| Current Conditions | ||
| Temp |  | 1.0 C |
| RH | | 84 % |
| Wind | | SE 6 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 730 FXHW60 PHFO 211326 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 326 AM HST Mon Aug 21 2017 .SYNOPSIS... Increasing moisture combined with an upper disturbance over the region will support heavy showers and a few thunderstorms through Tuesday. Drier and windier conditions will return Wednesday through the second half of the week as the ridge strengthens to our north and the upper disturbance shifts away from the region. && .DISCUSSION... Recently animated water vapor loop showed plenty of middle- to upper- level moisture streaming northward over the islands due to a broad upper low drifting southwestward just north of Kauai. This upper low combined with a sufficient amount of instability and moisture will keep the chance of heavy rain and thunderstorms in the forecast through Tuesday. The early morning upper air sounding at Lihue reflected this instability and depicted a relatively steep mid-level lapse rate with -9C at 500 mb. The latest satellite-derived precipitable water (PW) imagery showed values ranging from 1.5-2" over and around the islands, which is above normal for this time of year. As a result, localized flooding will remain a concern through Tuesday, mainly where the heavy showers become focused. Some of this activity will become focused over the leeward areas through the afternoon and early evening periods where sea breezes develop. For the extended period, the GFS and ECMWF both depict breezy trades and drier conditions returning across the state beginning Wednesday as the upper low shifts away from the region and high pressure builds north of the state. Model PWs support this and dip back down to below average for this time of year (near 1"). Showers will remain focused over windward and mauka areas each day. && .AVIATION... Surface high pressure to the north will maintain moderate east northeast surface winds across the area. These winds will carry very moist air over the area. A low aloft 150 nm north northwest of Kauai is forecast to move south southwest over the next 24 hours, then move off to the northwest. The low aloft will make the atmosphere unstable. VFR conditions prevail over southwest sections of all the islands this morning, but TEMPO MVFR and ISOL IFR conditions are expected over northeast sections of Maui, Molokai and Oahu. AIRMET SIERRA is currently in effect for MTN OBSC over the northeast section of Maui. MTN OBSC is expected to spread to the northeast sections of Molokai and Oahu before sunrise. Thunderstorms may develop again over land as the islands heat up today. && .MARINE... Winds are expected to remain in the moderate to fresh range through tonight, then strengthen into the fresh to locally strong category Tuesday through the remainder of the week. Increasing shower coverage will remain in the forecast through Tuesday as an upper trough drifts southwestward over the region. This upper trough will also support a slight chance of thunderstorms through Tuesday. Drier conditions will return Wednesday through the second half of the week. Surf will remain generally small all around. Small background long-period pulses from the south-southwest (210-220 deg) and south- southeast (130-160 deg) will keep the surf from being flat along south facing shores this week. A slightly larger south- southwest (220 deg) swell will be possible this coming weekend due to a recent gale over the Tasman Sea. Surf along east facing shores will remain generally small through tonight, then build Tuesday through the rest of the week as stronger trades return. Additionally, a long-period swell associated with tropical activity across the eastern Pacific will be possible Wednesday through Friday, especially along the exposed beaches of the Big Island. Expect higher than normal water levels due to a combination of an oceanic eddy moving through from east to west and the regular King Tide cycles. Water levels will slowly lower each day through the week. Please refer to the Special Weather Statement for more details. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Gibbs AVIATION...Donaldson MARINE...Powell Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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