Photo Credit: Braden Jarvis

West Kaua’i

Today: Mostly sunny with isolated showers. Highs around 85 near the shore to around 70 above 3000 feet. Southeast winds up to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows around 69 near the shore to around 59 above 3000 feet. East winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Monday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 86 near the shore to around 71 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

South Kaua’i

Today: Mostly sunny with isolated showers. Highs 77 to 84. East winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 68. Northeast winds 10 to 15 mph. Chance of rain 40 percent.

Monday: Partly sunny with scattered showers. Highs 78 to 84. East winds around 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Partly sunny with scattered showers. Highs 71 to 82 in the valleys to around 63 above 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 64 to 69 in the valleys to around 56 above 4000 feet. East winds up to 10 mph in the evening becoming light. Chance of rain 40 percent.

Monday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 72 to 82 in the valleys to 61 to 66 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

North Kaua’i

Today: Partly sunny with scattered showers. Highs 75 to 85. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with scattered showers. Lows 63 to 72. Southeast winds up to 10 mph. Chance of rain 40 percent.

Monday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 76 to 86. Southeast winds up to 15 mph. Chance of rain 50 percent.

East Kaua’i

Today: Mostly sunny with isolated showers. Highs 72 to 82. East winds up to 10 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy with scattered showers. Lows 63 to 72. Northeast winds up to 10 mph. Chance of rain 40 percent.

Monday: Partly sunny with scattered showers. Highs 73 to 83. East winds up to 10 mph shifting to the southeast in the afternoon. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Heavy rain and isolated thunderstorms are possible over all islands again today. The threat will diminish by tonight and a land and sea breeze pattern favoring clouds and showers over island interiors will continue into the middle of next week. Another low will bring the threat of heavy rain to the islands during the second half of next week.

Discussion

Local radar composite loop hints at the present state of the middle and upper atmosphere as heavy showers and thunderstorms rotate cyclonically around the area. Heavy rain up to 3″/hour lifting northeastward over northern Big Island has recently dissipated while a fresh round of heavy showers developed and moved over Kauai from the north this morning. In between, steering flow remains rather nebulous and slow-moving showers and thunderstorms have developed off the windward coasts of Oahu and Molokai. The onset of daytime heating will allow these showers to redevelop inland. Worth noting that all guidance has quite a poor handle on the current distribution of precipitation across the state this morning. The Flood Watch remains in effect for the islands through today.
Focus for today will shift toward another west-east progression of heavy rain potential as a shortwave embedded within the parent low aloft pivots through the area during the next 12-18 hours. The thermal trough associated with this second wave is of similar magnitude to yesterday's, if not slightly colder (-14 to -16C at 500mb). 700-500mb lapse rates over Kauai have almost certainly steepened from this morning's 12z sounding and are probably solidly in excess of 7.0 C/km. This favorable environment for deep convective growth leading to heavy showers and isolated thunderstorms will sweep eastward as the day progresses. Heavy rain leading to flash flooding will be a possibility for all islands once again today.
The resident upper low will very gradually reabsorb into the baroclinic westerlies during the next few days leaving the islands firmly within the trailing envelope of cyclonic flow aloft, though forcing and instability will diminish. In the near term (Sunday into Monday), the established airmass will remain in place with continued weak troughing over the islands supporting a light easterly gradient and the consequent land and sea breeze pattern. This will favor afternoon cloud build ups and interior showers of less intensity than those of yesterday and today.
Strong consensus exists among the global models that high amplitude mid-latitude troughing will beget another closed low in the vicinity of the islands during the middle of next week. The local gradient responds to the associated height falls as early as Monday-Tuesday when surface winds veer to southeasterly and advect higher dewpoints (near 70F) into the region. In addition to the lack of meaningful wind and increasingly uncomfortable humidity, this will supply plenty of boundary layer moisture for afternoon showers as the sea breeze pattern continues.
The second half of next week bears watching as the aforementioned closed low reaches its southernmost extent. Current consensus indicates PWATs rising toward 2″ as convergent southwesterly flow establishes over the islands leading to an elevated heavy rain threat. The inherited forecast handles this potential well, including the risk of thunderstorms.

Aviation

A strong upper level disturbance will continue to gradually move eastward across the state today. As a result, enhanced shower activity and isolated thunderstorms will produce periods of MVFR and local IFR. Light winds should allow for sea breeze development this afternoon, with scattered showers possible over the interior and sheltered leeward areas.
AIRMET Sierra for tempo mountain obscuration is in effect for all of Kauai and windward portions of Oahu this morning. This AIRMET will likely be needed on and off throughout the day for portions of island chain as showers develop and dissipate.
AIRMET Tango for moderate to isolated severe mid to upper level turbulence remains in effect as the upper level disturbance moves across the state. Conditions will likely improve heading into the evening hours.
AMD NOT SKED for PMDY continues due to equipment issues, with no timeline for restoration.

Marine

Thunderstorms, some of which could be strong to severe, will remain possible across Hawaiian coastal waters today as an upper disturbance moves across the area. Any thunderstorms that develop will have the potential to produce gusty winds, small hail, and frequent lightning. Marine interests should continue to monitor for any Special Marine Warnings that may be issued. Light and variable winds will prevail today, except within and near thunderstorms. Light to moderate easterly trade winds will begin building back into the region on Sunday. Winds will weaken again and become more southerly by Monday night as high pressure becomes established both far northeast and far northwest of the islands and a trough in between moves into the region.
Surf along south facing shores remains elevated, but will slowly decline through Sunday and then hold into early next week as a long-lived, small south swell continues to affect the islands. Although the swell size itself will remain largely unchanged through Sunday night, the period will gradually shorten from 15 seconds today to near 13 seconds by Sunday night, allowing surf heights to slowly trend down. Another series of overlapping, long-period south swells will begin moving in on Tuesday, with reinforcements giving another noticeable boost to south shore surf from Wednesday through the rest of next week.
A small, short-period north-northwest swell has increased north shore surf this morning, but this will decline later today into Sunday. A new small, long-period northwest swell will build Sunday night, giving north shore surf a boost early next week. Surf along east facing shores will be well below normal levels through the middle of next week.

HFO Watches/Warnings/Advisories

Flood Watch through this evening for all Hawaiian Islands.
Wind Advisory until 6 PM HST this evening for Big Island Summits.
Winter Weather Advisory until 6 PM HST this evening for Big Island Summits.

Data Courtesy of NOAA.gov