Typical moderate to locally breezy trade winds will continue through this weekend and bring mainly night and morning scattered showers to windward and mauka locations. By early next week, a deepening trough northwest of the state will pull up a more moisture rich tropical air mass over the state and increase precipitation chances. Some of the rain could be heavy, especially mid- to late next week.
Water vapor imagery loop from this morning showed very dry air in the mid and upper levels entrenched over and east of the state, aside from a thin layer of high clouds passing over the Big Island. This can also be noted on the latest 12Z RAOB soundings from both Hilo and Lihue, with a rather dramatic dry layer residing above a 6000 ft deep boundary layer temperature inversion. At the surface, an area of high pressure, anchored roughly 1000 miles north of Hawaii continued to drive moderate to locally breezy east northeasterly trades. Scattered low clouds caught up in this flow have been pushing towards windward portions of the islands for the last several hours. Several windward stations on each island picked of a few hundredths of an inch of accumulation, but nothing of significance.
Today through Friday, the pattern will change very little with moderate to locally breezy trades driving light scattered showers along mainly windward and mauka locations. The greatest shower activity can be expected to occur during the night and early morning hours. By this weekend, the aforementioned high well north of the area drifts east as a frontal system approaches. This should allow the local surface pressure gradient around the region to ease and trades to weaken slightly. Low clouds and showers will be isolated at best with a shallow boundary layer in the lower levels and dry air in place aloft.
By Monday, both the GFS and ECMWF begin to dig a broad upper trough well northwest of the state. Light background flow around the islands will become southeasterly, veering to southerly late Tuesday. With such light flow and slightly deeper lower level moisture in place, afternoon sea breezes could help initiate a few showers over interior portions of the islands.
Mid to late next week, models continue to deepen the upper level trough west of the islands and induce a low and cold front at the surface. Guidance has been consistent with this solution for the last several runs, so confidence is increasing. This would set the stage for tropical moisture being drawn northward across the Hawaiian Islands by deep layer southerly or south southwesterly flow out and a destabilizing atmosphere aloft. As it stands, the ECMWF generates a more compact and robust parent surface low compared to the GFS. Nonetheless, both models indicate the potential for gusty southerly winds and rounds of moderate to heavy rain. Thus, flooding may once again be a concern. The details for timing, location, and strength will need to be refined in the coming days.
Moderate to locally breezy ENE trades persist through the period, with a gradual weakening expected through the remainder of the week. Embedded isolated trade showers continue, primarily affecting windward and mauka areas, occasionally lowering ceilings and visibility to MVFR at times. Otherwise, VFR conditions are expected to prevail.
No AIRMETs are currently in effect.
Weakening high pressure north of the islands will shift eastward over the next few days, leading to a weakening of the trade winds into the moderate range. Winds could then veer out of the southeast next week ahead of an approaching front. A Small Craft Advisory is in effect for the typically windier waters around Maui County and the Big Island through tonight.
The current mid period northeast swell (7 ft, 9-11 seconds at 040 degrees) will gradually decrease today as the northeast trade winds slowly decrease. As a result, east shore surf will gradually decline today through the rest of the week, but will still retain a little size and chop due to the moderate to locally breezy trades.
The next small, medium-period northwest (310 degree) swell will arrive this weekend into next week and bring a small bump in surf for north and west facing shores. A larger mid period northwest swell is then possible by the middle of next week. Along south facing shores, expect small reinforcements to sustain small surf through the week until a slightly larger long period south swell fills in early next week.
Small Craft Advisory until 6 AM HST Friday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
