| Current Conditions | ||
| Temp |  | 6.8 C |
| RH | | 34 % |
| Wind | | SE 8 mph |
| Road | | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 166 FXHW60 PHFO 190137 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 337 PM HST Fri Aug 18 2017 .SYNOPSIS... Trade winds will decrease a bit as a weakness develops in the ridge to our north, allowing development of local sea breezes across leeward areas into early next week. Leeward areas will see an increase in clouds and showers each afternoon and evening, while diminished trade flow will continue to supply both to windward and mauka areas. Moisture from an old tropical cyclone will increase shower coverage and bring muggy conditions to the islands late Saturday through the first half of next week. Expect drier and windier conditions to return late next week as the ridge rebuilds. && .DISCUSSION... Upper troughing along 150W is beginning to put a dent in the surface subtropical ridge to our north and a recent ASCAT pass showed that winds have begun to subside across local waters. Satellite loop shows scattered to patchy broken low clouds embedded within trade flow moving across the islands, with little change noted upwind as far as 145W. Radar shows only isolated showers moving across the islands within this cloud cover. Latest soundings showed a dry and neutral airmass, with about an inch of PW and near zero lifted index values. Strong inversions persist between 6000 and 7000 feet. The upper trough to our north has yet to raise the inversion or significantly destabilize our airmass. Models show the upper trough to our north will dig southwestward over the next few days, placing the base of the trough over the islands, and the trough axis near Kauai and Oahu, by Sunday. This roughly coincides with the arrival of an influx of moisture from now-defunct Jova and a general decrease in background trade flow. While trade flow may rebound slightly Monday and Tuesday around the Big Island, the overall statewide result will likely be increased trade shower coverage across windward and mauka areas, the introduction of afternoon sea breeze clouds and showers across leeward areas, and an overall muggy feel to the air from late Saturday through Tuesday. There is still noticeable model disagreement concerning a potential thunderstorm threat on Monday. GFS continues to push enough cold air aloft southward to trigger this threat across the western islands on Monday, but ECMWF keeps this cold air too far west of the main islands to be threatening during the same time frame. We will keep thunderstorms and heavy rainfall out of the forecast for now, but will reconsider if and when model solutions converge. By Wednesday, both models show the subtropical ridge recovering, with trade winds increasing back into the moderate to locally breezy range once again. Drier conditions will move in as well. && .AVIATION... A ridge of high pressure north of the State will keep a moderate to locally breezy trade wind flow in place through tonight, with clouds and showers favoring windward and mauka areas. Brief MVFR cigs may affect windward areas as showers move in with the trades, particularly during the overnight and early morning hours. Predominantly VFR conditions are expected at the TAF sites through 00Z Sunday. No AIRMETs are currently in effect. AIRMET Sierra may be needed for mountain obscuration across some windward areas later tonight. && .MARINE... Winds and seas are expected to remain below Small Craft Advisory levels through the weekend as a trough aloft moves over the area and helps weaken the pressure gradient. The trough may make showers more active, and could even produce some thunderstorms early next week. Wind speeds will increase again later next week. Surf along all shores is expected to remain well below the High Surf Advisory threshold. Surf along south facing shores is up a bit due to a swell generated in the Southern Hemisphere. Surf along the south facing shores will gradually subside over the weekend. The peak high tides for the month will occur over the next few days. Water levels are running as much as a foot above the predicted levels due to an eddy moving westward through Hawaiian waters. Coastal flooding is possible around the times of the high tides through the weekend. See the Special Weather Statement for more details. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Powell AVIATION...Jelsema MARINE...Donaldson Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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