*******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 OCTOBER 18 2020 8:07 PM EDT...
See area of interest sections below for two areas being monitored for tropical development.
Elsewhere...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 upper vortex in the western Atlantic remains stationary and cut-off from the mid-latitude westerly flow while to the south and east of amplified north and west Atlantic upper ridges. The north Atlantic upper ridge will send a fragment of the current northeast Atlantic upper vortex southwestward into the western Atlantic upper vortex which will re-enforce it. The well-defined surface low pressure that previously formed a couple of days ago under the support of the eastern divergence zone of western Atlantic upper vortex has since whirled into the center of the upper vortex...and is currently positioned near 27N-56W. As such...the surface low has stopped strengthening for the time being while located directly under the upper vortex center where divergence is absent....with the showers and thunderstorms biased to the east of the surface low pressure center where divergence is present. Even though the surface low is over warm 27 deg C waters...the thunderstorm activity near the center remains 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 given a recent increase in thunderstorms just east of the surface low pressure center...I have kept my short term odds of development at 70%. By 48+ 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 will bring a zone of upper divergence over the surface low to strengthen it further...therefore by that time I begin to raise odds of development...with a very high 90% shown by days 3 to 5 as there are already increasing thunderstorms just east of the center tonight before these more favorable conditions arrive.
In the short term the surface low pressure will continue to drift southward as the current north Atlantic surface ridge continues to intensify under the convergence zone of the north Atlantic upper ridge...and from surface ridges over eastern North America that are being supported by the western convergence zones of upper troughs pivoting around the central North America upper vortex. The short term track forecast has been nudged southward due to the current position of the surface low. For the longer term...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. The longer term track forecast has been adjusted to a faster northwest speed as this idea has been holding in the model runs...and therefore the chances of Bermuda to see coastal sea swells and damaging winds by days 4 and 5 are increasing. Therefore on the home page bulletins of this site....I will be adding statements for Bermuda.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 19)...70% chance of subtropical cyclone formation (central Atlantic near 25N-56W)
IOH 48 Hr Outlook (1800Z Oct 20)...80% chance of subtropical cyclone formation (central Atlantic near 25N-56W)
IOH 72 Hr Outlook (1800Z Oct 21)...90% chance of subtropical cyclone formation (central Atlantic near 26N-57W)
IOH 96 Hr Outlook (1800Z Oct 22)...90% chance of subtropical cyclone formation (central Atlantic near 30N-60W)
IOH 120 Hr Outlook (1800Z Oct 23)...90% chance of subtropical cyclone formation (over Bermuda near 32N-64W)
AREA OF INTEREST #2...The latest round of scattered showers and thunderstorms in the western Caribbean Sea are the result of a tropical wave of low pressure passing through and the outflow of the southwestern part of the west Atlantic upper ridge now present over the area. The thunderstorms are currently not widespread as the upper ridge outflow is currently constricted by the regime of upper troughs that have been pivoting around the central North America upper vortex. However various models in the long range insist on an increase in thunderstorms and the development of a broad surface low pressure in the region. According to the 1200Z GFS...the upper ridge may be suppressed for the next two days until the central North America upper vortex breaks up...therefore I forecast odds of tropical development of 0% until days 3 to 5...which are still kept at a low 10% as two of the major global models today did not show development...the GFS and ECMWF. 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. The position of this area of interest is nudged northward to 15N latitude...where the 1200Z GFS showed the recovering upper ridge axis in days 3 to 5. Since its not clear that a tropical cyclone will form in this region...I continue to withhold 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 19)...0% chance of tropical cyclone formation (western Caribbean Sea near 15N-80W)
IOH 48 Hr Outlook (1800Z Oct 20)...0% chance of tropical cyclone formation (western Caribbean Sea near 15N-80W)
IOH 72 Hr Outlook (1800Z Oct 21)...10% chance of tropical cyclone formation (western Caribbean Sea near 15N-80W)
IOH 96 Hr Outlook (1800Z Oct 22)...10% chance of tropical cyclone formation (western Caribbean Sea near 15N-80W)
IOH 120 Hr Outlook (1800Z Oct 23)...10% chance of tropical cyclone formation (western Caribbean Sea near 15N-80W)
...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...surface low undergoes a counter-clockwise loop over the next few days while gradually strengthening...passing just east of Bermuda at 96 hours...and reaching 35N-62.5W at 120 hours. For area of interest #2..weak surface low forms in the western Caribbean Sea at 66 hours near 15N-80W...tropical cyclone formation over the Cayman Islands at 102 hours...compact and strong tropical cyclone drifting northward toward central Cuba by 120 hours.
1200Z 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-60.5W at 120 hours. For area of interest #2...no tropical cyclone formation shown.
1200Z GFS Model Run...For area of interest #1...surface low undergoes a counter-clockwise loop over the next few days while undergoing gradual strengthening...reaching Bermuda for a direct strike by 120 hours. For area of interest #2...no tropical cyclone formation shown.
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 31N-61.5W at 120 hours. For area of interest #2...weak surface low forms over western Cuba by 30 hours...no tropical cyclone formation shown in the long range as surface low moves into the central Gulf of Mexico.