Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Tuesday 20 October 2020

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while

Summary of Key Meteorological Variables
Summit temperatures will be near 8.5 C this afternoon and 3.5 C for the night. Winds will be light and variable, with seeing near 0.6-0.65 arcseconds for the night. Precipitable water is expected to exceed 4 mm throughout the night.

Discussion
Although lingering mid-level moisture may allow for periods of humidity in the 40-60% range, a well-defined inversion at or below 8 thousand feet will ensure the summit steers free of fog and precipitation through the next 5 nights. Some minor afternoon clouds are possible throughout the forecast period, particularly on Friday.

Scattered mid-level clouds will litter the eastern skies over the next 24 hours, while more broken high clouds are expected to rapidly fill in from the SW later this evening, contributing to extensive cloud cover for the remainder of the night and into tomorrow night. These clouds are set to pass eastward on Thursday, while more patchy high clouds develop on the northern skies for that night, then also shift eastward on Friday, leaving clear skies for that night. Scattered high clouds may eventually fill back in from the west again during the latter half of Saturday night.

Precipitable water is expected to linger near or exceed 4 mm throughout the forecast period.

An increase in mid and upper-level turbulence is expected to degrade seeing toward slightly poorer than average seeing through tonight and into tomorrow night. There is a good chance that seeing will improve as this turbulence subsides through Thursday night an dallow seeing to settle back in below 0.5 arcseconds for Friday night. However, another possibility for mid-level moisture could contribute to some degradation/variation in seeing for Saturday night.

The SW tip of the mid/low-level ridge will continue to sit over the Big Island, while the large/stationary low to the NNW begins to slowly weakens and eventually lifts off toward the north as the week progresses. Nonetheless, steady subsidence associated with the ridge will help maintain a well-defined inversion near 8 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. Relatively deep SW mid/upper-level flow bordering the low and the ridge will continue to advect bands of mid/upper-level clouds/moisture over the summit, contributing to PW > 4 and moisture extensive cloud cover over the next 2 nights. There is a good chance for poorer than average seeing as the sub-tropical jet along the southern flank of the low dips over the Big Island and strengthens during this time. Seeing and cloud cover are set to improve as the STJ shifts eastward and the low continues to fall apart and lift off toward the NE for Thursday and especially Friday night. Unfortunately, residual mid-level moisture in the area will likely keep PW near 4 mm, and there is a possibility for an increase in mid-level turbulence, which could negatively impact seeing for Saturday night.