WTPZ42 KNHC 160909

200 AM PDT TUE SEP 16 2014

Scatterometer data from earlier this evening indicated that the circulation of the low pressure area south of Mexico has become better defined. The ASCAT data showed several 35-kt wind vectors over the southeastern portion of the circulation. In addition, a ship about 50 n mi south-southeast of the center reported winds to near 30 kt, which was in good agreement with the ASCAT data. Based on these data and the increase in organization of the convection overnight, advisories are being initiated on Tropical Storm Polo. Polo becomes the sixteenth tropical storm in the eastern Pacific basin this season.

The center of the tropical storm is located near the northeastern edge of the deep convection due to about 15 to 20 kt of northeasterly shear. The shear is forecast to decrease somewhat during the next couple of days, which should allow for gradual strengthening. After 72 hours, the global models suggest that upper-level easterly winds could increase resulting in increasing shear. As a result, the NHC forecast shows little change in strength late in the forecast period. The NHC forecast is a little below the statistical guidance and is close to the intensity consensus, ICON.

The initial motion is a somewhat uncertain 295/8 kt. Polo is forecast to turn northwestward today and continue in that general direction during the next couple days while it moves around the southwestern portion of a mid-level ridge over the northern Gulf of Mexico. Although the global models generally agree on this scenario, there are differences in how close Polo moves to the southwestern coast of Mexico in a couple of days. The ECMWF takes Polo inland while most of the remainder of the guidance keeps the tropical storm offshore. The NHC forecast is near the middle of the guidance envelope during the first 2 to 3 days. After that time, a building ridge over northern Mexico should turn the cyclone west-northwestward. Confidence in the track forecast, especially late in the forecast period, is lower than normal.

Interests along the southwestern coast of Mexico should monitor the progress of Polo. A tropical storm watch may be required for a portion of the coast of southwestern Mexico later today.


INIT 16/0900Z 11.9N 98.4W 35 KT 40 MPH 12H 16/1800Z 12.8N 99.9W 40 KT 45 MPH 24H 17/0600Z 14.1N 101.3W 45 KT 50 MPH 36H 17/1800Z 15.5N 102.5W 50 KT 60 MPH 48H 18/0600Z 16.9N 104.0W 55 KT 65 MPH 72H 19/0600Z 18.8N 107.0W 60 KT 70 MPH 96H 20/0600Z 19.7N 108.8W 60 KT 70 MPH 120H 21/0600Z 20.5N 110.8W 55 KT 65 MPH

$$ Forecaster Brown