Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 14 September (0300 UTC Friday 15 September) 2017

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while a band of high clouds rapidly sets up along the western half of the Big Island through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this evening and 3.5 C tomorrow morning. Winds will be light and from the NE for this evening, switching to a more ESE direction through the night. Seeing will be near 0.5-0.55 arcseconds, while precipitable water is expected to be in the 1.75-2.25 mm range for the night.

Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 9 thousand feet and ensure the summit remains dry and stable through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

A band of high clouds is expected to rapidly set up along the western half of the Big Island beginning late this afternoon, contributing to periods of extensive cloud cover for much of tonight. This band is projected to shift westward through tomorrow, leaving predominately clear skies overhead probably for the remainder of the forecast period. However, there is a possibility that very light cirrus will litter the northern skies for Saturday night.

Precipitable water is set to linger near 2 mm for the next 2 nights, briefly slip below 1 mm for Saturday night, then increase probably toward 2 mm for the following 2 nights.

While calm skies and deep subsidence will prevail in the free atmosphere for the next 2 nights, there is a possibility for periods of variability induced by ground layer turbulence mainly during the second half of tonight. Still, better than average should prevail for tonight and there is a possibility for excellent seeing for tomorrow night. An increase in boundary layer and/or low-level turbulence will likely contribute to poor seeing for Saturday and Sunday night. There is a good chance that seeing will improve again as winds taper for Monday night.

Little change since the morning forecast...The relatively deep low to the north is expected to fall apart as the mid/low-level ridge and an upper-level ridge to the NE and SE, respectively, restrengthen and push westward over the next 12-18 hours. While the upper portion of the ridge will eventually fall apart in response to the development of an upper-level low to the NE, the lower portion will continue to promote strong/steady large-scale subsidence in the area well into next week. This subsidence will easily maintain a well-defined inversion at or below 9 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. Unfortuntely, SW flow along the western flank of the building upper-level ridge is expected to advect a band of high clouds primarily along the western half of the Big Island for tonight. This band is set to breakdown and shift off toward the west as the ridge broadens through tomorrow, leaving predominately clear skies for the remainder of the forecast period. Calm skies embedded within the upper-level ridge could also allow seeing to dip toward 0.4 arcseconds for tomorrow night. However, summit-level winds are set to increase as upper-level momentum is redistributed into the lower-levels for Saturday and Sunday night. This will likely result in an increase in boundary layer/low-level turbulence which will probably contribute to poor seeing for that night. Summit-level winds are expected to taper again as the upper-level ridge breaks down, which should allow seeing to improve once again for the early part of next week.