Latest Forecast for Mauna Kea Observatories
9 AM HST (1900 UTC) Monday 14 August 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 8.5 C this afternoon, 3.5 C this evening and 3 C tomorrow morning. Winds will be light and from the east, with seeing near 0.5 arcseconds. Precipitable water is expected to be in the 1.1-1.3 mm range for the night.

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 7-8 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.

Skies will overhead will remain predominately clear through the next 5 nights, but there is a possibility for light cirrus along the southwestern skies for Wednesday and Thursday night.

Precipitable water is expected to linger near or just above 1 mm through the next 5 nights.

A slight increase in upper-level shear/turbulence may degrade seeing toward 0.5 arcseconds for tonight and the opening half of tomorrow night. Seeing is expected to improve toward 0.4 arcseconds or less as deep subsidence and very calm skies build in for Wednesday and Thursday. Another round of upper-level shear/turbulence may fill in overhead and degrade seeing back toward 0.5-0.6 arcseconds again for Friday night.

A persistent mid/low-level ridge will continue to sit to the north of the state and stretch across most of the Pacific, negating the instability associated with the equally steady tropical upper-tropospheric trough (TUTT) to the north, through the week. This will ridge will allow strong large-scale subsidence in the area, which will easily maintain a well-defined tradewind inversion near 7-8 thousand feet and ensure a dry/stable summit throughout the forecast period. An upper-level low to the NE and embedded in the TUTT is expected to increase shear/turbulence in the free atmosphere and may degrade seeing a bit over the next 36 hours. Fortunately, the low will dig far enough south to gain access to any tropical high clouds during its tenure. The low will eventually shift further off toward the NE as an upper-level ridge fills in overhead between early Wednesday morning and Friday morning. This will not only augment the subsidence in the area, but diminish the shear/turbulence in the free atmosphere, and probably allow seeing to improve toward 0.3-0.4 arcseconds during that time. The departed low is set to be reinforced by a weak westerly trough later in the week. That may help to push the upper-level ridge off toward the west, increase shear in the free atmosphere and degrade seeing again for Friday night (still better than average seeing should prevail).