Trade wind weather will continue through the weekend with brief passing showers favoring windward and mountain areas of all islands. Winds begin to shift out of the south from Sunday night into Monday as a strong cutoff low pressure system approaches from the northwest. Strong southerly winds will draw deep tropical moisture northward increasing the threat for widespread heavy rainfall and severe weather through much of next week.
In the big picture, a cold front approaching the state from the northwest has stalled roughly 400 miles northwest of Kauai this morning. The high pressure ridge north of the state will remain in place and continue to produce easterly to east-southeasterly trade winds through the weekend. Bands of clouds developing across the region will produce brief periods of showers along windward slopes of all islands. These showers will favor windward and mountain areas, mainly during the overnight to early morning hours. Subsidence temperature inversion heights will range from 5,000 to 7,000 feet through Sunday, supporting this passing shower regime.
The next storm system continues to prowl across the northwest Pacific on its journey toward the Hawaiian Islands. The latest forecast model guidance shows increasing potential for several days of widespread heavy rainfall along with a few strong to potentially severe thunderstorms beginning early next week.
On Monday, the weather pattern will transition from trade wind conditions to southerly Kona wind flow as a strong cutoff low deepens and meanders roughly 800 miles northwest of Kauai. Although the center of this system will remain well northwest of the state, its influence will extend across the island chain. This strong upper low is expected to support the development of a deep surface low of around 990 mb.
From Monday night into Tuesday, strong southerly winds will develop across the state, drawing abundant moisture northward from the deep tropics. This low level moisture plume will move beneath much colder temperatures aloft, with the latest global model guidance suggesting 500 mb temperatures around -10 to -12 Celsius. Two additional troughs associated with this storm will provide the forcing mechanism for severe weather potential. The combination of a low level trough moving from west to east down the island chain and an upper level negatively tilted trough aloft will likely trigger periods of heavy showers and thunderstorms. The timing of these island by island impacts may evolve as the track of this system becomes better defined.
Expect several days of impactful weather with this next significant storm system. Potential impacts include strong gusty southerly winds over and along steep northern and eastern slopes of island mountains. These gusty winds may exceed Wind Advisory thresholds as southerly winds strengthen from Tuesday through Friday. Heavy rainfall in this unstable environment, combined with surface convergent and upper level divergent forcing, will lead to a significant flash flooding threat. These flooding concerns will begin across the western half of the state early next week, then spread statewide from Wednesday into Friday. A Hydrologic Outlook may be issued to highlight these flooding threats, and a Flood Watch may also be required by early next week.
High elevation ice and snow, along with increasing summit level winds, were included in the latest extended forecast for the highest summits of Mauna Kea and Mauna Loa on the Big Island above 12,000 feet elevation beginning Wednesday as the deepest showers move into the eastern half of the state. Winter weather and wind advisories or warnings may be needed to address these high elevation winter hazards.
The upper level negatively tilted trough will provide an efficient atmospheric lifting mechanism within this unstable environment characterized by ample wind shear. Overall, these conditions may support the development of deviant-moving severe thunderstorms during this event. Strong damaging winds, lightning, and extremely heavy rainfall will be the primary threats with the stronger thunderstorms across the Hawaiian Islands.
Please prepare now for the potential for severe weather impacts across the state of Hawaii lasting several days next week. The primary forecast challenge this far out in time will be pinning down the day to day impacts for each island. Many of these island by island hazards will develop on smaller scales and may evolve rapidly over shorter forecast time periods.
Trades will weaken slightly today, allowing for more of a land- sea breeze pattern through the first half of the period for most sites, before trades prevail once again by the afternoon into early evening hours. VFR is expected to prevail, however intermittent MVFR remains possible along windward sites due to low clouds and period light rain showers.
AIRMET Sierra remains in effect for east through south Kauai and Oahu. This is expected to prevail through the period.
Fresh to strong east to southeast winds associated with a surface ridge extending southwest into the region will remain in place through the weekend. Expect the strongest winds and roughest conditions within the channels, along the Hamakua Coast of the Big Island, windward waters, and south of the Big Island. The surface ridge will begin to weaken early next week as broad low pressure develops nearby to the west. This pattern will cause winds to veer out of the south to south-southeast, potentially remaining in the fresh to strong range next week.
Surf along east-facing shores will remain near seasonal average into early next week due to a large upstream fetch of fresh to strong breezes extending across the northeast Pacific. A downward trend is expected next week as this pattern weakens and local winds veer southerly.
Surf along north- and west-facing shores will remain relatively small through the period, with only a few west-northwest pulses expected over the weekend. A gradual increase is expected through the weekend, with Sunday potentially the largest day, reaching around head-high levels. A long-period westerly swell associated with a slow-moving storm-force low tracking eastward toward the Date Line from the far northwest Pacific is expected early next week. However, the westerly winds associated with this system near the islands are forecast to weaken by the weekend, which will limit the amount of swell generated toward the state. Additionally, Kauai will block some of this energy from reaching other coasts across the island chain.
Surf along south-facing shores exposed to southeast trade-wind seas will gradually increase through the weekend. Conditions could become rough early next week as winds veer southerly and strengthen. A long-period south-southwest swell associated with a recent system passing through our swell window near New Zealand is expected to arrive Tuesday and persist through midweek.
Small Craft Advisory until 6 PM HST Saturday for all eastern coastal waters and channels.
