West Side

Today: Breezy. Mostly sunny with isolated showers. Highs 72 to 83. East winds up to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 62 to 70. East winds up to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny early in the morning then becoming partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Haze through the day. Highs 71 to 83. East winds up to 10 mph. Chance of rain 30 percent.

South Side

Today: Mostly sunny. Highs 74 to 84. North winds up to 10 mph.

Tonight: Partly cloudy with isolated showers. Lows around 65. North winds up to 10 mph. Chance of rain 20 percent.

Monday: Partly sunny with isolated showers. Haze. Highs 73 to 83. Light winds becoming north up to 10 mph in the afternoon. Chance of rain 20 percent.

North Shore

Today: Breezy. Partly sunny with scattered showers. Highs around 77 near the shore to around 63 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 70 near the shore to around 51 near 5000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Mostly sunny early in the morning then becoming partly sunny. Isolated showers early in the morning, then scattered showers in the late morning and afternoon. Haze through the day. Highs around 77 near the shore to around 63 near 5000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Central Maui

Today: Mostly sunny. Breezy. Isolated showers in the morning. Highs 79 to 85. East winds 10 to 25 mph. Chance of rain 20 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows around 65. East winds up to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny early in the morning then becoming partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Haze through the day. Highs around 81. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 30 percent.

Upcountry

Today: Breezy. Partly sunny with scattered showers. Highs 50 to 67. East winds up to 20 mph. Chance of rain 50 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows around 42 at the visitor center to around 39 at the summit. East winds up to 15 mph. Chance of rain 50 percent.

Monday: Mostly sunny with isolated showers early in the morning, then partly sunny with scattered showers in the late morning and afternoon. Haze through the day. Highs 49 to 66. Southeast winds up to 10 mph in the morning becoming light. Chance of rain 40 percent.

East Maui

Today: Breezy. Partly sunny with scattered showers. Highs around 77 near the shore to around 63 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 70 near the shore to around 51 near 5000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Mostly sunny early in the morning then becoming partly sunny. Isolated showers early in the morning, then scattered showers in the late morning and afternoon. Haze through the day. Highs around 77 near the shore to around 63 near 5000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Lanai City

Today: Mostly sunny. Isolated showers in the afternoon. Highs 67 to 77. Light winds. Chance of rain 20 percent.

Tonight: Mostly cloudy with isolated showers in the evening, then partly cloudy after midnight. Lows 57 to 63. Light winds. Chance of rain 20 percent.

Monday: Mostly sunny early in the morning then becoming partly sunny. Isolated showers early in the morning, then scattered showers in the late morning and afternoon. Haze through the day. Highs 66 to 76. Light winds. Chance of rain 40 percent.

Kaunakakai

Today: Mostly sunny. Breezy. Highs 65 to 84. East winds 10 to 20 mph.

Tonight: Partly cloudy with isolated showers. Lows 54 to 70. East winds 10 to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny early in the morning then becoming partly sunny. Isolated showers early in the morning, then scattered showers in the late morning and afternoon. Haze through the day. Highs 65 to 83. East winds up to 10 mph. Chance of rain 40 percent.

Detailed Forecast

Synopsis

The high pressure ridge north of the islands will weaken through Monday as a strong cut off low pressure system deepens near the International Date Line. Wind directions will veer more southeasterly on Monday, then become southerly to southwesterly with strengthening winds from Tuesday onward. This next storm system appears extremely strong for the Hawaii region and dangerous impacts are likely to develop statewide through much of this week. A combination of flooding, strong and gusty southerly winds, thunderstorms possibly severe at times, and Big Island summit level snow and ice will threaten Hawaii from Tuesday into next weekend. Island by island impacts will be driven by both large scale troughs and smaller scale thunderstorm bands over the entire duration of this severe weather event.

Discussion

Looking into the satellite picture this morning we see unstable cumulus clouds riding into the eastern windward slopes of island mountains. Clouds and passing showers will continue through the morning hours for these mountain areas. Farther away near the International Date Line, a very large upper low with surface cold front continues to deepen and will slowly bring significant weather impacts to the Hawaiian Islands lasting for several days this week.
This strong low pressure system is already showing a long reach with associated weather impacts far from the center of the storm. A significant band of thunderstorms continues to develop roughly 500 to 700 miles east to southeast of the low's center. A band of high level clouds even farther away from the low, shows embedded thunderstorms forming just 150 to 250 miles west of Kauai. Strong divergence under an upper level trough and dynamic lift from a 150 MPH jet stream will slowly drive these heavy showers and thunderstorm bands into Hawaii lasting beginning Tuesday and continuing into next weekend.
The latest ensemble and operational model guidance continues to show good consistency with the track and intensity of this large storm into next weekend, adding confidence to our statewide impacts forecast. A combination of upper level divergence and low level convergence will produce deep convection from abundantly unstable moisture riding northward into the islands from the tropics. Passing upper level troughs, oriented in a negative tilt, will provide more efficient divergence aloft and a higher degree of lift for heavy showers and thunderstorms over the islands during this event.
The highest threats start with heavy rain that may cause flooding to develop over the western half of the state on Tuesday, these heavy rain threats then rapidly spread eastward becoming statewide flood threat impacts from Wednesday into the weekend. A statewide Flood Watch will likely be issued over the next couple of days to cover these heavy rain and potential flooding impacts. Storm total rainfall amounts from Tuesday through Saturday are forecast to range from 4 to 12 inches across the state during this five day time period. Locally higher amounts are possible, likely driven by slow moving strong thunderstorms or heavy rain bands. Expect very saturated soil conditions to develop over the first few days, accelerating the flooding threats by the second half of the week, as less intense rainfall on saturated ground will quickly produce water runoff that leads to flooding concerns. A Hydrologic Outlook remains in effect for these flood threats later this week.
The next impacts are related to the developing southerly to southwesterly kona winds. A deep surface low approaching the islands will cause the wind directions to veer from a southerly direction from Tuesday onward. Initially Wind Advisory threats are possible due to strong wind gusts developing north of steep mountain slopes of all islands. These gusty winds may knock down trees in saturated soil conditions that weaken root systems, making them vulnerable to atypically strong southerly winds. Southerly to southwesterly wind speeds will significantly increase into high end advisory or even exceeding warning thresholds from Friday into Sunday. Damaging winds are likely during this time period mainly over northern and easterly slopes of steep mountain ranges of all islands. Power outages should be expected mainly caused by falling tree branches on power lines.
Additional wind threats on the smaller scale will be driven by strong thunderstorms that will form rapidly in the southerly kona winds. These storms will tend to auto-convect, as this unstable tropical air converges and lifts, producing bands of heavy rain and thunderstorms. Significant atmospheric instability and wind shear aloft will set the table for some of these stronger thunderstorms to become severe. The main threats from severe thunderstorms will be strong damaging wind gusts, very high rainfall rates, and dangerous lightning. A Severe Thunderstorm Watch may be needed at some point during this event to cover the threat for damaging winds.
High elevation snow and ice with stronger winds are also expected over the highest elevations of Mauna Kea and Mauna Loa on the Big Island. These conditions will likely limit road access to the observatories at summit level.
Please plan ahead for these significant statewide weather impacts over a five day time period lasting into next weekend.

Aviation

East southeast flow across Hawaii is expected to slightly weaken tomorrow. Scattered clouds will move in and bringing SHRA over southeast areas. MVFR conds possible in SHRA, otherwise VFR conds prevail.
AIRMET Sierra for mtn obsc is in effect for windward locations of Kauai, Oahu, Maui and The Big Island.
AIRMET Tango in effect for mod turb blw 080 downwind slopes (south across northwest facing). Expect this AIRMET to drop off in the morning. Directional LLWS possible at the PHOG/OGG runway this afternoon.

Marine

Strong high pressure far northeast of the state will maintain fresh to strong trade winds today and tonight. A Small Craft Advisory (SCA) is now in effect for most marine zones through 6 pm today, and for most central and eastern waters through 6 am Monday. A front approaching from the west will begin to ease the trades and shift them southeasterly on Monday, with the winds then becoming southerly and increasing to fresh to strong levels Tuesday through late this week.
Surf along east facing shores will remain elevated and choppy today, trend downward on Monday, then fall below normal levels Tuesday through late this week.
A series of west-northwest swells will keep some small surf in place along north and west facing shores during the next 7 days, but surf will remain well below advisory levels. A small to moderate sized north swell is possible late this week.
Surf along south-facing shores will remain small, with the exception of areas exposed to trade wind swell wrap. Rough and choppy conditions will develop Tuesday and continue through late week as southerly winds increase in advance of a front. A series of small long period south swells will also move through Monday through late this week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Channel, Oahu Windward Waters.
Small Craft Advisory until 6 AM HST Monday for Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov