Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 16 September (0300 UTC Thursday 17 September) 2020
Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while isolated to scattered high clouds pass over and especially along the northern skies through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 8.5 C for today and 3.5 C for the night. Winds will be from the south at 5-15 mph, with seeing near 0.4-0.45 arcseconds. Precipitable water is expected to start out near 1 mm, but increase toward 3 mm by the end of the night.
The tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.
Isolated to scattered high clouds will continue to fill in from the SW, passing over and particularly along the northern skies for the next 3 nights. There is also a chance that a patch of slightly deeper clouds will spread in from the south and pass through the area on Friday. These sets of clouds are expected to shift off toward the NW on Saturday, opening up skies as the weekend progresses.
Precipitable water is expected to start out near 1 mm for this evening, but is expected to increase toward 3 mm by the end of the night and settle in near that value for tomorrow night. It will then slip below 2 mm for Friday night and drop back to 1 mm for the following 2 nights.
Mostly calm skies will prevail in the free atmosphere as weak/laminar SW flow switch to a more southerly direction as the forecast period progresses. This, combined with light summit-level winds and a stable air mass should allow seeing to linger primarily near 0.4 arcseconds over the next 3 nights. There is a chance for a slight increase in summit-level winds, which could contribute to light boundary layer turbulence and a minor degradation in seeing (toward 0.5 arcseconds) over the weekend.
Little change since the morning forecast...Although the mid/low-level ridge remain a bit weak/indistinct to the NE before strengthening over the weekend, an equally weak tropical upper-tropospheric trough will remain displaced to the NW while an upper-level ridge lingers off to the SE. This will allow predominately steady large-scale subsidence to prevail in the area, which will easily negate the minor affects of the TUTT, maintain a well-defined inversion near 7-8 thousand and ensure a dry/stable summit-level air mass throughout the forecast period. There is a chance that the STJ along the SE flank of the TUTT will persist over the next northern half of the state and gain access to mostly scattered high clouds coming out of the south over the next 3 nights. These clouds will disperse and lift off toward the north as the aforementioned upper-level ridge tries to build into the area over the weekend. The NW displacement and relatively weakness of the STJ will also minimize turbulence in the free atmosphere as light SW flow back toward more a southerly direction over the next 3-5 nights. Initially, this combined with a stable air mass and weak summit-level winds should allow seeing to settle in near 0.4 arcseconds over much of the next 3 nights. There is a chance that mid-level flow will begin to increase as the mid-level portion of the ridge strengthens and the upper portion builds in from the SE over the weekend. This could result in a minor increase in boundary layer turbulence, which may contribute to a slight degradation in seeing for Saturday and Sunday night.