A high pressure system far northwest of the Hawaiian Islands will slowly drift from west to east over the next few days. This passing system will increase easterly trade winds into the moderate to breezy range lasting into the middle of next week. Brief pulses of enhanced showers are forecast with three passing easterly waves during this time period. Increasing shower trends will favor the windward mountain areas in the overnight to early morning hours starting Friday night and lingering into Tuesday morning. The strongest of these three waves will pass through the islands from Sunday night into Tuesday morning.
The satellite imagery this morning shows a lingering band of unstable showers riding into the windward mountain slopes of each island on the trade winds. Periods of windward and mountain showers should be expected for the typical areas, more isolated showers are possible for drier leeward locations. Increased rainfall coverage in the morning forecast to account for the additional activity. These enhanced trade wind shower trends will diminish in coverage a few hours after sunrise due to the typical diurnal day time stability trends. Farther to the east, the satellite imagery verifies three short waves moving towards the islands within the easterly trade winds. These easterly waves will bring pulses of showers to all islands as they March across the island chain from Friday into early next week.
In the big picture, we continue to track a high pressure system currently located far northwest of the Hawaiian Islands drifting eastward over the next few days. This migratory high will increase easterly trade winds into the moderate to breezy range lasting into next week Tuesday.
Periods of wet weather remain in the weather forecast for this weekend as the three low level easterly waves will ride into the islands on the strengthening trade winds. Three pulses of higher shower coverage will favor Friday night, then late Saturday, and the last one unfolding from Sunday through Monday. The last of these easterly short wave troughs might be the strongest of the three with enhanced showers extending over the typically drier leeward areas at times. Forecasting rainfall amounts with these waves is always challenging due to variations in arrival timing and their intensity relative to each island.
A larger degree of uncertainty remains in the long range forecast from next week Wednesday onward. Both the American (GFS) and European (ECMWF) global forecast models continue to show diverging solutions for a passing middle latitude trough as the trough digs in and potentially approaches the islands from the northwest. The GFS model solution deepens this trough into a large cut off low north of Kauai and secondary upper low deepens northeast of the Big Island from Thursday into Saturday. The GFS shows us a wetter solution for sure as upper level cold core temperatures and additional divergence aloft will lift inversion heights and produce a surface trough likely producing southeasterly to southerly winds near the islands at some point. However, the ECMWF model presents a much more stable solution with two weaker upper lows north of the state. This european model solution will produce more stable conditions, as compared to the GFS model, with a typical trade wind shower pattern setup.
We will continue to closely monitor this long range forecast time period for potential island by island weather impacts as the model solutions begin to converge and the weather forecast becomes clearer over the next several days. Stay tuned.
Trades will strengthen to breezy levels today as a ridge builds north of the islands. A batch of moisture moving from east to west over the islands is bringing brief MVFR conditions to Maui County and the Big Island from time to time. Otherwise, VFR conditions are expected.
No AIRMETs are in effect, however as winds increase, AIRMET Tango may be needed for mechanical turbulence to the lee of the mountains.
Surface high pressure far northwest of the islands will move closer today, then build as it shifts north and then northeast of Hawaii on Friday. As a result, trade winds will gradually increase to fresh to locally strong levels over the next couple of days. A Small Craft Advisory (SCA) is in effect for the typically windy waters around Big Island and Maui beginning at 6 am this morning. A stronger surface high will arrive far north of the islands late Saturday and Sunday, likely increasing trade winds further, which may necessitate an expansion of the SCA to additional waters. Elevated trade winds will persist into early next week.
A small bump in the northwest swell will bring another peak of small surf to north and west-facing shores today that will fade tonight before a moderate north-northwest swell arrives Friday and continues through the weekend. Surf along north and west-facing shores should top out around seasonal average during the peak of the swell Friday afternoon and Saturday, then slowly decline late Saturday and Sunday as the dropping swell shifts more northerly and a smaller overlapping northwest swell arrives. Both swells will fade early next week.
A new long-periond south swell builds today and may push surf along south-facing shores back above seasonal average through Friday. The swell will slowly decline Saturday through early next week.
Surf along east-facing shores will begin to increase this afternoon into tonight as trade winds strengthen over the region. Rough trade wind swell will produce east shore surf above seasonal average Saturday through early next week.
Small Craft Advisory from 6 AM this morning to 6 PM HST Saturday for the typical windier waters and channels near Maui and the Big Island.
