Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Breezy. Showers and scattered thunderstorms. Locally heavy rainfall possible. Highs around 82 near the shore to around 69 above 3000 feet. South winds up to 15 mph shifting to the southwest in the afternoon. Chance of rain near 100 percent.

Tonight: Windy. Showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 61 to 67 near the shore to around 54 above 3000 feet. West winds 15 to 30 mph. Chance of rain near 100 percent.

Monday: Windy. Showers and scattered thunderstorms until late afternoon, then scattered showers late in the afternoon. Some thunderstorms may produce gusty winds until late afternoon. Locally heavy rainfall possible until late afternoon. Highs around 79 near the shore to around 66 above 3000 feet. West winds 25 to 30 mph with gusts to 50 mph. Chance of rain near 100 percent.

South Kaua’i

Today: Breezy. Showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 77 to 84. Southwest winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Windy. Showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows around 65. West winds 20 to 30 mph. Gusts up to 50 mph after midnight. Chance of rain near 100 percent.

Monday: Windy. Showers and scattered thunderstorms until late afternoon, then scattered showers late in the afternoon. Some thunderstorms may produce gusty winds until late afternoon. Locally heavy rainfall possible until late afternoon. Highs 74 to 81. West winds 20 to 30 mph with gusts to 50 mph. Chance of rain near 100 percent.

Kaua’i Mountains

Today: Breezy. Showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 70 to 77 in the valleys to around 61 above 4000 feet. Southwest winds up to 20 mph increasing to 10 to 25 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Windy. Showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 56 to 62 in the valleys to around 51 above 4000 feet. West winds 15 to 35 mph with gusts to 60 mph. Chance of rain near 100 percent.

Monday: Windy. Showers and scattered thunderstorms until late afternoon, then scattered showers late in the afternoon. Some thunderstorms may produce gusty winds until late afternoon. Locally heavy rainfall possible until late afternoon. Highs 65 to 73 in the valleys to around 57 above 4000 feet. West winds 25 to 35 mph decreasing to 15 to 35 mph in the afternoon. Gusts up to 60 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Breezy. Showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 72 to 81. South winds up to 15 mph shifting to the southwest in the afternoon. Chance of rain near 100 percent.

Tonight: Windy. Showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 57 to 65. West winds 15 to 30 mph with gusts to 60 mph. Chance of rain near 100 percent.

Monday: Windy. Showers and scattered thunderstorms until late afternoon, then scattered showers late in the afternoon. Some thunderstorms may produce gusty winds until late afternoon. Locally heavy rainfall possible until late afternoon. Highs 68 to 78. West winds 20 to 30 mph with gusts to 60 mph. Chance of rain near 100 percent.

East Kaua’i

Today: Breezy. Showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 70 to 80. Southwest winds 10 to 20 mph. Chance of rain near 100 percent.

Tonight: Breezy. Showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 57 to 67. Southwest winds 15 to 25 mph with gusts to 60 mph. Chance of rain near 100 percent.

Monday: Windy. Showers and scattered thunderstorms until late afternoon, then scattered showers late in the afternoon. Some thunderstorms may produce gusty winds until late afternoon. Locally heavy rainfall possible until late afternoon. Highs 65 to 77. West winds 15 to 30 mph with gusts to 60 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

Numerous showers and thunderstorms will continue through Monday. Some storms may be severe tonight and Monday. Any severe storms will be capable of producing strong winds and hail. Strong and damaging southwest, or kona, winds will develop tonight and continue through Monday. Periods of heavy rain throughout this time may result in areas of flash flooding.

Discussion

Satellite imagery shows an impressive mid-latitude cyclone deepening over the Central Pacific Basin as digs southward toward the islands. The latest data indicate the surface low has deepened to 988mb south of 40N with further deepening expected as it makes its closest approach on late Monday night.
Today. During the last several hours, the local gradient has begun responding to the approach of this feature as kona, or southwest winds, have emerged over the western end of the state. This is ushering an existing batch of moisture northeastward across island chain and touched off numerous showers and thunderstorms during the overnight period. Radar indicated rain rates have ranged from 1 to 3 inches per along with isolated signatures supportive of small hail. As noted in previous days, the supply of instability during the next 24 to 36 hours will be continuous and impressive courtesy of cold WNW flow aloft and strengthening southwesterlies in the lowest levels. As a result, the islands will likely remain convectively active through today with the greatest coverage and organization focusing along the western and northern portions of the moisture gradient, the latter of which will lift north of the area during the next several hours. Thus, after a potential lull in activity during the morning, the western end of the moisture gradient (presently draped over Kauai) will slowly work eastward supporting an increased coverage of showers and thunderstorms over Oahu and Maui during the daytime hours today. Throughout this time, background isentropic ascent will steadily increase ensuring frequent regeneration of convective cells as strengthening winds lead to more progressive storm motions. With multiple flood advisories already issued and numerous thunderstorms ongoing in the area, the Flood Watch beginning this morning at 6am remains in good shape.
This evening through Monday. Pressure falls spread across the island chain in response to impressive mid-level forcing and the southwesterly wind field responds accordingly. Sustained southwest surface winds of 20 to 30 mph will be common with locally stronger winds likely in areas favorable for local accelerations. Of greater concern is winds right off the deck, in the 925mb to 950mb layer, strengthening to 45 kts or so during the overnight. Windward zones and areas downwind of terrain will be highly susceptible to downsloping which will force damaging winds to the surface. Windward Oahu will be particularly susceptible. Convective instability will also be maximized during this time as 0-1km MLCAPE increases to at least 1500 j/kg. Any particularly vigorous showers will therefore have the potential to mix down severe winds up to 60 mph. For this reason, the High Wind Warning includes the entire area – not only favored downsloping areas.
Finally, a severe thunderstorm threat will exist tonight through through the time of frontal passage on Monday. Lapse rates aloft are modeled to remain fairly impressive, in the neighborhood of 7 to 7.5 km/C in the prefrontal environment. Strong, deep unidirectional wind profiles in the presence of moderate convective instability will contribute to an enhanced threat for thunderstorms to evolve into northeast-moving bowing segments capable of producing damaging straight line winds. 0-6km bulk shear in excess of 40 kts and cold temperatures aloft will also support a marginal risk for hail up to 1 inch in diameter. The potential for a Severe Thunderstorm Watch will be evaluated today and this evening.
Tuesday into midweek. Well-mixed, gusty westerlies build in behind the cold front and focus showers over leeward slopes into the mid- week period.

Aviation

A strong cold front located far northwest of Kauai will move rapidly toward the western islands through tonight. An upper- level trough is also digging down toward the state. As a result of these features, the atmosphere has already become unstable. In addition, the background low-level south to southwest flow is transporting enhanced moisture into the islands. This is causing the coverage of clouds and showers to increase this morning. Isolated thunderstorms are also in the vicinity of the islands. The clouds and precipitation along with the deep convection are expected to continue increasing today. This will likely produce widespread areas with MVFR ceilings and visibilities. In addition, as the front moves closer to the area, the southwest winds are also expected to strengthen from later this morning through this evening.
AIRMET Sierra for mountain obscuration has recently been issued for the higher terrain above 2500 ft on Kauai, Oahu, Molokai, and Maui. This AIRMET will likely be needed for most of these islands through this morning, and possibly this afternoon. In addition, as the gusty southwest winds increase later today, AIRMET Tango will likely be needed for TEMPO moderate mechanical turbulence northwest through southeast of the higher terrain on most islands. Once it is issued, this AIRMET will likely remain in effect into Monday.

Marine

A potent cold front will move SE across the islands tonight through Monday night, bringing a wide array of marine hazards. These include the potential for very gusty winds and hail in and around strong to severe thunderstorms, sustained near-gale to gale-force southwest to west winds, as well as rapidly increasing seas and surf. A Small Craft Advisory (SCA) for all Hawaiian waters today will be replaced by a Gale Warning tonight. Additionally, a High Surf Warning (HSW) for exposed N and W facing shores will go into effect at 6 pm HST tonight. Other less- exposed shorelines may need a High Surf Advisory as the swell builds.
Even after the front moves E of the islands, W winds are expected to remain strong to near-gale on Tuesday as low pressure passes a couple hundred miles N of the islands. Winds will decline Tuesday night into Wednesday as the low departs to the NE, and a ridge of high pressure builds over the area.
As discussed above, the powerful low sending the cold front over the islands will produce a long-lived and extra-large NW swell that will bring surf heights well above High Surf Warning (HSW) levels. There is high potential for wave run-up onto vulnerable roadways and coastal areas in the HSW areas, with the most likely time for these impacts being Monday night into Tuesday morning during high tide. This swell is expected to drop below warning thresholds Tuesday night into Wednesday, but a large NW swell is possible during the second half of next week. Surf along S and W facing shores will also see an increase in short-period wind waves Sunday evening into Tuesday due to the strong onshore winds.

HFO Watches/Warnings/Advisories

Flood Watch through late Monday 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, 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.
High Wind Warning from 6 PM this evening to 6 PM HST Monday 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, 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.
High Surf Warning from 6 PM this evening to 6 PM HST Tuesday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Olomana, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Windward Haleakala.
High Wind Warning until 6 AM HST Tuesday for Haleakala Summit.
High Wind Warning until 6 AM HST Wednesday for Big Island Summits.
Winter Storm Warning from 6 PM this evening to 6 AM HST Tuesday for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening for all Hawaiian waters,
Gale Warning from 6 PM this evening to 6 PM HST Monday for all Hawaiian waters,

Data Courtesy of NOAA.gov