Issued: Sep 21, 2014 2:30 AM HST
Based on data through 2:00 am Sep 21 2014 HST
Regional satellite imagery shows that low clouds have diminished over most of the main Hawaiian islands since Saturday evening, although cloudiness of varying types continues near and over the Big Island. Latest imagery in fact shows isolated cloud tops extending as high as 40 thousand feet over leeward Big Island waters, and also about 150 to 275 miles east of windward Big Island, where thunderstorms are likely occurring. Water vapor imagery shows a sharp trough aloft axis straddling the island chain near Maui, and the presence of this feature is likely adding instability to an area where increased low level moisture is noted.
Light winds prevail over the islands, but favor an easterly direction at cloud level. While the light easterly component of the wind at cloud level has pushed few to scattered low clouds and isolated showers over windward portions of the islands overnight, the overall trend has been toward clearing of skies over the islands since sunset on Saturday. Meanwhile, scattered to locally overcast layered clouds are over the Big Island, adjacent leeward waters, and over waters east and southeast of the Big Island. Increased moisture in the vicinity of the Big Island is associated with weak tropical disturbances passing to the south, with this moisture being enhanced due to a trough aloft in the vicinity.
Water vapor imagery shows that the axis of this sharp trough aloft is over Maui county, moving southeast near 10 mph. With the trough aligned in this manner, winds aloft over Kauai and Oahu are from the east-northeast, while winds aloft over the Big Island are from the southwest. A ridge aloft is seen to the distant southeast of the islands, and strong southwesterly winds aloft are between the trough and the ridge. This is resulting in upper level winds that are not conducive for the further development of a weak westward-moving tropical disturbance located about 425 miles southwest of Oahu. Associated showers and thunderstorms have diminished near this disturbance over the past 6 hours, but moisture extending north and east from the disturbance has helped to fuel the thunderstorms described earlier near the Big Island.
While most islands lie in an area of relatively few low clouds on the large scale, an east to west oriented cloud band, marking a stalled and weakening front, is about 500 to 600 miles north of Oahu. Another cluster of nearly stationary low clouds is about 300 to 400 miles north of Maui county, associated with a weak surface trough that extends southward toward Oahu.