Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Tuesday 12 January 2021

Warnings
Chance for fog and ice

Cloud Cover and Precipitation Forecast
There is a high risk for periods of fog, ice and flurries as summit-level humidity increase 80% or more for the night. Patches of summit/low-level clouds are also expected to spread in from the SE for the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this afternoon, -1 C this evening and -2 C tomorrow morning. Winds will be from the SSW at 10-20 mph, with seeing near 0.8-0.9 arcseconds. Precipitable water is expected to exceed 4 mm for the night.

Discussion
An influx of moisture below 15 thousand feet is set to erode the inversion through the day, raising summit-level humidity to 80% or more and signficantly increasing the risk for periods of fog, ice and light flurries at the summit for the next 2 nights. The air mass is expected to gradually dry out beginning late Thursday morning, which will help to diminish this risk for that night and eventually allow dry/stable conditions to return to the summit as the inversion becomes well-established near 7-8 thousand feet for Friday and Saturday night. Daytime clouds are expected to pick up today, turn extensive with the possibility for afternoon convection for tomorrow, then taper for Thursday and become minimal and short-lived for Friday and over the weekend.

Patches of summit/low-level clouds are expected to spread in from the SE and linger in the area through tomorrow night. These clouds will push back off toward the east and disperse through Thursday, leaving predominately clear skies for the remainder of the forecast period.

Precipitable water is expected to linger near or exceed 4 mm through Thursday night, trend toward 3 mm for Friday night and probably 2 mm for Saturday night.

An increase in moisture and an elevated/indistinct inversion will likely contribute to poor/variable seeing for the next 2 nights. Seeing is set to improve as the air mass dries out through Thursday, and eventually settle in near 0.55-0.6 arcseconds as moderate turbulence prevails in the free atmosphere as early as the second half of Thursday night as well as Friday and Saturday night.

The mid/low-level ridge will continue to move off to the east as a fast-moving developing trough/low develops to the NW digs toward the state. SE winds brodering the incoming trough and departing ridge will begin to advect mid/low-level moisture to the south into the area over the next 6-12 hours. This moisture combined with instability associated with the incoming low/trough will erode the inversion probably by this evening 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 over the next 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 mainly for tomorrow. 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 by that night. In addition, the mid/low-level ridge is projected to return over the state by that evening, re-establish strong widespread subsidence in the area and rebuild the inversion near 7-8 thousand feet over the weekend.