*******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.**********
...SATURDAY NOVEMBER 14 2020 8:10 PM EDT...
See Tropical Storm Theta and Tropical Storm Iota sections below for the two currently active tropical cyclones in the Atlantic basin. See area of interest section below for an area of disturbed weather developing northeast of the Lesser Antilles. Elsewhere...the GFS model has been forecasting development from the tropical wave of low pressure currently crossing the Lesser Antilles as the wave later enters the central Caribbean Sea in the wake of Iota and potentially taps into favorable upper winds in a Caribbean upper ridge cell to persist. However with Iota currently forecast to become a powerful hurricane...Iota’s low-level inflow and upper outflow may dominate and prevent development of this wave. Therefore not adding this wave as an area of interest for tropical development at this time.
TROPICAL STORM THETA...The deep-layered ridge west of Theta has pushed the central Atlantic upper trough that was previously supporting the tropical storm southward....and has overspread the storm with its eastern upper convergence zone. And combined with the cooler water temperatures in the low 20s of deg C...Theta has weakened and more quickly than forecast. Updated forecast track is nudged eastward due to the current position of Theta...and shows Theta drifting very slowly eastward in the next 24 hours in conflicting steering between the deep-layered ridge to the west and a surface ridge over Europe to the east being supported by the western convergence zones of upper troughs moving into Europe. Theta should also be weakening further to a remnant low pressure in the next 24 hours. After 24 hours...whatever is left of Theta will be pulled northward into the current northwest Atlantic frontal cyclone once that cyclone nears. As Theta has already substantially weakened...the potential for gusty winds in the Canary Islands has greatly reduced.
0 Hr Position (1800Z Nov 14)...40 mph maximum sustained wind tropical storm centered west of the Canary Islands at 31.7-19.2W
IOH 24 Hr Forecast (1800Z Nov 15)...Remnant low centered just southwest of Madeira Island at 31.5N-18.5W
TROPICAL STORM IOTA...Since yesterday afternoon...tropical depression 31 in the central Caribbean Sea became Tropical Storm Iota. The weak upper trough axis in the northern Caribbean Sea has been more disruptive to Iota while keeping the strongest thunderstorms to the southeast half of the circulation...perhaps with the upper divergence field southeast of the upper trough axis also making Iota more large and broad in size. However this good news has quickly come to an end as Iota has developed a core of thunderstorms this afternoon and briskly intensified to 60 mph maximum sustained winds as of 4 PM EDT.
Forecast track points of Iota are shifted south and east as the tropical storm has moved southwest while reforming towards its strongest thunderstorm activity squashed southward by the northern Caribbean upper trough axis. There is an opportunity for some increase in the west speed of Iota in the next 24 hours from the current eastern US surface ridge. After that time...the current upper trough over the northwest US is forecast to arrive into the eastern North America as a strong upper trough whose eastern divergence zone creates a strong frontal cyclone and whose western convergence zone creates a strong surface ridge over the south-central United States. Because the models insist on a northward angle in track...I bend the track slightly north by 48 hours in response to the ridge weakness created by the frontal cyclone...but the reality is the frontal cyclone will be too far north to recurve Iota completely northward. Instead Iota should largely continue westward but slow down and bend on a southward angle due to the blocking effect of the forecast south-central United States surface ridge.
The weak north Caribbean upper trough should be gradually weakening from the latent heat release of Iota’s thunderstorms... therefore the upper outflow of Iota should improve with time. Even though Iota is now on par with my previous intensity forecast...I have a slightly slower intensification rate initially given Iota is now larger and will tend to have a more lax surface pressure gradient such that surface winds would be a tad lower. However rapid intensification seems rather certain given the favorable upper wind outlook... strong computer model support which...and warm Caribbean waters. My intensity forecast has been increased further as the updated track keeps Iota over water for longer...and now I forecast a top end category 4 hurricane for the expected landfall on the Nicaragua coast.
Needless to say...this forecast is exceptionally terrible news as we are likely in for a second wave of catastrophe in Nicaragua and Honduras only recently after Hurricane Eta devastated this region with powerful winds and catastrophic rainfall flooding...and unfortunately interests here need to do whatever they can to prepare themselves against Iota. In addition to catastrophic wind and coastal storm surge near the landfall area...the mudslide and flooding potential is also high with Iota due to the slow forecast track...and now extends to Costa Rica and El Salvador given the now larger size of the storm.
0 Hr Position (1800Z Nov 14)...60 mph maximum sustained wind tropical storm centered in the central Caribbean Sea at 12.6N-76.4W
IOH 24 Hr Forecast (1800Z Nov 15)...85 mph maximum sustained wind hurricane centered in the central Caribbean Sea at 12.6N-79W
IOH 48 Hr Forecast (1800Z Nov 16)...130 mph maximum sustained wind major hurricane centered offshore of northeastern Nicaragua at 13.5N-81.5W
IOH 72 Hr Forecast (1800Z Nov 17)...155 mph maximum sustained wind major hurricane centered on the coast of Nicaragua at 13N-83.5W
IOH 96 Hr Forecast (1800Z Nov 18)...45 mph maximum sustained wind tropical storm centered over western Nicaragua at 12.5N-85.5W
IOH 120 Hr Forecast (1800Z Nov 19)...Remnant low pressure centered over the eastern Pacific just offshore of Nicaragua at 12N-87.5W
AREA OF INTEREST #1...An upper vortex northeast of the Lesser Antilles is becoming cut-off from the central Atlantic upper trough by the deep-layered ridge moving into the central Atlantic from the western Atlantic. A surface trough of low pressure is expected to form in the eastern divergence zone of the upper vortex over the next 48 hours...which should subsequently track northwest around the north side of the upper vortex and toward a surface ridge weakness from a frontal cyclone to exit eastern North America. A southwest track is likely by 120 hours as the surface ridge weakness closes and a strong surface ridge over the southern US approaches. I have marked this disturbance as an area of interest for tropical development even though none of the models show development at the present time...but at least a surface trough is shown in the models. This is due to potentially favorable low shear and upper outflow to occur northeast of the upper vortex and on the southwest side of the passing deep-layered ridge. I have low odds of development at 30% due to the current lack of computer model support...and drop them to 20% as this disturbance will likely be exposed to some southwesterly shear to be generated by the upper trough tied to the frontal cyclone that will exit eastern North America. A subtropical designation is used for the first 48 hours of the outlook due to entanglement with the cold core upper vortex...followed by a tropical designation as the surface trough disturbance decouples from the upper vortex due to the low-level steering.
IOH 24 Hr Outlook (1800Z Nov 15)...0% chance of subtropical cyclone formation (central Atlantic near 19N-55W)
IOH 48 Hr Outlook (1800Z Nov 16)... 15% chance of subtropical cyclone formation (central Atlantic near 20.5-55W)
IOH 72 Hr Outlook (1800Z Nov 17)...30% chance of tropical cyclone formation (central Atlantic near 22.5N-60W)
IOH 96 Hr Outlook (1800Z Nov 18)...20% chance of tropical cyclone formation (western Atlantic near 23.5N-65W)
IOH 120 Hr Outlook (1800Z Nov 19)...20% chance of tropical cyclone formation (western Atlantic near 22N-67.5W)
...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 Theta...stalls just west of the Canary Islands from 24 to 48 hours while weakening...circulation weakens to a remnant trough at 36N-19.5W at 72 hours. For tropical Storm Iota...makes landfall on Nicaragua/Honduras border as a strong hurricane at 66 hours...weakens to a remnant low over central Honduras at 84 hours.
0000Z ECMWF Model Run...For Tropical Storm Theta...stalls just west of the Canary Islands from 24 to 48 hours while beginning to weaken...circulation weakens to a remnant surface trough located north of the Canary Islands at 36.5N-19W at 72 hours. For Tropical Storm Iota...makes landfall at the Nicargua/Honduras border as a strong hurricane at 72 hours...weakens to a remnant low reaching the western Honduras/Nicaragua border by 96 hours.
0600Z GFS Model Run...For Tropical Storm Theta...stalls just west of the Canary Islands from 18 to 42 hours while weakening...circulation passed over Madeira Island at 54 hours...weakens to a remnant trough at 36.5N-14W at 72 hours. For Tropical Storm Iota...makes landfall over the Nicaragua/Honduras border as a strong hurricane at 78 hours...weakens to a remnant low over the central Nicaragua/Honduras border at 102 hours. Elsewhere...tropical wave currently at 60W becomes a tropical low pressure in the southern Carribean Sea at 13N-77.5W at 132 hours...tropical cyclone formation suggested in long range.
0600Z NAVGEM Model Run...For Tropical Storm Theta...stalls just west of the Canary Islands from 18 to 30 hours while weakening...weakens to a remnant trough at 37-19W at 66 hours. For Tropical Storm Iota...passes just north of the Honduras coast by 78 hours as a strong hurricane...weakens to a tropical storm while drifting southwest into southern Belize and northwestern Honduras by 114 hours...weakens to a remnant low over Guatemala by 132 hours.