Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Monday 19 October 2020

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while scattered to broken mid/high clouds flow overhead through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 8 C this afternoon and 3.5 C for the night. Winds will be from the NNW at 5-15 mph for today, switching to a more WNW direction through the night. Seeing will be near 0.55-0.6 arcseconds, while precipitable water is expected to exceed 4 mm through the night.

Discussion
Despite an influx of mid-level moisture and 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 afternoon clouds are possible, particularly through Wednesday then will become minimal and short-lived for Thursday, Friday and over the weekend.

A band of broken mid/upper-level clouds will continue to fill in from the SW and contribute to extensive cloud cover for tonight. This band will sag to the south and open up the northern skies for the early part of tomorrow night. However, more high clouds may move in from the SW and pass over the summit early Wednesday morning and will likely result in another round of extensive cloud cover for that night. These clouds will also shift southward on Thursday, while more patchy high clouds develop on the northern skies for that night. Both sets will eventually shift eastward on Friday, leaving predominately clear skies for that night.

Precipitable water is expected to exceed 4 mm for the next 3 nights, but could slip toward 3 mm for Thursday and Friday night.

Although predominately laminar westerly flow will prevail in the free atmosphere, residual mid-level moisture will likely keep seeing near 0.55-0.6 arcseconds for tonight. There is a good chance that an increase in mid-level free atmospheric turbulence will contribute to slightly poorer than average seeing for Tuesday and Wednesday night. This turbulence is set to pass on Thursday and likely allow seeing to improve toward 0.45-0.5 arcseconds as lighter/laminar WSW flow prevails at and above the summit for that night and Friday 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 gradually 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 3 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 for Tuesday and Wednesday night. Seeing and cloud cover is set to improve as the STJ and the low shifts off to the east and weakens, respectively, for Thursday and Friday night. Precipitable water will also see a very subtle improvement as the residual moisture slowly detrains into the surrounding atmosphere during those nights.