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Tropical Storm Flossie Winds: 45 kt
Pressure: 1001 mbar
Advisory 5 / 2:00 PM PDT Monday, July 29, 2019

Flossie is a compact tropical storm, with a small central dense overcast and minuscule spiral bands extending west and south of the center. The system is undergoing some moderate northeasterly wind shear, limiting convective activity in the northern and eastern semicircles. Satellite intensity estimates were T3.0/45kt from SAB and T3.6/57kt from UW-CIMSS ADT. Given the lack of an inner core on microwave imagery, I have elected to side with SAB in assigning 45kt as the initial intensity.

Flossie is moving rapidly westward and this is not expected to change for the next 24 hours. Thereafter, a weakness in the mid-level ridge to the storm's north should promote a west-northwesterly course for the remainder of the period. There is little uncertainty in this forecast.

Where there is an enormous amount of uncertainty is Flossie's intensity forecast. While ocean temperatures and mid-level relative humidity values are expected to remain favorable for the next 4 days or so, the storm is currently being plagued by moderate northeasterly wind shear. And unlike previous days, these upper-level winds are now actually forecast to increase and remain at marginal to unfavorable levels after 24 hours. The origin of this shear is a southward dive in a PV streamer, courtesy of outflow from Hurricane Erick. This change in shear has resulted in a scattering of model solutions. The GFS, LGEM, and SHIPS make Flossie a low-end Category 1 hurricane. The ECMWF, which yesterday showed a powerful hurricane, now maintains Flossie as a tropical storm. The only constant model is the HWRF which depicts a Category 4 hurricane still. In situations where upper-level winds are the deciding factor between a storm that struggles and a storm that rapidly intensifies, the forecast is especially hard. This is true because the interaction between the cyclone and the shearing mechanism is key and wholly subject to timing, placement, etc. For now, the best course of action is to maintain a down-the-middle forecast like had been implemented in previous forecasts. This is a low certainty forecast though.

FORECAST MAX WINDS

INIT 29/2100Z 45 KT 50 MPH

12H 30/0600Z 50 KT 60 MPH

24H 30/1800Z 55 KT 65 MPH

36H 31/0600Z 65 KT 75 MPH

48H 31/1800Z 80 KT 90 MPH

72H 01/1800Z 95 KT 110 MPH

96H 02/1800Z 90 KT 105 MPH

120H 03/1800Z 80 KT 90 MPH

Forecaster TAWX14
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