The flooding threat will remain through the day primarily across the eastern half of the state. A lingering surface trough lying over the islands will be the focus for return periods of generally light to moderate rain. The bulk of the rain is forecast to fall over Maui and Hawaii Counties through this afternoon, but the remaining islands will still experience showers. Conditions will begin improving from this evening through Friday. Trade weather returns this weekend with a frontal passage early next week significantly limiting rain chances while producing a few days of very comfortable, relatively drier and cooler weather.
The overall synoptic scene has undergone little change overnight. A diffuse, meandering low level trough of low pressure laying within a conditionally unstable moist air mass will be the channel in focusing return rain episodes through the afternoon. Shortwave energy rounding the base of the upper trough whose axis lies just west of 170W will pass over the state today. This may provide the extra lift needed, along with early day cloud breaks allowing more Sun to the surface and destabilizing the boundary layer, to initiate repeat rounds of light to moderate rainfall. While higher resolution NWP guidance has failed at properly placing where the higher precipitation will fall, ensemble modeling has been consisting telling the tale of primarily higher QPF being focused along more eastern-facing exposures. Low level trade flow will assist in keeping better organized rain anchored or pinned across windward and upslope east-facing mauka regions. Therefore, with antecedent 24 hour rainfall of over a foot across areas of eastern Maui, along with little change in the large scale of a southwest positioned upper level low and associated slowly eastern-moving low level trough located near the Alenuihaha Channel early this morning, the Flash Flood Watch has been extended through this afternoon for Maui and Hawaii Counties.
While it is hard to determine which areas will receive the most rain, little change in the synoptic pattern within this moist fairly unstable environment ensures that those that receive another inch or more on top of already soaked grounds will experience the greater probability of flooding. The highest low to mid layer moisture will travel east of the state Friday as the broad upper low southwest of the island chain begins to wobble west in response to the passage of a North Central Pacific upper trough this weekend. A cooler and drier air mass behind this trough will descend upon the state early next week. The associated surface front is forecast to reach the islands and back trades around to north northeast late Sunday or early Monday. In the meantime, lingering lower to middle layer moisture over the islands will ensure cloudier skies and periodic trade shower activity, especially over windward locales and upslope mauka, this weekend. The passage of this trough will usher in a much drier air mass where dew points are forecast to fall into the lower 60s by Monday afternoon. A backing near 1030 mb surface high north of the region will tighten the downstream pressure gradient over the islands enough to significantly strengthen early to mid week trade winds. Breezy to locally strong trades at higher elevation will persist, as will much drier conditions, through at least next Thursday.
A few brief periods of IFR cigs/vis have occurred early this morning with some heavier showers. Generally should see more VFR than MVFR until late morning/early afternoon when MVFR cigs are expected to become more prevalent. VCSH/SHRA chances remain for all, with trade winds slowly increasing through the day.
AIRMET Sierra for mountain obscuration remains in effect abv 020 for Oahu through the Big Island. Will likely be able to take Oahu out later this morning, but expecting the low clouds and -SHRA to continue elsewhere.
AIRMET Tango for Turbulence remains in effect from Oahu to the Big Island. Moderate turbulence between 100-FL200. AIRMET Zulu is in effect for light ice from Oahu to the Big Island in layer 140-FL200. Similar to AIRMET Sierra, will likely be able to remove Oahu later this morning.
This morning, a diffuse trough over the islands continues to support showers and isolated thunderstorm chances, particularly across the central and eastern waters. This trough will weaken near the Big Island today, allowing ridging to the north to drive moderate to fresh easterly winds over most waters, but southerly lighter winds will remain near the Big Island. The trough will move south of Big Island Friday, and moderate to locally fresh trades will prevail Friday night into the weekend. Stronger northeast winds will likely require a Small Craft Advisory late this weekend or early next week.
A small northwest swell will continue a slow decline today then fade on Friday, but a small, short-period north-northeast swell will maintain small surf along north-facing shores during the next few days. A larger, long-period northwest swell will arrive late Saturday and may push surf above the high surf advisory threshold Sunday. The swell will shift more northerly and may briefly build further on Monday before dropping Tuesday.
A series of overlapping south-southwest swells will maintain south shore surf above seasonal average through the weekend. Surf along south-facing shores will decline to around seasonal average early next week.
Surf along most east-facing shores will be dominated by wrapping north to north-northeast swell energy, producing periods of moderate surf. Strong northeast winds and a large, rough north- northeast swell are possible Sunday night and Monday.
Flood Watch through this afternoon for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast-Big Island Summits-Haleakala Summit-Kahoolawe-Kipahulu- Kohala-Kona-Lanai Leeward-Lanai Mauka-Lanai South-Lanai Windward- Maui Central Valley North-Maui Central Valley South-Maui Leeward West-Maui Windward West-Molokai Leeward South-Molokai North- Molokai Southeast-Molokai West-Molokai Windward-South Haleakala- South Maui/Upcountry-Windward Haleakala.
