Latest Forecast for Mauna Kea Observatories
9 AM HST (1900 UTC) Monday 19 June 2017

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while scattered high clouds continue to dissipate along the north and western skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 11 C this afternoon and 6 C for the night. Winds will be from the SE at 15-25 mph for today, decreasing to 10-20 mph for the night. Seeing will be near 0.4-0.45 arcseconds, while precipitable water is expected to be in the 0.8-1 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture primarily near 7-8 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Scattered high clouds will continue to dissipate along the northern and western skies through tonight, leaving primarily clear skies for the remainder of the forecast period.

Precipitable water is expected to linger near 0.8-1 mm for the next 2 nights, increase toward 1.5-2 mm for Wednesday night and 2-2.5 mm for the following 2 nights.

While there is a possibility for minor boundary layer turbulence for this evening, seeing should settle in near 0.4 arcseconds (or slightly better) as very calm skies and deep subsidence prevails at and above the summit over the next 2 nights. Seeing may trend back toward 0.45-0.5 arcseconds as the upper-level subsidence and westerly flow tapers and increase, respectively, for Wednesday, Thursday and Friday night.

The persistent mid/low-level ridge to the NE will continue to promote steady/strong subsidence in the area, which will easily maintain a well-defined inversion near 7-8 thousand feet and ensure a dry and stable summit-level air mass through the week. In addition, an upper-level ridge building in from the SE will help clear out the high clouds and augment the subsidence over the Big Island, before dissipating as a very weak sub-tropical jet (STJ) gradually builds in overhead beginning Wednesday. This enhanced subsidence will not only increase summit-level temperatures and push PW below 1 mm, but could also allow seeing to settle in near 0.35-0.4 arcseconds as calm skies prevail over the next 2 nights (though there is a slight possibility for minor boundary layer turbulence for early this evening). Seeing my trend back toward 0.5 arcseconds as the upper-level ridge dissipates and gives way to the building STJ for the following 3 nights. There is also a possibility that PW will increase toward 2 mm as the jet transports minor mid/upper-level moisture through the area during that time. Fortunately, the STJ will maintain a fairly zonal orientation, limiting its access to any tropical high clouds, allowing calm skies to prevail during its initial tenure (that could change next week).