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Hawaii Weather Outlook for May 3rd to 5th, 2019

Wet Weather along with High Winds at the summits of Maunakea and Mauna Loa is expected this weekend.

Synopsis

Trade winds will weaken and shift southerly tonight as an advancing upper level trough gradually destabilizes the atmosphere. An unseasonably strong front will approach the state on Friday, leading to an unstable southerly flow that will produce spotty showers over most islands. The front will move down the island chain Saturday into early Sunday, likely bringing a period of heavy showers, gusty winds, and possibly thunderstorms. Cooler northwesterly winds are expected behind the front into the middle of next week.


Update

The forecast was updated this evening, mainly for small adjustments to the rainfall coverage/chances. Although most of the showers that developed over leeward Oahu late this afternoon have diminished, moderate to heavy showers have been persistent over Kauai and parts of Maui County and the Big Island early this evening. Peak rainfall accumulations over the past three hours have occurred over Kauai with totals reaching 1.52 inches at N Wailua Ditch (through 7 pm HST). Outside of a few spotty showers remaining a possibility, a downward trend in the shower coverage is expected through the late evening and overnight hours. Otherwise, the previous forecast remains on track with no other updates anticipated this evening.


Discussion (issued at 3:51 pm Thursday afternoon)

Trade winds are weakening, and the atmosphere is destabilizing this afternoon. Trades are on the decline as a cold front centered 600 miles northwest of Kauai weakens the subtropical ridge north of the state. The upper level trough driving the cold front is eroding the inversion, which has lifted up to as much as 10,000 ft according to afternoon soundings and recent aircraft data. This has led to isolated showers across leeward terrain on most islands, while a narrow band of low level moisture continues to fuel showers over windward slopes.

The trades will continue to weaken and shift out of the southeast tonight as the atmosphere slowly destabilizes. The afternoon leeward showers will diminish around sunset, while windward showers persist. Windward showers should push offshore late tonight as the shifting background flow causes land breezes to develop. However, Oahu and Kauai could see a few showers moving up from the south late.


A somewhat unstable southerly flow will develop on Friday as the unseasonably strong front approaches the state. The upper trough supporting the front will continue to produce cooling aloft that will wipe out the inversion. Models show a developing prefrontal trough focusing moisture just west of Kauai, leaving the state under an unstable land and sea breeze regime with near normal precipitable water. Though there could be an isolated downpour at any time, showers over land will favor the afternoon hours.


The front will reach Kauai early Saturday, then bring unsettled weather down the island chain through Saturday night. We expect the front will be near Kauai in the predawn hours on Saturday, reach Oahu midday, move to Maui County Saturday afternoon and evening, then possibly advance to the Big Island by predawn hours on Sunday. During this time, a potent upper low will drop southward into the upper trough and approach the state from the north. A jet stream along the southern flank of the upper low should move over the islands and produce forcing that will generate heavy showers along the front. The eventual position and strength of the compact upper low will be critical, but at this time, the greatest amount of forcing and coldest air aloft look to remain just northeast of the state. However, a line of heavy showers or thunderstorms along the front could be capable of producing strong, or even damaging, winds. Another item of note is the potential for gusty downsloping southwest to west winds along the front. It does not appear that a wind advisory will be need, though localized wind gusts of around 40 mph are a strong possibility on Saturday, especially on Oahu and parts of Maui County. At higher elevations, the high summits on the Big Island will almost certainly experience warning level winds, while advisory level conditions are expected on summit of Haleakala.


On Sunday, the front will likely weaken near the Big Island as the compact upper level low passes north of the state. Latest runs of the GFS and ECMWF are hinting at a surface low developing below the upper level feature, potentially driving another band of showers and gusty northwest winds down the island chain.


Stable and rather cool conditions are expected to fill in on the northwesterly flow later Sunday and Monday. The latest guidance drops dew points into the upper 50s, which will provide rather cool conditions for early May. As the upper level low weakens and lifts off to the northeast, stable and rather dry northerly winds will likely lead to dry conditions through the middle of next week.


Weather Advisories

HIGH WIND WATCH IN EFFECT FROM FRIDAY EVENING THROUGH LATE SUNDAY NIGHT The National Weather Service in Honolulu has issued a High Wind Watch, which is in effect from Friday evening through late Sunday night. - WINDS: Westerly winds will strengthen across the high summits of the Big Island Friday night and will likely exceed warning levels of 60 mph by Saturday morning. - TIMING: Increasing Friday night and continuing through the weekend.

- IMPACTS: Travel to the summits will be difficult or dangerous. PRECAUTIONARY/PREPAREDNESS ACTIONS A High Wind Watch for the summits means there is the potential for a hazardous high wind event. Sustained winds of at least 56 mph or gusts of 66 mph or stronger may occur. Continue to monitor the latest forecasts.

Currently, Mauna Kea Access Road is closed to all traffic at Hale Pohaku Visitor Center due to high humidity, fog, snow flurries, hail, and saturated road conditions.

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