Latest Forecast for Mauna Kea Observatories
5 PM HST Monday 22 February (0300 UTC Tuesday 23 February) 2021

Warnings
Chance for fog/ice

Cloud Cover and Precipitation Forecast
There is a risk for periods of fog, ice and high humidity toward the end of the night; precipitation is not expected and skies will remain clear.

Summary of Key Meteorological Variables
Summit temperatures will be near -2.5 C, with winds from the ENE at 10-20 mph for the night. Seeing will be near 0.8 arcseconds, while precipitable water is expected to be in the 1-1.2 mm range for the night.

Discussion
A patch of moisture passing through the area is expected to lift the inversion and increase the risk for periods of fog, ice and high humidity toward the end of tonight. The inversion is set to recover and ensure a dry/stable air mass for Tuesday night and at least the early half of Wednesday night. However, building instability will quickly dismantle the inversion around sunrise on Thursday and allow the air mass below 20 thousand feet to become saturated, significantly increasing the risk for extensive fog/ice, high humidity and flurries at the summit for that night and Friday night. Daytime clouds could pick tomorrow, taper for Wednesday, then turn extensive with the possibility for isolated convection for Thursday, Friday and into the weekend.

Skies will remain predominately clear for tonight, but high clouds may approach along the northern skies for tomorrow night and could contribute to periods of extensive cloud cover for Wednesday night. These clouds will breakdown on Thursday, but there is a good chance that patches of mid/summit-level clouds will develop in the area for that night and Friday night.

Precipitable water is expected to linger in the 1-1.2 mm range for the next 2 nights, slip just below 1 mm for most of Wednesday night, then jump to 4+ mm for the following 2 nights.

A sharp vertical thermal gradient just above the summit will contribute to poor seeing for tonight. Calmer skies with light westerly upper-level flow should allow seeing to improve and settle in near 0.5-0.55 arcseconds for Tuesday and Wednesday night. However, an increase in instability and moisture will probably result in bad seeing for Thursday and Friday night.

Little change since the morning forecast...Although the mid/low-level ridge will continue to sit to th enorth of the state and promote large-scale subsidence in the area through at least Wednesday night, a residual patch of deep moisture (likely a remnant cold front) is set to the pass through around sunrise tomorrow. This patch could briefly lift the inversion toward 12-13 thousand feet and increase the risk for fog, ice and high humidity at the summit around that time. And while PW will remain near 1 mm for much of the next 3 nights, a sharp thermal gradient in the first 1-2 km above the summit will likely contribute to poor seeing for tonight. Seeing is set to improve and settle in near 0.5-0.55 arcseconds as this gradient subsides and deep/laminar westerly flow prevails in the free atmosphere for Tuesday and Wednesday night. An upper-level trough is expected to move in from the west, while a weak mid-level low forms to the south beginning early Thursday. The latter will help bring deeper moisture to the area and shove the ridge northward, while the former will destabilize the air mass until passing eastward around Friday night. However, there is a chance that another broader trough will take its place to the west of the state over the weekend. Nevertheless, this will likely result in a quick erosion of the inversion, which will allow the atmosphere below 20 thousand feet to become quite saturated and significantly increasing the risk for extensive fog, ice and flurries at the summit for Thursday and Friday night. Isolated convection (mainly along the eastern slopes) and short periods of moderate/heavy snow cannot be entirely ruled out during that time as well.