Trades will generally remain breezy through the week with a slight strengthening of winds later this week. Widely scattered shower activity will have many areas receiving measurable rain the next several days. The greatest amount of rain will occur where the most frequent showers will move across windward exposures and upslope mauka. There are low chances for an isolated thunderstorm and or a light snow shower near the Mauna Kea and Mauna Loa summits the next few days.
Wet trades are in full force this week and these overnight hours were another in a string of early pre-dawn mornings where passing showers upon these winds have wetted many statewide communities. Primarily progressive light showers that left behind less than a couple of tenths of an inch of rain since Sunday morning. The highest 12 hour rainfall accumulations of around three quarters of an inch have occurred in the highlands and windward Kauai locations (Wailua, Hanamaulu) and the leeward areas of Oahu (Manoa, Moanalua) and Big Island (Puu Waawaa).
A general weakness in the North Pacific height field, with a series of higher lobes of moisture riding in on moderate to breezy trade flow, will allow a more wet pattern to persist through most of the week. This weak height channel between an upper ridge settled northwest of the state and an upper low positioned a thousand miles off the Baja California Sur peninsula will linger through Friday, or until the northwest upper ridge finally becomes the dominant feature and advances east, traveling north of the islands this weekend. Areas of higher mid layer humidity northeast of Hawaii will travel within southwesterly steering flow in this channel and ride across the state. This will produce periodic lowering clouds and frequent return showers to most communities with the greatest precipitation activity remaining focused to the higher terrain. Weak regional troughing may destabilize the atmosphere just enough that, with the heating of the Big Island interior, late day into evening isolated thunderstorms are a possibility. A deeper boundary layer with weak lifted inversion heights reaching near 12k ft opens the door for the possibility of a light wintery mix to occur near the summits the next couple of days. 500 mb temperatures are not overly cold (around -8 C at coldest) with current freezing levels near 15k ft. The set up of the inversion and any available upper level moisture are in high question but, if the atmosphere destabilizes just enough later in the day, winter weather processes could briefly come into play and produce a brief light snow shower or rain/snow mix in the highest Big Island elevations.
Surface high pressure far northeast of the islands today will travel even further away from the state. The downstream pressure gradient between the ridge and equatorial low pressure will be tight enough to maintain breezy trades through mid week. A new surface high moving in from the west will fill in the wake of weaker surface pressure and establish itself north northeast of the state by this weekend. This will subtly tighten the gradient over the islands and strengthen trades back to breezy to locally windy (within higher elevations) Friday and Saturday. Associated upper ridging will expand north of the state, assist in advecting drier mid layer air around its eastern periphery and across the area. This will lead to a relatively drier weekend. A somewhat deep trough pass across the Central Pacific early next week. The trough does not appear strong enough to push a cold front into the region but it will veer winds more southeast, disrupting trade flow. Greater rain and isolated thunderstorm probabilities will exist over Kauai and Oahu due to the proximity of the base of the trough.
Moderate to locally breezy trade winds will continue to focus low clouds and showers over windward and mountain areas through the forecast period. A band of moisture riding in on the trades this morning has led to increased shower coverage across the island chain, bringing more frequent MVFR conditions to windward and mountain areas. As such, AIRMET Sierra for tempo mountain obscuration is now in effect for all windward areas. Shower coverage is expected to decrease later this morning.
Moderate to locally strong trades will persist through mid week as a weak, broad ridge remains north of the state. A small craft advisory (SCA) is in place for the typical windy waters around the Big Island and including the Alenuihaha channel through Tuesday. Trades will build further to moderate to strong speeds during the latter half of the week as a developing high far northwest tracks east just north of the state. As a result, the SCA could expand in coverage at that time.
A series of small to moderate, long-period northwest to north- northwest swells (320-340 degrees) will fill in throughout the week, keeping surf along north and west facing shores elevated. The first of these swells (320-330 degrees) have reached the offshore buoys and is currently filling in locally. Overnight swell energy has not pick up as much as previously thought, with much of the energy passing north of the state. Thus, have lowered expected swell heights for the day closer to guidance. This swell will peak today, producing near average surf along north and west facing shores before subsiding Tuesday. The next swell (330-340 degrees) will be similar in size and peak near average late Tuesday before subsiding Wednesday afternoon. A moderate, long- period swell (330-340 degrees) could produce advisory level surf for north and west facing shores late Wednesday into Thursday. Another slightly larger, moderate, long- period swell (330-340 degrees) will fill in Friday through Saturday, keeping surf elevated along north and west facing shores above advisory levels.
Background, medium- to long-period south to southwest swells will continue to filter into the area through the week, keeping surf along south facing shores from going flat. Small, choppy surf along east facing shores will hold through mid week. Rough, choppy surf will return for the latter half of the week as the trades strengthen.
Small Craft Advisory until 6 PM HST Tuesday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
