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Hurricane John Discussion Number 12

Wiki-Hurricanes Forecast Center

8:00 AM PDT Wed Aug 8 2018

It appears that Hurricane John's fate is sealed as it continues to degrade in satellite appearance this morning. On infrared imagery, the eye has been completely overrun by disorganized convective activity. A large rainband is attempting to surge north ahead of the hurricane and over the Baja California Peninsula, but nonetheless a large subsident air channel is evident in the space between the rainband and the central dense overcast. Despite the storm's large size, upper-level cloud motions in the central dense overcast are now despondent to the traditional circulatory flow, indicative of a decrease in winds and an inability for the hurricane to bring about cohesion vertically across the atmosphere. There have been no new microwave passes to probe the interior of John, but an 0656Z GMI supported the aforementioned conclusions, illustrating a western semicircle devoid of rainfall. SAB estimated T4.5/77kt while TAFB estimated T4.0/65kt; both estimates were in agreement across raw-T values and CI values. UW-CIMSS ADT values have fallen to that of a tropical storm, though persistence constraints have kept the final value to T4.1/67kt. SAB is evidently too high due to the use of an eye pattern, so good agreement bettween TAFB and UW-CIMSS suggest lowering the intensity to 65 kt this morning. However, given the storm's large size, I am willing to lag estimates at 70 kt for now.

There is nothing new to introduce to the track forecast as John continues to follow a marginal weakness towards the northwest. This weakness is slowly shallowing out, which will allow ridging over the United States to expand and curve John west. Models are and have been in agreement over this scenario. Despite favorable upper-level winds, high humidity values, and good lapse rates today, the system's poor organization and large size mean intensification is unlikely. Too much dry continental air has been pulled into the circulation, and we are more likely to see weakening as opposed to strengthening today despite favorable environmental parameters. In the late afternoon today, John is expected to cross a sharp gradient in sea surface temperatures that will shallow out convection further. Low wind shear during the remainder of the forecast window suggests that this weakening will be more gradual than what would be typically forecast, but the cold subtropical waters should do enough to degrade John into a remnant low in roughly 72 hours.

MAXIMUM WINDS FORECAST

INIT 08/1200Z 70 KT 80 MPH

12H 09/0000Z 60 KT 70 MPH

24H 09/1200Z 50 KT 60 MPH

36H 10/0000Z 45 KT 50 MPH

48H 10/1200Z 35 KT 40 MPH

72H 11/1200Z 30 KT 35 MPH...POST-TROP/REMNT LOW

96H 12/1200Z 25 KT 30 MPH...POST-TROP/REMNT LOW

120H 13/1200Z 20 KT 25 MPH...POST-TROP/REMNT LOW

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