Deep tropical moisture will continue to stream over the eastern end of the state through Wednesday. This moisture will interact with a surface trough to bring periods of heavy showers through Wednesday, especially for windward portions of Maui and the Big Island. The chance of heavy rainfall diminishes late Wednesday into Thursday, but breezy and showery trades will persist. A brief period of stable and drier trade wind conditions is expected on Friday, followed by more showery weather into the weekend as a cold front advances toward the island chain.
Visible satellite imagery continues to show thick mid to upper level clouds streaming in from the southwest over the Big Island this afternoon, with PWATs near 2.0 inches. Meanwhile, the latest surface analysis depicts a surface trough southeast of the Big Island moving towards the west. Guidance also continues to show an upper level trough northwest of the island deepening tonight as it moves east which will keep unstable conditions across the area. Expect these troughs to continue to interact with the deep tropical moisture moving across the Big Island to bring periods of heavy showers to the area. The latest guidance continues to show the potential for heavy rain across the Big Island will continue through Wednesday, but total accumulations are now on the lower side as guidance has started to back off. Nonetheless, some locally heavy rain remains possible and could produce nuisance flooding in the typical areas with 1 to 2 inches of rain expected across windward areas of the Big Island. Any potential threat for heavy rain will decrease late Wednesday.
The thick clouds over the Big Island summits produced some light snow accumulations last night, with precipitation quickly transitioning to rain this afternoon. Expect precipitation to become a mix of rain and snow overnight as snow levels fall to around 12,000 feet. Guidance doesn't show much change in summit conditions on Wednesday with an additional 1 to 2 inches of snow possible so decided to extend the current Winter Weather Advisory through Wednesday evening.
As for the rest of the state, water vapor imagery shows a small dry air slot filling in which explains the lack of clouds over the smaller islands. Radar imagery this afternoon show mainly scattered windward and mauka showers moving across the smaller islands. Moderate trades will continue across these areas as high pressure builds far north of the region.
Breezy and wet conditions are expected into Thursday. The strong surface high passing to the north will maintain breezy trade winds, and even though PWATs will be decreasing, both the GFS and ECMWF keep abundant moisture around the islands, pointing to a rather wet pattern.
A brief period of stable and drier trade wind conditions is expected on Friday, followed by the potential for more showery weather into the weekend and early next week as a cold front approaches the island chain. Once again models vary on timing and location so please pay attention for further updates.
Trade winds expected to strengthen by Wednesday. Layered clouds over the Big Island will bring light icing from 140-FL260 into Wednesday. AIRMET Tango posted for tempo moderate turbulence between FL250 and FL400 all islands. AIRMET Tango will likely be dropped tonight as guidance suggest a decrease in turbulence.
Showers expected to increase tonight into Wednesday as trades build. AIRMET Sierra likely to be issued for tempo mountain obscurations later tonight for some windward areas, particularly the eastern half of the state.
Moderate trade winds will steadily increase over the next 24 hours. Local trade winds are near a minimum this afternoon as a front, currently just beyond the offshore waters 240 nm northwest of Kauai, has eroded and pushed the subtropical ridge to the east. The weakening front is not expected to push much farther east, and northeast winds are already beginning to increase at NOAA buoy 51001 as robust surface high pressure behind the front is starting to exert influence. As the front dissipates and the surface high moves north of the state tonight and Wednesday, expect trade winds to ramp up, eventually reaching near gale strength in the Alenuihaha Channel. A Small Craft Advisory (SCA) has been issued for the typically windy waters around the Big Island and Maui tonight and expands to cover all waters for Wednesday and Thursday as trades rapidly build. The high will be pushed to the east by a North Pacific front late Thursday and Friday, leading to a gradual decline in the trades. Trades may decline further this weekend as the front passes north of the islands.
A small northwest swell peaking this evening will be overwhelmed by a larger northwest tonight and Wednesday. The first swell is up to 2 to 3 feet at 17 seconds at the PacIOOS Waimea Bay buoy this afternoon, while NOAA buoy 51001 is measuring the larger swell around 5 to 6 feet at 14 to 15 seconds. The larger swell will produce increasing surf tonight along exposed north and west showers from Kauai to Maui, with waves peaking around High Surf Advisory levels on Wednesday as the swell shifts out of the north- northwest (330 degrees). We have held off on issuing an advisory for now and will wait for additional data from buoy 51001. The northwest swell will be accompanied by a small to moderate short period north swell aimed primarily west of Kauai, while building trades produce increased wind waves. The combined seas from all of these sources will be in excess of 10 feet that will contribute to the need for the SCA. The north-northwest swell will gradually decline Thursday and Friday, with another northwest swell pushing surf back near the advisory level Saturday.
East shore surf will be on the rise Wednesday and Thursday, primarily from increased short period trade wind swell, though the western end of the island chain could also experience a brief short period north (350-010 degrees) swell of 3 to 6 feet at 10 seconds. As mentioned above, this north swell will be primarily aimed west of Kauai, but some wrapping energy could push east shores of Kauai near the High Surf Advisory level Wednesday. The north swell will rapidly fade on Thursday, when peaking trade wind swell will produce surf just below the advisory level on east shores of all islands. East shore surf will decline through the weekend as trades weaken over and upstream of the islands.
For south shores, tiny background south swell energy will persist.
Winter Weather Advisory until 6 PM HST Wednesday for Big Island Summits.
Small Craft Advisory from 6 AM Wednesday to 6 PM HST Thursday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters- Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Big Island Windward Waters.
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
