An upper level trough over the islands will set the stage for an unstable environment today. A low level trough riding into the islands from the north will trigger bands of moderate to heavy showers with possible thunderstorms through the afternoon hours. Cool moderate to breezy northeasterly trade winds will blow across the region with decreasing shower trends as high pressure builds in from tonight into Thursday. Winds begin to weaken and veer from a more southeasterly direction from Friday into the weekend ahead of the next cold front approaching the islands from the west.
This mornings satellite imagery shows an upper level trough lingering over the Hawaiian Islands as a low level trough moves into the islands from the north. The upper level trough provides the large scale instability, while the low level trough and strengthening trade winds provide the lift over island mountain ranges producing bands of moderate to heavy showers marching southward down the island chain today.
An elevated risk of flash flooding remains as the ground is already saturated from heavy rain that has fallen over the past 48 hours, and even additional rainfall rates in the 2 inches per hour range could lead to excessive runoff and trigger localized flash flooding. The main threat to watch for in this pattern will be along windward northern and eastern slopes from several storm cells training into one area or with terrain anchored thunderstorms. A Flood Watch remains in effect for most of the state through this afternoon. The latest short range model trends are suggesting shower intensity will strengthen along the low level trough from 8 AM to 2 PM HST. Flooding threats will diminish from north to south later today after clouds along the low level trough pass through each island.
Strong winds over the Big Island summits of Mauna Loa and Mauna Kea will continue to trend lower this morning. A Wind Advisory replaced the wind warning at summit level, this advisory was issued with the morning forecast and will end at noon HST.
Drier and much cooler air from the North Pacific will ride into the region on the northeasterly trade winds into Thursday. This drier atmosphere will allow additional radiational cooling into outer space for the next few nights with low temperatures dropping into the 50's and 60's depending upon elevation. This drier trade wind pattern will continue through Thursday before winds decrease and veer from a more east- southeasterly direction from Friday into the weekend. The wind pattern shifts as the ridge over the islands weakens in response to yet another cold front approaching the Hawaiian Islands from the west.
Periods of moderate to heavy showers will continue mainly along the windward sides of the islands through the day today, tapering off by sunset. Isolated thunderstorms remain in the forecast for all islands. MVFR conditions are possible under showers. Winds will remain primarily NE across most of the island chain.
AIRMET Sierra is in effect for mountain obscuration above 2 kft across the windward sections of most islands due to clouds and showers.
AIRMET Tango remains in effect for moderate upper level turbulence across the entire state between flight levels 27 and 35 kft.
A trough over the central coastal waters will slowly push southward today due to the strong high pressure far north of the islands. Currently, gentle westerly winds are south of the trough, while fresh to strong easterly winds prevail north of the trough. Scattered moderate showers and isolated thunderstorms can be expected near and along the trough. A Small Craft Advisory (SCA) is in effect for waters surrounding Kauai due to fresh to strong trade winds and will be expanded and extended this morning even though the winds should ease slightly tonight and Tuesday, but due to a large north- northeast swell expected to produce seas in excess of the 10 foot SCA threshold over most waters into Tuesday night. Moderate to locally fresh trades are expected on Wednesday, giving way to southeasterly winds Thursday or Friday as a front approaches.
Surf along north facing shores will build through the morning hours due to a north-east (010-030 deg) moderate long period swell that has begun to show forerunners at NDBC buoy 51000 early this morning. As the swell builds early this afternoon, it will produce breaking waves at High Surf Warning levels across most north and east facing shores during the peak tonight and Tuesday. Due to the direction of the large swell, exposed west facing shores of West Maui, western Molokai, and North Kohala on the Big Island will likely experience High Surf Advisory waves. A Coastal Hazard Message has been issued to highlight these conditions. The north- northeast swell will gradually decline Tuesday night through Friday. In addition to this swell, a small pulse of overlapping west- northwest swell is expected to build through the morning and hold through Wednesday.
Due to the direction of the large north-northeast (010-030) swell, some overwash will be possible along low-lying coastal roads and infrastructure in windward areas, and hazardous conditions and heavy surges are expected in north facing harbors, mainly in Hilo and Kahului. A Marine Weather Statement has been issued to highlight this threat.
Very small wintertime surf will continue along south facing shores this week.
Flood Watch through this afternoon for Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe and Maui.
High Surf Warning from noon today to 6 PM HST Tuesday for north and east facing shores of Niihau, Kauai, Oahu, Molokai, Maui and the Big Island.
High Surf Advisory from 6 PM this evening to 6 PM HST Tuesday for west facing shores of Molokai, Maui, the Big Island, and north facing shores of Lanai.
Wind Advisory until noon HST today for Big Island Summits of Mauna Loa and Mauna Kea.
Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel.
Small Craft Advisory until 6 PM HST Tuesday for Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters.
