Latest Forecast for Mauna Kea Observatories
730 AM HST (1730 UTC) Tuesday 21 March 2017

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain clear, dry and stable through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 7.5 C this afternoon, 3 C this evening and 2.5 C tomorrow morning. Winds will be from the NE at 5-15 mph for today, easing and switching to a more easterly direction through the night. Seeing will be near 0.5-0.6 arcseconds, while precipitable water is expected to be in the 1-1.5 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure the summit steers free of fog and precipitation through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Skies will remain predominately clear for tonight, but high clouds building in from the NW is expected to drift through the summit between late tomorrow evening and Friday morning, contributing to periods of extensive cloud cover mainly for much of Thursday. These clouds will shift eastward, leaving clear skies for most of Friday night and especially Saturday night.

Precipitable water is expected to slip toward 1.25 mm for the next 2 nights, briefly increase to 3-4 mm for Thursday night, then slip back toward 1.25 mm for Friday and Saturday night.

The decay of the low-level turbulence should allow for relatively calm skies at and above the summit and better than average (0.5-0.6 arcseconds) relatively steady seeing over the next 4 nights. There is a good chance that an increase in boundary layer turbulence will contribute to more poor-like seeing for Saturday night.

A well-defined mid-level ridge will continue to sit almost directly over the state for the next 2 days, then will weaken a bit before a new ridge fills in from the west and settles in to the north over the weekend. Nonetheless, the ridge will continue to promote strong/steady large-scale subsidence in the area, which will easily maintain the tradewind inversion near 8 thousand feet and ensure a dry/stable summit/mid-level air mass through the week. The weakening of the ridge will likely be in response to a passing of a weak upper-level trough to the north around Thursday/Friday. This trough will hardly affect the stability of the atmosphere, but is expected to bring broken high clouds to the area, which could contribute to periods of extensive cloud cover (and perhaps high PW) mainly for much of Thursday. Still, relatively uniform westerly flow aloft, combined with a very gradual change in potential temperature and the decay of low-level turbulence (associated with an elevated inversion) should allow for calm skies and better than average seeing over the next 4 nights. Summit-level winds are set to increase in response to the building of a new ridge from the west over the weekend. This could stir up boundary layer turbulence and contribute to poor seeing for Saturday night.