A powerful Kona storm will impact the state through this weekend resulting in potential flash flooding, damaging winds, and strong to severe thunderstorms. Impacts will generally be felt over the western end of the state first and then spread east with time. The peak of this event is forecast to be Friday and Saturday.
Tonight. NE-SW oriented moisture axis in place over the islands supports ongoing shower activity as the mid and upper levels slowly destabilize. These showers are anchored along a corridor of weakly confluent SW flow and will evolve eastward through tonight through Maui County tonight. Paltry convergence is likely to limit potential for organization and meaningful heavy rain tonight.
Thursday. The next wave of showers in the sequence is beginning to materialize along the same moisture axis well south of Kauai today. Stellar left exit jet support and steady dynamic forcing aloft will allow this developing band of showers to intensify as it reaches Kauai and possibly Oahu early Thursday. Lapse rates on the order of 7C/km and 500mb temperatures around -13C will favor embedded thunderstorms. Despite the increasingly favorable thermodynamic environment, low-level convergence will be lacking for much of the day Thursday. This will likely lead to poorly organized batches of heavy rainfall and/or thunderstorms through at least the first half of the day. Additionally, the left exit circulation and local pressure falls will force winds to steadily ramp up during Thursday with 850mb winds reaching 35 kts by Thursday evening and sustained southerlies in the vicinity of Kauai around 20-25kts. Strong southerly moisture transport may favor fairly widespread showers by afternoon, but gradually strengthening surface convergence will increasingly place the focus on the developing heavy rain band over Kauai. An increasingly organized band of heavy rain and thunderstorms is expected to be taking shape over Kauai no later than Thursday evening.
Thursday night through Saturday. This represents the period of greatest concern and the potential timing for considerable impacts to life and property. 850mb low-level jet further ramps up to nearly 50 kts on Thursday as sustained southerlies settle around 25 kts. As usual, downsloping potential exists but is uncertain. Suspect power outages and downed trees will be common by late Thursday night into early Friday, particularly where soil is water logged. Of note, a strong punch of mid level dry air and subsidence impinges on Kauai and Oahu during the day Saturday when the wind field still remains quite strong. This may represent even greater potential for downsloping winds to develop. Therefore, a High Wind Watch has been issued for the lower elevations beginning Thursday night and continuing through Saturday night for Kauai through Maui. An HWW for the Big Island has likewise been issued with a start time of early Friday. This Watch will later be converted to either a Wind Advisory or a High Wind Warning as downsloping potential becomes more clear, though the aforementioned dry intrusion increases confidence that strong downsloping winds will occur if not during the entire watch period, then at least during the tail end of it Sat-Sat night.
This period will also be characterized by intense rain rates, thunderstorms, and the potentially serious flash flooding. The most intense rainfall will be focused along the rapidly strengthening frontal zone as it slowly progresses eastward through the state. This band will develop over Kauai by late Thursday, reach Oahu during Friday and Maui County late Friday into Friday night. The heavy rain axis finally reaches the Big Island on Saturday, focusing heavy rain along leeward areas. Also suspect showers will ramp up during this time frame over the SE slopes of the Big Island potentially resulting in several inches of rain in advance of the heavy rain band. There is also a slight risk for severe thunderstorms. Wind profiles are largely unidirectional out of the SW which will favor a primary threat of wind. However, large hail (1" diameter or greater) is also not out of the question with any particularly intense thunderstorms. It should be noted that the heavy rain band is forecast to move slowly but still be rather progressive in nature. Should it stall over an island, the threat for severe flooding would drastically increase.
Finally, Big Island and Haleakala Summits will see winds ramp up Friday into Saturday. Gusts over 100 mph can be expected on the Big Island with lesser gusts of 80-90 mph developing on Haleakala Summit by early Saturday. This will coincide with the period of heaviest precip in these areas. Blizzard conditions are therefore anticipated on the Big Island Summits. The High Wind Watch and Winter Storm Watch for the summits remain in effect.
Sunday onward. Moist SW flow potentially favors continuing showers, though of diminished intensity over Leeward Big Island. The next round of mid-level height falls approaches during the middle of next week potentially reactivating the lingering band of moisture over the islands.
Shower activity has become broken this afternoon, with the expectation that showers will become more isolated for Maui County and the Big Island this evening, while a more scattered threat persists for Kauai and Oahu. Most sites currently read VFR this afternoon, however sites under showers may fall as low as LIFR at times. Showers are expected to spread eastward during the day Thursday, with isolated thunderstorms possible, however confidence remained too low to make mention in the TAFs themselves.
AIRMET SIERRA remains in effect for mountain obscuration for all of Kauai, Oahu and Maui Counties, as well as the south and southeast portions of the Big Island.
AIRMET ZULU remains in effect for light icing from Kauai through Maui County for FL 130-240.
AIRMET TANGO is in effect for moderate turbulence over Kauai and Oahu as winds are expected to be breezy out of the south over the next couple of days.
AIRMET TANGO is also in effect across the state for moderate turbulence associated with this ongoing Kona low. Upper level turbulence is generally expected to decrease towards the evening with the exception of areas with thunderstorms.
A powerful Kona storm will result in heavy rain, thunderstorms, and rough seas for the islands and adjacent coastal waters through the rest of this week. Today's scatterometer passes showed winds have significantly diminished around the Big Island, though they remain fresh near Kauai. As a result, the Small Craft Advisory (SCA) has been cancelled. A relative lull in winds is forecast tonight before strong southerly winds redevelop Thursday afternoon. Winds are likely to approach, or possibly reach, gale- force for portions of the area as early as Thursday night or Friday and last into the weekend in association with a band of heavy rainfall and thunderstorms pushing eastward across the forecast area.
A series of small, medium- to long-period west-northwest swells will bring small surf to north and west facing shores through the rest of the week. Kauai will block some of this swell energy from reaching Oahu and Maui, but it will make it into the west facing shores of the Big Island. The current swell will begin to diminish late Thursday before the next west-northwest swell builds Friday into the weekend. Additionally, a small, medium-period north swell originating from Alaska Peninsula gap winds is expected to arrive Thursday night and continue into the weekend.
Choppy surf along east-facing shores will continue to trend downward and remain below seasonal levels as south to southeast winds prevail. The southerly wind direction will contribute to rough and choppy surf along south-facing shores, but a bit of a lull can be expected on Thursday with a brief break in southerly winds before they reintensify Thursday night. South shore surf will build further Friday into the weekend, likely reaching advisory levels. Once southerly winds diminish early next week, a small to moderate, long-period south swell generated by a gale force low southeast of New Zealand will arrive along south-facing shores.
Flood Watch through Saturday 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-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.
High Wind Watch from Thursday evening through late Saturday night 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-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.
High Wind Watch from Thursday evening through Sunday afternoon for Haleakala Summit-Big Island Summits.
Flood Watch from 6 PM HST this evening through Saturday afternoon for Kona-Kohala-Big Island Interior-Big Island South- Big Island Southeast-Big Island East-Big Island North.
High Wind Watch from Friday morning through late Saturday night for Kona-Kohala-Big Island Interior-Big Island South-Big Island Southeast-Big Island East-Big Island North.
Winter Storm Watch from Thursday evening through Saturday afternoon for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Big Island Windward Waters-Big Island Leeward Waters.
