Latest Forecast for Mauna Kea Observatories
5 PM HST Friday 15 January (0300 UTC Saturday 16 January) 2021

Warnings
Chance for moisture late in the weekend

Cloud Cover and Precipitation Forecast
The summit will remain clear, dry and stable through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 1 C, with winds from the NE at 15-25 mph for this evening, easing a bit as the night progresses. Seeing will be near 0.6-0.65 arcseconds, while precipitable water is expected to be in the 1.5-2 mm range for the first half of the night and 1-1.5 mm range for the second half.

Discussion
A well-defined inversion will continue to cap low-level moisture near 7 thousand feet and ensure a dry/stable summit-level air mass through at least the next 2 nights. Building instability and an influx of moisture will begin to erode the inversion as early as Sunday afternoon, increasing the risk for extensive fog, ice and flurries for that night and especially the following 2 nights. Convection in the area, periods of heavy snow and perhaps blizzard-like conditions are also possible particularly between sunrise Monday and early Tuesday afternoon. Daytime clouds will be minimal and short-lived through tomorrow, then will pick up on Sunday and turn extensive for the early prt of next week.

Skies will remain predominately clear for the next 2 nights, but broken to overcast thick clouds will begin to fill in from the NW and/or develop in the area late Sunday, then stall out over the summit area for the early part of next week. These clouds will likely contribute to extensive cloud cover for Sunday night and will blanket summit skies for the following 2 nights.

Precipitable water is expected to start out near 2 mm for this evening, but will trend toward 1 mm as the night progresses, only to increase back to 2-3 mm for Saturday night and probably 4+ mm for the following 3 nights.

Despite relatively calm flow in the free atmosphere, a minor increase in boundary-layer turbulence will likely contribute to more average-like seeing for much of tonight. Seeing is set to briefly improve as light flow and a dry/stable air mass prevails at and above the summit for Saturday night. However, an influx of moisture, instability and turbulence will contribute to bad seeing for much of the next 3 nights.

No change since the morning forecast...The mid/low-level ridge will continue to sit over the state and stretch across most of the Pacific as the westerly jet retracts northward and maintains a fairly zonal orientation until early Sunday morning. This will allow strong steady large-scale subsidence to prevail in the area, which will help keep the inversion will established near 7 thousand feet and ensure a clear, dry and stable air mass over the next 2 nights. The northward displacement of the jet will also allow weak turbulence to prevail in the free atmosphere during this time, but there is a chance that summit-level winds will briefly pick up and disrupt seeing for as the ridge slips a tad northward for tonight; an improvement in seeing is likely for Saturday night. Unfortunately a sharp trough is slated to rapidly dig in from the NW and help develop a low with a distinct frontal band on Sunday. The leading edge of the frontal band is set to arrive late Sunday afternoon and will begin to erode the inversion through that night. Deeper moisture will follow suit and combined with cold mid/upper-level temperatures associated with the low will further dismantle the inversion by sunrise Monday, allowing the air mass to become very unstable and saturated as the frontal band stalls out over the Big Island into at least early Tuesday afternoon. This will likely contribute to inoperable conditions/skies as extensive cloud cover/fog, ice and flurries plague the summit for much of Sunday, Monday and Tuesday night. There is a possibility for blizzard-like conditions at the summit as banding convection sweeps through, perhaps desposting heavy snow, while winds may pick up to 30-35 mph between sunrise Monday and late Tuesday morning. While the parent trough of the low is set to depart off toward the NE on Tuesday, the residual low may continue to wobble to the north before lifting northward around Thursday. Latest model suggests that the frontal band may push just eastward by sunrise Wednesday, but may also return westward with the building trades later that night and into Thursday. As such, there is a good chance that poor/wet conditions into the latter part of next week.