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Menu of Text Products for the Hawaiian Islands and the Tropical Pacific/Atlantic Oceans: Select a text product: Hawaiian Islands Synoptic Discussion and Guidance Hawaiian Islands 7-day Zone Forecast Terminal Aerodome Foreasts (TAFS) Area Forecast for Hawaii and Adjacent Coastal Waters AIRMETs for IFR Conditions, Turbulence, and Icing Central/North Pacific Tropical Weather Outlook Oahu Surf Observation and Forecast/Coastal Forecast for Hawaiian Waters Offshore Forecast for Hawaiian Waters North Pacific High Seas Forecast South Pacific High Seas Forecast Hawaiian Islands Automated Surface Observation System Hourly Weather Roundup Hawaiian Islands Wind Data Hawaiian Islands Rainfall Summary

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Hawaiian Islands Synoptic Discussion and Guidance 492 FXHW60 PHFO 220649 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 849 PM HST Sat Oct 21 2017 .SYNOPSIS... Trade winds will diminish tonight with clouds and showers remaining focused over windward and mauka areas. Sunday, light and variable winds with afternoon clouds and sea breezes are expected. Some heavier showers over the eastern half of the Big Island are possible Sunday afternoon and night. Monday through Wednesday, southerly winds ahead of a front will bring deep tropical moisture with the potential for flooding for mainly the eastern half of the state. Cool and dry air will fill in behind the front. && .DISCUSSION... After a long stretch of breezy to windy conditions across the islands, trades are forecast to quickly trend down over the upcoming 24 hrs as a cold front approaches and moves into the area from the northwest Sunday through midweek. The latest surface analysis showed this frontal boundary north of Midway extending westward from a developing storm-force low. A weak surface trough was analyzed east- northeast of the islands, marking the leading edge of a broad area of higher moisture. The latest satellite-derived precipitable water (PW) imagery reflected this and showed a large area of 1.75 to 2" PWs a few hundred miles east of the Big Island tracking toward the islands. Drier than normal conditions, however, currently remain in place across the state (PWs < 1.25"), which should hold through the overnight hours into Sunday with quiet conditions continuing. The latest short-term (Sunday-Tuesday) guidance remains in good agreement and indicates increasing moisture advancing westward across the islands as the area of higher moisture to the east moves into the area and a cold front approaches from the northwest. Model PWs are forecast to rise into 1.75 to 2" range late Sunday through Monday, with the highest moisture focused over the eastern half of the state as a pre-frontal convergence boundary sets up through the day Monday. The low-level flow is forecast to shift toward the south through the day Sunday, then toward the south-southwest and strengthen Monday and Monday night (850 mb SW winds 25-35 mph) ahead of the approaching front. The front is forecast to reach Kauai Monday night, then progress eastward over the rest of the island chain Tuesday through Tuesday night. A combination of falling upper heights associated with an upper trough digging southward over the region, increasing instability and isentropic lift, shear (effective 0-6 km bulk shear up to 35 kt), and plenty of deep tropical moisture pulling northward over the islands will support heavy rain and thunderstorms developing. At this time, guidance supports the best chance for heavy rain and thunderstorm development late Sunday night through Monday night from Oahu to the Big Island, then shifting east of Oahu late Monday night through Tuesday and focusing over Maui County and the Big Island. Impacts associated with this front and upper trough expected across the state will be most likely Monday through Tuesday and include: the threat for flash flooding, strong thunderstorms (an isolated severe storm can't be ruled), gusty downslope winds mixing down over windward areas, and a wintry mix over the summits Monday night into Tuesday. As a result, a flash flood watch will be possible in the late night package tonight for much of the state for this period. North winds behind the front will bring cool and dry conditions to Kauai Tuesday, and possibly Oahu Wednesday; while the convergence band keeps things moist and humid over Maui County and the Big Island. Model guidance is a bit cloudy for the second half of the week with the GFS maintaining a weak trough east of the state, while the ECMWF places a stronger surface low southeast of the Big Island. The forecast maintains light to moderate northeast trade winds with windward showers in the extended forecast. && .AVIATION... Moderate trade winds are trending down, as the high pressure north of the islands moves closer to the islands. Scattered clouds and showers continue to be carried in on the trade wind flow, and will primarily impact the windward sides of the islands overnight. VFR conditions are expected to prevail, with some isolated MVFR conditions in passing showers. With lighter winds expected Sunday, expect localized afternoon sea breezes that will allow clouds and shower to form over the interior of the islands. Isolated MVFR conditions are possible with these showers as well. No AIRMETs anticipated at this time. && .MARINE... A ridge north of the area will produce gentle to fresh easterly trade winds overnight. The ridge will shift south over the weekend ahead of a front approaching from the northwest. Winds will weaken further and shift out of the southeast tomorrow as the ridge moves over the area. Winds may shift out of the south Monday and could become locally fresh again as the front approaches. The front is expected to reach the Kauai waters Monday evening and push down to the Big Island waters by Tuesday evening. Heavy showers and thunderstorms are possible ahead of and along the front. Fresh to locally strong north winds will spread over the islands behind the front. The winds will slowly veer to northeast by the end of next week. A northwest swell expected tomorrow and Monday, will likely produce surf near the advisory threshold along north facing shores. That swell will diminish Monday night into Tuesday, but a bigger northwest swell Tuesday and Wednesday will likely require a High Surf Advisory for north and west facing shores. The swell will also boost combined seas above the 10 foot level, so a Small Craft Advisory will likely be needed. An even larger northwest swell is forecast to arrive next Friday. That swell could produce warning level along north and west facing shores. A low east of New Zealand generated a swell which will produce surf near the advisory level along south facing shores Sunday and Monday. See the Collaborative Surf Discussion for details. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...GIBBS AVIATION...BALLARD MARINE...DONALDSON Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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