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Hurricane Hector Discussion Number 12

Wiki-Hurricanes Forecast Center

8:00 AM PDT Fri Aug 3 2018

Hector shows signs of becoming a more compact tropical cyclone this morning, but the overall thermal structural character has remained relatively stable since the last advisory, though there are signs that are suggestive of weakening weakening. The hurricane features a modest rainband to the northwest and a partial annulus of deep convection within its central dense overcast. A slight warm spot indicating some eye penetration of the upper-cloud tops has appeared intermittently but more recently has clouded over. The eye is apparent at the mid-levels in 89GHz AMSR2 microwave imagery from 0945Z revealing a small pinhole eye spanning just 10-12 nmi across. However, the eyewall encompassing this eyewall is only stout halfway about the eye's circumference, with microwave data revealing a concentrated area of subsidence to the eyewall's immediate north. The hurricane's water vapor profile has become more diffuse this morning, suggesting a noticeable slackening of effective moisture transport and a weakening of the hurricane. The system's internal structure has generally outpaced infrared analyses, so the T4.5/77kt consensus from TAFB and SAB and T3.8/61kt final value from UW-CIMSS ADT do not appear to be truly reflective of the storm's actual intensity. A microwave-derived value of 85kt from CIMSS SSMIS seems more apt given the storm's small size, and thus the intensity for this advisory has been lowered to 85 kt.

While Hector made a valient attempt to fully shape its inner core yesterday in its brief ramp up to a major hurricane, its inability to fully round off an eyewall and fortify its inner core raises questions over how much it can reap from the atmospheric conditions that lay ahead. Sea surface temperatures are expected to remain supportive of a decent deep-layer thermal differential hovering around 80°C for about four days before the tropopause begins to warm as Hector pulls away from the tropics. As a small system, Hector's intensity forecast features a high degree of volatility given the dry conditions indicated by SHIPS diagnostic outputs that sharpen to unfavorable values particularly after 48 hours. Small tropical cyclones like Hector can insulate themselves from a subsident environment, but can also quickly fall apart at the slightest dry air intrusion. Luckily for Hector, upper-level winds are expected to grow more favorable due to a lack of localized upper-air disturbances in its path. In addition, the PV anomaly that has been injecting 10-20 kt northeasterly wind shear appears to have been dampened by Hector's outflow as seen on GOES-16 airmass RGB imagery. Hector is expected to resume intensification today and should continue to do so before slowly tapering off after day four as the slow thermal tapering in light shear conditions should allow for the acquisition of possible annular characteristics and slow decay.

The subtropical ridge that has steered Hector west is now southwest of California and continues to induce westerly motion. A deep mid- to upper-level low over the Gulf of Alaska is moving southeastwards and is expected to briefly settle west of the Pacific Northwest towards the latter half of the forecast period, inducing some west-northwesterly motion. Motion-wise, guidance is in good consensus out to 120 hours and shows a slight west-northwesterly turn. The Hawaiian Islands are potentially implicated in this scenario, and while impacts are outside the scope of this forecast, those on the islands should keep close tabs on the progress of Hurricane Hector has it approaches the island chain.

FORECAST POSITIONS AND MAX WINDS

INIT 03/1500Z 85 KT 100 MPH

12H 04/0000Z 90 KT 105 MPH

24H 04/1200Z 105 KT 120 MPH

36H 05/0000Z 110 KT 125 MPH

48H 05/1200Z 115 KT 130 MPH

72H 06/1200Z 115 KT 130 MPH

96H 07/1200Z 110 KT 125 MPH

120H 08/1200Z 95 KT 110 MPH