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 and mauka areas in the overnight to early morning hours starting tonight and lingering into Tuesday morning. The strongest of these three waves will pass through the islands from late Sunday through late Monday.
Radar shows scattered windward and mauka showers across the state this morning. A large surface high continues to strengthen to our north. Aloft, a weak upper level low to our east will become a trough that stretches from southern CA all the way out to us. The trough will be weak, and apart from a slight increase in instability, we won't see much from it.
Several impulses of increased moisture will move toward and over the state, especially the eastern end, over the coming week. The first will move through this afternoon and evening, and reach all islands. The second should move over the area late Sunday through late Monday, boosting showers mainly across the eastern end of the state. Finally, another area may reach us next Thursday, but at this time range confidence is low. Models currently indicate the focus of this one would be on the eastern end.
Trade winds will remain moderate to breezy through the coming week, with the a slight increase possible Tuesday evening. A slight weakening may develop next Thursday, but as mentioned in the previous paragraph, at seven days out, confidence in that level of detail is low.
Finally, guidence indicates that the upper level trough extending from southern CA over to us may join with energy moving in from the northwest and strengthen toward the middle of next week. It is unclear at this time whether it will develop into a closed low or a stronger trough. Either way, it may draw tropical moisture northward toward the state and present a heavy rain threat. Details should become clearer in a few days.
A high pressure system far north of the Hawaiian Islands will drift slowly eastward, increasing easterly trade wind speeds today. An easterly wave riding into the islands on the trade winds will bring increasing showers spreading from east to west across the islands near the Big Island today then spreading across the state by Saturday. Periods of MVFR conditions and showers will develop over windward mountain areas.
AIRMET Sierra remains in effect for tempo mountain obscuration abv 020 for north and east sections of the Big Island.
AIRMET Tango is in effect for moderate low level turbulence downwind of island terrain.
Surface high pressure far north of the islands will build as it shifts northeast of Hawaii through Saturday. A Small Craft Advisory (SCA) remains in effect for the typically windy waters around Big Island and Maui through Saturday, but this may need to be expanded in area as the high building to the north results in strengthening trade winds. Elevated trade winds will persist into early next week.
A small northwest swell will continue to fade today as a moderate north-northwest swell arrives. Surf along north and west-facing shores should build quickly today and top out near or slightly above seasonal average during the peak of the swell this afternoon into tonight, then slowly decline Saturday and Sunday as the dropping swell shifts more northerly. A smaller overlapping northwest swell arrives on Sunday which will bolster surf briefly. Both swells fade early next week.
A new long-periond south swell will peak today, pushing surf along south-facing shores back above seasonal average through tonight. This swell will slowly decline Saturday through early next week.
Surf along east-facing shores will gradually increase today as trade winds strengthen over the region. Expect some wrap from the moderate north-northwest swell Friday afternoon into Saturday. This swell along with developing rough trade wind swell will produce east shore surf above seasonal average Saturday through early next week.
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
