*******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 JUNE 21 2020 5:24 PM EDT...
See area of interest section below for the potential of subtropical cyclone development in the northwest Atlantic later tonight and into Monday.
Elsewhere...the surface low pressure that has been forming in the open central Atlantic near 30N-50W will experience an increase in northwesterly shear and upper convergence that will weaken this feature once its parent upper vortex shifts eastward and away due to the strength of the central Canada upper vortex that is pushing the upper ridging over eastern North America and the western Atlatnic eastward. And combined with the fact the surface low has become disorganized and lost its thunderstorm activity on satellite imagery...subtropical cyclone formation here is not expected.
And finally...the configuration of the Madden Julian Oscillation (MJO) favorble for tropical cyclone activity appears to be shifting eastward toward the eastern Pacific and western Atlantic regions (https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml). Computer models by next week also show a large upper ridge building over the eastern Pacific and western Gulf of Mexico region which would favor tropical development...but at this time the computer models favor development on the eastern Pacific side instead of the western Gulf of Mexico.
AREA OF INTEREST #1...The cut-off upper vortex that has been over the eastern United States is finally shifting eastward into the northwestern Atlantic while the upper vortex over Central Canada has finally had enough strength to push the blocking upper ridge that has been west of the upper vortex eastward. The eastern upper divergence maximum of the upper vortex and resultant surface low pressure that was offshore of the Carolinas yesterday have shifted quickly into the northwestern Atlantic offshore of the northeastern United States. The surface low has been broad in nature with multiple cloud swirls until recently as satellite imagery seems to suggest a better defined center coming together near 38N-68W...and this is confirmed by recent ASCAT-A and ASCAT-B descending passes which show the maximum winds southeast of this center at depression force. Shower and thunderstorm bands have also become better organized in the north half of the circulation...wrapping now into the west side of the circulation. Per the language in the National Hurricane Center tropical weather outlook and observations that this system is both supported non-tropically by the divergence zone of the upper vortex and tropically by the warm swath of Gulf Stream waters and upper outflow of the adjacent upper ridging to the east...it appears the surface low pressure is subtropical in nature. Therefore if a more impressive thunderstorm burst develops near the consolidating center at 38N-68W...we could easily have a subtropical depression declared within the next 24 hours...or perhaps a minimal subtropical storm should the surface circulation strengthen a bit further. However I only have 50% odds of this happening as the more north position of the consolidating center and latest model runs which have very strong agreement on the track of the surface low over the next 24 hours force me to adjust my forecast points northwest of my previous assessment...which puts the surface low just past the north edge of the warm Gulf stream waters at 24 hours and over cooler waters away from the Gulf stream by 48 hours at which point I drops the odds of subtropical cyclone formation to 0%. My 48-hour forecast point is a compromise between the NAVGEM which still shows the surface low being more tangled with the upper vortex and staying at a more south position...and the GFS-CMC-ECMWF consensus which still shows the surface low escapign the upper vortex while caught by the southwestelry steering of the Atlantic surface subtropical ridge.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 22)...50% chance of subtropical cyclone formation (northwestern Atlantic near 40.5N-66.5W)
IOH 48 Hr Outlook (1800Z Jun 23)...0% chance of subtropical cyclone formation (northwestern Atlantic near 42N-61W)
...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 pressure shown reaching peak strength near 40N-67W at 24 hours...drifts northeastward and dissipates by 60 hours at a location south of Nova Scotia.
1200Z ECMWF Model Run...For area of interest #1...weak surface low reaches 40N-67W by 24 hours...shifting northeast and dissipating south-southwest of Newfoundland by 72 hours.
1200Z GFS Model Run...For area of interest #1...surface low pressure area reaches 40N-67W by 24 hours...shifting northeast and weakening to a surface trough southeast of Nova Scotia by 72 hours.
1200Z NAVGEM Model Run...For area of interest #1...surface low pressure area reaches 40N-67W by 24 hours...drifts east-northeastward and dissipates southeast of Newfoundland by 102 hours.