Trade winds will strengthen to become moderate to locally breezy this week. The main focus for this week is on the increasing shower trends and the potential for isolated thunderstorms as an unstable upper low approaches the islands from the north and bands of deep moisture filter across on the trades. Late this weekend into early next week, conditions should improve as the upper low lifts away from the state.
This afternoon, satellite and radar show a typical trade wind pattern with limited clouds and showers moving into windward and mauka areas of the smaller islands and more extensive cloud cover over the Big Island. In addition, an upper level low can be clearly identified in the water vapor satellite imagery, located about 350 miles to the northwest of Kauai.
Trade winds will continue to build back over the state through the week as the surface trough northwest of the state dampens out and high pressure to the distant northeast builds. Generally light to moderate easterly trades are expected through this evening, then as the high builds and tightens the local pressure gradient, they'll strengthen to become moderate to locally breezy for the rest of the week.
The focus for this week's forecast remains on the increasing shower trends as the upper low approaches the islands from the north. This feature will bring instability aloft, and batches of moisture will be transported across the area in the trade wind flow, increasing the potential for enhanced trade wind showers and isolated thunderstorms. The first band of moisture (remnant moisture from an old front) is expected to arrive Wednesday night, with models showing precipitable water values (PWats) increasing to 1.6-1.8 inches. This band will be quickly followed by a second on Thursday night into Friday. While the low level moisture and instability aloft are notable, the surface "lifting mechanism" to force that moist air to tap into the instability aloft looks weak. Orographic lift as the trades interact with island terrain will likely focus the bulk of the heavier shower activity over windward and mauka areas through the period, though weak surface convergence bands may also provide an additional source of lift. Additional batches of moisture will filter through on the trades while the low lingers over the western end of the state through the weekend, prolonging the potential for enhanced trade wind showers and isolated thunderstorms.
Storm total rainfall estimates for this multi-day event may exceed 2 inches in terrain favored locations. It remains too early to pin down any islands for flooding, however a Flood Watch may be needed at some point as this event unfolds. Any flooding potential may tend to favor the western half of the state, as will the thunderstorm potential, as these islands will be closer to the unstable upper low center. The Big Island may also see isolated thunderstorms from late Thursday morning to early evening as the low level convergent band drifts through the island.
Sunday into early next week, global model guidance begins to diverge, but it looks like mid level ridging will try to briefly build in from the southeast and establish a more stable trade wind weather regime. Light showers will focus over windward and mauka areas, particularly during the overnight and early morning hours. Then long range weather models are hinting at yet another unstable upper low drifting towards the Hawaiian Islands early to midweek next week, potentially triggering yet another round of wet weather across the island chain. Stay tuned.
Moderate easterly trade winds will strengthen to moderate to locally gusty levels tonight into Wednesday. An upper level low will sag south over the western end of the island chain, increasing shower trends by early Wednesday morning. Brief periods of MVFR conditions are possible in passing showers mainly over windward and mountain areas overnight and early morning hours. Iso IFR conditions are possible in heavier showers.
AIRMET has been issued for moderate turbulence below FL250, but turbulence will be most likely at the lower levels farther south near the Big Island and at the upper portion of the range farther north near Kauai.
Light to moderate trades become moderate to fresh by Wednesday as weak surface troughing over the western end of the state advances west and dissipates. The Small Craft Advisory (SCA) for elevated seas will be allowed to expire this afternoon while the SCA for winds around Maui County and the Big Island remains in effect. An approaching upper low brings a chance for thunderstorms to area waters tonight into the weekend.
Moderate to long period NW (310-320) swell brought an early morning peak to N and W facing shores as indicated by the PacIOOS Buoy at Waimea Bay where observations hovered near the High Surf Advisory (HSA) threshold for much of the day. During the last few hours, energy has begun to consolidate in the lower energy bands even as the observed significant wave height remains rather steady. As such, the HSA will also be expired this afternoon.
Small, medium period S swell continues through the week with minor longer period pulses arriving tonight and Saturday. E shores remain small through the near term, but will see increasing short period action as trades strengthen mid-week.
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
