Latest Forecast for Mauna Kea Observatories
830 AM HST (1830 UTC) Friday 21 April 2017

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while scattered to broken fill in from the west, contributing to periods of extensive cloud cover particularly as the night progresses.

Summary of Key Meteorological Variables
Summit temperatures will be near 7 C this afternoon, 2 C this evening and 1.5 C tomorrow morning. Winds will be light and from the SE, with seeing near 0.7-0.8 arcseconds. Precipitable water is expected to be in the 1.25-1.75 mm range for the night.

Discussion
Although the inversion will remain rather indistinct before significantly restrengthening near 6-7 thousand feet late in the weekend, the lack of moisture in the area will ensure the summit steers free of fog and precipitation through the next 5 nights. Minor afternoon clouds are possible through Saturday, but will become minimal and short-lived thereafter.

Scattered to broken high clouds will continue to fill in from the west and SW, contributing to periods of extensive cloud cover through late Sunday afternoon. The bulk of these clouds are set to shift off toward the east by that evening, but there is a possibility for lingering patches of high clouds to the north well into next week. A stream of cirrus is also expected to set up along the southern for Monday and Tuesday night.

Precipitable water is expected to linger primarily in the 1.5-2 mm range through the next 5 nights.

Mid-level turbulence will likely contribute to poorer than average seeing through at least tomorrow evening. This source of turbulence is expected to dissipate, which should help to improve seeing early Sunday morning. Relatively calm skies/laminar flow aloft should allow seeing to settle in near 0.5-0.55 arcseconds for the following 3 nights.

While the mid/low-level ridge will remain somewhat indistinct and displaced to the SE before restrengthening to the north later in the weekend, moisture will be fairly hard to come by in the area as it is drawn toward a low and its associated stalled cold front to the north of the Big Island over the next 2 nights. This will ensure the summit steers free of fog and certainly precipitation during this time, despite the lack of a prominent inversion. Unfortunately, very light summit winds combined with mid-level turbulence out ahead of the front could contribute to poor/variable seeing during this time. Cirrus filling in from the west and southwest will also result in periods of extensive cloud cover throughout most of this period. The inversion is set to strengthen near 6-7 thousand feet as the new ridge builds in to the north and shoves the decaying cold front westward on Sunday and into the early part of next week. This will not only ensure a dry air mass, but also should allow calm/stable skies to prevail over the summit, which should help seeing to settle in near 0.5-0.6 arcseconds during that time. Skies are also set to open up as the bulk of the high clouds shift eastward on Sunday, but there is a possibility that a stream of high clouds will develop within a rather broad sub-tropical jet just south of the Big Island for the early part of next week.