A stationary front will remain parked over Maui County through Thursday. This feature is expected to bring periods to showery weather to mainly Maui County and to a lesser extent windward portions of the Big Island and Oahu. More unsettled weather is possible Friday through the weekend for the western islands as a plume of deep moisture develops ahead of another approaching front. Drier weather and improving conditions are forecast by Tuesday as ridging builds across the region.
Water vapor imagery this afternoon showed a complex of lows spinning roughly 1000 miles north and northwest of the state. A deep upper level trough can be noted south of these low, over the islands, with abundant mid and upper level moisture/clouds streaming over the region from the southwest. Near the surface, the cold front that affected the smaller islands last night and into this morning has stalled near Maui County. Gusty northerly flow immediately behind the front, combined with ample low level moisture, allowed for rather impressive rainfall rates and daily totals. Since midnight, Mt. Kaala on Oahu received near 5 inches of rainfall, generally 1 to 2 inches of accumulation on the Ko'olau range, close to 6 inches on the northern slopes of Molokai, and 2 to 3 inches along the northern slopes of Haleakala according to reporting stations.
Short range model guidance keeps the aforementioned front stalled over Maui County through Thursday. Lift from this boundary, along with moderate northeast flow, may confine shower activity to mainly to windward and mauka areas of Maui County over the next 24 hours and to a lesser extent windward Big Island and Oahu. Various pulses of shortwave energy embedded in the longwave upper trough will act to enhance shower activity over the next several days. One of these pulses is progged to move across the central islands Thursday morning through the afternoon. During this time, guidance is hinting at heavier shower development. Have opted to introduce isolated thunderstorms to the forecast for Maui County and adjacent coastal waters Thursday morning through the afternoon when instability, though still relatively marginal, is at its greatest. Confidence is not high enough to issue a Flood Watch, but would not be surprised if some higher rain rates occur, leading to minor flooding and ponding of water on roadways.
Through early this weekend, both the GFS and ECMWF keep the long wave upper trough anchored over state with periods of shortwaves rotating through. The combination of added lift and remnant moisture hanging over the islands will keep us in an active weather pattern with periods of showers. Northeasterly flow will begin to veer easterly Friday night, then southeasterly by Saturday as the next system begins to approach the islands from the northwest.
Sunday into Monday, long range models agree on pushing a front down the island chain, though there still remains differences in the exact timing and strengthen of the system. The ECMWF pushes a rather weak front down the island chain late Sunday, while the GFS pushes a more robust system down through Monday. Current forecast philosophy is to splitting the difference until further details become clearer in the coming days. Post frontal, northerly surface winds should advect in drier air, then veer to northeasterly as transient deep layer high pressure moves north of the state.
Scattered showers over portions of the area will persist into the overnight with increasing coverage across windward areas. The weakening front will keep chances for MVFR to IFR conditions on Maui, Lanai, and Molokai with isolated thunderstorms possible Thursday morning.
AIRMET Sierra remains in effect for mountain obscuration for Maui, Lanai, and Molokai.
AIRMET Tango remains in effect.
Strong post-frontal NE winds have weakened considerably this afternoon, giving way to moderate NE winds along and behind the front and gentle to moderate E to SE winds ahead of the front around the Big Island. Gentle to moderate trades hold into Friday night. Winds then veer to southerly over the weekend as a front approaches the western waters. A disturbance aloft could trigger a few heavier showers and possibly thunderstorms during this time.
New medium period NW (310) swell has arrived at the Hanalei PacIOOS Buoy this afternoon and and will continue building into area waters tonight. This swell will quickly peak just shy of the High Surf Advisory threshold tonight and fade during Thursday. Overlapping this swell will be a smaller NNW (340) swell that is forecast to arrive early Thursday peaking Thursday night into early Friday. A sharp downward trend for N shore surf is anticipated during the weekend followed by a large NW swell late Monday night and Tuesday. E shore surf remains below the seasonal average through the week. Surf along S shores remains very small through the period.
None.
