A deepening Kona low north of Kauai will shift southwest of the state through Monday. Light to breezy southeastly flow will gradually strengthen from east to west across the state through tonight as the low sags just west of Kauai. This will place the associated rainband over the state where thunderstorms and embedded heavy showers will impact the region for the next couple of days leading to a threat of flash flooding. Winds may also strengthen to Wind Advisory levels Monday for lower elevations and possibly High Wind Warning levels on the Big Island summits. In addition, significant snow and ice accumulations are expected on the Big Island Summits over the next couple days. The low will weaken southwest of the state Tuesday through mid-week allowing chances of thunderstorms and heavy rain to taper off. However, lingering moisture and weak upper level troughing could still keep chances of rain over the state through the week.
A deepening Kona low 400 miles north of Kauai can been seen in satellite imagery this morning. The developing rainband from the low has been taking shape across the state from south the north centered around Oahu and Maui County where winds are converging along the circulation. A few thunderstorms have been observed mainly over the offshore waters north of Oahu and west of the Big Island within the converging flow. Light southerly flow has developed over the western half of the state with breezy southeasterly flow over the eastern half of the state. Persistent light to moderate rainfall has been observed mainly along southeast exposures of Maui County and the Big Island overnight. Peak overnight rainfall totals range from 0.5 inches to 1.5 inches in these areas. A Flood Watch remains in effect through Monday afternoon with the highest amount of multi-day rainfall total expected to occur along the southeast to south- facing mountain slopes of Big Island and Maui County due to persistent southeasterly flow over this area forcing deep southerly moisture over these areas. Heavy rain and thunderstorms will also be possible state wide as the thick cloud band associated with the parent Kona low develops over the state. The American GFS model guidance continues significant upper level instability over the state into Tuesday before declining Wednesday which could keep the flood threat over the state through Tuesday. The European model deterministic solution has the low petering out earlier and further west of the state ending significant impacts earlier.
A Winter Storm Warning for Big Island Summits was issued today and will last through early Tuesday morning as upper level temperatures are expected to plummet as the low deepens nears the state and moisture from the south is advected within the southerly flow. Snow accumulations of greater than a half of a foot and heavy ice accumulations are possible. Gradient winds over the region will strengthen to High Wind Advisory for for lower elevations and High Wind Warning for the Big Islands summits. Thus, a High Wind Watch is now in effect for Big Island Summits for the Monday time period.
Extended global models show the upper level support for the Kona weakening and dissipating far west of the state late Tuesday through Wednesday. A weak upper level trough is then expected to move west across the state bringing a low level frontal boundary over the region late into the week and next weekend. This pattern will allow strong southeasterly winds to weaken to light and variable speeds during this time. A hybrid of convergent winds along the weak boundary and day time sea breezes will keep chances of showers over the state through the rest of the week although not expecting any significant weather at this time.
Light and variable winds become more southerly today as the weather conditions turn unstable. Deep tropical moisture with a passing Kona low will produce periods of MVFR conditions, with local IFR conditions possible, in moderate to heavy showers with a few thunderstorms across the state through Monday. Light icing in cloud will continue through Monday and high and middle level clouds stream across the island chain.
AIRMET Sierra for mountain obscurations remains in effect for north through southeast sections of all islands. This AIRMET may expand to all island mountains later today and tonight, as clouds and shower trends increase across the region.
A Kona low, currently located north of Kauai, will slowly drop to the southwest over the next several days and bring periods of heavy rain, thunderstorms, and strengthening winds to both the offshore and coastal waters. As the low passes just west of the state Monday into Tuesday, east to east-southeast winds are forecast to increase to strong to locally gale force, then slowly weaken by mid-week. In addition, combined seas will be building to 8 to 14 feet across most waters this evening and remain elevated through the event. A Small Craft Advisory (SCA) has been issued for the more typical zones surrounding the Big Island and Maui windward waters starting this morning, with the remaining zones going into effect this evening. A Gale Watch has been issued for Monday morning to Monday evening for near gale to gale force winds developing in the Alenuihaha Channel.
A moderate, medium-period north to north-northeast swell (360-020 deg) will decline slowly through Monday and produce moderate surges for north facing harbors, especially for Kahului and Hilo. Thus, a Marine Weather Statement has been extended through this afternoon to highlight the threat. A small, long period northwest (310-320 deg) swell will arrive late Tuesday and persist through the end of the work week.
Aside from areas exposed to wrapping north-northeast swell, surf along east facing shores will remain rather subdued today. However, a fetch of strong to locally gale- force winds will significantly increase easterly wind swell tonight through the first half of the week, which will likely exceed High Surf Advisory thresholds for east facing shores.
Surf along south facing shores will remain small through the forecast period.
Peak astronomical monthly tides with water levels running roughly 0.5 ft above normal is expected to produce minor coastal flooding through Monday morning. This flooding may be enhanced along north facing shores due to the current north northeast swell. A Coastal Flood Statement has been extended through Monday morning to highlight this flooding potential, especially during the daily peak high tide cycle.
Flood Watch 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 Warning until 6 AM HST Tuesday for Big Island Summits.
High Wind Watch from Monday morning through late Monday night for Big Island Summits.
Small Craft Advisory from 6 PM this evening to 6 AM HST Tuesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters- Kaiwi Channel-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel.
Small Craft Advisory until 6 AM HST Tuesday for Maui County Windward Waters-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.
Small Craft Advisory from noon today to 6 AM HST Monday for Alenuihaha Channel.
Gale Watch from Monday morning through Monday afternoon for Alenuihaha Channel.
