Photo Credit: Brendan Stephen

West Kaua’i

Rest Of Today: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs around 85 near the shore to around 72 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 70 near the shore to around 60 above 3000 feet. South winds around 10 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs around 84 near the shore to around 71 above 3000 feet. Southwest winds 10 to 15 mph. Chance of rain 50 percent.

South Kaua’i

Rest Of Today: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 79 to 85. Southeast winds up to 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 70. Light winds becoming southwest around 10 mph after midnight. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 78 to 84. Southwest winds around 10 mph. Chance of rain 50 percent.

Kaua’i Mountains

Rest Of Today: Mostly sunny with scattered showers in the morning, then mostly cloudy with occasional showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 73 to 80 in the valleys to around 65 above 4000 feet. South winds up to 10 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows 62 to 68 in the valleys to around 57 above 4000 feet. Southwest winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 50 percent.

Wednesday: Mostly cloudy with scattered showers. Highs 74 to 81 in the valleys to around 64 above 4000 feet. Southwest winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Rest Of Today: Mostly sunny with scattered showers in the morning, then mostly cloudy with showers likely in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 74 to 85. Light winds. Chance of rain 70 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 63 to 71. South winds up to 10 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 74 to 84. Southwest winds up to 15 mph shifting to the west in the afternoon. Chance of rain 50 percent.

East Kaua’i

Rest Of Today: Mostly sunny with scattered showers in the morning, then mostly cloudy with occasional showers in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 74 to 84. South winds up to 10 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows 64 to 73. Southwest winds around 10 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 72 to 84. Southwest winds around 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A lingering upper level low northeast of the islands is keeping the local airmass relatively moist and unstable. Spotty, hit-or- miss heavy downpours will continue to be possible over the next couple of days. A late season kona low will develop several hundred miles north of the islands Tuesday night, and then meander around well northwest of the islands the rest of the week. The kona low will turn our winds southerly as early as Tuesday, and eventually tap into deep tropical moisture lurking south of the islands. This will cause a slow-moving band of showers, some heavy, and a few embedded thunderstorms over some parts of the state for the second half of the week. Excessive rainfall and flash flooding will continue to be a concern.

Discussion

Water vapor imagery continues to show an upper level low located approximately 350 mi NE of Honolulu. This feature, along with a relatively large area of relatively moist air (TPW 1.6-1.7 in the predawn balloon soundings) is helping to enhance showers around the islands. Winds at the surface are mostly light land breezes early this morning but become S-SE in the lower levels just off the surface, which is pulling showers from the downwind Big Island convergence zone northward Oahu and Maui county.
The various convection-allowing-models all have their own ideas about the details of the next couple of days, but the overall theme seems to be that the general unsettled weather pattern will continue through Wed, with clouds and showers being drawn up northward over the smaller islands, as well as the possibility for some afternoon convective development over the interiors and north shores (where enough morning sunshine might allow).
The very unusual weather pattern for mid-May continues for latter half of the week. Another strong shortwave digs into the mean longwave trough position near the islands to generate a kona low about 500 mi N of Kauai (near 30N 160W) Tue night. This low is then forecast to strengthen and meander around several hundred miles NW of the islands for the rest of this week. A large reservoir of deep, moist, tropical air with TPW of greater than 2 inches is lurking about 400 mi or so SW of the islands, and the models focus some of this moisture northward along a developing convergence band over Oahu and Maui county Wed night. After that, the models slowly take the very moist convergence band slowly westward, with another surge of very high TPW air riding up the boundary as we approach the weekend.
Although the model QPFs are not incredibly impressive, this general pattern, with a relatively slow moving band of moisture convergence over the islands could lead to locally excessive rainfall. Flash flooding will be a concern, especially given the wet antecedent conditions from recent heavy rainfall. Have nudged QPF toward the highest NBM percentile values for Wed night into Thu, where confidence is highest for heavy rain timing and placement in the near term.
As the band of convergence shifts westward toward the weekend, even more copious moisture (TPW near or possibly even in excess of 2 inches) will be pulled northward toward Kauai and Oahu. A shortwave aloft rotating around the kona low and favorable jet diffluence along may help to enhance rainfall even more. This scenario is less confident, however, since the band may slow to a crawl and intensify mostly west of the Garden Isle over the weekend. Also, there is the possibility for one or more thunderstorm complexes to develop SW of the islands that might rob us of some moisture and instability. Nevertheless, we want to stress that this is the type of pattern that can lead to some pretty serious flooding. We will continue to monitor the situation and expect that details will become more clear as we get closer.
It is worth noting that as all the action shifts west, the Big Island and perhaps Maui county will be under increasing subsidence and warming aloft, with some dry air possibly trying to make inroads from the E, and a mid- level ridge located E of the islands. So the weather there by the weekend may end up being drier, with locally breezy E or ESE winds thanks to a nearly stationary surface high far to the NE of the Big Island.

Aviation

VFR prevails this morning as land breezes have scoured out low clouds and convective debris from yesterday afternoon and evening lingers as sct-bkn cigs around 10kft. Showers have largely refocused offshore where they will remain for the remainder of the early morning, save for a few locales along the immediate coast, before refocusing over island interiors again late this morning into this afternoon. Light to moderate southeasterlies will veer to southerly during this time.
No AIRMETs in effect.

Marine

Light to moderate east southeast to south winds will persist today into Wednesday as an area of low pressure develops 450 nm north of the state. A frontal boundary extending from the low will move over the western islands early evening Wednesday then stall near or over the central islands Thursday. The front will then weaken into a trough and shift westward Friday. Scattered heavy showers are possible through the week with the threat of thunderstorms developing Wednesday and persisting into the weekend.
A series of overlapping, long-period south swells will begin to fill in this afternoon, keeping surf along south facing shores near to just below average. A second set of southerly swells will fill in late Wednesday and could bring above average surf Thursday. Surf should remain elevated through the weekend.
A small, medium-period northwest swell will slowly decline today. A new small, medium period north-northwest swell will fill in later today, and peak Wednesday. As the low mentioned above develops near Hawaii, this source may send a moderate, short period northwest swell Wednesday night, peaking near to just below advisory levels Thursday into Friday. Model guidance tends to under-forecast swell heights with sources developing at a close proximity. Therefore, there is a potential for surf to reach High Surf Advisory criteria and will continue to monitor all available guidance.
West facing shores will see a bump in surf through the week due to the overlapping northwest and south swells wrapping into some exposed areas. Surf along east facing shores will remain well below normal levels through the weekend.

HFO Watches/Warnings/Advisories

Flood Watch from Wednesday morning through Friday evening for all Hawaii islands,

Data Courtesy of NOAA.gov