Latest Forecast for Mauna Kea Observatories
5 PM HST Monday 11 January (0300 UTC Tuesday 12 January) 2021

Warnings
Moderate winds
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a chance for fog, high humidity and perhaps ice, particularly as the night progresses. Broken high clouds will continue linger over the summit, but will slip southward and break up toward the end of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 0 C this evening and -1 C tomorrow morning. Winds will be from the ESE at 20-35 mph for today, switching to a more SSE direction and easing a bit through the night. Seeing will be near 0.8-0.9 arcseconds, while precipitable water is expected to be in the 1.5-2.5 mm range for the first half of the night and 2.5-3.5 mm range for the second half.

Discussion
While the inversion will remain intact for another 12-18 hours, an abrupt influx of mid-level moisture from the south is expected to push summit-level humidity beyond 80% toward the end of tonight and eventually erode the inversion for at least the following 2 nights. This will increase the risk high humidity, fog, ice and light flurries at the summit for the second half of tonight and particularly following 2 nights. The inversion is set to slowly recover beginning Thursday, which will help to diminish this risk for that night and especially Friday night. Daytime clouds could turn extensive with the possibility for afternoon convection over the next 3 days, only to taper again for Friday and over the weekend.

Broken high clouds will continue to linger over/near the summit through at least the first half of the night, but will begin to slip southward and break up through the second half. However, mid/summit-level clouds are set to quickly move in from the south for the following 2 nights, then will disperse around sunrise Thursday, leaving clear skies for the remainder of the following 2 nights.

Precipitable water is expected to start out near 1.5 mm for this evening, but will trend toward 3.5 mm by the end of the night and probably linger near or exceed 4 mm for the remainder of the forecast period (there is a chance that it could dip toward 3 mm again through Friday night).

Boundary layer and low-level turbulence will contribute to poor seeing for tonight. While winds are expected to quickly subside through tomorrow, an influx of mid-level moisture and an elevated/indistinct inversion will prohibit any improvement in seeing until Thursday night. Seeing should return to more normal values as the air mass stabilize/dries and moderate/strong turbulence prevails in the free atmosphere for Friday night.

No change since the morning forecast...The mid/low-level ridge is set to quickly shift off toward the east as a fast moving trough/low develops to the NW of the state through the night. While this will help taper summit-level winds during this time, SE winds along the SW flank of the ridge will begin to advect mid/low-level moisture left behind by a mid-level low to the south of the Big Island toward the end of the night. This moisture combined with instability associated with the incoming low/trough will erode the inversion through tomorrow and allow the atmosphere below 15-16 thousand feet to become mostly saturated through Thursday morning. This will significantly raise the stakes on extensive fog, high humidity, ice and light flurries at the summit later tonight and especially the following 2 nights. While the bulk of the dynamic instability associated with the low/trough will pass to the north, there is an chance that calmer winds at and below the summit will limit mixing and allow for afternoon convection along the Big Island sloes for Tuesday and Wednesday. The lack of winds will also slow the detrainment process, once the trough/low passes to the NE on Thursday, which could allow for more period of fog, high humidity and light ice at the summit for that night. The westerly trough is set to dip further southward on Friday, which will help to further dry out the air mass, especially as the mid/low-level ridge returns over the state for that night.