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Hurricane John Discussion Number 6

Wiki-Hurricanes Forecast Center

8:00 PM PDT Mon Aug 6 2018

While John rapidly developed during the morning and early afternoon in exceptionally favorable conditions, there are signs that the rapid intensification has been severely undermined by intrusion of subsident continental air exacerbated in part by the hurricane's large circulation size. It is possible that the intensification period has come to an unexpected screeching slowdown, at least for now. In stark contrast to the deep convection observed this morning, this subsidence appears to have overspread throughout the storm's rainbands, and as a result the storm's cloud tops are quite warm even when considering diurnal minimum. A very clear channel between the outside environment and the storm's ragged eye is evident to the north. More recently, a singular new curved band has developed about the eye's east with some cold cloud tops, but this is the only clear core feature as microwave data suggests an incomplete eyewall limited only to the southeast. SAB and TAFB have stalled to T4.0/65kt, while raw T-values from UW-CIMSS ADT have fallen to suggesting a strong tropical storm though they seem to be misreading the storm's eye. While it appeared that the storm leveled off earlier, a bolstered eye and eastern eyewall give some confidence in raising the storm's intensity slightly to 75 kt.

Models remain in good agreement over the future track of John, continuing its northwesterly track in response to a weakness in subtropical ridging now tracking over the Gulf of California. Over the next five days, the Rocky Mountain ridging should gradually strengthen and result in a gradual western turn at around 28°N latitude. While conditions remain as favorable as they've ever been, the hurricane's large size and lanky core raises questions over its potential. We've seen this sort of scenario historically, and John's subsident-prone structure and detached core makes it increasingly difficult for John to reach Category 4 strength as previously forecast, let alone major hurricane intensity. SHIPS and LGEM have scaled back to showing Category 2 hurricane intensity, agreeing largely with dynamical guidance and ECMWF-IFS and UKMO-G. The latest forecast has scaled back intensification, but still depicts strengthening as conditions remain favorable for intensification, dampened only by John's large size. Rapidly cooling waters after 48 hours should weaken John considerably, though its large size may allow it to maintain intensity for a bit longer than what would typicall be anticipated.

FORECAST POSITIONS AND MAX WINDS

INIT 07/0300Z 75 KT 85 MPH

12H 07/1200Z 85 KT 100 MPH

24H 08/0000Z 95 KT 105 MPH

36H 08/1200Z 100 KT 115 MPH

48H 09/0000Z 90 KT 105 MPH

72H 10/0000Z 75 KT 85 MPH

96H 11/0000Z 50 KT 60 MPH

120H 12/0000Z 35 KT 40 MPH...POST-TROP/REMNT LOW

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