Latest Forecast for Maunakea Observatories
10 AM HST (2000 UTC) Wednesday 07 April 2021

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, but there is chance for patches of mid/upper-level clouds passing through the area mainly for this evening.

Summary of Key Meteorological Variables
Summit temperatures will be near 2 C this afternoon and -2 C for the night. Winds will be from the NNW at 20-35 mph for today, easing to 10-20 mph through the night. Seeing will start out near 0.8 arcseconds, but should improve toward 0.6 arcseconds as the night progresses. Precipitable water is expected to start out near 1.5 mm, but will trend toward 0.6 mm through the night.

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure a dry/stable summit-level air mass through Saturday night. There is a chance that the inversion will begin to weaken, while mid-level moisture builds into the area on Sunday, which could increase the risk for short-lived fog, high humidity and/or ice at the summit for that night; precipitation is unlikely. Daytime clouds will be minimal and short-lived through Saturday, but could pick up on Sunday and early part of next week.

There is a chance that patches of mid-level clouds will pass through the area for this evening, but mostly clear skies will prevail for tomorrow morning/night and into Friday evening. Thin high clouds will begin to fill in from the west during the latter night, then could thicken and become more widespread, contributing extensive cloud cover for Saturday and Sunday night.

Precipitable water is expected to start out near 1.5 mm for this evening, but will trend toward 0.6 mm through the night, subtly increase toward 0.7-0.8 for tomorrow night, 1-1.5 mm for Friday night, then jump to 4+ mm for Saturday and Sunday night.

A mixture of low-level and boundary layer turbulence will likely contribute to mostly poorer than average seeing throughout most of the forecast period. There is a slight possibility for periods of average (or slightly better) seeing mainly near sunrise tomorrow and again around Friday evening. An elevated inversion and/or an influx of moisture may contribute to a further degradation in seeing for Sunday night.

Although a quasi-stationary low will persist to the NE probably well into next week, the southern tip of the mid-level ridge will linger in the area and promote strong/steady large-scale subsidence in the area into the weekend. This subsidence will help maintain a well-defined inversion at or below 8 thousand feet and ensure a dry/stable summit-level air mass through early Sunday morning. Unfortunately, a tight wind gradient along the SE flank of the low will linger the Big Island likely keeping winds near 20 mph (or more) throughout most of the forecast period. This will likely stir up low-level and/or boundary layer turbulence and contribute to mostly poorer than average seeing over the next 5 nights. There is a possibility for brief easing of the winds in response to a inward retraction of the low mainly around tomorrow morning and again Friday evening. However, the latter event may be followed by a broadening/strengthening as well as westward progression of the low over the weekend. Initially, this will result in a strengthening of the sub-tropical jet over the area, which could bring in widespread clouds to the area particularly for Saturday night. The jet may deepen vertically as the weekend progresses, which may result in an increase in winds and mid-level moisture early Sunday. This combined with the decay of the ridge in response to the westerd progession of the low could erode the inversion and increase the risk for moisture at the summit for Sunday night.