Mountain View, HI Weather

Synopsis

High pressure far north of the 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 shower trends this weekend.

Discussion

The satellite imagery this morning shows bands of clouds drifting towards the islands on the brisk easterly trade winds, while a large band of high cirrus clouds are developing aloft near the Big Island associated with the subtropical jet stream. Expect brief periods of showers this week, favoring the windward mountain areas in the overnight to early morning hours.

In the larger scale, a high pressure system north of the Hawaiian Islands will slowly drift eastward over the next few days and a mid latitude trough will drop in from the north and deepen just northeast of the islands. This large scale pressure pattern change will disrupt the trade winds this weekend and draw up some unstable tropical moisture from the south into the state.

Expect warm and humid conditions for all islands starting this weekend with additional cloud cover and periods of shower activity as temperature inversion heights rise to above the 10,000 foot elevation level. There does not appear to be strong enough forcing to drive heavy rain during this time period. Instead we are anticipating rainfall in the light to moderate range.

Long range European (ECMWF) and American (GFS) models are both suggesting an unusual set up around the middle of next week as several tropical circulations may drift northward into the Hawaii region. If this scenario develops as these global models are suggesting, then we are looking at the potential for very unstable conditions with heavy rain and thunderstorms at some point. Still too far out in time to have good confidence, yet the pattern is worth mentioning, and we will continue to monitor this developing situation for potential weather impacts to the Hawaiian Islands.

Aviation

Breezy trades will persist through the forecast period. Passing showers will mainly be concentrated along windward and mauka areas. Some of 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.

Marine

High pressure 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 Wednesday morning. 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.

For north facing shores, expect tiny surf over the next few days before a small northwest swell arrives giving a small bump 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.