Light east to southeast winds and mostly dry conditions will prevail through early next week, with a few showers possible along windward slopes and coasts at night and over the island interiors each afternoon. A series of cold fronts will approach from the northwest next week, bringing an increase in showers, particularly over the western end of the state for the latter half of the work week.
This afternoon's radar and satellite imagery show a few clouds and isolated showers moving into windward areas of the eastern end of the state on the light to moderate east-southeasterly trades. Over the western half, the low level clouds and isolated showers slow and take on a more southeasterly direction as a weak cold front approaches from the northwest and relegates the island chain to the southwestern periphery of a surface ridge. In addition, sea breezes have developed across most of the state, increasing clouds over leeward and interior areas and bringing the occasional light shower.
With a progressive pattern to our north, a series of three fronts will approach the state from the northwest over the next week, but will likely dissipate just north of the area or only briefly brush Kauai. The first will near today into tomorrow, the second on Tuesday, and the third on Thursday into Friday, with each front seeming to nudge slightly closer to the state. As each front nears, the local pressure gradient will relax, bringing light background winds that will allow for land and sea breezes to become more prevalent. The surface ridge centered to the far northeast will try to reestablish itself after each front passes to our north, but the quick succession of fronts will only allow trades to return briefly to the area before the local pressure gradient weakens yet again and land and sea breezes dominate.
The fluctuations in the prevailing wind speed and direction will determine where any showers are likely to develop each day. With the lighter wind days expected today, Tuesday and Thursday as each front nears, a land and sea breeze pattern featuring windward showers at night and interior showers during the day will dominate, with a slight uptick in showers over the western end of the state possible. On Sunday into Monday, moderate easterly trades will try to build back in, with clouds and showers favoring windward and mauka areas, particularly overnight, although isolated sea breeze showers will be possible in the afternoon. On Wednesday, winds will turn southerly and increase boundary layer moisture which should cause inversion heights to rise, bringing an increase in showers to mainly leeward areas, particularly over the western end of the state. On Friday into Saturday, moderate east- southeasterly trades should return, with showers favoring windward and mauka areas over the eastern end of the state and sea breezes bringing showers to island interiors of the western half of the state.
With all of this said, mostly dry and stable conditions are expected to persist through Tuesday due to limited available moisture and increasing stability as mid-level ridging builds in from the west. So any showers that move in on the trades or develop across inland areas with the sea breeze should be brief and light. With the southerly flow on Wednesday into Thursday, low level moisture will increase and 500 mb heights are expected to gradually fall into the weekend, allowing for a slightly more active shower pattern ahead of and near the frontal boundary as it approaches on Thursday into Friday.
A ridge just north of the islands will bring light and variable winds statewide through tonight. Sea breezes resulted in interior cloud build ups and mountain obscurations this afternoon and will likely linger until around sunset. AIRMET Sierra is posted for tempo mountain obscurations above 2500 feet for Kauai, Oahu and Molokai through 04Z today. VFR conditions will prevail across the rest of the state. Land breezes will clear out skies tonight, followed by the return of light trade winds Sunday.
High pressure remains over the northeastern Pacific, with a ridge extending southwest over the islands. The ridge will keep light and variable winds in place for the western end of the state, and light to moderate southeasterly winds for the eastern part of the state through tonight. Light to moderate easterly trade wind flow will return Sunday into Monday as high pressure briefly rebuilds north of the state. A front is forecast to pass just north of the islands on Monday, weakening wind speeds over the islands again into Tuesday. A very short period of east to east-southeast trades returns for Tuesday afternoon into Wednesday, before quickly shifting out of the south Wednesday afternoon ahead of a front approaching from the northwest.
The current west-northwest swell will hold through this evening before a long-period reinforcement builds in tonight through Sunday. Surf heights are expected to reach advisory levels after midnight tonight and peak Sunday afternoon along north and west facing shores. A High Surf Advisory (HSA) has been issued for north and west facing shores of Niihau, Kauai, Oahu, Molokai and north facing shores of Maui through Sunday night. If this swell comes in larger than expected, may need to add west facing shores of the Big Island to the HSA, as well as issue a Small Craft Advisory for hazardous seas of 10+ feet. This swell will slowly lower through the first part of next week. A slightly larger northwest swell is expected to build in late Tuesday and linger through Wednesday, resulting in peak surf heights near- warning level for north and west facing shores. Small surf is expected along south and east facing shores.
High Surf Advisory from midnight tonight to 6 AM HST Monday for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North-Windward Haleakala.
