Daily Weather

March 1, 2015 3:43 AM HST

Synopsis
Moist and unstable conditions will persist across the eastern end of the island chain through Monday as a surface trough weakens nearby and a mid to upper level trough remains over the state. A moderate to locally breezy northeasterly flow will focus clouds and showers on windward slopes over the western portion of the state. Wet and somewhat unstable trade wind weather will likely continue through at least mid week, especially for the eastern end of the state.

Discussion
Isolated heavy showers and thunderstorms continued over and around the Big Island and Maui overnight. Pooled moisture associated with a surface trough near the Big Island has been activated by a mid to upper level trough overhead. Meanwhile, shallow moisture within the northeasterly low level flow over the western end of the state has been producing some showers along windward slopes. The overnight soundings captured the contrast. The Lihue sounding revealed an inversion based around 7000 ft with precipitable water /pw/ values of less than an inch, and the Hilo sounding showed no inversion and a nearly saturated atmosphere below 25000 ft with PW values over 1.6 inches. Lingering vertical wind shear has caused at least one thunderstorm cell nearby to become briefly strong.

Moist an unstable conditions will continue across the Big Island and possibly Maui into Monday. The surface trough will stubbornly hang on near and north of the Big Island, keeping enhanced low level moisture in place. A jet stream aloft will remain over the islands, but the diffluent portion of the jet will shift slowly eastward, pushing high clouds and greatest instability east of the state. However, a mid level trough will remain stalled across the central portion of the state, and with 500 mb temps remaining very cold at -12 to -14C, the atmosphere will continue to be unstable near the Big Island and possibly Maui. As a result, isolated thunderstorms and heavy showers remain a threat, and some thunderstorms could be strong today. Winter weather also remains a threat on the Big Island summits above 12500 ft. The main concern is freezing fog this morning followed by the possibility show showers this afternoon.

Across the western half of the state, expect continued moderate to locally breezy northeasterly winds to deliver showers to windward slopes through Monday. The northeast winds are being generated by surface high pressure about 1400 miles northwest of Kauai.

Heading into the middle of the work week, wet trade wind weather is expected. A weakened surface trough will linger north of the Big Island, allowing strong high pressure northwest of the islands to exert greater influence and cause trade winds to spread across the entire state. The greatest amount of low level moisture will initially be over the Big Island and Maui, but the GFS suggests that some increase in moisture over the western end of the state, likely producing a wet trade wind pattern. Mid to upper level troughing will also linger, raising the possibility of continued instability, but confidence is not high enough at this time to include heavy showers.

Uncertainty increases toward weekend as the GFS and ECMWF models diverge. The GFS is developing a rather strong low-latitude surface trough west of the islands on Saturday and pulling it overhead on Sunday. We have opted to go with the more conservative ECMWF which shows a drier trade wind flow.

Broken clouds are clouds which cover between 6/10 and 9/10 of the sky while scattered clouds is a sky condition when between 1/10 and 5/10 of clouds are covered.
Advisory and Warning Criteria
Location Advisory Warning
North-Facing Shores 15 Feet (8') 25 Feet
West-Facing Shores - Big Island 8 Feet (4'+) 12 Feet
West-Facing Shores - Remaining Islands 12 Feet (7') 20 Feet
South-Facing Shores 8 Feet (4'+) 15 Feet
East-Facing Shores 8 Feet (4'+) 15 Feet

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