Guillermo weakened below hurricane intensity on Monday and the Central Pacific Hurricane Center issued a Tropical Storm Watch for some of the Hawaiian Islands.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Guillermo was located at latitude 18.1°N and longitude 147.4°W which put it about 515 miles (830 km) east of Hilo, Hawaii.  Guillermo was moving toward the northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 65 m.p.h. (100 km/h) and there were gusts to 75 m.p.h. (120 km/h).  The minimum surface pressure was 991 mb.  A Tropical Storm Watch has been issued for Hawaii County and Maui County including the islands of Maui, Molokai, Lanai and Kahoolawe.

An upper level trough north of Guillermo is causing strong southwesterly winds over the top of the circulation.  Significant vertical wind shear is affecting the tropical storm and is weakening it.  The southwesterly winds are also inhibiting upper level divergence on the western side of the storm.  Most of the thunderstorms are occurring in rainbands east of the center of circulation.  The vertical wind shear is expected to increase later this week and Guillermo will be moving over cooler Sea Surface Temperatures.  The weakening trend is expected to continue.

A subtropical ridge is pushing Guillermo toward the west-northwest and the southwesterly winds in the upper levels are turning it more toward the northwest.  That general motion is expected to continue for the next day or two.  On its anticipated track Guillermo is expected to pass north of Hawaii.  However, if it weakens a little faster than expected and the circulation is not as tall, then it could be steered by winds lower in the atmosphere.  Those winds could steer it more toward the west which would bring it nearer to Hawaii.  That possibility prompted the issuance of a Tropical Storm Watch for some of the Hawaiian Islands.