Photo Credit: Jason Weingardt

West Kaua’i

Today: Mostly cloudy. Breezy. Scattered showers in the morning, then numerous showers in the afternoon. Highs around 83 near the shore to around 70 above 3000 feet. Southwest winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 60 percent.

Tonight: Breezy and showers. Lows around 69 near the shore to around 59 above 3000 feet. Southwest winds 15 to 25 mph with gusts to 45 mph. Chance of rain near 100 percent.

Monday: Breezy and showers. Highs around 82 near the shore to around 68 above 3000 feet. Southwest winds 10 to 25 mph with gusts to 45 mph. Chance of rain near 100 percent.

South Kaua’i

Today: Mostly cloudy. Breezy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 80 to 85. Southwest winds 10 to 25 mph. Chance of rain 60 percent.

Tonight: Breezy. Cloudy with numerous showers. Lows around 67. Southwest winds 15 to 25 mph with gusts to 50 mph. Chance of rain 70 percent.

Monday: Breezy. Cloudy with showers. Highs 78 to 83. Southwest winds 10 to 25 mph. Gusts up to 50 mph in the morning. Chance of rain 90 percent.

Kaua’i Mountains

Today: Mostly cloudy. Breezy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 77 to 83 in the valleys to around 65 above 4000 feet. Southwest winds up to 25 mph increasing to 10 to 25 mph with gusts to 45 mph in the afternoon. Chance of rain 70 percent.

Tonight: Windy and showers. Lows 60 to 67 in the valleys to around 57 above 4000 feet. Southwest winds 15 to 30 mph with gusts to 50 mph. Chance of rain near 100 percent.

Monday: Windy and showers. Highs 74 to 80 in the valleys to around 62 above 4000 feet. Southwest winds 10 to 30 mph with gusts to 50 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Partly sunny. Breezy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 74 to 86. Southwest winds up to 25 mph. Gusts up to 45 mph in the afternoon. Chance of rain 60 percent.

Tonight: Breezy. Mostly cloudy with numerous showers. Lows 62 to 69. Southwest winds 15 to 25 mph with gusts to 50 mph. Chance of rain 70 percent.

Monday: Breezy. Showers in the morning, then scattered showers in the afternoon. Highs 72 to 83. Southwest winds 10 to 25 mph with gusts to 50 mph. Chance of rain near 100 percent.

East Kaua’i

Today: Breezy. Partly sunny with scattered showers. Highs 75 to 85. Southwest winds up to 25 mph. Gusts up to 45 mph in the afternoon. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with numerous showers. Lows 62 to 71. Southwest winds 15 to 20 mph. Gusts up to 45 mph increasing to 55 mph after midnight. Chance of rain 70 percent.

Monday: Breezy. Cloudy with showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 72 to 82. Southwest winds 10 to 25 mph. Gusts up to 55 mph in the morning. Chance of rain 80 percent.

Detailed Forecast

Synopsis

Strengthening southwesterly winds will increase prefrontal shower band activity today in the unstable warm sector ahead of the approaching cold front. A cold front will move down the island chain from early Monday morning through Wednesday bringing increasing rainfall coverage and moderate to windy southwesterly winds. Strong wind gusts may develop along northern and eastern slopes of island mountains with this system. A second cold front will sweep through the islands from Thursday to Friday with yet another round of soaking rainfall for the state. Drier conditions develop rapidly as trade winds fill in after the second front passes through each island. Breezy to windy trade winds with passing showers over the typical windward and mountain areas are expected from Friday through next weekend.

Discussion

The morning satellite imagery around the Hawaii region shows a large band of clouds roughly 200 miles northwest of Kauai associated with an approaching cold front. Prefrontal shower bands are developing in along convergence bands in the stronger southwesterly winds just ahead of the frontal trough. The first of these prefrontal shower bands is passing through the islands of Niihau, Kauai and Oahu this morning. Mountain waves are also present northeast of Kauai, Oahu and Molokai due to strong southwesterly winds blowing across island mountains.
The weather pattern begins to change towards the wet side today as southwesterly winds strengthen ahead of the approaching cold front. These winds will be stronger above the surface in the 30 to 50 MPH range. Prefrontal convective showers can reach up to higher levels in the atmosphere and bring down these stronger winds in the form of wind gusts. These strong gusts blowing from a southwest direction also tend to accelerate on the downslope sides of island mountains. This means we will likely see some of the stronger wind gusts from 40 to 50 MPH developing along north and east slopes of island mountains from Kauai to Maui by this evening. This southwesterly wind direction is unusual and these strong gusts may cause trees to fall or cause damage to buildings and a wind advisory may be issued later today to cover the north and east slopes of most islands for this event. The latest model guidance has weakened these upper level winds by about 5 to 10 MPH, which caused us to hold off on issuing the wind advisory with this mornings forecast package. The strongest wind gusts are expected to last from Sunday night through Monday night, after that the winds aloft appear to weaken rapidly as the frontal system diminishes. The Big Island will see limited wind impacts, mainly over the summit areas of Mauna Kea and Mauna Loa where high elevation wind impacts will continue into Tuesday. A High Wind Warning remains in effect for the summits of Mauna Kea and Mauna Loa today.
Rainfall will spread across the island chain starting today with prefrontal showers that have already enhanced rainfall near Kauai and are spreading into Oahu this morning. These prefrontal shower bands will only increase through the day as stronger southwesterly winds sweep eastward affecting all islands from Kauai to Maui. Periods of scattered to numerous showers should be expected for all these islands by Monday, especially as the cold frontal cloud band passes through the state. Timing on the frontal passage starts with Kauai just after midnight on Monday morning, then reaching Oahu later in the day on Monday, then the islands of Maui County from Monday night through Tuesday morning. Rainfall amounts through Tuesday night will cover all islands, except for the Big Island where enhanced showers will be delayed until Wednesday morning as the frontal trough weakens and stalls over the eastern half of the state. Rainfall amounts will be moderate with this next system and chances for flash flooding remains low at this time.
A weak secondary cold front will rapidly sweep eastward through the islands from Thursday through Friday. Another brief round of soaking rainfall is expected for all islands. A passing high pressure system north of the state will cause breezy to windy trade winds to fill in rapidly with drier trends statewide starting on Friday and possibly strengthening to 'blasting trade winds' from Friday night through much of next weekend.

Aviation

Moderate to breezy southwest winds ahead of an approaching front will persist through Monday. Bands of pre-frontal showers will impact mainly Kauai and western Oahu. Elsewhere, diurnal heating will drive cumulus development over island interiors in the afternoon and evening hours. Weak land breezes will encourage clearing across the Big Island in the late night and morning hours.
VFR conditions will prevail statewide today with a couple of exceptions. Over Kauai and Oahu, brief MVFR CIG and VIS are to be expected in passing showers both today and tonight. To the east, areas of radiative stratus are causing patchy reports of MVFR ceilings. Stratus is expected to lift and dissipate by early afternoon. Mostly clear skies will predominate across Maui and the Big Island into Monday.
AIRMET Tango remains in effect for moderate turbulence below 8000 feet north through east of terrain across all main Hawaiian Islands. Bumpy conditions are expected to persist into Monday. No other AIRMETS are in effect, nor anticipated.

Marine

Moderate to fresh southwest winds will reach the fresh to strong category later today through Monday night as a front approaches. This combined with a west-northwest swell arriving will lead to the Small Craft Advisory being expanded down the island chain for exposed waters due to a combination of winds and seas in later packages. The winds will quickly ease as the front advances down the island chain and stalls over the eastern end of the state Monday night through Tuesday night, likely enough for localized land and sea breeze conditions to form near the coasts (light northerly background flow trailing the front Tuesday into Tuesday night). Light east to southeast background flow is forecast for a brief period Wednesday as the next front approaches. This next front will move through Thursday through Friday with strong northerly winds trailing the front Thursday night through Friday. Strong trade winds are depicted Friday through next weekend as high pressure builds north of the area.
Surf along exposed north and west facing shores will remain rough through Monday due to a mix of building short-period wind waves associated with the increasing southwest winds and an arriving medium-period west-northwest swell. Surf heights may near advisory levels for north and west facing shores of the smaller islands Monday. A larger west-northwest swell arriving Monday night into Tuesday will drive the surf heights to near warning levels late Tuesday through Tuesday night as it peaks. Similar to the past several west-northwest swells over the last few weeks, surf along the leeward side of the Big Island will likely reach advisory levels Monday, then near their warning level Tuesday through Tuesday night.
As this west-northwest swell eases Wednesday through Thursday, guidance shows a mix of a moderate north to north-northeast and west-northwest swell moving through. The north-northeast component will become the dominant swell Thursday night through Friday in response to a broad gale forming in the northeast Pacific Tuesday through midweek – likely enough for a return of advisory-level surf for exposed coasts (010-020 deg swell).
Surf along east facing shores will remain small through most of the week due to the lack of trades locally and upstream. An upward trend is expected later in the week through next weekend as the large north-northeast swell and strong trades return.
Surf along south facing shores will remain rough through Monday night due to the strengthening southwest winds. A downward trend is expected Tuesday through midweek as the winds diminish.

HFO Watches/Warnings/Advisories

High Wind Warning until 6 PM HST this evening for Big Island Summits.
Small Craft Advisory until 6 PM HST Monday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters.

Data Courtesy of NOAA.gov