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Tropical Storm Hector Discussion Number 1

Wiki-Hurricanes Forecast Center

2:00 PM PDT Tue Jul 31 2018

The area of disturbed weather about 850 miles south-southwest of the southern tip of the Baja California Peninsula has acquired sufficient organization to be declared Tropical Storm Hector. Organized, deep convection has persisted for at least the past 12 hours, and low-level cloud lines suggest the presence of a coherent center of circulation within that convection. I hesitated to upgrade straight to a tropical storm in the absence of ASCAT, but a prominent spiral band extending north and west from the center yielded an intensity estimate of T2.5/35kt from SAB. That is the initial intensity used for this advisory.

Hector is moving steadily west-northwest. A broad subtropical ridge exists to the north of the storm, but a small mid-level low is offering a break in the southeastern periphery of the ridge. This feature is expected to persist for the next 36 hours or so before the low dissipates, allowing the subtropical ridge to become the prominent steering feature for the remainder of the period. In fact, a dip to the west-southwest may take shape after 96 hours given the prominence of the high pressure.

The track forecast is pretty straightforward, but the intensity forecast is anything but. At face value, the environment Hector finds itself in is quite conducive, with SHIPS analyzing ocean temperatures near or above 27C for the next five days, wind shear of 15kt or generally less for the next four days, and mid-level relative humidity values above 50 percent through the remainder of the period. However, point soundings from the GFS indicate a narrow layer of shear upwards of 20kt around 600mb. These mid-level winds relax in a few days, but by then on the model Hector has not built the inner core it needs to sufficiently wall off the more marginal relative humidity values; the system ultimately peaks as a low-end tropical storm on the GFS. This is in stark contrast to its parallel model which predicts a moderate Category 1 hurricane, and in even starker contrast to the recent ECMWF and HWRF runs, which show a Category 2 hurricane within five days, with more significant intensification thereafter on the former model. The SHIPS and LGEM indicate peaks of 71kt and 62kt, respectively. For now, the forecast shows a slow rate of intensification over the next 3 days, with a quicker ramp-up thereafter. This is obviously a low confidence forecast.

FORECAST POSITIONS AND MAX WINDS

INIT 31/2100Z 35 KT 40 MPH

12H 01/0600Z 35 KT 40 MPH

24H 01/1800Z 40 KT 45 MPH

36H 02/0600Z 40 KT 45 MPH

48H 02/1800Z 45 KT 50 MPH

72H 03/1800Z 55 KT 65 MPH

96H 04/1800Z 65 KT 75 MPH

120H 05/1800Z 75 KT 85 MPH