Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 07 December (0300 UTC Friday 8 December) 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 1 C, with winds from the NNW at 15-25 mph for this evening, easing to 5-15 mph through the night. Seeing will start out near 0.7-0.8 arcseconds, but will likely settle in near 0.5-0.6 arcseconds as the night progresses. Precipitable water is expected to be in the 3-4 mm range for the first half of the night and 2.5-3.5 mm range for the second half.

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 6-7 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 overhead will remain predominately clear for tonight, but scattered high clouds may begin to pass along the northern skies for tomorrow evening and briefly drift overhead later that night. More patches willl continue to drift by along the northern skies for the remainder of the weekend. There is a chance that another more organized band of broken high clouds will pass overhead again for Monday night.

Precipitable water is expected to linger near 2.5-3 mm for the next 2 nights, slip toward 1 mm for Saturday night, then rebound back to 2 mm for Sunday night and perhaps 3+ mm for the early part of next week.

Residual (light) boundary layer turbulence may contribute to poorer than average seeing for this evening, but seeing is set to improve as this turbulence and winds subsides through the night and especially for tomorrow night. However, another round of boundary layer turbulence will likely result in bad seeing for Saturday and Monday night. While there is a chance that the boundary layer turbulence will briefly subside on Sunday night, an increase in mid-level turbulence will limit much improvement in seeing for that night.

Little change since the morning forecast...While a large quasi-stationary trough to the NNE continues to prohibit the eastward progression of the mid/upper-level ridge to the west, the SE tip of the ridge will continue to linger over the state and promote strong/steady subsidence in the area well into next week. This subsidence will easily maintain a well-defined inversion near 6-7 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. Winds are also set to decrease as an short-wave trough embedded in the large trough lifts northward through today. This will help reduce the boundary layer turbulence and improve seeing as the night goes on and especially for tomorrow night. However, a couple of short-wave troughs are expected to pass to the north, buring more patches of high clouds to the northern skies and may increase winds once again on Saturday night and for the early part of next week. The winds will likely stir up boundary layer turbulence and contribute another round of poor/bad seeing during those nights. While there is a chance that winds will briefly subside between the troughs, mid-level turbulence and/or gravity waves in the wake of the first trough will not allow for much improvement in seeing. Fortunately both short-wave troughs are expected to pass too far to the north to affect the stability of the atmosphere during those nights. That could change as a third short-wave digs in immediately behind the second short-wave and enhances the large trough around the middle part of next week.