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Hawaii Weather Outlook Update for May 4th to 5th, 2019



Synopsis

An out of season cold front, nearing the Kauai waters this evening, will advance down the island chain beginning late tonight on Kauai, then to the rest of the islands through the day Saturday and Saturday night. Instability associated this front and a potent upper low dropping south toward the islands along with plenty of moisture will bring a period of heavy showers, gusty winds, and possibly a few strong thunderstorms along and ahead of the front. Drier and cooler northwest winds will fill in behind the front Saturday night into Monday, with trade winds unlikely to make a return until Thursday.


Discussion

Water vapor imagery showed an anomalously potent upper low dropping south toward the islands this evening (associated cold pool around -20 C aloft). At the surface, the attendant cold front is nearing the Kauai waters this evening quickly tracking eastward. Isentropic lift combined with plenty of moisture (dewpoints in the upper 60s; above normal PWs) to work with out ahead of this approaching boundary and upper low are already triggering showers around Kauai. As the night progresses and the forcing for ascent and instability increase, conditions will deteriorate quickly over the western end of the state. The latest hi-res model reflectivities remain in good agreement and show the main line of showers along and ahead of the front reaching Kauai and its surrounding waters between midnight and daybreak leading up to the frontal passage. The potential for localized flooding and a few strong storms with rotating updrafts (0-6 km effective bulk shear values up to 35 to 45 knots) will become a possibility through these late night hours, especially over Kauai. Additionally, strong straight-line winds up to 40 mph can't be ruled out from any storm that does materialize.


The front will quickly advance down the island chain through the day Saturday and Saturday night bringing the potential for heavy showers, gusty winds and thunderstorms. Guidance suggests the boundary will move through Oahu around mid-day Saturday, then Maui County late Saturday afternoon through the evening hours. As the boundary continues eastward across the eastern end of the state, some weakening is anticipated, especially Saturday night into Sunday as it moves through the Big Island. The potent upper low is forecast to drop south to an area just north of the islands Saturday through Saturday night, then slowly shift east-northeastward and away from the area Sunday through Monday. A secondary vort lobe wrapping around the back-end of this upper feature will likely translate to a band of clouds and showers moving through from west to east late Saturday night into Sunday.

Very cool conditions by May standards are expected to fill in on the northwesterly flow Saturday night through Monday. The latest guidance drops dew points into the upper 50s, and daytime high temperatures will likely remain in the upper 70s, which is highly unusual for early May.


There is no real sign of trade winds until Thursday. As the upper level low fills and lifts off to the northeast Tuesday and Wednesday, it will leave a weak area of low pressure north of the state. This will produce generally weak west to northwest winds over the islands and will likely lead to somewhat dry conditions.

Weather Advisories


HIGH WIND WARNING IN EFFECT FROM 6 PM THIS EVENING TO 6 PM HST SUNDAY The National Weather Service in Honolulu has issued a High Wind Warning, which is in effect from 6 PM this evening to 6 PM HST Sunday. The High Wind Watch is no longer in effect.

- WINDS: West increasing to 40 to 60 mph tonight, then strengthening to 50 to 75 mph Saturday. - TIMING: Increasing tonight, continuing through Sunday afternoon, and possibly holding through Sunday night.

- IMPACTS: Travel to the summits will be difficult or dangerous. PRECAUTIONARY/PREPAREDNESS ACTIONS A High Wind Warning means that winds at the summits are expected to be at least 56 mph, with gusts over 66 mph. Take necessary precautions to protect property. Travel to summit areas should be delayed until winds subside.


Mauna Kea Access Road remains closed to all traffic at Hale Pohaku Visitor Information Center due to high winds in excess of 60 mph and gusting to over 70 mph. The National Weather Service has issued a High Wind Warning for Big Island summits until late Sunday evening.

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