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Hurricane Hector Discussion Number 14

Wiki-Hurricanes Forecast Center

8:00 PM PDT Sat Aug 4 2018

Last night, Hector made an impressive run at intensification that culminated in its upgrade to a Category 4 hurricane on the basis of good agreement among Dvorak intensity estimates and adjusting upwards due to the system's small size and the presence of a non-interfering eyewall replacement cycle. Since that time, Hector has looked slightly less impressive on satellite imagery, but remains a stout tropical cyclone nonetheless. Despite earlier indications that the eyewall replacement cycle identified last night was progressing quite quickly and not impinging on the system's organization, there may be signs this morning that the process is beginning to materialize in terms of hurricane intensity and core structure. The latest infrared images show a gradual progression of the deepest convection radially outwards from the center occuring over a series of convective pulses. An AMSR2 microwave pass at 1028Z revealed the presence of a partial eyewall fragment rotating about a larger complete eyewall, with yet another third competing band just outside the most prevalent eyewall. The process should result in an expanded eye measuring about 15-20 nmi across, whcih could be seen later today. SAB and TAFB have each observed a degradation of the storm's appearance but have maintained a current intensity of T5.5/102kt which has persisted through the night. Corrected UW-CIMSS ADT values are in close agreement at T5.8/110kt. Given a slight overall decrease in the last six hours, the current intensity has been set at 110 kt.

Hector continues a straightforward westerly track this morning thanks to an elongated 594 dm subtropical ridge that extends westward out to 150°W. However, a deep-layer low pressure system west of British Columbia is currently tracking towards the south and is expected to settle in the vicinity of 40°N, 140°W over the next few days before reconnecting with the jet stream. The presence of the low should dampen the subtropical ridge and provide a marginal weakness to allow for a west-northwest turn that could inititate as soon as tonight. The weakness is expected to be large enough to split the ridge into two lobes, with the westerly focus settling north of the Hawaiian Islands around day 4 and day 5. This should restore a westely heading as Hector nears Hawaii. Currently, model consensus depicts Hector tracking 50-200 nautical miles south of the Big Island in 120 hours, but a direct strike on any of the Hawaiian Islands remains well within the realm of possibility, and residents should remain vigilant.

After an initial period of time sneaking past infrared detection, the ongoing eyewall replacement cycle appears to be at last changing Hector's core structure and satellite appearance. Current environmental conditions are somewhat supportive for intensification within a rather low shear environment and a marginally favorable deep-layer thermal differential, which supports modest convective development. While upper-level winds and sea surface temperatures are expected to be fairly consistent as Hector nears Hawaii over the next five days, the big variable appears to be how Hector deals with a drying overall environment. After about 60 hours, Hector could lose the umbilical support of the intertropical convergence zone and find itself in a particularly dry environment with relative humidity values in the mid-levels near 40%. Following the eyewall replacement cycle, the next 48 hours present an opportunity for Hector to develop annular characteristics and fortify itself a bit longer against the drying conditions. Following a period of stability, the HWRF eyes a period of intensification in around 36-48 hours, as does its cousin model HMON. COAMPS-TC also eyes this region of favorability, though SHIPS and LGEM both observe a steady weakening trend through all five days in the forecast. Global models are also in some agreement with this general idea, and thus it seems probable that peak intensity would occur during this period. Afterwards, unfavorable atmospheric conditions should chip away from Hector towards the end of the forecast run, and the system's proximity to Hawaii at the end of the forecast window may influence whether Hector tapers off or regains some muster beyond 120 hours.

FORECAST POSITIONS AND MAX WINDS

INIT 04/1500Z 110 KT 125 MPH

12H 05/0000Z 110 KT 125 MPH

24H 05/1200Z 120 KT 140 MPH

36H 06/0000Z 120 KT 140 MPH

48H 06/1200Z 115 KT 130 MPH

72H 07/1200Z 100 KT 115 MPH

96H 08/1200Z 90 KT 100 MPH

120H 09/1200Z 85 KT 100 MPH

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