Daily Weather

Feb 26, 2015 3:43 AM HST

Synopsis
Winds will become light and variable over the smaller islands today, with southeast winds around the Big Island, as a surface trough settles over the state. The trough will remain near Kauai and Oahu tonight and Friday before drifting east during the weekend. The nearby trough will act to focus clouds and showers over the islands, and locally heavy rain will be possible from Friday night into the weekend as the atmosphere becomes increasingly moist and unstable. A rather unsettled wet trade wind pattern appears likely to continue from the weekend into next week.

Discussion
Shower activity has increased southeast of the Big Island and over the windward sections and adjacent waters of the smaller islands during the night. Rain gauge data show scattered mostly light amounts thus far, with a few gauges on Maui approaching a half inch of rain. Surface analysis depicts a weak trough near Maui, high pressure centered over the northeast Pacific, about 1700 miles NE of the state, and a stationary front about 300 miles NW of Kauai. The high to our northeast has been supporting trade winds across the state, but these are currently being disrupted by the local trough. Aloft, the region lies near a col point between ridges to the northeast and south, and troughs to the southeast and northwest. The 12Z Hilo sounding depicts increased moisture and an eroding inversion, with pwat up to 1.27 inches. The 12Z Lihue sounding remained drier and more stable, with an inversion based near 6000 feet and pwat of 1.14 inches.

A generally wet and unsettled pattern is shaping up across the state through the next several days, with a complex evolution of features occurring both at the surface and aloft. The surface trough will settle over the western half of the state today and tonight, with the remnants of the frontal band to our west drifting eastward and merging with the trough. This will greatly weaken trade winds across the state, with seabreezes likely developing across many areas this afternoon. The mid/upper level trough over the central Pacific will also drift slowly eastward during this time, with associated cooling aloft resulting in gradual destabilization over the islands. Ample moisture and instability will be in place, but with no apparent strong forcing and only weak low level convergence along the trough it is difficult to predict just how widespread shower activity will be. For now the existing forecast of generally scattered showers with highest rain chances windward and mauka seems reasonable.

The low level trough will drift slowly back eastward Friday through Saturday, with stronger northeast winds and a drier airmass edging into far western parts of the state. Cold temperatures aloft (ranging from -11C to -14C) should provide ample instability, which combined with plentiful moisture across most areas will yield an enhanced potential for some locally heavy downpours. Have added mention of locally heavy rains to the forecast for all areas except Kauai Friday night and Saturday. Isolated thunderstorms cannot be ruled out, but confidence remains too low to include in the forecast at this time.

The low level trough should edge to the east of the Big Island Saturday night or Sunday, allowing a wet trade wind pattern to develop over the state and persist into early next week. Continued troughing aloft over the central Pacific will maintain unusually cold mid level temperatures, suppressing the typical trade wind inversion. With model consensus maintaining above normal moisture over the state, locally heavy downpours and isolated thunderstorms will remain possible during this time. Have not added such wording to the forecast for the later periods beyond Saturday yet, but later shifts will need to consider doing so if current model trends persist. Highest rain chances should focus over windward areas, but with deeper than usual moisture in place would expect some showers to spill into leeward areas as well.

Aviation
Moist east to southeast flow will keep scattered SHRA moving across the islands over the next 24 hours. Predominantly VFR conditions are expected, but periods of MVFR cigs will be possible in -shra.

Airmet sierra is in effect for mtn obsc across Oahu, Molokai and Maui, with conditions likely to continue through mid to late morning. Airmets for mtn obsc may also be needed across some of the other islands later this morning.

Marine
Westerly swell continues to run around 6 feet and 14 seconds at Barbers Point buoy, and has been slow to subside during the night. We still expect this swell to decrease later on today, but high surf will remain possible across the west facing shores of the Big Island this morning, and have extended the high surf advisory for that area through noon. Elsewhere, short-period wind waves at windward buoys such as Pauwela north of Maui have increased during the past 24 hours in response to increased trade winds over and east of the islands. Although local winds will be diminishing today, the lingering fetch of E winds upstream of the islands will maintain elevated, but just below advisory- level, surf along E facing shores through Friday. Not much surf is expected through the weekend. A NNW swell is expected to arrive late Sunday, peak on Monday, and lower on Tuesday, with peak surf heights just below advisory levels along N facing shores. The latest Oahu surf discussion /srdhfo/ contains details on the sources of the swells.

The overnight ascat pass found winds of 15 to 20 kt across the coastal waters. With seas below 10 feet across the area, and winds expected to drop further today, the small craft advisory has been discontinued for all waters. Lighter winds will prevail today through Friday as a surface trough lingers over the islands. Trade winds should build back in across the state during the weekend into early next week, with some chance that winds may approach advisory levels across the windier zones.

Watches/Warnings/Advisories
High surf advisory until noon HST today for west facing shores of the Big Island.



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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