*******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 JUNE 18 2020 7:22 PM EDT...
See area of interest sections below for the potential of tropical cyclone development in the northwest Atlantic in the coming days. Elsewhere...the 0600Z GFS model run from yesterday and now the 1200Z ECMWF run described in the computer model summary below show a surface low pressure spin forming in the open central Atlantic by the timeframe that is now 72 hours away in response to a string of upper vorticity that currently extends northeast from the large upper vortex over the eastern United States...as that vorticity later dives south and becomes a cut-off upper vortex in the central Atlantic in response to the upper ridge currently building in the western Atlantic. However not considering this feature as an area of interest for tropical development as models are not consistent in showing the formation of a low pressure spin...and the forecast location of this disturbance is over waters just below the 26 deg C threshold for tropical development...with the forecast upper vortex also being to warm to aid in tropical development at those water temperatures.
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). It also appears computer models by next week 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 and CMC have joined the GFS in favoring 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 a pair of surface troughs of low pressure as of 1800Z...one in the vicinity of south Florida and the second to the southwest of Bermuda. These features maybe the remnants of the cold front that was driven into the western Atlatnic by the frontal low that moved into North Carolina and Virginia yesterday. In between these surface troughs...an impressive flare up of showers and thunderstorms has occurred this afternoon with the support of the same upper divergence. Watching to see if this thunderstorm activity later becomes a tropical cyclone 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. In the updated outlook below...my short-term forecast points are shifted westward due to the current location of the thunderstorm activity and as the models have dropped the idea of a surface low developing further east over Bermuda. 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. However I keep odds of development at 0% until 48 to 120 hours when the forecast points pass over a warm swath of Gulf stream waters. I have raised odds of tropical cyclone development to 15% as the CMC and NAVGEM models have joined the GFS in showing a well-defined low pressure spin developing in the northwestern Atlantic. The GFS in the longer range is now 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...of course whether that materializes or not depends on exactly how much warm air advection occurs out ahead of the slow-moving central United States frontal system. But with the CMC and NAVGEM also showing similar ideas...with the surface low becoming entagled with the upper vortex and thus drifting only slowly eastward also...I have also adjusted my updated outlook forecast points toward 120 hours to be further southwest from my previous assessments. But my 120-hour point is a compromise between the CMC-NAVGEM consensus and the GFS which 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. Thus I am forced to drop my odds of development to 5% due to the cooler water temperatures at the 120-hour forecast point. On a final note...this system by 96 and 120 hours may resemble a more subtropical system should it indeed stay entangled with the upper vortex.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 19)...0% chance of tropical cyclone formation (west of Bermuda near 33N-71W)
IOH 48 Hr Outlook (1800Z Jun 20)...5% chance of tropical cyclone formation (northwestern Atlantic near 35N-70W)
IOH 72 Hr Outlook (1800Z Jun 21)...15% chance of tropical cyclone formation(northwestern Atlantic near 38N-66W)
IOH 96 Hr Outlook (1800Z Jun 22)...15% chance of tropical cyclone formation (northwestern Atlantic near 39N-62W)
IOH 120 Hr Outlook (1800Z Jun 23)...5% chance of tropical cyclone formation (northwestern Atlantic near 40N-60W)
...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...mature surface low pressure shown forming in the northwest Atlantic near 39N-65W at 90 hours...drifts eastward to 39N-63W by 120 hours.
1200Z ECMWF Model Run...For area of interest #1...no tropical cyclone formation shown...but an east-west surface trough shown to form just offshore of the mid-Atlantic coast of the United States by 72 hours...shifting east-northeast into the open northwest Atlantic and losing its definition by 120 hours. Elsewhere...defined surface low pressure spin shown developing in the open central Atlantic near 30N-48W from 72 to 120 hours.
1200Z GFS Model Run...For area of interest #1...weak surface low pressure area shown to form west of Bermuda near 33N-69W at 60 hours which then moves into the open northwest Atlantic where it reaches peak strength near 40N-62W by 102 hours...begins weakening by 120 hours while located near 41N-59W.
1200Z NAVGEM Model Run...For area of interest #1...surface low pressure shown forming in the northwest Atlantic near 39N-66W at 78 hours...strengthens further while drifting eastward to 39.5N-60.5W by 120 hours.
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