829 FXHW60 PHFO 301942 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 942 AM HST Fri Aug 30 2019 .SYNOPSIS... Gentle trades will maintain through the holiday weekend with localized afternoon sea breezes setting up over some of the blocked leeward areas. Increasing moisture from the east will bring the potential for brief heavy showers Saturday and Sunday. It is also expected to be very humid through the weekend. Drier air with gentle trades will begin the week with a possible strengthening of the trades late next week. && .DISCUSSION... Water vapor imagery this morning shows a tropical upper tropospheric trough west of the main Hawaiian Islands. Scattered to broken cirrus clouds associated with the subtropical jet are passing over the state, which will likely continue at least through the next 24 hours given the current satellite/model trend. At the low levels, a weak ridge is located northeast of the islands, with a weak stationary front to the north and a stronger high even further to the north. The highs are driving gentle to moderate trade winds. Expect another warm summer day today with highs around 90 F plus and lows in the upper 70s. Sea surface temperatures near the islands are around 82 degrees F. A surface trough 200 miles east of the Big Island will drag a plume of tropical moisture across the state beginning early this evening in the east and then across the smaller islands by Saturday morning. Guidance shows precipitable water (PW) values jumping from the current normal 1.4 inches to around 1.9 inches. We may also see a slight weakening of the winds through the weekend as the trough passes over the islands. Both GFS and ECMWF models show gentle trades maintaining through the weekend and the winds should just be strong enough to focus more of the clouds and showers windward, however, localized sea breezes may set up Saturday and Sunday afternoons in blocked leeward locations leading to shower development both windward and leeward. Dew points will increase to the mid 70s through the weekend which could push the afternoon heat index over 100 in a couple of spots. With PWs this high, locally heavy rainfall and will be possible this weekend. Highest confidence is across the interior Big Island Saturday and Sunday afternoons. It's possible that other areas of the state could get brief heavy showers too. As the trough departs on Labor Day, relatively drier air will fill in leading to a more comfortable and somewhat less showery weather pattern. Winds will remain in the gentle category into midweek as a slowly-moving weak disturbance passes northeast of the islands. Expect the warm summertime temperatures to continue through the week. Winds may strengthen by the end of the week as long range models hint at a possible disturbance passing south of the islands next weekend, though a bit early to tell this far out in the forecast period. && .AVIATION... Gentle to moderate trade winds will deliver limited amounts of low clouds to most windward areas through the day today. However, an area of increased moisture moving in from the E is expected to fuel increased windward showers over windward Big Island by this evening, spreading to the other islands overnight into Saturday. Cloud and shower coverage could potentially increase to the point that mountain obscuration may occur, but for now no AIRMETs are posted. High clouds streaming over the area from the SW will have little impact on sensible weather. VFR will prevail at most TAF sites, with MVFR CIG expected at PHNY through mid-afternoon, and MVFR CIG/VIS in SHRA possible at PHTO later today. && .MARINE... A weak trough will move through from east to west over the weekend while high pressure remains north of the area. The trough will bring an increase in showers mainly over windward waters during the weekend. Small long-period southwest swells will hold into the weekend. This will keep surf heights near the seasonal average through Saturday before dropping Sunday. A compact low, south of Easter Island on 8/26, had a fetch of gale force winds aimed at Hawaii that may bring a small boost to southeast facing shores Monday night through the middle of next week. A south- southwest swell will be possible through for the second half of next week. A small north swell is expected over the weekend with its peak likely on Sunday. A low in the NW Pacific is expected to strengthen this weekend as it passes south of the Aleutian Islands. This may bring a small to moderate north-northwest swell starting Tuesday. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Foster AVIATION...Birchard MARINE...Morrison