Daily Weather

Dec 19, 2014 4:00 AM HST

Synopsis
Moderate trade winds will keep mostly light showers focused over windward slopes into the weekend. From Sunday night into Monday, a front will spread rain, cold air, and strong northerly winds across the state from the northwest. Unsettled weather will continue through about Tuesday, before returning gradually to trade-wind conditions by the end of next week.

Discussion
Surface observations around the main Hawaiian islands reported mostly moderate trade winds. Guidance strongly supported a decrease in wind speeds through the day, but a few outlying observations suggested that this process may take longer than currently expected. For now, we will follow prior thinking.

The inversion height and strength, cloud cover, and shower activity still approximated climatology. Radar detected slightly fewer showers over the coastal waters, but orographic lifting continued to squeeze out widespread, if generally light, accumulation over rain gauges, despite persistently low precipitable water. Aloft, strong and broad ridging kept cirrus layers from the ITCZ south of the Big Island, but only just.

Guidance still agreed fairly well that a long-wave mid-level trough will dig toward the state from the northwest through the weekend, effectively displacing the ridge by Sunday evening, and then sharpening toward the southwest into Tuesday. This development aloft will support the approach of a cold front at the surface. Through about Sunday, the subtropical ridge will recede toward the east ahead of the front, causing surface winds around the islands to diminish and become more southeasterly. From Sunday night into next week, the front will work its way southeast across the state. With significant support from the digging trough aloft, the front will have the opportunity to produce widespread rain. The push of cold air at low levels behind the front also will bring strong northerly winds.

Perhaps most importantly, a very cold pool of air aloft will accompany the mid-level trough. This cold pool will destabilize the atmosphere significantly, introducing a non-trivial Risk of thunderstorms around Monday, mainly from Molokai northwest. A slight chance of thunderstorms has been introduced into the forecast following the local tool run off of model consensus. Strong vertical wind shear will improve the ventilation of any thunderstorms that do develop, potentially allowing some to become severe. It remained much too early to consider a severe thunderstorm watch, but possible high impacts required continued vigilance with respect to this matter.

The cold pool also will introduce the possibility of winter weather over higher elevations of Maui and the Big Island. This far out, much remained uncertain, but generally speaking the available moisture appeared limited largely to below the summits through most of the forecast period. Where both cold temperatures and available moisture coincided, a slight chance of light snow has been included in the forecast as a place holder. Deeper moisture, or thunderstorms developing further south than expected, could increase the Risk of winter weather over the summits dramatically.

From Wednesday into Friday of next week, the trough aloft will lift out toward the northeast, and conditions will return toward climatology. Recent solutions tended to draw out this process, so the relatively quick return of trade-wind weather in the forecast may be too optimistic, and should be reevaluated as more runs become available. On the whole though, trade-wind weather should return by next weekend.

Aviation
Windward and mauka areas will continue to experience passing showers with embedded isol MVFR conditions. Heavier cloud cover and showers remain over Kauai and Big Island this morning, therefore airmet sierra continues. Elsewhere, airmet sierra has been cancelled.

Moderate and gusty trade winds will start to ease today as a ridge begins to build just north of the islands. This will likely allow for the cancellation of airmet tango for low level turb.

There is also mention for possible upper level turb between fl280 and fl420. A jet, with Max winds to 100 kt, sits just south of Hawaii. At this time have not received any pireps, this will be upgraded to an official airmet if reports are received.

Marine
With winds expected to diminish today, and the large northwest swell slowly passing the state, waters around Kauai and most of Oahu should fall below the criteria for a small craft advisory /sca/ this morning. Elsewhere, the advisory will remain in effect through 6 pm HST today. Conditions should subside further this evening, but then another large northwest will produce SCA conditions again by Saturday night.

An even larger NW swell is expected to arrive on Sunday, likely pushing surf heights into warning levels over both north- and west-facing shores of most islands. Deep low pressure far north of the state may continue to produce swells keeping surf elevated to near the advisory levels through the second half of next week.

Watches/Warnings/Advisories
High surf advisory until 6 pm HST this evening for Niihau, Kauai windward, Kauai leeward, Waianae coast, Oahu north shore, Oahu Koolau, Molokai, Maui windward west, windward Haleakala.

Small craft advisory until 6 pm HST this evening for Kaiwi Channel, Maui county windward waters, Maui county leeward waters, Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island windward waters, Big Island leeward waters, Big Island southeast waters.






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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