National Weather Service United States Department of Commerce

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Issued: Feb 17, 2019 3:47 AM HST

Synopsis
Breezy northerly winds and showery conditions will continue across the western end of the state into tonight as low pressure lingers to the north. Drier conditions along with a transition to light and variable winds are anticipated for the eastern end of the state through early Monday. A return of wet and unsettled conditions are expected late Monday through midweek, mainly over Maui County and the Big Island where a plume of deep tropical moisture is forecast to setup. Improving conditions will be possible through the second half of the week and upcoming weekend.

Discussion
Satellite imagery showed a plume of deep tropical moisture streaming northward several hundred miles east of an upper low centered near the islands. Surface low pressure associated with this upper feature was analyzed just north-northeast of the islands. Although lighter winds and slightly drier conditions have resulted over the far eastern end of the state, moist and breezy northerly winds are holding across Molokai, Oahu and Kauai. Satellite-derived precipitable water (PW) imagery reflects this and depicts higher values (1.25-1.5 inches) over the western end of the state this morning. This has translated to the best shower coverage setting up over the northern and windward areas of these islands. Farther east, mostly quiet conditions should continue today.

The latest short-term guidance is lining up well the current pattern and supports low pressure at the surface and aloft previously discussed slowly shifting westward to a position west to northwest of the area come Monday. Breezy northerly winds and showery conditions will likely hold today for Oahu and Kauai, before the winds trend down tonight from east to west. Elsewhere, light winds along with drier conditions will prevail. On Monday, falling upper heights just west of Kauai combined with light winds and a sufficient amount of moisture will continue to support showers for the western end of the state, especially on Kauai where the better instability will exist. Model reflectivities support this instability and show heavy showers triggering around Kauai through this time. Although confidence remains too low at this point, a thunderstorm or two can't be ruled out around Kauai through the day Monday.

A wet pattern is expected to return late Monday through midweek as the plume of deep tropical moisture currently lurking to the east shifts westward over the eastern half of the state. Model PWs within this plume show values up to 2 inches setting up near the Big Island by Tuesday, then continuing through midweek. Periods of heavy rainfall will likely result and translate to an increasing risk of flash flooding for these areas Monday night through Wednesday. In addition to the increasing rainfall chances, a few thunderstorms will remain possible with the upper low remaining positioned near the western end of the state through Tuesday. A flash flood watch will be a possibility for these areas (Maui County and the Big Island) by the end of the weekend covering Monday night through Wednesday.

Although this plume of moisture may linger over the far eastern end of the state into the latter half of the week, conditions should begin to gradually improve across the state. Guidance suggests the upper/surface low to the west ejecting northward with a weak surface ridge nosing in to the north of islands. PWs will likely hover around an inch to 1.4 inches near the Big Island. Winds could transition back out of the north to northeast Thursday through Friday in the light to moderate range.

Aviation
A surface low pressure area sits some 205 nm northeast of Maui (24n154w) this early Sunday morning. This is a much closer location compare to the evening position, having becoming more evident in the past couple of hours. Another low lies further north at 29n154w, making this a complex area of low pressure. The low pressure area is forecast to move west with the northern low becoming the dominant feature later today. A trough extends south to the Big Island, which will be moving slowly westward through today, reaching Maui this afternoon. Light southeast winds lies to the east of the trough, and locally strong northerly winds lies to the west of the trough. With this said, the Big Island will be under a southeast wind flow, while Maui County will have winds becoming westerly. Kauai will remain under a moderate to strong northerly wind flow through at least today, while Oahu will be light to moderate.

The radar derived low level winds over Kauai, is rather strong, clocking 30kt at 4k feet. Thus, AIRMET Tango will continue at least through 2200 UTC.

The large field of more robust looking showers is currently blowing through the smaller islands, from north to south. The showers appears to disperse and weaken as they near the smaller islands, with max tops around 15k feet, compare to 20k feet further upwind of the islands. Kauai and Oahu will continue to experience passing showers today, and the lighter winds over Molokai, Maui, and Lanai, will lead to some spotty afternoon showers. The Big Island is nearly clear this hour, with a dominant land breeze. Do expect clouds to return later this morning, with isolated afternoon showers expected on both sides of the island. The summit area of the Big Island will have some clouds, but nothing more is expected.

Isolated thunderstorms are expected beyond 40 nm north of our northern coastline.

The disperse shower rolling through Kauai and Oahu, in particular, does not warrant an AIRMET Sierra. Thus the airmet for Oahu will be taken down for the 16z aviation package.

No other aviation advisories are expected, and we will be keeping an eye out for AIRMET Sierra and mountain obscuration.

Marine
A surface low about 210 nm northeast of Maui is forecast to track west through Monday. A trough extends southwest of the low to the Alenuihaha Channel. There will be fresh to strong north winds west of the trough and light to moderate variable winds east of the trough. As the trough shifts west over the next few day, fresh to strong southeast winds will spread over the eastern offshore waters, but are not expected to reach the Hawaiian Coastal Waters.

Seas at NOAA Buoy 51000 northeast of the islands have subsided to 14 ft, 11 seconds as of 2 AM Sunday. Surf from this swell is expected to remain at High Surf Advisory (HSA) levels along north facing shores through Sunday and at High Surf Warning (HSW) levels along east facing shores (where the threshold is lower). A new west northwest swell should peak today. Surf from this swell is expected to remain below HSA levels.

A Small Craft Advisory (SCA) is in effect through late tonight for most Hawaiian Waters, mainly for the large seas associated with the northeast swell, however winds will remain above SCA levels today in waters around Kauai County. With the new west northwest swell building today, expect seas to remain above the 10 ft SCA threshold in many locations at least through tonight.

Watches/Warnings/Advisories
High Surf Advisory until 6 PM HST this evening for Niihau, Oahu North Shore, Molokai Leeward, Maui Central Valley.

High Surf Warning until 6 PM HST this evening for Kauai Windward, Oahu Koolau, Olomana, Molokai Windward, Maui Windward West, Windward Haleakala, Big Island North and East.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Southeast Waters.


DISCUSSION...Gibbs

AVIATION...Lau

MARINE...Donaldson