Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 13 January (0300 UTC Thursday 14 January) 2021

Warnings
Chance for fog and high humidity

Cloud Cover and Precipitation Forecast
There is a small chance for periods of fog and high humidity throughout the night; precipitation is not expected. Scattered high clouds are also expected to pass along the northern skies and occasionally over the summit through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 1 C, with winds from the north at at 10-20 mph for this evening, easing and switching to more easterly direction through the night. Seeing will be near 0.6-0.7 arcseconds, while precipitable water is expected to be in the 2-3 mm range for the night.

Discussion
The mid-level moisture will continue to disperse and allow the inversion to rebuild, diminishing the risk for fog and high humidity at the summit for tonight. Dry and stable conditions are ensured as the inversion becomes well-established near 7 thousand feet for the following 4 nights. Daytime clouds will be minimal and short-lived through the weekend, but could pick up for the early part of next week.

Scattered high clouds are expected to pass along the northern skies and occasionally over the summit for tonight and early part of tomorrow night. Clear skies will prevail for the remainder of the forecast period.

Precipitable water is expected to slip into the 2-3 mm range for tonight, briefly increase back to 3-4 mm for tomorow night, then settle back in near 2-3 mm for the remainder of the forecast period.

While residual moisture may contribute to slightly poorer than average seeing for this evening, seeing will continue to improve and settle in near average-like values as moderate/strong turbulence prevails in the free atmosphere for tonight. A drying air mass combined with a building inversion and relatively laminar northerly flow at and above the summit should allow seeing to improve toward 0.5-0.55 arcseconds for the following 3 nights. However, an increase in boundary layer and perhaps upper-level turbulence will likely degrade seeing through Sunday night.

Lots have changes have been made since the morning forecast, due to the early departure of the moisture...Mid/low-level moisture intially drawn toward a trough/low passing to the north earlier today, will continue to disperse and now shift eastward, while the mid-level ridge fills back into the area through the night. This will help re-establish large-scale subsidence in the area, which will rebuild the inversion near 7 thousand feet and dry/stabilize the air mass near the Big Island over the next 24 hours. That will help reduce the risk for moisture at the summit, PW back toward 2 mm and likely allow seeing to settle back in near average-like values as moderate/strong turbulence prevails in the free atmosphere for tonight. The inversion will continue to strengthen through tomorrow allowing dry/stable conditions to return to the summit for the remainder of the forecast period, and probably help seeing to settle back in near 0.5-0.55 arcseconds as the upper-level turbulence lifts northward for the weekend. However, a rather tight mid/upper-level low is expected to dig into toward the state and could increase summit-level flow and thus boundary layer turbulence on Sunday night. Previous model runs suggested that this low could also bring instability and moisture to the summit for Monday night.