Issued: Feb 06, 2016 8:30 AM HST
Based on data through 8:00 am Feb 06 2016 HST
Satellite images centered over the main Hawaiian islands show that sky cover varies wildly from island to island, ranging from completely clear to entirely overcast. Initial visible images of the day indicate that sunny skies prevail over Niihau, Kauai and the Big Island. Mostly sunny skies are over Maui county, but scattered to broken showery low clouds are noted over near shore waters south of Maui, and west of the Big Island. Meanwhile, skies are completely overcast over Oahu and surrounding near shore waters, due to the presence of a slow-moving, northeast to southwest oriented, band of low clouds.
Not only does sky cover vary drastically from place to place across the state, animation of satellite imagery shows that strong gravity waves moving rapidly northwest to southeast across the area are dramatically changing cloud cover over any particular area. One example is the sky cover over Oahu, and its evolution over the past 6 hours. Skies were nearly clear over the island around 2 am but then low clouds quickly increased between Oahu and Kauai by 4 am, only to see a brief period of clearing around 6 am transition to the current overcast conditions. Low clouds are moving toward the west near and south of the Big Island, are nearly stationary near Maui and Oahu, and are moving toward the southeast near and northwest of Kauai. These cloud motions highlight the weak low-level convergence that is likely to be at least partially responsible for the showery low clouds near Oahu this morning.
The leading edge of a 150 to 200 mile wide band of broken to overcast showery and stable low clouds lies about 175 miles northwest of Kauai, which is moving southeast at about 20 mph. This cloud band marks a cold front that extends from near 30°N 153°W to northwest of Kauai, to 18°N 180. Northwest of this boundary, broken to overcast showery cumulus and towering cumulus clouds prevail, indicative of an unstable and cold air mass.
Water vapor imagery shows that a persistent ridge of high pressure in the mid and upper levels of the atmosphere continues to prevail south and southwest of the islands, but also shows that it has weakened and been suppressed southward from where it was earlier in the week. This appears to have occurred in response to a southward dip in the jet stream to the north of the islands, with a trough aloft supporting the surface front described above. Although this trough is not very sharp south of 30°N, it lies along a line from 30°N 155°W to 25°N 156°W to near Kauai and Oahu to 21°N 166°W, and is moving rapidly east across the area at speeds between 60 and 70 mph.