Photo Credit: Brendan Stephen

West Kaua’i

Today: Mostly sunny. Breezy. Highs 87 to 92 near the shore to around 78 above 3000 feet. East winds up to 20 mph.

Tonight: Mostly clear. Lows around 74 near the shore to around 63 above 3000 feet. Northeast winds up to 15 mph.

Thursday: Sunny. Highs 87 to 92 near the shore to around 77 above 3000 feet. Northeast winds up to 15 mph.

South Kaua’i

Today: Mostly sunny. Breezy. Isolated showers in the morning. Highs 82 to 90. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows around 74. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Sunny. Isolated showers in the morning. Highs 82 to 90. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Kaua’i Mountains

Today: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 76 to 88 in the valleys to around 67 above 4000 feet. East winds up to 15 mph. Chance of rain 30 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 65 to 72 in the valleys to around 60 above 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Thursday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 76 to 87 in the valleys to around 68 above 4000 feet. East winds up to 15 mph. Chance of rain 40 percent.

North Kaua’i

Today: Mostly sunny with isolated showers. Highs 78 to 90. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 67 to 74. Southeast winds up to 10 mph. Chance of rain 20 percent.

Thursday: Mostly sunny with isolated showers. Highs 78 to 89. East winds up to 15 mph. Chance of rain 20 percent.

East Kaua’i

Today: Mostly sunny with isolated showers. Highs 75 to 87. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 66 to 77. North winds up to 10 mph. Chance of rain 20 percent.

Thursday: Mostly sunny with isolated showers. Highs 74 to 87. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Gentle to breezy east trade winds will continue the next several days. Rainfall will be limited with more cloudiness and brief showers favoring windward exposures and higher terrain. The lone exception will be along Kona region slopes of Big Island where thicker cloud cover and showers will develop during daytime hours.

Discussion

A large ridge of high pressure orientated parallel to the middle latitudes is this week's main Central Pacific synoptic scale feature. The pressure gradient from the associated 1022 mb surface high centered between 30N and 40N latitude and along 150W longitude will continue to be the driver of gentle interior and breezy coastal and mauka east trades into October. Low pressure moving east across the Gulf of Alaska will ride atop this ridging and nudge it slightly closer to the island chain. Overall, this will have little to no effect on these ongoing gentle to breezy east trades the next several days. The only subtle effect would be a minor slackening of the gradient that could result in a hardly noticeable weakening of late week easterlies to more gentle magnitudes. The low will progressively move far northeast of the region this weekend, allowing the ridge to rebound to the north. This will likely correlate to the return of more statewide breezy east trades early next week.
Mid to upper level ridging will build in over and north of the islands during the remainder of the week. A vast majority of the state's 24 hour rainfall observations were zero this morning with those that did pick up rain only measuring a few hundredths of an inch (windward Big Island's Mountain View was the winner with slightly over a quarter of an inch since yesterday morning). Mid layer moisture advection will not be overly abundant the next few days with 850-700 mb relative humidity values consistently coming in under 50%. This will translate to clearing out skies and ultimately result in a dry weather pattern. Windward Big Island and upper elevation Kauai will experience more periods of rain through Friday. Pockets of greater than 90% mid layer RH passing through the next few days will warrant high end chance to likely rain probabilities in the Hilo and Puna Big Island districts and generally above 2,000 ft on Kauai. The rest of the state will experience brief shower activity that will generally favor the windward slopes and highest terrain, especially overnight. More frequent ribbons of high mid level moisture moving through from the northeast and east early next week will increase areawide rain chances and coverage.

Aviation

Moderate to locally breezy trade winds will continue due to high pressure to the north of the area. Upper level high pressure moving over the islands from the east will maintain a stable airmass during this time. Some high clouds are streaming over Kauai today from the south, but should shift to the west as the mid-level features shifts west. Limited clouds and showers will be carried in on the trade winds, focusing what few showers make it to windward sides of the islands. Some afternoon clouds and showers are possible over the Kona slopes of the Big Island. VFR conditions are expected to prevail.
No AIRMETs are currently in effect and none are anticipated at this time.

Marine

High pressure far north of the state will maintain moderate to locally strong trade winds through this afternoon. Latest observations and model guidance shows that winds will hover near to just above Small Craft Advisory criteria through today with conditions possibly dipping below advisory levels tonight. The SCA remains in effect for the typical windy waters around Maui County and the Big Island through today and will continue to be monitored to see if an extension is warranted later this afternoon or allowed to expire. Forecast remains on track with winds further weakening this Friday and over the weekend as a front pass north of the state, weakening and shifting the high far northeast of the state. Moderate to locally strong trades may rebound as early next week as the front dissipates and a new high quickly rebuilds north of the state. Have removed chances of thunderstorms from the offshore forecast as it appears the any thunderstorm activity will stay just south and west of the area.
Latest offshore buoys 51001 and 51101 show a slow decline of the moderate long period northwest swell (320 to 330 degrees) energy that peaked locally overnight and early this morning. Should see a gradually downtrend this afternoon through the rest of the work week. A smaller north- northwest swell could provide a small, short lived boost this weekend.
Surf along south facing shores will remain elevated through today as a small, long period southwest swell peaks today with a gradual decline tonight. In addition an overlapping short period southwest swell is expected to fill in today from a fetch of strong southwest winds that occurred Monday around 10N170W. A smaller long- period southwest swell is possible Thursday night into Friday before slowly declining over the weekend. Forerunners of a small, long- period south- southwest swell is expected arrive late over the weekend and peak early next week. East facing shores should remain small through the rest of the week and into the weekend due to the lack of strong trades locally or upstream of the state.

Fire weather

It is that time of year where Keetch-Byram Drought Index (KBDI) values are greater than 600 in tandem with afternoon minimum sub-50% relative humidity readings. As we are in a drier weather pattern this week, fire weather conditions will worsen going into late September. Short and mid term sustained winds at the Honolulu Airport (PHNL) will likely remain below the 20 mph Red Flag Warning criteria. Thus, critical fire weather conditions are not expected for the rest of the week. However, many leeward areas are in a moderate to severe drought and this alone elevates the fire weather threat.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov