textproduct: Honolulu

SYNOPSIS

Showers and isolated thunderstorms will spread over the western half of the state becoming heavy over central portions of the state through tonight. The heaviest rain bands are anticipated over Oahu and Molokai, and possibly spread westward over Kauai. Because of the latest rainfall over the last few days, flash flooding is possible. Light and variable winds are expected to prevail through the first half of the week, with light trades returning to the state thereafter.

DISCUSSION

Latest radar observations have brought another band of showers and thunderstorms into Kauai and rapidly approaches Oahu and Molokai, while continuing to meander northeastward. Much of this activity is stemmed from the broad area of low pressure northwest of the Hawaiian Islands and an associated quasi-stationary trough trickling through the middle of the state. Latest balloon soundings at Hilo and Lihue from 12z (6 AM HST) suggest PWAT values of around 1.82 inches and 1.67 inches, respectively, implying a moisture rich environment remains over the state, courtesy of the abundant tropical moisture continuing to be pulled in over the state.

Model guidance of the GFS and ECMWF remain in fairly strong agreement with the timing of the rain bands, suggesting shower activity will continue to move across the state early this morning with the heaviest band developing over Oahu and potentially much of, if not all, of Molokai, though there is some indication that the band may anchor over the Kaiwi Channel between Oahu and Molokai.

Showers that do develop over Oahu and portions of Molokai will likely persist for numerous hours, accumulating several inches over rain during that time. Models are hinting, however, that the rain band begin to slowly retrograde shortly thereafter back westward potentially reaching Kauai tonight, but that remains quite uncertain at this point in time. Conversely, confidence is increasing that the Big Island will remain largely dry apart from the typical afternoon showers over upslope areas.

That said, opted to retain the current Flood Watch across the entire state through Monday afternoon, maintaining the "considerable" tag to account for the many areas that are moisture-laden from rainfall earlier this week and are now more prone to significant flash flooding impacts. Furthermore, opted to leave the Big Island into the Watch, as any rainfall can still cause issues in areas where the ground is saturated.

In the longer-range, model guidance shows the rain band moving west of Kauai by mid-week, allowing light trades to reestablish toward thereafter. A more typical trade shower pattern will also return as a result, with showers across windward and mauka areas persisting through the end of the period.

AVIATION

An unstable band of deep tropical moisture will move up from the south under an upper level disturbance. Weather conditions will destabilize later tonight, producing widespread MVFR and isolated IFR conditions statewide with low clouds, showers and thunderstorms from early Sunday morning through Monday. Light and variable winds will prevail at all airfields.

AIRMET Sierra remains in effect for mountain obscuration above 1500 ft for Oahu, Molokai and Lanai. Mountain obscuration coverage will likely be expanded overnight as showers develop.

MARINE

A surface trough will linger in the vicinity of the central islands into early next week, keeping moderate to fresh southeasterly winds to its east, and light and variable winds to its west. This weak surface boundary along with an upper level disturbance moving into the region brings another round of showers and isolated thunderstorms today into Monday. As the surface trough lifts north and high pressure to the northeast begins to takes over at the surface, gentle to moderate easterly trades will gradually develop across the Hawaiian coastal waters by the end of next week.

A moderate, short to medium-period north-northwest swell (340 degrees) generated by the formerly-gale force low north-northwest of the main Hawaiian islands arrives along north and west facing shores today. This will maintain small to moderate surf through the beginning of the week, followed by a decline in northwest swell energy for the rest of the week.

A medium-period south swell will continue to decline through Monday. By Tuesday, however, a new south swell will arrive, once again providing a boost to surf along south facing shores through mid week before gradually subsiding into the weekend.

Surf along east facing shores remains below seasonal average with lighter than average trade winds near and upstream of the islands forecast through next week.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through Monday afternoon for all Hawaii islands-