Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 23 July (0300 UTC Friday 24 July) 2015

Warnings
None

Cloud Cover and Precipitation Forecast
There is a slight risk for short-lived fog and/or periods of high humidity, mainly as the night progresses. Thin high clouds may also pass over/near the Big Island through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4.5 C, with winds from the NNE at 5-15 mph and seeing around 0.4-0.45 arcseconds. Precipitable water is expected to be in the 3-4 mm range for the night.

Discussion
The tradewind inversion is expected to lift toward 10-11 thousand feet, which may allow for short periods of fog and high humidity over the next 3 nights (the odds are fairly small though); precipitation is not expected. The inversion is set to recover near 8 thousand feet on Sunday, ensuring dry/stable conditions at the summit for the following 2 nights. Daytime clouds will be minimal and short-lived for today, but could pick up for tomorrow and perhaps Saturday, then taper for Sunday and Monday.

There is a chance that thin high clouds dissipating along the western skies will occasionally pass over/near the summit area through Saturday night. These clouds will shift further westward and/or decay on Sunday, leaving relatively clear skies for the remainder of the forecast period.

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

Minimal turbulence will persist in the free atmosphere, which should allow for better than average seeing through the next 5 nights. There is a possibility for periods of excellent seeing mainly over the next 2 nights.

Although the mid/low-level ridge will continue to sit to the north of the state and an upper-level ridge lingers to the south probably throughout the forecast period, a patch of relatively unstable low-level clouds is expected to pass through the area and lift the inversion toward 10-11 thousand feet over the next 3 nights. This, combined with persistent easterly trades may allow moisture to make its way up the summit and perhaps produce short-lived periods of fog and high humidity during that time. I suspect this moisture/cloud field is overdone and have kept the odds on fog quite low, particularly with a fairly deep ridge in the area. However, westerly winds on the upper portion of the ridge may send decaying high clouds over the summit area during these nights. On the other hand, the extra subsidence provided by this ridge will keep turbulence in the free atmosphere to a minimum and should allow for good/excellent seeing particularly over the next 2 nights. Seeing may deteriorate a tad (but still remain near better than average values) as the upper portion of the ridge slides off toward the SW and allows a zonal sub-tropical jet to redevelop to the north of the state over the weekend. But this should also help take most of the decaying clouds off toward the west and allow for clear skies for Sunday and Monday night.