Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 20 February (0300 UTC Thursday 21 February) 2019

Warnings
Fog/ice and high humidity
Moderate/strong winds
Chance for light flurries

Cloud Cover and Precipitation Forecast
Extensive fog, ice, high humidity, patches of mid-level clouds and/or light flurries will likely plague the summit through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -1 C this evening and -2 C tomorrow morning. Winds will be from the SW at 35-50 mph for today, diminishing to 30-45 mph through the night. Seeing and PW are expected to exceed 1 arcseconds and 4 mm, respectively, through the night.

Discussion
While the air mass is expected to briefly stabilize as a weak inversion rebuilds near 6-7 thousand over the next 12 hours, the atmosphere between 10-17 thousand feet will remain saturated, contributing to extensive fog, ice, high humidity and periods of light flurries for tonight. The inversion is set to fall apart again tomorrow, allowing the air mass below 15-16 thousand feet to become fairly saturated into Sunday morning. This will not only allow for a high risk for moisture at the summit during that period, but raise the stakes on afternoon convection along the Big Island slopes for Thursday, Friday, Saturday and perhaps into Sunday. The air mass is expected to begin to stabilize and dry as the inversion rebuilds near 8 thousand feet, decreasing the risk for moisture at the summit through Sunday night. Extensive daytime clouds are expected probably through Sunday, then will taper for the early part of next week.

After a brief break and eastward migration of the bulk of the thick clouds, more mid/low-level clouds are set to spread in from the south, contributing to extensive cloud cover for tonight and probably most of tomorrow night. These clouds may shift toward the eastern half of the Big Island thereafter, but there is a possibility for lingering patches of low-level clouds and afternoon/evening clouds probably into Sunday evening. Skies are expected to open up by Monday morning.

Precipitable water is expected to linger near or exceed 4 mm probably through Saturday night, then slip toward 2 mm for Sunday night.

A mixture of boundary layer and free atmospheric turbulence, as well as instability, moisture and/or an elevated inversion will contribute to poor/bad seeing throughout the forecast period.

Little change since the morning forecast...Although the low to the north of the state will continue to lift off toward the NE, allowing the low-level air mass to stabilize a bit, a secondary influx of moisture between 10-17 thousand feet is set to move in from the south, which will contribute to another round of inoperable/saturated conditions for tonight. In addition, a large-scale mid/upper-level trough lingering to the west will slowly shift eastward, bringing colder upper-level air to the area and destabizing the air mass in the process over the next several days. And while winds are set to diminish over the next 12-18 hours, a fairly tight wind gradient along the southern flank of the trough will likely keep winds near 20-30 mph over the remainder of the remainder of the forecast period (perhaps longer). Furthermore, the departing low's residual low-level front (that shift eastward this morning) is set to slip back over the Big Island, allowing the air mass below 16 thousand feet to become fairly saturated between tomorrow afternoon and Sunday morning. This will likely not only allow the inoperable conditions to continue to at the summit during that time, but could raise the stakes on afternoon convection over the Big Island slopes for Thursday, Friday, Saturday, and possibily as the large-scale trough passes overhead around Sunday. Drier more stable conditions should begin to return to the summit as the trough shifts eastward and a new ridge builds in from the west for the early part of next week.