Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Wednesday 20 August 2008

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain clear, dry and stable throughout 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 from the east at 10-20 mph for today, increasing to 15-30 mph by the end of the night. Seeing will likely be around 0.55-0.6 arcseconds. Precipitable water is expected to be in the 0.8-1 mm range for the first half of the night and 1-1.2 mm range for the second half.

Discussion
The tradewind inversion will continue to cap the fairly limited supply of low-level moisture near 8-9 thousand feet and keep the mid/summit-level air mass dry and stable through the next 5 nights. Daytime clouds will also be minimal and short-lived throughout the forecast period.

Clear skies will prevail through the next 5 nights.

Precipitable water will likely start out near 0.8 mm for this evening, but is expected to increase toward 1.2 mm throughout the night, and then peak at 2 mm for tomorrow night. It will probably drop back toward 1.25 mm for the remainder of the forecast period, though there is a chance it will increase to 2+ mm late Sunday night.

Lighter flow at the summit, combined with weak shear/turbulence in the free atmosphere should result in better than average seeing through most of the night. Unfortunately, some degradation and variability may occur as summit-level winds start to pick up during the latter half of the night. In addition, mesoscale turbulence moving through the area, as well as strong boundary layer turbulence will likely result in poor/bad seeing for at least Thursday and Friday night. Seeing may improve toward 0.6-0.7 arcseconds, though boundary layer turbulence may still produce large fluctuations for the remainder of the weekend.

The mid/low-level ridge will continue to sit to the north of the Islands and promote strong large-scale subsidence in the area and maintain the tradewind inversion near 8-9 thousand well into next week. This should be enough to ensure the summit remains dry and stable, despite mid/upper-level lows passing through the region over the next several days. In fact, the upper-level low will be too weak to affect the stability of the atmosphere, but will push mesoscale turbulence through the area and thus disrupt seeing. The mid-level low, on the other hand, is slated to pass to the south over the next several days, and help strengthen summit-level winds, particularly for Thursday and Friday night. This will obviously result in another round of boundary layer turbulence and probably poor/bad variable seeing during that time. While both lows are slated to pass westward early Saturday morning, an upper-level ridge is expected to build in from the east and north for the weekend. The extra subsidence provided by the ridge will redistribute upper-level momentum into the mid-levels and probably keep summit-level winds near 20-30 mph during that time.