*******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 28 2020 9:00 PM EDT...
See area of interest sections below for areas currently being monitored for tropical development in the Atlantic basin. Elsewhere...the current upper trough over the northeastern US will enter the northwest Atlantic over the next day and re-enforce the currently weak northwest Atlantic frontal low such that the frontal low strengthens and absorbs area of interest #1. After that time...a warm deep-layered ridge to form in the warm sector of a forecast strong eastern US frontal cyclone will cut-off the south part of this upper trough into a central Atlantic upper vortex...which could produce yet another subtropical disturbance. So far the GFS...NAVGEM...and ECMWF present a forecast subtropical surface low that stays north of warm 26 deg C waters and the upper vortex (forecast to measure around 1205 dekameters in height at 200 mb) will likely not be cold enough to aid in generating strong enough thunderstorm activity for subtropical cyclone formation. Therefore have not added another area of interest in the central Atlantic.
And finally the upper-level anticyclonic center of the deep-layered ridge to form in the warm sector of the forecast eastern US frontal cyclone is expected to settle over the southern Caribbean Sea where it could enhance the upper outflow and thunderstorms in the region. The southwesterly flow ahead of the frontal cyclone’s upper trough may also enhance the poleward upper outflow in the region as well. The GFS model run continues to suggests possible tropical cyclone formation in the southern Caribbean in about 5 days. However the GFS has a recent history of incorrectly forecasting tropical development in this region nearly two weeks ago...therefore not adding a new area of interest in the southern Caribbean at the present time.
AREA OF INTEREST #1...The meandering western to central Atlantic surface low pressure that we have been monitoring for several days for tropical development has moved more east and less north than previously forecast to 26N-55W while being swung by a northern lobe of low surface pressure and also a weak northwest Atlantic frontal low supported by the northeastern divergence zone of a tilted upper trough in the region which has recently merged with a shortwave upper trough in the northwest Atlantic. There has been a recent flare up of thunderstorms just northeast of the surface low’s center as the surface low has entered the divergence zone of the tilted upper trough. However as of 1 PM EDT...the NHC in their tropical weather outlook indicated the surface low has lost its closed circulation thanks to the developing northern lobe of low pressure and also the weak northwest Atlantic frontal low...and therefore a subtropical cyclone has not been declared. The current northeastern US upper trough and its strong eastern divergence zone will soon emerge into the northwestern Atlantic...causing the currently weak northwest Atlantic frontal low to intensify and finally absorb this surface low pressure system in the next 24 hours. Therefore I have dropped odds of subtropical cyclone formation to 0%...and this is my final statement on this area of interest on this blog unless it remains in the NHC tropical weather outlook by my next update.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Nov 29)...0% chance of subtropical cyclone formation (dissipated near 32.5N-52.5W)
AREA OF INTEREST #2...The current northeast Atlantic upper trough is evolving into a strong and rather cold cut-off upper vortex thanks to the strength of the deep-layered ridge to the west. The tremendous amount of upper divergence on the east side of the upper vortex has triggered a frontal low in the last 24 hours which is explosively intensifying into a frontal cyclone centered at 44N-19W as of 1800Z. The upper vortex is forecast to dive south toward Madeira Island under the influence of the deep-layered ridge...and the surface frontal cyclone will follow suit while whirling into the center of the vortex. Even though water temps in the region are currently running at 18 to 20 deg C...the upper vortex is expected to be cold enough (measuring 1160 dekameters in height at 200 mb) to aid in the generation of thunderstorms...and the strong surface convergence of the surface cyclone will also aid in generating activity. Given the surface cyclone already exhibits a band of shower and thunderstorm activity wrapping into the center...I have raised peak odds of subtropical cyclone formation to 50%. Forecast positions in the outlook below are based on the position of the upper vortex in today’s 1200Z GFS model run. I begin to taper the odds of development down by 72+ hours as the surface cyclone (if it fails to develop a thunderstorm core with mid-level warm core upper outflow) could begin to weaken beneath the center of the upper vortex where there is a lack of divergence as we see with typical post-mature mid-latitude storms. See home page bulletins of this site for expected impacts to the Canary Islands and Portugal in the coming days. In particular...the biggest concern is for Madeira Island where the center of the surface cyclone and upper vortex is currently forecast to nearly stall over or nearby by 24 to 72 hours...which could result in a very long period of gusty potentially damaging winds starting tomorrow. After 72 hours...the deep-layer ridge which will be responsible for keeping this system cut-off from the mid-latitude westerlies will have moved into Europe...and there will be two competing mechanisms that could move this system away from Madeira Island. One is the approach of the current northeastern US upper trough which would push this system east...and the other is another deep-layered ridge to form to the west in the warm sector of a strong frontal cyclone to eject from the eastern US which would push this system to the southwest. The model runs have shifted to a long range southwest drift in the forecast track of the surface cyclone and upper vortex...and the updated forecast track below reflects this. However it should be cautioned that the two competing influences could cancel each other out and the surface cyclone and upper vortex remain stalled near Madeira Island...which would unfortunately extend impacts beyond 72 hours.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Nov 29)...50% chance of subtropical cyclone formation (north-northwest of Madeira Island near 35.5N-17.5W)
IOH 48 Hr Outlook (1800Z Nov 30)...50% chance of subtropical cyclone formation (just northwest of Madeira Island near 34.5N-17.5W)
IOH 72 Hr Outlook (1800Z Dec 1)...40% chance of subtropical cyclone formation (west-northwest of Madeira Island near 34.5N-19W)
IOH 96 Hr Outlook (1800Z Dec 2)...20% chance of subtropical cyclone formation (eastern Atlantic near 34.5N-21W)
IOH 120 Hr Outlook (1800Z Dec 2)...10% chance of subtropical cyclone formation (eastern Atlantic near 30.5N-22W)
...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 area of interest #1...loses identity to strengthening frontal low to the north in the next 24 hours. For area of interest #2...center of surface cyclone stalls just northwest of Madeira Island from 30 to 60 hours...afterwards drifts west-southwest while weakening and reaches 29N-23.5W by 120 hours.
0000Z ECMWF Model Run....For area of interest #1...loses identity to strengthening frontal low to the north in the next 24 hours. For area of interest #2...center of surface cyclone stalls just northwest of Madeira Island from 48 to 72 hours...afterwards drifts southward while weakening and reaches 31N-19W by 120 hours. Elsewhere...subtropical low pressure becomes well-defined at 29.5N-56W by 96 hours...dissipates near 26N-61W just after 120 hours.
1200Z GFS Model Run...For area of interest #1...loses identity to strengthening frontal low to the north in the next 24 hours. For area of interest #2...center of surface cyclone stalls just northwest of Madeira Island from 30 to 60 hours...afterwards core maintains strength from central thunderstorms activity while cyclone drifts southwest and cyclone center reaches 28N-20.5W by 120 hours. Elsewhere...subtropical low pressure becomes well-defined at 30N-53.5W by 72 hours...dissipates near 28N-57.5W at 102 hours. In addition...tropical low forms in the southern Caribbean Sea near 10.5N-79.5W at 84 hours...compact tropical cyclone formation suggested just east of southern Nicaragua at 120 hours.
0600Z NAVGEM Model Run...For area of interest #1...loses identity to strengthening frontal low to the north in the next 24 hours. For area of interest #2...center of surface cyclone stalls just northwest of Madeira Island from 36 to 54 hours...afterwards drifts west-southwest while weakening and reaches 30N-24.5W by 120 hours. Elsewhere... subtropical low pressure becomes well-defined at 30.5N-54W by 72 hours...reaches 31N-59.5W at 120 hours.