Easterly trade winds will slowly decrease through the Christmas holiday weekend. Shower activity will be limited through Sunday. An approaching cold front and upper level disturbance on Sunday will spread significant clouds and showers across the state next week as the system marches eastward into the Hawaii region. This upper disturbance may significantly increase rainfall for all islands next week in a light to moderate wind pattern.
This mornings infrared satellite imagery shows a low pressure system far northwest of the state with a stalled front lingering roughly 500 miles west of Kauai. More stable stratocumulus clouds are found just northeast or upstream of the Hawaiian Islands. These more stable clouds are consistent with a high pressure ridge building over the region, producing more stable conditions by warming and drying the middle atmosphere through downward moving air compression or subsidence. Subsidence temperature inversion heights, as measured by upper air balloon soundings launched at 2 AM HST (12Z) from Lihue and Hilo, range from around 6,000 to 7,000 feet elevation respectively. These inversion heights are significantly lower than previous observations, matching well with our increasing stability trend forecast due to the building ridge. One wrinkle noted in this mornings satellite imagery is a few thunderstorms drifting eastward into the Northwest Kauai Waters, these thunderstorms will likely fade out around sunrise.
Expect a drier and more stable weather pattern lasting into the weekend with diminishing easterly trade winds, veering from a more east to southeast direction from Friday into the weekend. Easterly trade winds are hanging on a little longer than we originally forecast, however drier trends with only brief passing showers will prevail in the short run. Temperature inversion heights during this time period will lower, ranging from around 4,000 to 6,000 feet elevation, capping vertical cloud development and reducing shower potential statewide. Lighter wind speeds will also limit orographic lifting of passing clouds over windward mountain ranges.
By Sunday, another shallow cold front approaches Kauai from the northwest. The latest model consensus suggests this weak frontal trough will stall out near Kauai. However, a more interesting pattern is brewing in the upper levels, as a passing and strong upper level trough may deepen into an upper low, and sweep eastward into the islands. The latest American (GFS) extended range model solution continues to show this upper low moving across the entire state during this time period. However, the European (ECMWF) weather model is projecting a much weaker upper level trough passing through the region, a much drier solution. If this long range GFS solution works out as predicted, then all islands will see several days of enhanced rain shower activity in a light to moderate wind pattern through the middle of next week. Island by island rainfall activity will heavily depend on the strength and track of this upper level cold core low pressure system. Please stay tuned to this evolving weather pattern as the potential for heavy rain from this system may develop during this time period.
Trade winds will decrease over the next few days as more stable trends move into the islands under the influence of a building high pressure ridge. VFR conditions will prevail.
AIRMET Sierra remains in place for mountain obscuration affecting most islands, as low clouds continue to bank up against windward mountain ranges this morning. These low cloud ceilings will likely diminish in coverage after sunrise.
Surface high pressure located far north of the coastal waters will continue to drift southeast as a large area of low pressure far northwest of Kauai will attribute to fresh to locally strong easterly trade winds through this afternoon and gradually weaken through Thursday. A Small Craft Advisory (SCA) has been scaled back to the windier waters and channels around Maui County and the Big Island through tonight. The SCA may need to be extended for another day or two as trades slowly ease. The low will move to the east Friday into the weekend, pushing a surface ridge near the islands and causing winds to further decline and eventually shift out of the southeast and south.
Surf along north facing shores will gradually decline through the afternoon before a moderate, medium period north-northeast (010-030 degrees) swell is expected to fill in this evening, peak Christmas Day near High Surf Advisory levels, before declining Friday. Forerunners of this swell should start to reach offshore buoys through the morning hours with most of the energy to be focused over the eastern end of the state when it arrives. A Marine Weather Statement will likely be issued this afternoon for moderate harbor surges at Kahului and Hilo tonight through Christmas Day. A combination of the declining north northeast swell and a small long period west northwest swell will keep small surf through the weekend. West facing shores will see a tiny, moderate period west swell hold through the day. The next large long period northwest swell is expected over the first half of next week but stay tuned as the system develops northwest of the islands.
Rough and choppy surf along east facing shores will remain elevated today and gradually decline as trade winds ease slightly today. Seas remain elevated just below SCA criteria over windward coastal waters this morning but are expected to trend down through the day. With the building north-northeast swell expected this evening, exposed shorelines will see wrapping energy through Friday. Surf along south facing shores will remain very small through the week.
Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
