Photo Credit: Brendan Stephen

West Kaua’i

Today: Cloudy with showers likely and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Highs around 79 near the shore to around 68 above 3000 feet. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy with showers likely. Lows around 67 near the shore to around 58 above 3000 feet. Southeast winds around 10 mph. Chance of rain 70 percent.

Saturday: Cloudy. Showers likely in the morning, then scattered showers in the afternoon. Highs 78 to 83 near the shore to around 70 above 3000 feet. Southeast winds around 10 mph. Chance of rain 70 percent.

South Kaua’i

Today: Cloudy with showers likely and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Highs 75 to 80. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy. Showers likely and slight chance of thunderstorms in the evening, then showers and slight chance of thunderstorms after midnight. Some thunderstorms may produce heavy rainfall. Lows around 66. Southeast winds around 10 mph. Chance of rain 90 percent.

Saturday: Cloudy. Showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Some thunderstorms may produce heavy rainfall. Highs 78 to 83. Southeast winds around 10 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Breezy. Showers and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Highs 69 to 74 in the valleys to around 61 above 4000 feet. East winds 10 to 20 mph. Chance of rain near 100 percent.

Tonight: Showers and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Lows 58 to 65 in the valleys to around 56 above 4000 feet. Southeast winds 10 to 15 mph. Chance of rain near 100 percent.

Saturday: Showers and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Highs 70 to 76 in the valleys to around 63 above 4000 feet. Southeast winds 10 to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Cloudy and breezy. Showers and isolated thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Some thunderstorms may produce heavy rainfall. Highs 71 to 79. East winds 10 to 20 mph. Chance of rain 90 percent.

Tonight: Cloudy. Showers likely and slight chance of thunderstorms in the evening, then showers and slight chance of thunderstorms after midnight. Some thunderstorms may produce heavy rainfall. Lows 60 to 69. Southeast winds around 10 mph. Chance of rain 90 percent.

Saturday: Showers and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Highs 72 to 80. Southeast winds around 10 mph. Chance of rain near 100 percent.

East Kaua’i

Today: Showers likely and isolated thunderstorms in the morning, then showers and slight chance of thunderstorms in the afternoon. Some thunderstorms may produce heavy rainfall. Highs 68 to 79. East winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Showers and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Lows 60 to 70. South winds 10 to 15 mph. Chance of rain near 100 percent.

Saturday: Showers and slight chance of thunderstorms. Some thunderstorms may produce heavy rainfall. Highs 70 to 81. Southeast winds around 10 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

Light to moderate rain may be locally heavy at times over Kauai and Oahu while periods of heavy rain will result in much greater flash flood potential over windward and southeast-facing slopes of the Big Island and Maui. Showers will decrease in coverage over western areas by tonight, but will continue over favored portions of Maui and the Big Island well into next week.

Discussion

Satellite imagery shows an upper low centered just west of Kauai, the attendant surface low orphaned and drifting westward as it spins down. Deep southeasterly flow around the eastern flank of this low has drawn north a plume of tropical moisture characterized by PWATs around 2″. Most of the overnight period has featured steady light to moderate stratiform rain with a few embedded convective elements and 12-hour rainfall totals as of 2am generally about 1.5″ or less everywhere. Thunderstorms have been common in the vicinity of Kauai where steep lapse rates exist near the center of low, but this unstable mid-level airmass will only tend to drift further from the islands and modify which will significantly limit thunderstorm potential going forward. Of note from last night, a 47kt wind gust was recorded at Barking Sands on Kauai, the result of enhanced downsloping easterlies around a mesoscale low that was observed on radar. This was highly localized and transient and is unlikely to materialize elsewhere.
With the low-level flow finally veering to the southeast, rain has filled in over the southeast slopes of the Big Island, though rates mostly remain under 1″/hr at press time. It is worth noting that as of 2am HST this morning, hyperactive convection in the hi-res suite has caused those models to overestimate 12-hour rainfall over southeast and windward sections of the Big Island by an average factor of around 3-5x and locally as much as 10x. The same can be said for the HREF.
IR imagery during the last few hours does show a distinct erosion of cold cloud tops occurring around 300 miles east of the Big Island while cloud top cooling is observed within the moisture plume itself. This is visual confirmation of model depictions of upper- level divergence developing in response to an amplifying downstream ridge, and it will offer some background synoptic support to ongoing rainfall. However, the lack of a large scale mechanism for organization, such as a progressive shortwave aloft or a cold front, means heavy rain potential will be almost entirely reliant on orographic forcing. For this reason, shadowed southeast flow downstream of the Big island is making for a rather tenuous Flood Watch for Kauai, Oahu, and Molokai. Some island interaction was observed overnight and probably represents the most realistic flooding scenario for these islands. If said threat fails to materialize today, then consideration will be given to canceling the watch for western portions of the state.
For windward & southeast Big Island and Windward Maui, rain will be essentially constant through at least Saturday. Rainfall intensity has underachieved thus far, though this is at least partially due to delay in low-level winds veering to southeasterly. However, poor lapse rates and weak to moderate mid- level static stability are also contributing factors, and they will not improve during the course of the event. Nonetheless, with the orographic contribution now in full swing and the imminent the onset of diurnal heating, an increase in rates is forecast during the next several hours. Strengthening southeasterlies are also forecast to develop today as surface high pressure builds to the northeast. The convergent lead edge of the building high may give a boost to showers over east-facing zones. The GFS and the high-res HRRR both key in on this potential this morning. Forecast rainfall amounts have been lowered for the balance of the event and the footprint of the heaviest rain through the near term has been refined to focus more over interior and upslope zones. The Flood Watch itself remains in effect and is especially good shape over the eastern end of the state with several waves of rain expected during the next 48 hours.
The deep southeast flow pattern will be a fixture through the middle of next week. Even as large scale forcing diminishes and showers become less organized by late this weekend, the resident band of tropical moisture will stagnate over or near the area. Consensus among the global models remains in tact that a deep mid-latitude trough will develop to the west early next week. This will in turn re-activate the moisture band further west (i.e. around Kauai) and strengthen low-level southeasterlies bringing another round of rain to windward and southeast-facing slopes of the Big Island and Maui.

Aviation

A low pressure system southwest of Kauai will continue to move westward and away from the state over the next 24-hours. However, in its wake, trailing moisture and instability will lead to numerous showers and isolated thunderstorms areawide. Periods of MVFR conditions, due to lowered cloud bases and reduced visibility, will likely accompany any of the heavier showers or storms.
Moderate southeasterly flow will persist through the forecast period.
AIRMET SIERRA remains in effect for mountain obscuration for the Big Island, Maui, Molokai, and Oahu above 2000 feet due to low clouds and showers.
AIRMET TANGO remains in effect over all islands for tempo moderate turbulence between FL120 and FL420.
AIRMET ZULU remains in effect above all islands for moderate rime ice in precipitation in layer FL120 to FL250.

Marine

Strong high pressure will remain anchored northeast of the state through the middle of next week. This will keep persistent strong to near-gale force east to southeast winds in place over most windward waters through the middle of next week, with lighter winds over the leeward waters. A Small Craft Advisory (SCA) remains in effect through 6 PM Saturday for all windward waters due to winds/seas or a combination of the two. This SCA will likely need to be extended for most windward waters through the middle of next week.
No significant northwest swells are expected during the next 7 days, with only small surf expected along north and west facing shores. East shore surf will build today as east-southeast winds strengthen. A High Surf Advisory remains in effect from 6 AM this morning until 6 PM Saturday, and this advisory may need to be extended through the middle of next week as winds over and upstream of the islands will continue to support large and rough surf. Elevated early morning high tides may bring some seawater onto vulnerable coastal roadways this weekend.

HFO Watches/Warnings/Advisories

Flood Watch through Saturday afternoon for all Hawaiian Islands.
High Surf Advisory through Saturday afternoon for east, facing shores of Kauai, Oahu, Molokai, Maui, and Big Island.
Winter Storm Warning through Saturday afternoon for Big Island Summits.
Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov