| Current Conditions | ||
| Temp |  | 0.6 C |
| RH | | 20 % |
| Wind | | ESE 8 mph |
| Road | | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 876 FXHW60 PHFO 200648 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 848 PM HST Sat Aug 19 2017 .SYNOPSIS... Light to moderate trade winds will prevail through early next week as a weakened ridge remains to our north. Expect local sea breezes along with increased afternoon clouds and showers across leeward areas. Low clouds carried by the trades will continue to affect windward and mauka areas. Tropical moisture from the east will increase shower coverage and keep muggy conditions around through Tuesday. Instability will introduce a slight chance of thunderstorms Sunday through Tuesday. Drier and windier conditions will return Wednesday as the ridge strengthens to our north. && .DISCUSSION... The subtropical ridge far north of the main Hawaiian islands continues to weaken as upper troughing between 140W and 165W takes its toll. The ridge is strong enough to drive light to moderate trade winds across local waters, but we expect trade flow will be too weak to defeat development of local leeward sea breezes each afternoon Sunday through Tuesday. Satellite loop shows patchy broken low clouds embedded within trade flow moving across windward and mauka areas of the islands, while radar shows scattered showers across these same areas. Increased low cloud cover is noted east of about 152W, along the leading edge of an expected increase in tropical moisture. Afternoon soundings show either no inversion or one much weaker than at this time yesterday, another sign of increased troughing aloft. Hilo had more moisture than Lihue, with 1.32 versus 0.98 inches of PW. Models show the upper trough over and north of the islands will dig to the southwest through early next week, keeping the ridge weak and dragging a pool of cold air over the western half of the chain. Combined with an expected increase in tropical moisture from the east, this colder air aloft will introduce a slight chance of thunderstorms to our forecast from Sunday afternoon through the day Tuesday. This threat will start with the potential for thunderstorms across leeward portions of the Big Island Sunday afternoon, then morph into a broader threat including Kauai and/or Oahu Sunday night through Monday night. As the pool of cold air begins to pass west of the islands, the thunderstorm threat will shrink once again to only leeward portions of the Big island Tuesday afternoon before vanishing altogether. Outside of the thunderstorm threat, the general weather pattern Sunday through Tuesday will consist of windward clouds and showers merging with afternoon and evening leeward clouds and showers, thanks to the hybrid light trade/sea breeze setup. By Wednesday, the upper trough, with its associated cold air pool, will be far west of the main islands. This will allow the ridge to our north to rebound and drive stronger trade winds across local waters. In short, drier and windier conditions are expected Wednesday through the rest of the forecast period. && .AVIATION... VFR conditions will continue at all TAF sites with brief MVFR conditions expected mainly for the windward sites this evening. As an area of enhanced moisture from the east reaches the eastern islands overnight, MVFR conditions will become more widespread over windward areas. No AIRMETS are currently in effect but one may be needed overnight for mountain obscuration over eastern portions of the state. && .MARINE... Winds and seas are expected to remain below Small Craft Advisory levels through early next week as a trough aloft helps weaken the pressure gradient. The trough may make showers more active, and could even produce some thunderstorms Sunday through Tuesday. Wind speeds will increase again later next week. Surf along south facing shores will continue to lower overnight as a south-southeast swell that filled in and peaked late Friday eases. Looking ahead, mainly small background long-period reinforcing pulses out of the south-southwest (210-220 deg) and south-southeast (130-160 deg) will keep the surf from going flat each day along south facing shores. For more detailed surf information, see the latest Collaborative Nearshore Swell and Wind Forecast at www.prh.noaa.gov/hnl/pages/SRF.php#Discussion. Water levels this afternoon ended up coming in and exceeding the record water level measured at Honolulu back at the end of April and May (the Honolulu station recorded a preliminary water level of 3.284 ft relative to MLLW this afternoon). These extreme water levels are in response to an oceanic eddy moving through from east to west and the regular King Tide cycles. Water levels will steadily trend down Sunday through the upcoming week. See the Special Weather Statement for more details. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Powell AVIATION...Hui MARINE...Gibbs Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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