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Hurricane John Discussion Number 8

Wiki-Hurricanes Forecast Center

8:00 AM PDT Tue Aug 7 2018

John currently possesses an active structure with multiple areas of active convection as it tries to organize this morning. A microwave eye---or at least a warm spot encircled by deeper eyewall convection--has appeared intermittently on infrared satellite imagery. An AMSR microwave pass from 0919Z provided a clearer picture, revealing an eye with an eyewall open to the southwest, though the eyewall was sound for only about 50-60% of the eye's circumference, primarily to the east. Numerous sprawling rainbands are evident in all quadrants, though it does appear that continental dry air does have acess at the lower and mid-levels towards the system's north. SAB estimated T5.0/90kt at 12z, while SATCON estimates have increased to around 92-96 kt depending on the metric used. UW-CIMSS ADT has been fluctuating quite wildly as it tries to evaluate the scene type, though a blend of recent data T-values would seem to suggest T5.5/102kt as its most reasonable estimate. Given the rapidity of convection likely pushing strong winds to the surface and the warm ocean waters, the planetary boundary layer is likely lower and thus I have opted to go with a more aggressive 100 kt intensity for this advisory.

John is moving northwest as anticipated in response to a weakness in ridging over the western United States. As the system weakens over cooler waters, it will begin to flow west in response to the more widespread easterly lower-level winds. This will also be concurrent with a restrengthening of the subtropical ridge, which should ensure a curve to the west. While intensification seems likely, the degree to which John intensifies remains difficult to precisely ascertain. All environmental parameters are favorable for rapid intensification, and model guidance does pick up on these favorables. The HWRF shows a strong Category 2 hurricane while SHIPS and LGEM show good agreement on a Category 3 hurricane. The only major drawback to intensification, as discussed previously, is the system's large size, which makes it potentially susceptible to subsident and stable air drawn from Mexico. However, there should be enough juice to be reaped from the warm ocean waters to moisten the system enough to support a major hurricane. Understandably, large tropical cyclones have more difficulty rapidly intensifying at the upper echelons of hurricane intensities due to both the areal extent needed to support strong winds and the potential for dry air, so a slowdown in the intensification rate is anticipated as John reaches a crest in intensity. As it continues northwestward, John will reach cooling subtropical waters, which in concert with the large amounts of continental air pulled from Mexico should work to degenerate the tropical cyclone into a remnant low.

FORECAST POSITIONS AND MAX WINDS

INIT 07/1500Z 100 KT 115 MPH

12H 08/0000Z 105 KT 110 MPH

24H 08/1200Z 110 KT 125 MPH

36H 09/0000Z 100 KT 115 MPH

48H 09/1200Z 90 KT 105 MPH

72H 10/1200Z 65 KT 75 MPH

96H 11/1200Z 40 KT 45 MPH...POST-TROP/REMNT LOW

120H 12/1200Z 30 KT 35 MPH...POST-TROP/REMNT LOW