A wet trade wind pattern begins developing early Thursday morning and matures this weekend into week. Thunderstorms will be possible at times, particularly at night, during this stretch.
Closed low positioned about 300 miles NE of Honolulu is slowly advancing southwestward toward the western half of the island chain where it will reside through at least the first half of the weekend. Observed soundings indicate a trade wind inversion sloping from around 6.5kft at Hilo to 13kft at Lihue, indicating loss of stability over the westernmost islands. Beneath the closed low itself, over the open ocean, visible imagery indicates that showers are not particularly widespread but are topping out around 15kft to go along with a few thunderstorms where lapse rates are steepest, around 6.5C/km.
This environment will spread over the islands late tonight into early Thursday, particularly over Oahu and Kauai where the mid-level low is modeled to reside by tomorrow morning. The first wave of deeper moisture will reach the islands during this time bringing an increase in shower coverage and intensity. Low stability and breezy trades suggest a high likelihood of showers penetrating leeward over Oahu. Lapse rates will gradually steepen to 7C/km by Saturday night suggesting a tendency for shower intensity, but not necessarily shower coverage, to increase during the same time period. Isolated thunderstorms will also be possible, particularly where low-level convergence is maximized within island plumes over the waters west of the islands. Thunder cannot be ruled out elsewhere, but is comparably less likely over the islands themselves. The exception may be the Big Island this weekend, where potential for afternoon thunderstorms will be worth monitoring as mid-level lapse rates steepen. All told, expect a transition to a wetter trade wind pattern with embedded heavy showers and isolated thunderstorms beginning early Thursday morning. Breezy trade winds will favor progressive, fast-moving showers that will not be prone to causing flooding issues.
Renewed upper low development early next week sets the stage for a continued wet trade wind pattern through much of next week.
High pressure far northeast of the islands will maintain moderate to locally breezy easterly trade winds through tomorrow night. An upper low to the northeast will drift southwest and position itself just to the west of state. This upper low will partially destabilize the atmosphere tonight through tomorrow, allowing some bands of passing showers to become more intense, and cloud tops to become higher.
Most of the showers are expected over windward and mountain areas, with the exception of leeward Big Island, where afternoon cloud and shower development is expected. Although there will be increased instability from the upper low, the presence of dry mid level air should inhibit widespread shower coverage, keeping most TAF sites with prevailing VFR and occasional MVFR conditions. Isolated thunderstorms are possible, primarily for the northwest half of the state tomorrow.
No AIRMETs are in effect, however mtn obsc is possible early tomorrow morning due to increasing clouds and showers. AIRMET Tango will likely be needed later this afternoon as the upper low increases the chances of mid to upper level turbulence.
Moderate to fresh trade winds have filled across the Hawaiian coastal waters as a weak surface trough over the northwest waters advances west of the area and dissipates. A Small Craft Advisory (SCA) for winds around Maui County and the Big Island has been extended through 6 PM HST Friday. An approaching upper low may bring isolated thunderstorms to the northern waters from late tonight into the weekend.
Moderate to long period NW swell energy will generally persist over the next couple of days as a series of NW swells filter across the area, maintaining near to above average surf along north-facing shores through Friday. A brief, NNW swell may move through late Friday, but should taper off quickly on Saturday. Surf along north-facing shores will trend down through the weekend.
A series of small, medium to long period S swells will continue to filter into the area through the rest of the week and into the weekend, keeping south-facing shores from going flat. East-facing shores will remain small through the near term, but will see increasing short period action as trades strengthen over the latter part of the week.
Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
