The downstream pressure gradient from a near stationary area of surface high pressure far northeast of the islands will maintain light to breezy trades the next several days. Lessened stability from a weak upper low northwest of Kauai, along with higher moisture pooling north of the state, will increase shower frequency and coverage primarily north of the island chain through Monday. There is a decent chance for periods of heavy rain on or around Kauai with low probabilities for isolated weekend storms. Stable, drier weather is expected to return early next week.
A relatively quiet early Friday morning precipitation-wise as the islands lay under weak ridging and drier air advects in on moderate trades. High level clouds are rotating clockwise across the area as upper convective cirrus blow off southwest of the chain gets caught up within the upper level flow around the ridge. Thursday evening's highest rainfall focused along windward Big Island where a few South Hilo sites picked up a quick quarter to half of an inch. Generally, light trades rule with the exception of the windier windward exposures/interior channels and wind-sheltered leeward slopes where drainage breezes blow offshore.
A pattern change developing today will increase weekend rainfall over the more western islands of Oahu and Kauai and waters generally north of 21N latitude. Stability will begin to erode today with the evolution of a weak mid to upper level low forming just north northwest of the state in tandem with a mid level trough moving in from the east. These features will likely provide a boost in rain chances mainly north of the islands with higher precipitation passing into Oahu and Kauai later tonight through Sunday. The pros for more widespread rainfall will be falling heights, enhanced upper level vorticity, and a surge of unseasonably high moisture advecting in on modest trades, deepening the resident boundary layer. The majority of the GFS and EC ensemble guidance QPF members still depict a distinct swath of higher rain paralleling 20 to 25 N. While confidence is moderately high that more western islands will experience a wetter weekend (than the eastern half), the chance of thunderstorms remains fairly low. Upper levels should cool enough to provide the needed instability required to place isolated storms in northern Kauai and surrounding waters from late Saturday through early Monday. The proximity of the low across the far northern maritime will likely focus organized convection over the far northern offshore waters this weekend. The timing of the heaviest rain will depend upon the evolution of the upper low in conjunction with the greatest moisture.
A stable summer-like trade wind pattern will return by late Monday as lingering moisture advances west and drier air filters in its wake. The upper level low will no longer be an issue from Tuesday onward as it meanders off to the northwest and ridging re-establishes itself over the islands.
Moderate to locally breezy trade winds will prevail across the islands with occasional passing showers favoring the typical windward and mountain locations. Periods of MVFR ceilings and visibility in SHRA will be confined to windward areas of all islands, as well as the South Kona slopes of the Big Island during the afternoon and evening hours. Otherwise, VFR conditions are expected.
Due to the locally breezy trades, AIRMET Tango for low level turbulence below 7,000 feet will continue through the day before wind speeds gradually decline tonight. AIRMET Sierra is also in effect for mountain obscuration over the N through E sections of the Big Island through the morning hours.
High pressure will remain nearly stationary far northeast of the state through the middle of next week. Fresh to locally strong trades will persist through the day then drop slightly as a weak low- level trough moves through the state from the east through the weekend. The Small Craft Advisory (SCA) is currently in effect through this afternoon for the windier waters and channels around Maui County and the Big Island. The SCA will likely be scaled back to just the Alenuihaha Channel through tonight. Trade winds will continue to gradually decline to gentle to locally fresh through the forecast period.
A mixed southerly swell will provide average surf along south facing shores through early Saturday. A new small, long period southwest swell will fill in slowly Saturday, peak over the weekend before declining slowly through early next week.
Surf along east facing shores will continue to remain choppy due to the fresh to locally strong trade winds before declining slightly over the weekend. Surf along north facing shores will remain nearly flat through the forecast period.
Peak high tides associated with the lunar cycle could lead to minor coastal flooding along shorelines and low-lying coastal areas next week Monday through Wednesday during the daily peak tide each afternoon.
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
