hurricane force. The southward-adjusted track forecast has bigger consequences at 120 hours...as the leading upper trough from the central US and trailing upper trough over central Canada lift out of the region...closing the surface ridge weakness such that Eta misses the opportunity to accelerate northeast into the surface ridge weakness. Thus for now I stall Eta by 120 hours offshore of the northwestern Florida peninsula. As the upper troughs lift out...the southwesterly upper shearing winds should relax...but I do not raise the intensity forecast back up as Eta would be stalling at the 26 deg C sea surface temperature isotherm and potentially begin to upwell waters that are below 26 deg C as it stalls.
Given these forecast updates:
*Eta has already crossed central Cuba...but western Cuba may see some tropical storm conditions by Tuesday if Eta tracks far south enough after crossing south Florida. Interests here should prepare for such conditions.
*Parts of the western Bahamas and south Florida including the Keys are experiencing tropical storm force conditions (gusty winds with damage potential...coastal sea swells...heavy rain). Time to prepare in this region is over and you must remain sheltered until the storm passes.
*Although there is some long term uncertainty in Eta’s forecast track...there remains a chance that sometime next week Eta crosses northern Florida as a tropical storm or at least bring such conditions onshore should it track slowly just offshore of the northwest Florida peninsula coast. Interests here should gather storm preparation supplies early to avoid last minute shopping crowds amid the COVID-19 virus outbreak. When in public...wearing a mask and frequent hand sanitizing will protect you from the COVID-19 virus. If you live in an area prone to coastal storm surge impacts...think of a family/friend residence or a hotel you can relocate to in case you may have to do so in the coming days...as a public storm shelter may not be as ideal in controlling your exposure to COVID-19.
0 Hr Position (1800Z Nov 8)...65 mph maximum sustained wind tropical storm centered just west of Andros Island of the western Bahamas at 23.5N-79.2W
IOH 24 Hr Forecast (1800Z Nov 9)...65 mph maximum sustained wind tropical storm centered just offshore of the southwestern Florida peninsula coast at 25.2N-81W
IOH 48 Hr Forecast (1800Z Nov 10)...65 mph maximum sustained wind tropical storm centered over the southeastern Gulf of Mexico at 24N-85W
IOH 72 Hr Forecast (1800Z Nov 11)...75 mph maximum sustained wind hurricane centered in the eastern Gulf of Mexico at 26N-85W
IOH 96 Hr Forecast (1800Z Nov 12)...60 mph maximum sustained wind tropical storm centered offshore of the northwestern Florida peninsula coast at 28N-84W
IOH 120 Hr Forecast (1800Z Nov 13)...60 mph maximum sustained wind tropical storm centered offshore of the northwestern Florida peninsula coast at 28N-84W
AREA OF INTEREST #1...A surface low pressure in the open central Atlantic centered at 27.5N-47W is producing better organized thunderstorm activity as its supporting upper trough has become more amplified in response to an amplifying deep-layered ridge over the eastern United States and western Atlantic. Models have shown the upper trough moreso amplifying compared to yesterday...which will reduce the shear over the surface low while also producing an upper divergence maximum for a consolidated surface circulation to strengthen...and therefore I have notably raised short-term odds of subtropical cyclone formation. The updated forecast track below is based on following the upper divergence maximum from today’s 1200Z GFS model run. I drop odds of development in the longer term as the surface low tracks into water temps below 26 deg C...and the upper trough is not forecast to be cold enough to help drive instability (in the vicinity of the surface low...200 mb height is forecast to be 1212 dekameters...would like to see heights toward 1200 or less for cold enough upper air temps for instability at cooler water temps). By 96 hours...the upper trough will have a high chance of merging with the current upper trough over the northwest Atlantic...and due to the strength of the deep-layered ridge to the west the merged upper trough will likely take on a southwest-to-northeast orientation. This orientation would produce southwesterly shear across the surface low...thus I drop odds of subtropical development to 0% by that time.
IOH 24 Hr Outlook (1800Z Nov 9)...40% chance of subtropical cyclone formation (central Atlantic near 27.5N-42W)
IOH 48 Hr Outlook (1800Z Nov 10)...30% chance of subtropical cyclone formation (eastern Atlantic near 30N-39W)
IOH 72 Hr Outlook (1800Z Nov 11)...20% chance of subtropical cyclone formation (eastern Atlantic near 32N-36W)
IOH 96 Hr Outlook (1800Z Nov 12)...0% chance of subtropical cyclone formation (south of the Azores near 34N-27W)
AREA OF INTEREST #2...A tropical wave of low pressure that was analyzed to be at 57.5W as of 1200Z and now crossing the Lesser Antilles will experience increasingly favorable upper level winds for tropical development as it moves beneath the Caribbean upper ridge cell to persist in between the upper vortex forming near Tropical Storm Eta and central Atlantic upper trough associated with area of interest #1. Previously the CMC model was the only model forecasting development...but today the GFS and NAVGEM have joined in...therefore it makes sense to now introduce this tropical wave as an area of interest for tropical development as the NHC has done today. With the deep-layered ridge over the eastern United States to shift into the western Atlantic...a steady west motion of the tropical wave across the Caribbean is expected for much of the forecast period. However I gradually slow the west speed of the tropical wave through 96 hours...as the models suggest a drop in surface pressures further to the east by then from an upper divergence maximum to occur between the southwest side of a cut-off upper vortex to originate from the central Atlantic upper trough and east side of the Caribbean upper ridge cell. I forecast
a north bend in track by 120 hours toward the low pressure field of Tropical Storm Eta forecast to be in the eastern Gulf of Mexico by then. Due to the forecast track...I have begun to mention Jamaica in relation to this wave on the home page bulletins on this site. I agree with the NHC peak 5-day odds of development of 30% as the models have only begun to pick up on showing development...and show significant development mostly occurring just after the 5 day window.
IOH 24 Hr Outlook (1800Z Nov 9)...0% chance of tropical cyclone formation (eastern Caribbean Sea near 15N-65W)
IOH 48 Hr Outlook (1800Z Nov 10)...0% chance of tropical cyclone formation (central Caribbean Sea near 15N-69W)
IOH 72 Hr Outlook (1800Z Nov 11)...10% chance of tropical cyclone formation (central Caribbean Sea near 15N-73W)
IOH 96 Hr Outlook (1800Z Nov 12)...20% chance of tropical cyclone formation (central Caribbean Sea southeast of Jamaica near 15N-76W)
IOH 120 Hr Outlook (1800Z Nov 13)...30% chance of tropical cyclone formation (just southwest of Jamaica near 17N-79W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z CMC Model Run... For Tropical Storm Eta...crosses the Florida Keys at 24 hours...reaches southeastern Gulf of Mexico at 22.5N-87.5W at 54 hours...drifts slowly northward to 27.5N-86.5W at 120 hours. For area of interest #1...surface low elongates while located near 28N-40W at 54 hours...strengthens while still elongated as it passes south of the Azores by 90 hours...by 120 hours accelerates rapidly northeast to waters northwest of Spain while merging with cold front tied to frontal cyclone to the northwest. For area of interest #2...tropical wave currently at 57.5W longitude organizes into a tropical low pressure near 15N-72.5W at 102 hours...tropical cyclone formation suggested at 13.5N-75W at 126 hours...significant strengthening suggested in long range.
0000Z ECMWF Model Run...For Tropical Storm Eta...passes over the Florida Keys between 24 and 48 hours while accelerating westward...reaches hurricane strength in the southeastern Gulf of Mexico near 22.5N-86W at 72 hours...drifts northward to 27.5N-87W by 120 hours while weakening to a tropical storm. For area of interest #1...becomes elongated near 30N-40W by 48 hours...becomes consolidated by 120 hours near 31N-25W. For area of interest #2...no tropical cyclone formation shown.
1200Z GFS Model Run... For Tropical Storm Eta...crosses Florida Keys at 18 hours...curves southwest to the west tip of Cuba by 42 hours...drifts northward to 26N-84W while becoming a hurricane at 84 hours...weakens back to a tropical storm while drifting northwest to 26N-85W by 120 hours. For area of interest #1...surface low becomes elongated by 30 hours but a tight center is forecast at the southwest end of the elongated circulation at 28N-41.5W...this tight center strengthens near 29N-37.5W by 54 hours...reaches 31.5N-21.5W by 120 hours. For area of interest #2...tropical wave currently at 57.5W longitude becomes a surface low near 15N-71W at 96 hours...tropical cyclone formation suggested in the long range.
0600Z NAVGEM Model Run...For Tropical Storm Eta...passes over the Florida Keys at 24 hours...whirls southwest to waters just offshore of western Cuba by 54 hours...while drifting northward to waters offshore of the northwest Florida peninsula strengthens into a hurricane by 84 hours...moves across north Florida and southeast Georgia through 102 hours...reaches the eastern Carolinas by 114 hours...reached North Carolina Outer Banks by 120 hours. For area of interest #1...gradually strengthens while reaching 28.5N-40W at 48 hours...reaches hurricane strength south-southeast of the Azores at 120 hours. For area of interest #2...tropical wave currently at 57.5W longitude becomes a surface low south of Haiti and near 16N-73W at 102 hours...tropical cyclone formation suggested near 15.5N-74W at 132 hours...significant strengthening suggested in long range.