Daily Weather

Mar 02, 2015 4:00 AM HST

Synopsis
Showers over much of the state, and thunderstorms around the Big Island, will diminish slowly through mid-week. A very moist air mass surging from the south, and cool air lingering overhead and to the north, will spread stronger showers over the islands again from Thursday into the weekend. Trade winds will persist over most of the state through the forecast period.

Discussion
Deeply stacked troughing lingered around the Big Island. At the surface, the trough represented the tail end of a mostly dissipated front. Aloft, loose, baggy troughing extended over most of the state from the deeper vortex located mostly over the gulf of Alaska. The most significant dynamics now occurred just to the northeast of the offshore waters, but sharp temperature and humidity gradients kept conditions unstable over northeast through south parts of the offshore waters, and to a lesser extent over Maui and Hawaii counties.

These differences in air mass occurred in two different ways. Vertically, despite its poor dynamics, the shallow mid-level trough overhead included an impressive cold pool. Similarly, though to a lesser extent, the air mass at low levels over the northwest half of the state still retained some of its cool and dry polar character. The extremely moist air mass over the southeast end of the state contrasted sharply with both the air above and to its northwest. Short waves flowing around the edge of the low aloft also contributed to the thunderstorm development over the offshore waters, but simple thermodynamic instability seemed like the most significant ingredient this morning.

That being the case, the convection likely will continue through the early part of the week even if the trough aloft fills considerably as guidance suggested, especially once solar heating becomes important again later this morning. Thunderstorms as such will develop further southeast with time, becoming limited mainly to around the Big Island today, but towering cumuli may continue to produce locally significant showers through the first half of the week.

The Risk of afternoon convection also motivated extending the winter weather advisory over the Big Island summits through this afternoon. With the cold pool firmly in place, any convective showers forming over the Big Island slopes will produce mostly frozen precipitation there.

By Wednesday, guidance predicted the trough aloft to reform and sharpen along a zonal orientation, providing marginally increased dynamic support again. Although consensus was somewhat indifferent, some solutions showed a significant surge of water vapor from the deep tropics as well. The air mass over the northwest end of the state likely will modify by that time, but still should remain substantially cooler and drier than this surge from the south, especially if another weak frontal remnant reaches the state as most solutions showed. Thus, by Thursday, convection, or at least significant shower development, should start spreading north across the state again, even though winds will remain northeasterly.

Depending on how strong the moisture surge from the south turns out to be, increased showers may well continue through the second half of the week and into the weekend. Recent runs of the GFS have predicted the area of maximum pops to retreat to the south again a little more quickly than earlier runs did, so clouds and showers have not been increased for the weekend over earlier thinking. However, further adjustments may be needed with time.

Marine
A trough, the south end of a largely dissipated front, languished near the Big Island, keeping winds light and variable there, but moderate to strong from the northeast around Kauai. More widespread windy conditions are expected by mid-week.

A large north-northwest swell should produce surf above the threshold for a high surf advisory today, so a high surf advisory remained in effect. The swell also built seas to the threshold for a small craft advisory over most of the coastal waters. Smaller north-northwest swells are likely to arrive later in the week.

Watches/Warnings/Advisories
High surf advisory until midnight HST tonight for Niihau, Kauai windward, Kauai leeward, Waianae coast, Oahu north shore, Oahu Koolau, olomana, Molokai, Maui windward west, Maui central valley, windward Haleakala.

Winter weather advisory until 6 pm HST this evening for Big Island summits.

Small craft advisory until midnight HST tonight for Kauai northwest waters, Kauai windward waters, Kauai leeward waters, Kauai channel, Oahu windward waters, Oahu leeward waters, Kaiwi Channel, Maui county windward waters, Maui county leeward waters, Pailolo channel, Alenuihaha channel, Big Island windward waters.





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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