High pressure far north of the main Hawaiian islands will maintain a moderate to locally windy trade wind pattern over the region into the middle of the week. An area of showers moving in on the trades tonight will maintain passing showers over the windward and mountains into Tuesday. Moderate trades and fewer showers are expected during the second half of the week, with an increase in showers during the weekend.
The forecast for tonight has been updated to increase PoPs and associated fields for the windward sides of the islands, particularly over Maui County and the Big Island with an area of showery low clouds riding in on the trade wind flow. Satellite derived precipitable water (PW) shows values 1.3 inches upstream of the islands. The afternoon soundings at Lihue and Hilo had 1.19 and 1.06 inches of PW respectively. The 1.3 is near normal for this time of year, so just expecting a few more showers than we have been seeing in the last couple of days.
High pressure far to the north will linger into Wednesday, with a tight pressure gradient over the islands maintaining the moderate to locally breezy trades. A few sites in the windiest locations (along ridges) have seen winds briefly reach advisory levels, but not expecting any widespread or persistent advisory conditions so have held off on any headlines. There is good model agreement with the ECMWF and GFS models that the high will weaken Wednesday as a strong low, currently far to the northwest in the Western Pacific, moves to the east and sets up far north-northwest of the islands. The high will weaken and move to the east, as the low deepens, and an associated front moves closer to the islands over the weekend.
With the high weakening, the pressure gradient over the islands will weaken, and winds are expected to return to moderate levels. A drier airmass moving over the islands will result in fewer showers during the Wednesday through Friday time period.
As the above mentioned front moves closer to the islands this weekend, moisture will be drawn northward over the islands, increasing shower activity. And then the second half of the weekend and into next week, the tail end of a front may impact the northern islands by increasing shower activity.
Breezy trades will persist through tomorrow. Passing showers will mainly be concentrated in windward and mountain areas. These showers may produce short-lived MVFR ceilings and visibility.
AIRMET Tango remains in effect for tempo moderate turbulence downwind of terrain below flight level 090.
High pressure centered far north of Hawaii will maintain fresh to strong trade winds for the next few days, and a Small Craft Advisory (SCA) remains in effect for all waters except windward Big Island through Tuesday afternoon. Trades will steadily decrease early Wednesday morning as the high weakens and is displaced eastward by an approaching low far northwest of the islands. The SCA will likely be trimmed back to the typically windy waters around Big Island and Maui at that time. Trade winds will lower to gentle strength, or may become disrupted entirely, as the low passes to the north Thursday and Friday. Trades could then turn more northeasterly this weekend.
Declining north-northwest and west-northwest swells will keep small to tiny surf in place over the next few days before a small northwest swell gives a slight bump to north shores on Thursday and Friday. This will then be followed by a slightly larger north- northwest swell this weekend and a potentially larger long period west- northwest swell early next week from a forecasted storm force low developing northeast of Japan in the next few days.
Small background south swells will keep relatively small surf for south facing shores through much of the week. A moderate south- southwest swell is expected this weekend, and could bring surf heights above seasonal averages.
Rough trade wind swell will produce east shore surf just above seasonal average through Tuesday, then decline later in the week as trade winds decrease.
Small Craft Advisory until 6 PM HST Tuesday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Kaiwi Channel-Kauai Channel-Kauai Leeward Waters-Kauai Northwest Waters-Kauai Windward Waters-Maalaea Bay-Maui County Leeward Waters-Maui County Windward Waters-Oahu Leeward Waters-Oahu Windward Waters-Pailolo Channel.
