Haiku, HI 96708 Weather

Synopsis

High pressure far north of the Hawaiian islands will begin to weaken, allowing winds to decrease to moderate levels for the remainder of the week. Passing showers will be carried in on the trade wind flow over the next several days, with an increase in shower activity this weekend. A more significant change to our weather pattern continues to evolve for early next week from Monday evening into Wednesday. Deeply unstable tropical moisture moving across the state will bring the potential for heavy rain and thunderstorms affecting all islands.

Discussion

The satellite imagery this evening continues to show unsettled cloud bands riding into each island on the trade winds. The water vapor satellite imagery shows a weak upper low that initializes well with the current global weather models. This upper low will continue to keep subsidence temperature inversion heights elevated in the 7,000 to 8,000 foot range, allowing for passing showers favoring the overnight to early morning hours through Thursday. Trade winds will continue to weaken from Thursday to Friday as a passing mid latitude low pressure system will weaken the pressure gradient over the Hawaii region lasting through the weekend.

A significant change to our weather pattern continues to develop by early next week. The long range global models continue to show for several model runs now a large band of deep tropical moisture driving northward into the islands. Several tropical low pressure circulations appear to be embedded within this synoptic scale deep tropical cloud band. These lows would provide the forcing needed to produce a few days of heavy rain and thunderstorms across the state. This band of unstable tropical moisture is over 1,000 miles long and models tend to handle these larger weather systems with a higher degree of long range accuracy. Our confidence in this heavy rain and thunderstorm pattern developing over the state of Hawaii is fair at this point in time. Stay tuned.

A Hydrologic Outlook may be needed to cover the rising threats for heavy rain and flash flooding potential during this time period. Please continue to closely monitor weather conditions including the potential for flash flooding early next week as this weather pattern will likely evolve over time.

Aviation

VFR. Moderate trades will occur through the morning. Winds will gradually weaken through the day as a trough passes north of the state. A fairly dry atmosphere will limit shower activity across most island air fields. Increasing moisture streaming north of the state, in tandem with more instability brought on by the trough, may increase the frequency of showers over LIH later today. Brief MVFR conditions will be possible within more organized showers.

AIRMET Tango will be in effect this morning for tempo moderate turbulence downwind of terrain below flight level 090.

Marine

High pressure north of Hawaii has begun a weakening trend, and will continue to weaken through the day while shifting east through Thursday. Moderate to locally strong trades will hold through today. The Small Craft Advisory (SCA) has been trimmed back to the typically windy waters around Big Island and Maui County. Trade winds will further ease to light to moderate speeds Thursday through Friday as shifts north of the state, disrupting the trade wind flow. Moderate to locally strong northeasterly trades will return this weekend as a new high builds in from the northwest and the low sags southeast. Next week's wind outlook remains uncertain due to vast differences between the location and intensity of a low developing southwest of the state.

Tiny surf along north facing shores will persist through today. A small northwest swell will arrive Thursday giving a small bump Thursday afternoon and Friday. This will then be followed by a slightly larger mid period north- northwest swell this weekend. A potential storm force low in the process of developing north of Japan could send a larger long period west- northwest swell early next week. This swell is still uncertain and will depend on the evolution of the developing low.

South shores will remain small before a series of moderate, south- southwesterly swells boost surf heights near to above seasonal average this weekend into early next week.

Rough trade wind swell will gradually decline later today as trade winds decrease, leading to a down tick surf through the remainder of the week. A small, medium period east swell is possible Thursday into the weekend that will help keep surf elevated for east facing shores.