textproduct: Honolulu

SYNOPSIS

Upper-level troughing over the Hawaiian Islands will begin to lift northeastward, leaving behind lingering low-level moisture resulting in continued isolated shower activity over Maui County. Unsettled weather resumes this weekend as moisture moves over the islands ahead of the next approaching frontal system. Trades return briefly by Tuesday as a surface high passes north of the islands. By the end of next week, another frontal boundary is anticipated to move over the Hawaiian Islands, potentially leading to more unsettled weather.

DISCUSSION

Upper-level troughing will begin to lift northeastward, diminishing any remaining boundary over the Hawaiian Islands and leaving behind lingering low-level moisture later today. Northeasterly flow will begin to veer easterly tonight, then southeasterly early tomorrow morning as a weak surface high builds just north of the islands, allowing isolated shower activity to predominately impact windward and mauka areas through the early portion of the weekend.

By the latter half of the weekend leading into next week, model guidance hints at another trough supporting another frontal boundary advancing northwest to southeast toward the islands. PWAT values have come down a bit compared to the previous 24 hours, now depicting upwards of 2 standard deviations above average. Model guidance continues to showcase noticeable differences regarding the timing of the frontal boundary, with the European Model (ECWMF) arriving later than that of the American Model (GFS). Regardless, general consensus suggests the deepest moisture will remain north of the islands, which would ultimately limit any heavy rainfall and flooding concerns.

Conditions will begin to improve by late Monday into Tuesday as the aforementioned frontal boundary exits to the east and a drier air mass quickly fills in its wake. As a result, trades will make a brief and transient return, persisting for a short period before weakening again by midweek. By the end of the forecast outlook, latest model guidance indicates another frontal boundary approaching from the northwest, potentially ushering in a return to more unsettled weather. Additional analysis will be required to better determine the timing and impacts of this system.

AVIATION

Isolated showers continue over portions of Maui County, with most other locations remaining generally dry. Brief periods of MVFR conditions are possible within any passing showers. Winds will remain light and mostly out of the northeast, but will begin to veer easterly by tonight, and southeasterly by the end of the period. Otherwise, expect VFR conditions to prevail.

AIRMET Tango remain in effect for moderate turbulence between 110 and FL380.

AIRMET Zulu remains in effect through the morning for light to moderate rime icing between 120 and FL200.

MARINE

A stalled trough over central waters this morning will dissipate later today, keeping light to gentle east-northeast winds in the forecast. Easterly trade winds will continue through tonight as a high pressure system just north of the Hawaiian Islands drifts eastward. By Saturday wind directions will veer from a more southeasterly direction as the high moves away from the islands on its eastward journey, and a cold front approaches the region from the northwest direction. A weakening cold front moves near the islands from Sunday night through Monday. Larger differences in weather model forecast guidance on this next system may produce changes in the wind portion of the forecast. By the start of next week our weather conditions will briefly become easterly trade winds, the strength of the winds may change as this pattern evolves over time and the next cold front approaches from the west.

A medium period north-northwest (330-340 degree) swell will continue to build into the region today, producing advisory level surf heights for exposed north and west facing shores. The High Surf Advisory (HSA) was extended in time through this afternoon for north and west facing shores of Niihau, Kauai, and Oahu. Offshore buoy observations this morning at buoys 51001 and 51101 are observing a 6 to 8 ft swell in the 11-13 sec period band. These swell observations support continued HSA conditions for affected shores lasting into the afternoon hours.

Several additional northwest swells will move through the Hawaii region over the next seven days. The current north-northwest swell will peak today, then begin to decline by tonight into Saturday. The next small long period northwest (310-320 degree) swell will fill in over the weekend, keeping surf heights elevated. As this swell fades by Sunday night, another medium period northwest (310-320 degree) swell builds into the region, peaking on Wednesday near advisory thresholds. Another overlapping large long to medium period northwest (310-320 degree) swell builds in from Wednesday night onward, peaking out above warning levels for the second half of next week.

Surf along east-facing shores will remain small into next week due to the lack of persistent trade winds locally and upstream. South facing shores will also remain on the seasonably small side.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 PM HST this evening for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Kauai North.