Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

339 am hst Sat apr 13 2024

Synopsis

A low pressure system northwest of the Hawaii and a surface frontal trough moving into the state from the northwest will produce southerly winds and periods of heavy showers and thunderstorms for the western islands. The initial threat for flash flooding with this system is highest for the islands of Kauai and Niihau from later tonight through Sunday night. These showers will then decrease in intensity as the front weakens and moves across the eastern half of the state from Monday through Tuesday. Improving weather trends will develop after the frontal trough passes through each island. Light to moderate trade winds will fill in across the Hawaii region from Wednesday onward with drying trends forecast for most areas.

Discussion

This mornings satellite imagery shows a low pressure center lingering 700 miles northwest of Kauai and an associated surface frontal trough roughly 100 miles northwest of Barking Sands on Kauai. The surface front and a passing upper level trough will produce periods of significant rainfall over the western half of the state over the next few days. Satellite imagery this morning also shows a subtropical jet stream over the island chain producing some high elevation cirrus clouds over Maui and the Big Island. Snow showers over the summits of the Big Island have faded and the Winter Weather Advisory was cancelled this morning.

Little has changed in the upcoming weather pattern over the past 12 hours. Threats for heavy rain, thunderstorms and flash flooding will continue mainly over the western half of the state. The latest forecast guidance continues to show southerly winds developing ahead of the front with good chances for heavy rainfall and thunderstorms from late tonight through Sunday over Kauai and Niihau. A Flood Watch was issued for Kauai and Niihau this morning starting late tonight and lasting through early Monday morning. Heavy rainfall and thunderstorms will then spread eastward along the surface front to Oahu from Sunday afternoon through Monday morning. The Flood Watch may be need to be extended in coverage to include the island of Oahu at some point as the forecast time period grows closer. The surface front will continue to drift eastward and weaken spreading rainfall to all islands in Maui County from Sunday evening through Monday afternoon. Wet weather conditions spread to the Big Island from Monday afternoon into Tuesday.

Rainfall amounts will be highest over the western half of the state from Kauai to Molokai, with decreasing rainfall totals over Maui and the Big Island as the upper level trough weakens as it passes the eastern islands. This means the highest threat for heavy rain with thunderstorms will focus initially on Kauai later tonight, and then spread to Oahu by Sunday afternoon. Our storm total rainfall forecast ranges from 3 to 6 inches on Kauai, 2 to 5 inches on Oahu, 2 to 4 inches on Molokai, with lesser amounts forecast for Lanai, Maui and the Big Island. Understand that these rainfall totals will highly depend on less predictable smaller scale heavy rain band and thunderstorm formation areas, which may increase these island by island storm total rainfall estimates.

Wet weather conditions will quickly improve from west to east next week as the surface trough passes through each island and diminishes near the Big Island. High pressure will begin to filter in north of the state as the low moves northeast away from the island chain. Expect a return to trade winds in the light to moderate range from late Wednesday onward.

Aviation

Light to moderate south to southeast flow will allow for daytime sea breezes in overnight land breezes in many areas. A few showers will be possible over south and southeast facing slopes and coasts this morning, with a few showers then developing over interior and windward areas in the afternoon. Showers will likely pick up over the western islands tonight as a front approaches from the west.

AIRMET Sierra remains in effect for mountain over south and southeast facing sections of Kauai. Conditions should improve here later this morning.

AIRMET Tango remains in effect for moderate upper level turbulence between FL180 and FL380. These conditions will likely hold through much of the day.

Marine

Light to moderate southerly winds will prevail over the western islands today, while moderate to fresh southeast winds hold over the eastern end of the state. A deep low pressure system to the distant north, will track southeast with the attendant cold front approaching the western end of the state tonight into Sunday, ramping up the southeast to south winds into the fresh to strong range. The front will slowly push southeast through the islands late Sunday through Monday, before stalling out and slowly dissipating near the Big Island through Wednesday. Moderate to fresh south winds will prevail through Monday ahead of the front, with light to moderate west to northwest winds filling in behind it. Winds will become rather light by Monday night and continue through next Wednesday, with moderate trades potentially returning for the latter part of next week.

North shore surf will remain small through the weekend. Surf will trend up Sunday night and Monday as a small long-period north- northwest swell coincides with a more significant pulse of medium- period northwest swell. Surf could reach low end advisory levels Monday night and Tuesday, with the swell then on the decline Wednesday through late next week.

Surf along east facing shores will decline over the weekend, with only small very small surf expected through the upcoming work week.

A small, long-period south-southwest swell will hold through the weekend. A new small, but slightly larger, south swell will give south shore surf a nice boost Tuesday and Wednesday, with another southwest swell giving surf another bump for late next week.

Watches, Warnings, Advisories

Flood Watch from late tonight through late Sunday night for Niihau-Kauai-

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