Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 22 July (0300 UTC Thursday 23 July) 2015

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, but there is a chance that isolated/scattered thin high clouds will pass over/near the Big Island through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C, with light northerly winds and seeing around 0.4-0.5 arcseconds. Precipitable water is expected to be in the 3.5-4.5 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 9 thousand feet and ensure a dry/stable summit-level air mass through at least Thursday evening. There is a chance that the inversion will breakdown and lift toward 13-14 thousand feet early Friday morning, increasing the risk for fog and light rain at the summit for much of that night. The inversion is set to rebuild again near 8 thousand feet by late Saturday morning, ensuring dry/stable conditions for the remainder of the weekend. Some afternoon clouds are possible for today, taper for tomorrow, only to turn extensive for Friday, then subside again over the weekend.

There is a chance that thin isolated to scattered high clouds will fill in from the southwest and pass over the Big Island or just along the southern skies through tomorrow evening. These clouds will shift further southward by early Friday morning, but there is a possibility for summit-level clouds for that night. Skies overhead will remain clear for Saturday and Sunday night, through there is a chance for thin high clouds along the far northern skies during those night.

Precipitable wate is expected to linger near or exceed 4 mm through the next 5 nights.

Minimal turbulence will persist in the atmosphere, which should allow for better than average seeing through at least tomorrow evening. A brief influx of mid/upper-level turbulence and instability may degrade seeing for Friday night, but seeing is expected to improve again as this source of turbulence diminishes for the remainder of the weekend.

Little change since the morning forecast....The mid/low-level ridge will continue to sito the north of the state well into next week, while an upper-level ridge persists over or just south of the Big Island for the next 48 hours. This deep ridge will promote strong/steady large-scale subsidence to prevail in the area, which will not only help maintain the tradewind inversion and virtually ensure a dry/stable air mass, but also keep turbulence in the atmosphere to a minimum and allow for better than average seeing over the next 2 nights. However, there is a possibility that very thin clouds high embedded in the upper-level ridge will pass over or just south of the Big Island through tomorrow evening. An upper-level low/trough is slated to develop just east of the Big Island and shove the ridge off toward the SW early Friday morning. This trough is expected to ehance a patch of relatively unstable low-level cloud field riding toward the Big Island with the low-level trades through that day. This patch is projected to reach the Big Island early Friday afternoon and could potentially lift the inversion toward 15 thousand feet and briefly increase the risk for fog/rain and clouds at the summit mainly for Friday afternoon/evening. Fortunately, this cloud field is expected to quickly shift westward, allowing the mid-level ridge to dry out and stabilize the air mass for the remainder of the weekend. A rather zonal sub-tropical jet is slated to develop to the north of the state, which will help shift the trough eastward and keep turbulence in the atmosphere to a minimum once again during that time. That should allow for clear skies and better than average seeing to prevail for Saturday and Sunday night.