National Weather Service United States Department of Commerce

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Issued: Feb 23, 2018 4:15 AM HST

Synopsis
A low pressure system west of the islands will continue to draw deep and unstable tropical moisture northward across the islands into the weekend. Unsettled weather conditions are expected to continue with good chances for rain along with locally heavy rain and thunderstorms possible. Expect a drying trend next week as the low center drifts slowly westward and high pressure builds in from the east.

Discussion
Widespread rain showers with isolated thunderstorms are expected to develop across the state today in an unsettled weather pattern forecast to last into the weekend. Expect an elevated threat for flash flooding to continue across the state with a Flash Flood Watch in effect for all islands through this afternoon. The Flash Flood Watch may need to be extended beyond the afternoon time period as this dynamic weather situation evolves. A Winter Storm Warning also remains in effect for the Big Island Summits above the 12000 foot elevation level.

A slow moving low pressure system west of the islands and a strong 1040 mb high pressure center roughly 1800 miles northeast of the state will dominate the weather pattern for Hawaii over the next few days. These two systems will continue to advect deep and unstable tropical moisture northward producing widespread rain showers across the state with unsettled weather lingering into the weekend. An upper level jet stream over the islands complicates the pattern by lifting the atmosphere and assisting convection due to upper level divergence forcing.

The challenge with this pattern lies in predicting where the smaller mesoscale surface convergence and upper level divergence features will interact with the islands to produce deeper convection with locally heavy showers and thunderstorms. The latest forecast models suggest the best chances for showers today will develop along the eastern slopes of Maui and the Big Island. However due to deep moisture amounts, strong instability and forcing aloft all islands remain at an elevated risk for flash flooding today.

In the short term, the American (GFS) and European (ECMWF) models are in fair agreement with an overall unstable weather pattern producing widespread shower activity with locally heavy showers and thunderstorms possible into the weekend. As the low slowly drifts westward today and tomorrow, the deep tropical low level moisture and upper level jet stream forcing will remain locked in place.

On Sunday, a negatively tilted upper level trough rotating around the western low passes over Kauai. This trough may bring a second round of heavy shower and thunderstorm potential to the western islands of Kauai and Oahu by Sunday afternoon. Model solutions on the intensity of this trough remain less certain at this point. We expect Sundays weather impact details to come into focus as the time period gets closer.

The long range forecast for next week shows the high pressure system moving westward with higher pressure building in over the state. Downward moving subsident air under this ridge will stabilize the air mass and produce a drying trend. However, east to southeast wind speeds will also increase across all islands from Monday night into Tuesday morning. These elevated wind speeds are forecast to continue through the rest of the week.

Aviation
High pressure northeast of the state and a trough of low pressure to the west, will keep an east to southeast flow in place across the island chain through 12Z Saturday. A moist and unstable airmass will remain over the island chain, with the most unsettled weather expected across the eastern islands, particularly the Big Island, where widespread MVFR and isolated IFR conditions are expected over the next 24 hours. Across the smaller islands daytime heating will likely lead to some shower development over interior and leeward areas, along with some brief MVFR cigs and MVFR and IFR vsbys. Conditions should improve across most of the smaller islands tonight, with some more showery weather possibly lingering over windward sections of Kauai.

AIRMET SIERRA remains in place for mountain obscuration across interior terrain and NE to SE facing slopes of Oahu Molokai Maui and the Big Island. Little change is expected through the early morning hours, with some improvement possible later this morning, particularly over Oahu and Molokai. Light icing will be possible in the thick mid and high clouds blanketing the state through the early morning hours. This is mentioned in the AIRMET bulletin, but conditions are expected to remain below the AIRMET threshold.

Marine
An upper level disturbance west of the islands will maintain an unstable atmosphere over the region today, with heavy showers and some thunderstorms possible. Although the threat is for all waters, the greatest threat will be over the waters south and west of all islands, as well as the waters near Kauai due to the proximity to the upper level low. Expect reduced visibility, gusty winds and rough seas with the heavier showers and thunderstorms. The threat for heavy showers and thunderstorms will gradually shift north and west into Saturday.

Persistent strong high pressure northeast of the state will continue to produce an area of strong winds directed towards the area. A lingering surface trough, just west of the offshore waters will help to force a more southeast wind pattern for the state. The east-southeast/southeast winds have continue to increase over the last 12 hours, and some additional increase is likely today, with winds remaining elevated through most of next week. Model guidance suggests winds will increase further by the middle of next week, possibly reaching gale force, with the strongest winds around the Big Island and windward waters of the smaller islands, with the southeast flow being maintained through much of the event. The swell generated from these winds will continue to produce elevated surf along east-facing shores.

A High Surf Advisory for east facing shores today will continue through the weekend, and will likely continue into the middle of the week when surf could potentially reach warning levels. A Small Craft Advisory is posted across windward waters and around the Big Island for the combination of the elevated seas and increasing winds. As mentioned above, gale force winds are possible with the increasing winds next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional details on surf and swell.

Watches/Warnings/Advisories
Flash Flood Watch through this afternoon for Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui, Kona, South Big Island, Big Island North and East, Kohala, Big Island Interior.

Winter Storm Warning until 6 PM HST this evening for Big Island Summits.

High Surf Advisory until 6 AM HST Monday for Kauai Windward, Oahu Koolau, Olomana, Molokai Windward, Maui Windward West, Windward Haleakala, South Big Island, Big Island North and East.

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


DISCUSSION...Bohlin

AVIATION...Jelsema

MARINE...M Ballard