Issued: Jul 28, 2015 2:30 AM HST
Shower activity has increased over parts of the island chain during the past few hours, most notably over and in vicinity of Oahu and over eastern Big Island. The showers that are passing through Oahu now came from an area of cloudiness situated over the windward waters of Maui and Molokai at sunset. Primarily the Koolau mountains and the adjacent windward districts are affected, and from time to time, a shower will stray downwind into the leeward communities. The last of the showers with this area is about to leave Oahu and begin threatening Kauai. Kauai has been mostly dry up to now with scattered to locally broken low clouds. Narrow band of showers has formed while on its approach to the east facing shoreline of the Big Island. This band is about to move ashore. As of 2 am HST, it is mainly fair and dry across Maui county and the Hamakua side of the Big Island. There are still some lingering scattered to broken low clouds stretched across the leeward side of the Big Island. Scattered low clouds extended for 600 miles east of the Big Island and Maui before there are any significant amounts.
The general low level wind flow in vicinity and over the islands is from east to west with a slight south component over the western half of the island chain. The speed is clocked at 15 to 20 mph.
Outside of the main Hawaiian islands, a field of scattered cumulonimbus clouds /cb/ and towering cumulus /tcu/ lies northwest to north of the islands between 30°N and 25°N from 156°W to 180°W. The highest CB top is 44000 feet. The likely cause for these cumulus based clouds is an upper level low or a shortwave trough located 735 miles west of Kauai at coordinates 23°N 171°W. This feature is moving east at 10 to 15 mph. The shortwave trough extended northeastward to 30°N 163°W. Water vapor imagery shows a 250 mile wide band of thin cirrus grazing both Kauai and Oahu, or 125 miles either side of a line from 20°N 180°W to 20°N 171°W to 25°N 155°W.
Infrared satellite imagery shows several low level troughs over the open waters in the central north Pacific. First trough is located 710 miles east of the Big Island with the coordinates of 30°N 145°W to 19°N 143°W moving west at 12 mph. This trough has become more evident in the past few hours. The second trough is closer to the islands, about 400 miles east of Maui with the coordinates of 25°N 150°W to 20°N 150°W. Third trough is placed 350 miles south of the Big Island from 14°N 155°W to 06°N 153°W. The trough is surrounded by scattered convection about 100 miles west and 150 miles east of the trough. Some of this convection has spilled into our offshore waters. Lastly, the fourth trough is located 380 miles west of Kauai with the coordinates of 27°N 165°W to 20°N 166°W, and is moving west at 12 mph.
In the tropics south of the islands, a cluster of moderate intensity cb's is located between 14°N and 08°N east of 147°W. Highest CB top is 53000 feet.