Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Thursday 12 October 2017

Warnings
Fog, high humidity and ice
Chance for convection and snow/rain

Cloud Cover and Precipitation Forecast
Fog, high humidity, thick clouds and perhaps periods of ice will plague the summit through the night. There is also a chance for convection along the eastern skies and light rain/snow.

Summary of Key Meteorological Variables
Summit temperatures will be near 3 C this afternoon, 1 C this evening and 0 C tomorrow morning. Winds will be from the SSE at 5-15 mph, with seeing near 0.9-1 arcsecond. Precipitable water is expected to exceed 4 mm through the night.

Discussion
The atmosphere near the Big Island will remain quite saturated and unstable, likely allowing high humidity, extensive fog and perhaps periods of ice to plague the summit through at least Sunday evening. There is also a good chance for convection in the area and a mixture of rain and snow at the summit at virtually any time during this period (Friday and Saturday, in particular). The inversion is expected to rapidly recover near 8 thousand feet on Monday, ensuring the summit remains dry and stable for that night. Extensive daytime clouds are expected through the weekend, then will taper for the early part of next week.

Thick clouds will develop along the eastern skies and move in over the summit area contributing to extensive cloud cover for tonight. These clouds will likely become more widespread through tomorrow, likely blanketing skies for Friday and Saturday night, then will shift westward, slowly opening up skies through Sunday night. However, there is still a possibility for thin high clouds streaming along the southern skies for late in the weekend and early part of next week.

Precipitable water is expected to exceed 4 mm through Sunday night, then will slip toward 2 mm for Monday night.

Instability, turbuence and moisture will contribute to inoperable conditions and likely poor/bad seeing through at least Sunday evening. There is a chance for a moderate improvement in seeing through Monday, but persistent shear/turbulence in the free atmosphere will likely keep seeing near 0.6-0.7 arcseconds for that night.

An upper-level trough will continue to deepen to the NW over the next 12-24 hours, then persist in the area before shifting off toward the east as an upper-level trough builds in from the west late Sunday night. Unfortunately, instability associated with the trough combined with an influx of deep low-level moisture supplied by the low-level ridge to north will keep the atmosphere near the Big Island quite saturated through the weekend. This will result in mostly inoperable conditions as extensive fog, high humidity, clouds and periods of ice plague the summit over the next 4 nights. There is also a good chance for convection in the area and a mixture of snow/rain at the summit mainly through Saturday night. The inversion is set to rebuild near 8-9 thousand feet as the trough shifts eastward and allows ridge to north to regain control of the air mass early Monday morning. This help clear out the thick clouds, fog and high humidity, ensuring more normal conditions to return to the summit for that night. However, there is a chance that scattered bands of high clouds will stream along the southern skies, via the sub-tropical jet, for the early part of next week.