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Tropical Storm Hector Discussion Number 2

Wiki-Hurricanes Forecast Center

8:00 PM PDT Tue Jul 31 2018

Hector has shown progressive signs of organization this afternoon, developing a more stout central dense overcast with cloud tops largely below -70°C near the center. The overall structure seems a little misshapen about the center of circulation, but this is typical of newly developing tropical cyclones. A strong rainband has existed in the western semicircle throughout the day and continues to feed moisture into the storm. Microwave data from a 2036Z GPM pass and 2200Z SSMI pass reveal a classical curved band structure beneath the cloud cover composed of the central shower activity and the dominant western rainband. Cirrus-marked outflow is expanding over the western semicircle of Hector, but remains in its nascent stages east of the storm. SAB estimated T2.5/35kt, with objective ADT values slightly lower. However, raw values from objective ADT are nearing 40 kt. Given the improved appearance of Hector this evening, the intensity for Hector has been raised to 40 kt.

A steady west to west-northwestward trajectory continues with Hector sailing along the southern periphery of a broad 595 dm mid-level ridge centered over the southwestern United States, and for the most part this is not expected to change in the next five days. Some northerly ascent is being induced by a weakness carved out by a weak mid-level trough northwest of Hector, but will give way to a restrengthened ridge as it moves northwest and away from Hector. Stronger ridging could potentially induce some west-southwesterly motion, but the overall movement of Hector should be generally westward as is typical of East Pacific storms. Intensity and steering solutions at 120 hours indicate the possibility of a setup which would allow Hector to approach the Hawaiian islands, but this is well beyond the forecast window.

Hector lies in a environment generally suitable for intensification, characterized by low deep layer wind shear values, good deep-layer thermal differential, and supportively moist air through at least the next two days. At present, the upper-level anticyclone is offset to the northeast of Hector, which is producing a slightly irritant but divergent 200 hPa flow over the tropical storm which is leading to some easterly shear and suppressing outflow to the system's east. Most global guidance supports gradual intensification over the forecast period. The main inhibitory factor appears to be the orientation of the upper-level ridge centered over the California/Mexico border, which if oriented too meridionally could accelerate a PV anomaly currently emerging off the coast of southwestern Mexico towards Hector, producing unfavorable narrow-band wind shear. Regardless of the degree to which this feature impacts Hector, it is expected to interact with the system over the next two days, which could dampen development. Depending on how Hector manages during this first leg of its lifetime, it may or may not be able to stave off more marginal moisture content beginning in around two to three days.

The GFS shows gradual strengthening out to about Friday before depicting a steady-state Category 2 hurricane at the end of the forecast period, while the ECMWF-IFS shows a storm of similar intensity but gradually strengthens Hector throughout the entire forecast window. HMON appears to be in line with GFS forecasts. Despite the storm's progress today, I am a bit skeptical of the HWRF's prognosis as it shows a Category 2 hurricane by tomorrow morning. SHIPS/LGEM have increased in strength and now assess an 82 kt hurricane in around four days. The current intensity forecast respects the PV feature in the first two to three days by showing a more slower and gradual intensification before environmental favorables become more dominant, allowing for faster strengthening after flow associated with the PV/upper-level low curves parallel with respect to Hector.

FORECAST POSITIONS AND MAX WINDS

INIT 01/0300Z 40 KT 45 MPH

12H 01/1200Z 40 KT 45 MPH

24H 02/0000Z 45 KT 50 MPH

36H 02/1200Z 50 KT 60 MPH

48H 03/0000Z 55 KT 65 MPH

72H 04/0000Z 65 KT 75 MPH

96H 05/0000Z 80 KT 90 MPH

120H 06/0000Z 85 KT 100 MPH

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