An old remnant frontal boundary moves swiftly through the islands today with a brief boost in showers and breezy trade winds. Easterly trade winds will quickly decrease tonight through Wednesday with a hybrid light to moderate east-southeasterly wind and sea breeze pattern setting up over the islands from Wednesday night through the Christmas holiday weekend. Shower activity will be limited from Wednesday onward after the remnant cloud band passes through the state. An approaching upper level disturbance on Sunday will spread significant high level cirrus clouds from west to east across the region. This upper disturbance will increase statewide shower trends in a light southeasterly wind pattern for the first half of next week.
In the big picture satellite imagery, we see a high pressure ridge in place north to northeast of the Hawaiian Islands, a deep low and cold front far northwest of the state tracking northeastward,and a narrow low level cloud band drifting through the islands this morning. Breezy trade winds will push this narrow remnant cloud band through the islands today enhancing shower trends for all islands, especially over the typical windward mountain slopes.
The cold front trailing the low northwest of the islands will approach the Hawaii region from the west and stall long before reaching the western most island of Kauai. However, the main impact to island weather will be a change in wind directions as the high pressure system far north of the state drifts east, parking a low level ridge directly over the island chain. Wind speeds will weaken and veer more from a east to southeast direction from Wednesday night through the Christmas holiday weekend. Another impact of this ridge over the state will be felt in the large scale subsidence, or stabilizing downward vertical motions that will warm and dry the air over each island. Subsidence temperature inversion heights will lower, decreasing vertical cloud growth and keeping shower activity to minimum during this time period. Only brief passing showers remain in the forecast and any rainfall amounts will be light. American model (GFS) cross sections show this trend well with temperature inversion heights hovering around 5,000 to 6,000 feet elevation from Wednesday night into Sunday.
By early next week, another shallow cold front approaches Kauai from the northwest. The latest model consensus guidance suggests this weak frontal trough will stall out long before reaching the islands. However, a more interesting pattern is brewing in the upper levels as a passing and strong upper level trough will deepen into an upper low near the islands. Extended range weather model solutions project this upper low to pass from west to east across the entire state during this time period. If this long range solution works out as predicted, then all islands will see rain shower enhancement in a light wind sea breeze wind pattern. Rainfall activity will favor island mountain and interior sections with decent rainfall potential for even the normally drier leeward sections. Stay tuned as this extended range forecast period will certainly evolve over time.
An old remnant frontal boundary moves swiftly through the islands today with a brief boost in breezy trade winds. Easterly trade winds will quickly decrease tonight with a hybrid light to moderate east-southeasterly wind and sea breeze pattern setting up over the islands with decreasing shower trends from Wednesday night onward.
AIRMET Sierra has been posted for tempo mountain obscurations for all islands as this low level cloud band passes swiftly through the islands. This AIRMET will likely be trimmed down in coverage for some islands later today as more stable air moves in from the northeast.
AIRMET Tango remains in effect for tempo moderate turbulence downwind of terrain on all islands below 8000 feet will likely continue through tonight.
Surface high pressure far north-northwest of the waters will drift east through Wednesday as low pressure far to the northwest of Kauai remains nearly stationary and weakens. Easterly trade winds will strengthen over majority of the coastal zones today into the fresh to strong range as the Small Craft Advisory (SCA) has been extended through tonight due to a combination of winds and seas. The SCA will be trimmed to the windier waters and channels around Maui County and the Big Island Wednesday. The high pressure north of the waters will weaken as it passes north of Hawaii on Wednesday, coinciding with the weakening low, thus gradually weakening the winds and veering to the southeast to south by the end of the week. Area of low pressure looks to redevelop and push a surface ridge near Kauai by the weekend.
The current small north swell will continue to gradually decline into Wednesday. A moderate, medium period north-northeast (010-030 degrees) swell will arrive late Wednesday and peak Thursday and Friday. Surf along north facing shores could approach the High Surf Advisory level during the peak, with the largest swells favoring the eastern end of the island chain, and a Marine Weather Statement will likely be needed tonight for harbor surges at Kahului and Hilo. The north swell will decline over the weekend as a small northwest swell arrives.
Surf along east facing shores have increased overnight coinciding with the short period energy from the building trade wind energy. Strong trades will produce rough and choppy conditions through Wednesday, mainly over the western islands, peaking near or briefly at High Surf Advisory levels for east facing shores before declining. Short period trade wind swell will decline by Christmas, but wrapping northerly swell will affect exposed eastern shores into Friday. Surf along south facing shores will remain very small through the week.
Small Craft Advisory until 6 AM HST Wednesday for most Hawaiian Coastal Waters due to a combination of winds and seas.
