*******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.**********
...THURSDAY OCTOBER 31 2019 11:20 AM EDT...
See Rebekah section below for an update on the only active subtropical cyclone in the Atlantic basin. See area of interest section below for details on a new tropical disturbance emerging along the Intertropical Convergence Zone in the eastern tropical Atlantic. Elsewhere...the south part of the elongated upper vortex over Rebekah is expected to be cut-off by a western Atlantic deep-layered ridge into a seperate vortex to be located north of the Lesser Antilles in 3 to 4 days. Computer models agree the eastern divergence zone of this upper vortex will trigger a surface trough between the Lesser Antilles and Bermuda during this timeframe...and will watch to see if a subtropical or tropical disturbance emerges here.
SUBTROPICAL STORM REBEKAH...The frontal cyclone that has moved into the north-central Atlantic from the northwestern Atlantic has developed persistent thunderstorm bands at its core...thus being upgraded to Subtropical Storm Rebekah within the last 24 hours. This has occurred despite the cyclone being over 20 to 21 deg C waters...with the instability to drive thunderstorms boosted by the cold temperatures of the overhead elongated upper vortex in the region. Off to the west...a deep-layered ridge is developing in the western Atlantic due to warm air advection ahead of the frontal cyclone forming over the eastern US with the ridge's surface-level expansion pushing Rebekah eastward...and also a new northwestern Atlantic frontal cyclone southwest of Greenland with its cold air advection is expanding upper vorticity from Canada southeastward towards the elongated upper vortex over Rebekah...with the approaching upper vorticity helping to shift the elongated upper vortex over Rebekah also eastward. However the surface circulation of Rebekah has so far moved to the east faster than the elongated upper vortex...introducing Rebekah to supportive divergence on the east side of the vortex. The narrative that continues to be shown in the computer modeling is that the western Atlantic deep-layered ridge will eventually capture Rebekah...slowing the eastward progress of the subtropical storm and increasing the southward angle in the storm's track such that it falls behind the eastward speed of the overhead upper vorticity...eventually getting trapped into the western convergent side of the vorticity that would weaken Rebekah into dissipation. However given that Rebekah is now on the eastern divergent side of the upper vorticity and thus will take longer before becoming aligned with the western convergent side of the upper vorticity...I have delayed the inevitable weakening in my latest forecast and now keep Rebekah as a subtropical storm in an early weakening phase when it passes over the Azores tomorrow...and show Rebekah as a remnant surface trough of low pressure by Saturday. Interests in the Azores can expect gusty winds and coastal sea swells from the outer circulation surrounding Rebekah for today...with these impacts extending into tomorrow when Rebekah itself moves across the islands.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Oct 31)...45 mph maximum sustained wind subtropical storm centered west of the Azores at 39.7N-36W
IOH 24 Hr Forecast (1200Z Nov 1)...40 mph maximum sustained wind subtropical storm centered just north of the western Azores at 40N-29W
IOH 48 Hr Forecast (1200Z Nov 2)...Surface trough located east of the Azores and north of the Canary Islands at 38N-18W
AREA OF INTEREST #1...The National Hurricane Center in their tropical weather outlook has introduced a quickly expanding area of thunderstorms at the surface Intertropical Convergence Zone (ITCZ) in the vicinity of 5N-30W. This activity is supported by divergence and upper outflow beneath an upper ridge in the eastern Atlantic that has been supported by warm air advection (northward warm air transport) ahead of the outer non-tropical circulation surrounding Subtropical Storm Rebekah. Because wind shear is low in this upper ridge...there in fact appears to be an opportunity for a tropical cyclone to form in this disturbance. Subtropical Storm Rebekah is expected to weaken...allowing the surface ridges in the eastern and western Atlantic to bridge and steer this disturbance west-northwestward during the entire forecast period. For now my peak odds of tropical cyclone formation are kept low at 15% till a surface low pressure spin becomes more apparent in this disturbance. I begin dropping the odds of development at 96+ hours when the 0600Z GFS model run shows this disturbance lining up with stronger westerly shear on the east side of a cut-off upper vortex expected to form and persist near the northern Lesser Antilles.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Nov 1)...5% chance of tropical cyclone formation (tropical Atlantic near 6N-35W)
IOH 48 Hr Outlook (1200Z Nov 2)...15% chance of tropical cyclone formation (tropical Atlantic near 8N-40W)
IOH 72 Hr Outlook (1200Z Nov 3)...15% chance of tropical cyclone formation (tropical Atlantic near 9.5N-45W)
IOH 96 Hr Outlook (1200Z Nov 4)...10% chance of tropical cyclone formation (tropical Atlantic near 11N-50W)
IOH 120 Hr Outlook (1200Z Nov 5)...0% chance of tropical cyclone formation (tropical Atlantic near 12N-55W)