A developing Kona low north of Kauai will travel west of the islands through Monday. East winds will transition to southeast to south tonight and draw up a more moisture rich air mass across the state the next several days. The vicinity of the low will also cool and destabilize the upper atmosphere. These elements brought on by the low equate to high probabilities of several days of rain with locally high rainfall and possible isolated storms that will initiate flash flooding. Winds may also strengthen to advisory levels Monday along with Big Island summits receiving significant snow accumulation the next few days. As ridging builds back in over the islands by the middle of the week, rain should begin to taper off by Wednesday night.
This afternoon's water vapor imagery depicts a developing low about 550 miles north northwest of Kauai. This Kona low settling in west northwest of Kauai through Monday will be the impetus to a wet pattern across the Hawaiian Islands that may persist through Wednesday of next week. Many windward exposures have picked up another half an inch to over two inches of rain since yesterday at this time with various upper terrain leeward spots adding a few tenths to their early year totals. It will not take much more rain over those areas with saturated grounds and swollen streams in creating pure runoff and flooding in future rain. The location of a slow-moving low west of the islands pulling up a very moist air mass (precipitable waters greater than a very unseasonably high 1.5 inches) signals a higher threat for a multiple day flood event for the state. A Flood Watch remains in effect through Monday afternoon to account for return periods of thick cloud bands coming onshore and putting down a few inches of rain in just a matter of a couple of hours. The highest amount of rain is forecast to occur along the southeast to south-facing exposures of Big Island and Maui County due to the southeast-to-northwest orientation of these cloud bands training onshore over multiple days. Cooler upper level air over the state in association with the proximity of the low to the islands will increase the chances for thunderstorms through early Wednesday.
A Winter Storm Watch is in effect from Sunday morning through early Tuesday morning to account for snow accumulations of greater than a half of a foot. Higher resolution numerical weather prediction model guidance are showing favorable conditions for a potential high snow event atop Big Island summits the next couple of days. A cooling atmosphere to below freezing above 10k feet early Sunday will create prime conditions for possibly high snowfall rates in tandem with moisture rich air ascending Mauna Kea and Mauna Loa's southern slopes. Gradient winds far downstream of the Kona low Monday are forecast to strengthen to near advisory magnitudes Monday. Strong winds at the summits will also create low visibility conditions within blowing snow.
As the low continues its westward journey away from Hawaii and weak ridging builds in over the region, breezy east to southeast winds will weaken Tuesday and Wednesday. A weak troughing pattern will linger over the state throughout the week so uncertainty exists as to the extent of any persistent mid week precipitation. A passing boundary later next week may strengthen southerly winds ahead of it and increase (primarily western island) return rain probabilities late Thursday and Friday.
Winds will become southerly tonight as a Kona low develops just north of Kauai. Deep tropical moisture will continue to move into the state over the next few days bringing periods of unstable weather. Expect MVFR conditions to continue to impact portions of the state, with local IFR possible as heavier showers develop on Sunday.
AIRMET Sierra for mountain obscurations remains in effect for windward portions of all islands. This will likely need to be expanded to cover leeward portions as heavier bands of showers develop late tonight into Sunday.
AIRMET Zulu for light to moderate icing is possible for the next few days as mid and high clouds move into the state.
No significant changes to the marine forecast this afternoon. A developing Kona low will bring the chance for heavy rain, thunderstorms and strengthening winds over the next couple of days to both the coastal and offshore waters.
The developing Kona low to the north of the islands is expected to drift southwestward tonight. As the low passes just west of the state, strong to near gale-force east to east-southeasterly winds are expected to build Sunday evening, then continue through the first half of the new week.
A moderate, medium-period north to north-northeast swell (360-030 deg) is on the decline, and will continue to slowly decrease through the remainder of the weekend. This swell has likely produced moderate surges for north facing harbors, especially for Kahului and Hilo, but those impacts should subside tonight. Meanwhile, a small, medium- period northwest (310-320 deg) swell will gradually decline through Monday. Another small, long period northwest (310-320 deg) swell is expected to arrive on Tuesday.
Aside from areas exposed to wrapping north-northeast swell, surf along east facing shores is expected to lower this weekend as trade winds weaken in response to the developing Kona low mentioned above. However, a fetch of strong to near gale- force winds will significantly increase easterly wind swell from Sunday night through the first half of the week, which should exceed advisory thresholds for east facing shores.
Surf along south facing shores will remain tiny through the forecast period.
Peak astronomical monthly tides with water levels running roughly 0.5 ft above normal could produce minor coastal flooding through the weekend, and may be enhanced along north facing shores due to the incoming north swell. A Coastal Flood Statement will remain in effect to highlight this flooding potential, especially during the daily peak high tide cycle.
Flood Watch from midnight HST tonight through Monday afternoon for Niihau-Kauai Leeward-Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Lanai Mauka- Kahoolawe-Maui Windward West-Maui Leeward West-Haleakala Summit- Kona-Kohala-Big Island Interior-Kauai North-Kauai East-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward- Koolau Leeward-Molokai-Lanai Windward-Lanai Leeward-Lanai South- Maui Central Valley North-Maui Central Valley South-Windward Haleakala-Kipahulu-South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast-Big Island East-Big Island North.
Winter Storm Watch from Sunday morning through late Monday night for Big Island Summits.
