*******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 JUNE 20 2020 11:25 AM EDT...
See area of interest sections below for the potential of tropical cyclone development in the northwest Atlantic for tomorrow and Monday.
Elsewhere...the GFS and ECMWF runs described in the computer model summary below show a surface low pressure spin forming in the open central Atlantic near 30N-50W over the next day or so. A disturbance is already materializing here as a surface trough of low pressure has already formed...with showers and thunderstorms increasing in concentration and organization in response to a string of upper vorticity that originated from the much larger eastern US upper vortex diving south and becoming cut-off into its own smaller upper vortex thanks to the upper ridge currently building in the western Atlantic. Although water temperatures in the area are at 24 deg C (below the 26 deg C typical threshold for tropical development) and the new upper vortex is a bit warm to aid in generating instability...thunderstorms appear strong enough on infrared satellite for subtropical or tropical development...probably due to strong enough upper divergence and resulting lift being induced on the east side of the upper vortex. So although I think this disturbance could become further organized in the next 24 hours...a study of GFS model upper-level winds shows that the upper vorticity driving the central US frontal system will finally be able to push upper ridging over eastern North America and the western Atlatnic eastward...which will cause the upper vortex to also shift eastward and away and expose this disturbance to unfavorable westerly wind shear after 24 hours. Therefore still not considering this feature as an area of interest for subtropical or tropical development.
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 NAVGEM...GFS...and CMC favor tropical development on the eastern Pacific side instead of the western Gulf of Mexico.
AREA OF INTEREST #1...The southeastern divergence zone of a cut-off upper vortex over the eastern United States is supporting showers and thunderstorms located offshore of the southeastern United States coast...and a new weak 1017 mb surface low pressure center has formed offshore of the South Carolina coast at 32N-75W according to the 0600Z NHC TAFB surface analysis as shown in the above birdseye view chart. Further evidence of the surface low's existence is shown by the cloudiness beginning to form a hook around the surface low's coordinates. Watching to see if this surface low becomes a tropical cyclone over the next couple of days while shifting northeastward into the northwestern Atlantic where the zone of upper divergence will continue as the upper vortex itself also moves into the northwest Atlantic. Tropical cyclone development may also occurr with the support of anticyclonic outflow beneath western Atlantic upper ridging being generated by the warm southerly surface flow on the west side of the Atlantic surface subtropical ridge. Vertical wind shear values unfavorable to tropical development will be on the lower side due to the circular shape of the upper vortex as it moves into the northwest Atlantic...and due to the amplified nature of the western Atlantic upper ridging to be located just east of the upper vortex. I have adjusted my forecast points in the outlook below to be further southwest compared to the previous as the surface low has formed southwest of my forecast point I had for today...and also because models take a bit longer now to show a surface low forming in the northwest Atlantic...presumably the current already-formed 1017 mb low is probably the feature the models are showing form in the northwestern Atlantic by 36 to 48 hours. I drop odds of development for June 21 (24 hours) to 10% as the weak 1017 mb low will need time to mature further to have any chance of becoming a tropical cyclone. But I have raised the odds to 15% for June 22 (48 hours) as the southwest-adjusted forecast point for that timeframe is over the north edge of warm Gulf stream waters. I still have not raised odds of development above 15% as the computer models are now all showing a weaker surface low. The GFS toward the end of the 72-hour forecast period conitnues showing the upper vortex being cut-off from the jet stream for longer...thus moving more slowly eastward...as it shows the current warm upper ridging over southeastern Canada to the north being stronger...courtesy of warm air advection ahead of the slow-moving central United States frontal system. However the GFS also still shows the surface low escaping the upper vortex and accelerating northeastward in the surface southwesterly flow on the northwest side of the Atlantic subtropical ridge...with the CMC and ECMWF agreeing...only the NAVGEM now shows the surface low staying entangled with the upper vortex on a slow and more easward drift. So due to the majority model opinion at the present...the 72 hour forecast point lies to the northeast and outside of the warm Gulf stream waters...so I maintain 0% odds of development for June 23. On a final note...this system by 48 and 72 hours may resemble a more subtropical system should it indeed stay entangled with the upper vortex as the NAVGEM still shows.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Jun 21)...10% chance of tropical cyclone formation(northwestern Atlantic near 35.5N-70W)
IOH 48 Hr Outlook (1200Z Jun 22)...15% chance of tropical cyclone formation (northwestern Atlantic near 40N-65W)
IOH 72 Hr Outlook (1200Z Jun 23)...0% chance of tropical cyclone formation (northwestern Atlantic near 41N-59W)
...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...weak surface low pressure shown forming in the northwest Atlantic near 40N-65W at 54 hours...drifts northeastward and dissipates by 78 hours at a location south of Nova Scotia.
0000Z ECMWF Model Run...For area of interest #1...no tropical cyclone formation shown...but a weak surface low shown to form in the northwestern Atlantic near 37.5N-68W at 48 hours...shifting northeast and dissipating south of Nova Scotia by 96 hours. Elsewhere...defined surface low pressure spin shown developing in the open central Atlantic near 30N-50W at 48 hours...drifts west along south side of Atlantic surface subtropical ridge and dissipates into surface trough by 96 hours near 30N-54W.
0600Z GFS Model Run...For area of interest #1...no tropical cyclone formation shown...but a weak surface low pressure area shown to form in the northwestern Atlantic near 39N-66W at 36 hours...shifting northeast and dissipating south of Nova Scotia by 72 hours. Elsewhere...weak surface low pressure spin shown developing in the open central Atlantic near 31N-48W at 30 hours...stationary with a slow east drift at times through 72 hours...moves north-northwestward around west side of Atlantic surface subtropical ridge and disspates near 35N-51W by 108 hours.
0600Z NAVGEM Model Run...For area of interest #1...surface low pressure shown forming in the northwest Atlantic near 38N-66W at 36 hours...drifts east-northeastward and dissipates near 41N-58.5W by 120 hours.
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