Hilo, HI 96721 Weather

Synopsis

High pressure far north of the Hawaiian islands will begin to weaken, allowing winds to decrease to moderate levels for the remainder of the week. Passing showers will be carried in on the trade wind flow over the next several days, with an increase in shower activity this weekend.

Discussion

No significant changes to the forecast with the afternoon package. Main changes were to align the later parts of the forecast with the latest National Blend of Models (NBM). There remains uncertainty with the differences in the global models, so aligning with the NBM seems the prudent choice at this time.

A high pressure ridge remains to the north of the islands, maintaining a moderate to locally windy trade wind pattern over the region. As the associated high weakens, we expect the pressure gradient over the islands to relax, allowing trades to return to moderate levels across the islands.

Passing showers will continued to be carried in on the trade wind flow this evening, but with satellite derived precipitable water (PW) showing a drier than normal airmass upstream of the islands, we expect shower activity to taper off overnight, with isolated shower activity expected then through Friday.

Good model agreement continues between the ECMWF and GFS models that the high mentioned above will weaken Wednesday as a strong low, currently more than 2000 miles to the northwest, moves to the east and sets up far north of the islands Thursday. The low is forecasted to strengthen and sink southward into the weekend, with an associated front then moving towards the islands over the weekend. Currently both global models are in good agreement with showers increasing during the weekend and into next week.

There are indicators in the global models that winds atop the Big Island summits could reach warning levels during the first to middle part of the next week. The GFS is currently more aggressive that the ECMWF. The forecast for Monday through Wednesday has been nudged to the GFS solution, but tapered back a bit to account for the conservative nature of the ECMWF at this time.

An increase in dewpoints is expected during the second half of the weekend and into the early part of the new week, which will bring warm and humid conditions back to the islands.

Aviation

No major changes from the previous forecast. Trades are still expected to be a bit lighter heading into Wednesday. Brief MVFR conditions will be possible with any passing showers.

AIRMET Tango remains in effect for tempo moderate turbulence downwind of terrain below flight level 090.

Marine

High pressure far north of Hawaii will maintain fresh to strong trade winds through tonight, and a Small Craft Advisory (SCA) remains in effect for all waters except windward Big Island through tonight. Trades will steadily decrease early Wednesday 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. By next week, model discrepancies exist regarding a low pressure system moving in from the southwest which could affect wind speeds and direction.

Small surf for north facing shores is expected through tomorrow before a small northwest swell arrives giving a small bump Thursday afternoon and Friday. This will then be followed by a slightly larger mid period north- northwest swell this weekend. A potential storm force low in the process of developing north of Japan could send a larger long period west- northwest swell early next week. This swell is still uncertain and will depend on the evolution of the developing low.

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 gradually decline later tomorrow morning as trade winds decrease, leading to smaller surf through the remainder of the week. A small medium period east swell is possible Thursday into the weekend.