*******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.**********
...TUESDAY OCTOBER 13 2020 12:57 PM EDT...
See area of interest sections below for a pair of tropical waves of low pressure being monitored for tropical development. Elsewhere...the current central North America upper trough is forecast to remain settled in place while amplifying further into a vortex under the cold air advection (southward cold air transport) on the west side of the current frontal cyclone its supporting. A series of upper troughs will pivot around the south side of the upper vortex over the next several days...one of which is forecast to break away from the vortex and move into the open central Atlantic. It is possible this upper trough fragment triggers a surface low pressure capable of acquiring tropical characteristics if it amplifies enough...and whether or not this happens depends on how much the western Atlantic upper ridge amplifies enough in the warm sector of frontal systems to be supported by other upper troughs pivoting around the central North America upper vortex.
And finally by next week...the current western Atlantic to Gulf of Mexico upper ridge is forecast to shift southward into the western Caribbean Sea in response to the series of strong upper troughs to pivot around the central North America upper vortex. This will increase the upper outflow over the western Caribbean...potentially dropping surface pressures and increasing the coverage of thunderstorm activity as some of long range model runs suggest. This could result in the formation of a tropical cyclone in the western Caribbean region in the long range.
AREA OF INTEREST #1...In defiance of the wind shear induced by the upper trough in the region...the vigorous tropical wave of low pressure in the central tropical Atlantic continues to have a well-defined spin...located near 13N-55W as of 1200Z...and has seen a recent increase in thunderstorms located just east of the spin despite the shear. It could be argued that the thunderstorms are being aided by the eastern divergence zone of the upper trough. This system should continue tracking west-northwest toward a surface ridge weakness induced by an old central Atlantic front which has now decayed into surface troughs...as well as the northwestern Atlantic front and front driven by the remnant circulation of Delta moving into the region. This path will soon take the tropical wave toward the axis of the upper trough where there is less upper divergence to support thunderstorms...and even after passing through the upper trough axis the wave may have to deal with a zone of suppressive upper convergence between the west side of the upper trough and southeast side of the western Atlantic upper ridge. Thus I forecast a 0% chance of development by 24 hours. However given the NHC outlook still has 20% odds of development...I have included a special 12 hour forecast point showing odds above 0% in case some last minute development occurs before the upper-level wind conditions become more hostile. In fact my 12-hour odds are higher than the NHC...at 35%...given the recent increase in organized thunderstorms. Not mentioning the Lesser Antilles in regards to this tropical wave on the home page bulletins of this site as the upper trough axis where there is a lack of divergence needed for thunderstorms has settled over the islands.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 12 Hr Outlook (0000Z Oct 14)...35% chance of tropical cyclone formation (central tropical Atlantic east of the Lesser Antilles near 13.5N-57W)
IOH 24 Hr Outlook (1200Z Oct 14)...0% chance of tropical cyclone formation (central tropical Atlantic east of the Lesser Antilles near 14N-59W)
AREA OF INTEREST #2...Based on the latest satellite imagery...a tropical wave of low pressure has reached the western coast of Africa near 12.5N-15W...and continues to feature organized thunderstorm bands on its west side. As this tropical wave continues into and then across the eastern tropical Atlantic...a tropical upper ridge with low shear and upper outflow will persist and may potentially aid in developing this wave. Based on the persistence of organized thunderstorm bands...I have raised my peak 5-day odds of tropical cyclone formation to 30% even though the development signal from the CMC and GFS models remains weak...showing the development of a broad low pressure area instead of explicitly showing tropical cyclone formation. By 3+ days...less favorable wind shear over this tropical wave will likely increase as the central Atlantic upper trough currenly affecting area of interest #1 shifts eastward toward this tropical wave while pushed by an upper trough fragment to eject from the current central North America upper trough. Therefore odds by days 4 and 5 are slightly lowered from the 30% peak. Forecast track of this tropical wave remains close enough to the Republic of Cabo Verde Islands for me to continue statements on this wave in regards to those islands on the home page bulletins of this site.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 14)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-20W)
IOH 48 Hr Outlook (1200Z Oct 15)...20% chance of tropical cyclone formation (just south of the Republic of Cabo Verde Islands near 13.5N-25W)
IOH 72 Hr Outlook (1200Z Oct 16)...30% chance of tropical cyclone formation (eastern tropical Atlantic near 14N-30W)
IOH 96 Hr Outlook (1200Z Oct 17)...20% chance of tropical cyclone formation (eastern tropical Atlantic near 14.5N-35W)
IOH 120 Hr Outlook (1200Z Oct 18)...20% chance of tropical cyclone formation (eastern tropical Atlantic near 15N-40W)
...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...no tropical cyclone formation shown. For area of interest #2...tropical wave organizes into a tropical low pressure just southeast of the Republic of Cabo Verde Islands at 48 hours...opens back to a tropical wave shortly thereafter. Elsewhere...upper trough fragment ejecting from central North America upper vortex amplifies in the open central Atlantic...generating a surface low pressure and possible subtropical cyclone formation in the vicinity of 28N-55W from 90 to 126 hours.
0600Z GFS Model Run...For area of interest #1...current surface low pressure spin weakens back to a tropical wave while moving into the Lesser Antilles at 30 hours. For area of interest #2...tropical wave maintains a weak low pressure spin and reaches 14N-41.5W at 126 hours.
0600Z NAVGEM Model Run...Today's run not available at above-mentioned source.