Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 27 August (0300 UTC Thursday 28 August) 2008

Warnings
Moderate winds

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

Summary of Key Meteorological Variables
The summit will be near 3 C, while winds will be from the ENE at 15-30 mph (perhaps slightly stronger later in the night). Precipitable water is expected to be in the 0.7-0.9 mm range, with seeing likely around 0.7-0.8 arcseconds.

Discussion
The tradewind inversion will continue to cap low-level moisture near 9-10 thousand feet and keep the mid/summit-level air mass dry and stable through at least Thursday night. There is a chance the inversion will lift toward 11-12 thousand feet, increasing the risk for short-lived fog as early as Friday night and into the weekend; precipitation is not expected. Daytime clouds will also be minimal and short-lived through Thursday, but could pick up a bit on Friday and over the weekend.

Clear skies will prevail through the next 5 nights.

Precipitable water is expected to hang around 0.8 mm for the next 2 nights, then increase toward 1-1.5 mm for the remainder of the forecast period.

Minor shear will persist in the free atmosphere as winds aloft remain rather light throughout the forecast period. Unfortunately, average to slightly poorer than average seeing will likely prevail as summit-level winds continue to stir up boundary layer turbulence for the next 2 nights. Mesoscale turbulence is also slated to build in the area, disrupting seeing for much of the weekend.

There is a chance that the low will start to destabilize the atmosphere and bring fog to the summit as early as Friday now...The very persistent mid-level ridge will continue to sit to the north of the Islands and maintain predominately easterly flow and strong large-scale subsidence in the area well into next week. An upper-level ridge will also linger in the area, enhancing both summit-level winds and the subsidence in the area over the next couple of nights. Unfortunately, the former will continue to stir up boundary layer turbulence, likely resulting in slightly poorer than average seeing during this time. On the other hand, the extra subsidence will keep PW near 0.8 mm over the next 2 nights. Winds are slated to subside (and PW may increase a tad) as the upper-level ridge breaks down in response to a developing upper-level low over or just east of the Islands early Friday morning. This low is expected to meander through the southern half of the State, placing minor instability in the area over the weekend. While I suspect the instability will be too weak to overcome the mid-level ridge, I still put a slight risk for fog for during that time. Nevertheless, the low will also place mesoscale turbulence in the area, which will likely keep seeing near average-like values during that time.