A broad ridge north of the Hawaiian Islands will dominate our weather into Saturday, with moderate to locally strong easterly trade winds and only subtle day to day wind speed changes. A weak and narrow upper level trough will remain over the region, keeping periods of trade wind showers in the forecast.
A combination of satellite and radar imagery this morning shows a fairly wet pattern for this time of year with periods of shower bands passing through the islands favoring windward mountain areas. The 2 AM HST (12Z) upper air balloon sounding taken at Hilo this morning shows subsidence temperature inversion heights around 6,000 feet elevation. This inversion height supports the passing shower regime across the Hawaii region.
Not much change in the short and mid range forecast guidance as we will see moderate to breezy trades through much of this week. Trade wind speeds begin to weaken this weekend as the ridge north of the islands breaks down due to a passing cold front and an upper level low moving into the Hawaii region from the west. Expect a hybrid light to moderate trade wind and daytime sea breeze wind pattern over all islands by Saturday afternoon. Clouds and a few showers will develop more over island mountains and interior sections each afternoon to evening time period on Saturday and Sunday. Wetter and more unstable trends possibly by early next week.
Looking into the extended range model solutions there are some early hints at an unstable cloud band, the remnants of a dissipated East Pacific front, moving through the islands next weekend. Additional lift for these unstable clouds along this band will be supported by a deepening upper level low drifting over the islands. The combination of low level instability from the remnant frontal band and upper level forcing from the low aloft will likely increase cloud cover and shower activity statewide with possible thunderstorms at some point early next week. The surface ridge weakens as the upper low appears strong enough to deepen a surface trough directly over the islands, creating a light wind regime. Stay tuned as these wet weather impacts for the extended forecast period will likely evolve as the forecast time period grows shorter.
Moderate to breezy trades will continue for the next few days. Low cigs and SHRA possible over windward and mauka locations. MVFR conds possible in SHRA, otherwise VFR prevails.
AIRMET Sierra is in effect for mtn obsc for windward locations of Kauai, Oahu, Molokai, Maui and the Big Island.
AIRMET Tango remains in effect for mod low-level turb over and downwind of terrain.
Fresh to strong easterly trades will persist through the first half of the week as the ridge remains north of the state. This will allow the Small Craft Advisory to continue, particularly across exposed waters and channels. Expect a gradual downward trend through the second half of the week and next weekend due to a weakness forming in the ridge as a front passes far to the north.
Surf along south facing shores will gradually lower today as a medium-period south swell lingers. A fresh long-period south swell will arrive Tuesday, then build to near the seasonal average through midweek before slowly easing Thursday. A more significant long-period south-southwest swell is expected by next weekend due to a storm-force low that passed southeast of New Zealand over the weekend. Satellite data showed a large fetch of 40 to 50 kt winds generating seas of 35 to just over 40 ft, focused toward Hawaii along the 190-degree directional band. Expect surf to begin building locally Friday with 20+ second forerunners, then peak above/around the advisory level over the weekend. For the long range, expect a similar trend to persist through the first week of June as the active pattern persists within our swell window down around New Zealand.
Surf along exposed north and west facing shores will lower today as a small north-northwest swell lingers, but will trend up once again on Tuesday as the late season North Pacific activity continues. Although the bulk of the energy from this next swell will be focused northeast of the islands, expect long-period forerunners to arrive Tuesday, with this source gradually building down the island chain thereafter. Above-average surf is likely by daybreak Wednesday near the peak before lowering Thursday.
Surf along east facing shores will remain rough through midweek, then gradually lower later in the week as the trades ease.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Oahu Windward Waters-Maui County Leeward Waters- Big Island Windward Waters.
Small Craft Advisory until 6 PM HST Tuesday for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
