*******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 OCTOBER 17 2020 7:53 PM EDT...
See area of interest sections below for two areas being monitored for tropical development.
Elsewhere...a circular thunderstorm complex in the central tropical Atlantic near 14N-52.5W is associated with a surface trough of low pressure enhanced by upper divergence between the eastern tropical Atlantic upper ridge axis and upper vortex associated with the subtropical low pressure mentioned in area of interest #1...and it is possible this surface trough is a fracture of the tropical wave currently at 61W longitude. However development here is unlikely due to the southwesterly wind shear in the region and dominating inflow of the subtropical surface low pressure.
Widespread showers and thunderstorms in the eastern tropical Atlantic is caused by the combination of tropical waves of low pressure from western Africa enhanced by the outflow of the tropical upper ridge in the region. However the upper ridge is likely to remain suppressed to the southeast by the current northeast Atlantic upper vortex and western Atlantic upper vortex tied to area of interest #1. This will likely expose the tropical waves to westerly wind shear on the north side of the upper ridge and prevent tropical development.
AREA OF INTEREST #1...The current western Atlantic upper trough has amplified into an upper vortex due to adjacent amplification of the current western and northern Atlantic upper ridges in the warm sector of frontal systems supported by upper troughs pivoting around the central North America upper vortex. A fragment of the current northeast Atlantic upper vortex is also expected to retrograde southwestward into the western Atlantic upper vortex which will re-enforce it. The organized surface low pressure that has formed over the last day under the support of the eastern divergence zone of western Atlantic upper vortex has whirled into the center of the upper vortex...at a position near 30N-56W. As such...additional strengthening in the short term will likely cease as divergence directly under the upper vortex center is absent. In addition...even though the surface low is over warm 27 deg C waters...the thunderstorm activity has been limited on colorized infrared satellite pictures...a combo of the lack of divergence at the upper vortex center and the upper vortex not being cold enough to help with the instability. However only a small increase in thunderstorms may be enough for the NHC to declare a subtropical cyclone...so my short term odds of development remain high at 70%...but is still not as high as the NHC outlook to reflect the above observations. By 72+ hours...the cold packet of energy to arrive from the northeast Atlantic upper vortex may drop the upper air temps to increase the instability...and bring a zone of upper divergence over the surface low to strengthen it further...therefore by that time I raise odds of development to 80%.
In the short term the surface low pressure will drift southward as the current north Atlantic surface ridge intensifies under the convergence zone of the developing north Atlantic upper ridge...and from surface ridges over eastern North America that will be supported by the western convergence zones of the other upper troughs to pivot northeastward around the central North America upper vortex. The NAVGEM and CMC have joined the ECMWF in keeping the surface low pressure further east in response to the chunk of energy to arrive from the northeast Atlantic upper vortex....and since I was not as far west as many of the models my updated forecast track is only nudged eastward. The GFS remains the only holdout for a more westward track as of 1200Z. By later in the forecast period...as the packet of energy from the northeast pivots to the west of the surface low pressure and into the western Atlantic upper vortex....the surface low will be coaxed northwestward as it whirls around and then toward the packet of energy. If the models and current forecast are correct...the northwest track may allow the surface low to pass close enough to Bermuda to bring coastal sea swells in a few days. Therefore I will begin to mention Bermuda in regards to this system on the home page bulletins of this site if the forecast holds into tomorrow.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 18)...70% chance of subtropical cyclone formation (central Atlantic near 27.5N-56W)
IOH 48 Hr Outlook (1800Z Oct 19)...70% chance of subtropical cyclone formation (central Atlantic near 25.5N-56W)
IOH 72 Hr Outlook (1800Z Oct 20)...80% chance of subtropical cyclone formation (central Atlantic near 25N-56W)
IOH 96 Hr Outlook (1800Z Oct 21)...80% chance of subtropical cyclone formation (central Atlantic near 26N-57W)
IOH 120 Hr Outlook (1800Z Oct 22)...80% chance of subtropical cyclone formation (central Atlantic near 28N-58.5W)
AREA OF INTEREST #2...Showers and thunderstorms in the western Caribbean Sea have diminished as the small upper vortex and its divergence zone have dissipated. However various models in the long range show an increase in thunderstorms and the development of a broad surface low pressure in the region in a few days in response to the outflow of the western Atlantic upper ridge whose southwest extent has shifted into the western Caribbean out ahead of southward digging upper troughs thar are pivoting around the central North America upper vortex. According to the 1200Z GFS...the upper ridge may be suppressed for the next three days until the central North America upper vortex breaks up...therefore I forecast odds of tropical development of 0% until days 4 and 5...which are still kept at a low 10% as two of the major global models...the ECMWF and NAVGEM. In addition the GFS and CMC that do show development depict a weaker tropical cyclone than shown in previous days. It is possible the inflow of the large sized surface low pressure in area of interest #1 may interfere with the ability for inflow and thunderstorms to accumulate in the western Caribbean. Since its not clear that a tropical cyclone will form in this region...I continue to withold putting statements for land areas in regards to this area of interest on the home page bulletins of this site.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 18)... 0% chance of tropical cyclone formation (western Caribbean Sea near 13N-80W)
IOH 48 Hr Outlook (1800Z Oct 19)...0% chance of tropical cyclone formation (western Caribbean Sea near 13N-80W)
IOH 72 Hr Outlook (1800Z Oct 20)...0% chance of tropical cyclone formation (western Caribbean Sea near 13N-80W)
IOH 96 Hr Outlook (1800Z Oct 21)...10% chance of tropical cyclone formation (western Caribbean Sea near 13N-80W)
IOH 120 Hr Outlook (1800Z Oct 22)...10% chance of tropical cyclone formation (western Caribbean Sea near 13N-80W)
...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 area of interest #1...surface low undergoes a counter-clockwise loop over the next few days while gradually strengthening...ending up at 30.5N-60.5W at 120 hours. For area of interest #2..weak surface low forms in the western Caribbean Sea at 108 hours near 16.5N-80W...possible weak tropical cyclone formation as disturbance lifts northward over central Cuba and the central Bahamas in the long range. Elsewhere...tropical wave organizes into a tropical low pressure near 9N-47.5W at 168 hours.
0000Z ECMWF Model Run...For area of interest #1...surface low undergoes a counter-clockwise loop over the next few days while gradually strengthening...ending up at 31.5N-59.5W at 120 hours. For area of interest #2...no tropical cyclone formation shown.
1200Z GFS Model Run...For area of interest #1...surface low drifts southwest and then northwest while undergoing gradual strengthening...reaching 25N-61W by 120 hours. For area of interest #2...weak surface low forms in the western Caribbean Sea near 17N-80.5W by 150 hours...possible weak tropical cyclone formation as disturbance lifts northeastward over eastern Cuba and the eastern Bahamas in the long range.
1200Z NAVGEM Model Run... For area of interest #1...surface low undergoes a counter-clockwise loop over the next few days while gradually strengthening...ending up at 31.5N-59W at 120 hours. For area of interest #2...weak surface low from western Caribbean lifts northeast into western Cuba by 150 hours...no tropical cyclone formation shown in the long range as surface low moves into south Florida and the western Bahamas.