Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Friday 22 January 2021

Warnings
Moderate/strong winds

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while more scattered mid-level clouds return from the south for the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -1 C this afternoon and -5 C for the night. Winds will be from the east at 30-45 mph for today, decreasing to 25-40 mph through the night. Seeing will exceed 1 arcsecond, while precipitable water is expected to be in the 3-4 mm range for the night.

Discussion
A well-defined inversion will continue to cap low-level moisture near 7 thousand feet and ensure a dry/stable summit-level air mass for tonight. The inversion will begin to weaken/lift as deep moisture scrapes by to the south, increasing the risk for fog, ice and high humidity through Saturday night. This risk will further increase, as well as the possibility for flurries as the atmosphere turns saturated for the following 3 nights. There is also a possibility for convection in the area as well as periods of moderate/heavy snow at the summit mainly for Monday and Tuesday night. Some afternoon clouds are possible for today, then will likely pick up tomorrow and turn extensive for the remainder of the forecast period.

More scattered mid-level clouds are set to slowly spread back in from the south for tonight and into tomorrow night. A band of broken to overcast thick clouds will eventually quickly fill in from the south and develop overhead, blanketing summit skies for Sunday, Monday and Tuesday night.

Precipitable water is expected to slip into the 3-4 mm range for tonight, increase back to 4 mm for Saturday night, then jump to 8+ mm for the following 3 nights.

A mixture of boundary layer turbulence and/or instability/moisture will contribute to mostly bad seeing throughout the forecast period.

A tight mid/low-level ridge will continue to promote strong large-scale subsidence in the area, which will help maintain a well-defined inversion near 7 thousand feet and ensure a dry/stable summit-level air mass through at least late tomorrow morning. Unfortunately, a tight wind gradient along the southern flank of the ridge will also result in stronger than normal easterly flow, which will stir up boundary layer turbulence and contribute to bad seeing for tonight. Winds may subside a bit as the ridge weakens slightly and retracts northward in response to a trough building to the west of the state beginning tomorrow. Although initially the instability associated with the trough will remain off toward the SW, the displacement of the ridge will allow a band of deep moisture to fill in back into the area and will begin to scrape the southern portion of the Big Island by tomorrow evening. This band will start to erode the inversion, increasing the risk for fog, ice and high humidity and even cloud cover as that night progresses. Eventually, the air mass will turn completely saturated as this band of moisture further deepens and spreads across the Big Island by late Sunday afternoon and lingers overhead well into next week. This will likely contribute prolong period of inoperable conditions for Sunday, Monday and at least Tuesday night. In addition, there is a chance that convection will begin to develop in the area, which could deposit moderate/heavy snow as the trough shifts eastward and destabilizes the air mass between early Monday morning and Tuesday night. Winds may also pick up again as the trough closes in and tightens the wind gradient on Monday night, but should quickly subside as the ridge to the far NE weakens on Tuesday and into Wednesday.