Showers and isolated thunderstorms will spread over the western half of the state tonight and become heavy over central portions of the state Sunday into Sunday night. Flash flooding is possible. The heaviest rain is currently anticipated over Oahu and Molokai, and possibly Kauai.
Visible satellite indicates abundant high clouds that have limited surface heating and kept interior convection in check over island interiors. Low level clouds indicate persistent troughing draped over Maui County. This boundary supported scattered light showers this morning, but those have dissipated with diurnal warming. About 500 miles southwest of Kauai an extensive band of showers and thunderstorms is seen lifting northeastward toward the island. This activity is maintained by rather weak convergence and abundant tropical moisture characterized by PWAT values around 2.25". This moisture rich environment is in the process of returning to the islands where sea level dewpoints are already hovering in the low to mid 70s over Oahu and Maui County. This activity is estimated to reach the western end of the state around midnight.
Upper troughing approaches from the west tonight supporting additional pressure falls and strengthening of low-level convergence further east than the aforementioned band of showers upstream of Kauai. Further supporting this idea is a slight eastward shift by the 18z guidance with respect to the heavy rain axis. This is perhaps partly due to existing surface troughing drifting further east than anticipated. As right entrance forcing ramps up tonight into Sunday, model cross sections are actually impressive with how quickly a deep, convergent frontal structure redevelops over central Hawaii. The front is shown to extend from the surface and tilt northwestward with height until it reaches jet level west of the state. An appreciable frontal circulation is also shown to accompany this developing feature and increases confidence in the idea of development and maintenance of a weak-to-moderately forced axis of heavy rain. Primary forecast uncertainty lies with the position of this heavy rain axis. Although surface troughing has drifted eastward and settled over Maui County this afternoon, suspect convergence will tend to reorganize slightly further west as larger scale forcing manifests. Strong surface-700mb convergence then establishes over or in the vicinity of Oahu and slowly drifts westward with time.
Several inches of rain is likely over Oahu, and potentially part or all of Molokai. From a forecast accuracy standpoint, there is some concern that the heavy rain band could hang up in the Kaiwi Channel between Oahu and Molokai, but the expectation for a slow westward drift suggests Oahu will ultimately receive several hours of heavy rainfall one way or another. The heavy rain band potentially reaches Kauai on Sunday night, but that remains quite uncertain at this juncture. On the other hand, confidence is increasing that the Big Island will remain largely dry save for typical afternoon showers over upslope areas.
Given the flood response that was observed yesterday, there is concern for significant flash flooding impacts on Sunday especially since yesterday's rainfall further saturated the ground.
The existing Flood Watch is "considerable" and that is where it will stay with this forecast issuance. Overnight shift will determine the necessity of continuing the Flood Watch for the Big Island, but will leave in place for now.
A light seabreeze pattern prevails today over the western half of the state, while moderate S-SE winds occur over the eastern half of the state. Expect winds to trend lighter statewide, as a narrow band of moisture forms over the central portions of the island chain overnight, and potentially last through Monday morning. VFR to MVFR conditions are likely prior to the formation of the moisture band, and MVFR to isolated IFR conditions are expected to occur within showers once the moisture band forms. Oahu has the highest likelihood of receiving impacts, but shower coverage could reach Kauai and Maui county.
AIRMET Sierra is in effect for mountain obscuration above 1500 ft for Molokai, and windward Oahu. It is also in effect for IFR conditions on Lanai. Lanai will likely continue to see IFR conditions due to lingering showers and low ceilings. Mountain obscuration coverage will likely be expanded overnight as showers develop.
Icing threat is diminished for now, but light icing may be possible tomorrow as high clouds fill in over the state.
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 may help to bring another round of showers and isolated thunderstorms tonight through Sunday. 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 medium-period south swell will gradually 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.
A moderate, short to medium-period northwest swell generated by the gale force low to the northwest filled in early this morning and is providing small to moderate surf to north and west facing shores this afternoon. Another northwest reinforcement will arrive on Sunday and maintain small to moderate surf through the weekend, followed by a decline in northwest swell energy next week.
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
Flood Watch through Monday afternoon for all Hawaii islands-
