A powerful Kona storm will near the state producing periods of hazardous weather impacts across the Hawaiian Islands through the weekend. Expect a combination of threats ranging from significant flash flooding, damaging winds, strong to severe thunderstorms, and snow and ice over the highest Big Island summits. Southerly to southwesterly winds will be strongest from Friday to Saturday and will produce damaging wind gusts. The highest threats for damaging winds remain along north and east of the island mountains. Unsettled wet weather conditions with decreasing southwesterly wind trends will continue into early next week. This Kona storm begins to lift northward on Monday, and unsettled weather will start to ease across the Hawaii region. High pressure will build back in north of the state on Wednesday allowing trade winds to return spreading from north to south down the island chain reaching Maui and the Big Island by Thursday morning.
A powerful Kona storm remains in place northwest of the Hawaiian Islands this morning. The big picture satellite imagery continues to show several upper level lows and troughs rotating around the larger low pressure complex. Satellite cloud top temperatures for shower bands south and west of the state over the past few hours are trending colder and appear more strongly organized. Cooling cloud tops means stronger storms reaching higher levels of the atmosphere. These trends will continue through the morning hours as southerly wind speeds ramp up.
The large scale weather threats will pulse up and down as each of these upper level troughs dig into the islands, producing smaller scale (mesoscale) convective heavy shower and thunderstorm banding features. The heaviest rainfall rates will fall along these developing small scale heavy rain bands. Additionally, any thunderstorms developing within these bands are capable of reaching up into the upper atmosphere and pulling down strong winds aloft down to the ground, producing strong and damaging wind gusts potentially ranging from 50 to 70 mph at times.
The Flash Flooding threats will continue into the weekend due to both saturated soil conditions that more easily lead to ground runoff and flooding from even more moderate continuous rainfall rates. The highest flood threats across the state will favor southern and western slopes of all islands. These are typically the drier sides of the islands, less accustomed to heavy rain and flooding. Driving under these heavy shower bands will become challenging with ponding of water on area roadways. Current weather trends in this pattern will change rapidly, these threats to life and property will develop swiftly and can affect any location in the Hawaiian Islands. Postpone outdoor activities or travel if possible until early next week as the Kona storm begins to drift northward away from the islands and the threat level across the state diminishes. A Flood Watch continues for all Hawaiian Islands into Saturday, this Flood Watch may need to be extended in time as the heavy rainfall threat may linger into Sunday.
Weather conditions become vastly more severe today, as stronger southerly to southwesterly winds and a negatively tilted upper level trough will produce damaging wind gusts along island mountain slopes, favoring northern and eastern slopes of steep mountain terrain. Saturated ground conditions and trees unaccustomed to south to southwest winds will fall and likely cause blocked roadways and lead to power outages. These strong southwesterly winds will continue through Saturday afternoon. These stronger southwest winds speed up into the 50 to 70 MPH range around the 5,000 to 10,000 foot level. Southerly winds will also converge at the lower levels, lifting the atmosphere and producing strong convective heavy shower and thunderstorm bands. As these clouds reach higher and higher heights in the atmosphere they will reach up and convectively pull down these stronger winds to the surface, yielding wind gusts in the 50 to 70 mph range in the strongest thunderstorm cells. This means even outside of terrain enhancing wind gusts any strong thunderstorms are capable of producing hurricane force wind gusts over islands or coastal waters through Saturday. A Wind Advisory was issued this morning for the lower elevation areas, below summit level, of the Big Island. A High Wind Warning remains in effect to cover these stronger damaging wind threats for the rest of the state.
Some of these stronger thunderstorms may become severe with the heaviest rainfall rates coupled with strong damaging winds. High levels of instability, wind shear and atmospheric forcing ingredients are all coming into focus over the islands through Saturday for a potential severe weather event. A Severe Thunderstorm Watch may be needed to cover this threat through Saturday time period. However, we will likely wait until we see these severe thunderstorm developing over the local area before pulling the trigger on this Severe Thunderstorm Watch with a short lead time measured in hours.
Colder temperatures below freezing and developing showers over the Big Island summits of Mauna Kea and Mauna Loa are producing heavy snowfall and periods of icing at times. A combination of ice, snow and strong winds will continue to produce extremely treacherous conditions above the 12,000 foot elevation level through today. A Winter Storm Warning remains in effect for this winter weather threat and may need to be extended into Sunday. The Mauna Kea Access road to the summit remains closed to the public.
From Saturday night to Sunday another upper level short wave trough passes over the islands leading to another upward pulse in moderate to heavy shower activity. Thunderstorms will also threaten the islands and become more of a flooding threat as decreasing upper level and surface wind speeds diminish the damaging wind gust potential.
Monday and Tuesday, the large Kona storm begins to weaken and lift northward, slowly drifting away from the Hawaiian Islands. This means wet weather continues, however moderate to heavy rainfall rates will decrease in coverage and southwest winds will gradually weaken through the first half of next week.
From Wednesday to Thursday, the latest long range guidance shows a high pressure system building back into position just north of the island chain. Trade winds will build back into the islands from north to south with more typical brief passing showers over windward mountain areas, reaching Maui and the Big Island by Thursday morning.
Widespread showers, some heavy, along with isolated thunderstorms, will continue through tonight. Expect widespread MVFR to IFR with local LIFR. Winds will remain primarily out of the south and increase through the day. Widespread gusts up to and over 35 kts are forecast.
AIRMET Sierra remains in effect all islands due to mountain obscuration. This is expected to remain in effect through tonight.
AIRMET Tango remains in effect for moderate upper-level turbulence between FL240 and FL360. This is expected to remain in effect through tonight.
AIRMET Zulu is also in effect, as high clouds from these showers present icing concerns between 120 and FL260. This is also expected to continue through tonight.
Prepare for gusty winds, steep and fast building seas, and low visibility in heavy showers across the entire marine waters as this strong Kona storm event continues to impact the coastal and offshore waters through the weekend. Strong to gale southerly winds will continue through the weekend as a Gale Warning is in effect all waters around Kauai and Oahu through tonight. Winds are expected to near Maui County Saturday and Saturday night to near gale- force, so a Gale Watch has been posted for those areas during that time. These strong to gale- force winds will last into the weekend as the band of thunderstorms push eastward across the forecast area through Saturday. The winds by early next week will shift westerly and diminish as the storm gains increasing distance far to the north of the islands.
A small, short to medium period west to west-northwest swell will bring small to moderate surf to north and west facing shores through the weekend. 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. Additionally, a small, medium period north swell is expected to build through the day and peak Saturday before gradually declining.
Rough and choppy surf will continue along south-facing shores due to the south to southeast wind direction. These south winds will intensify this morning causing south shore surf to build into the weekend, likely just below advisory levels. Once southerly winds diminish early next week, a small to moderate, long-period south swell generated by a gale force low east- southeast of New Zealand will arrive along south-facing shores and build into midweek. Surf along east-facing shores will remain below seasonal levels as south to southeast winds prevail.
Flood Watch through Saturday afternoon for all Hawaiian Islands.
High Wind Warning until 6 AM HST Sunday for Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, and Maui.
High Wind Warning until 6 PM HST Sunday for Haleakala Summit-Big Island Summits.
Wind Advisory until 6 AM HST Sunday for the Big Island lower elevation slopes.
Winter Storm Warning until 6 PM HST Saturday for Big Island Summits.
Gale Warning until 6 AM HST Saturday for Kauai Northwest Waters- Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel.
Gale Watch from Saturday morning through Saturday evening for Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters- Maui County Leeward Waters-Alenuihaha Channel.
