Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Thursday 20 September 2018

Warnings
Chance for afternoon convection

Cloud Cover and Precipitation Forecast
There is a small risk for fog and high humidity, mainly during the first half of the night; precipitation is unlikely. There is also a possibility that patches of mid/upper-level clouds will pass through/near the summit area throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 7 C this afternoon and 2.5 C for the night. Winds will be from the east at 5-15 mph for today, increasing to 10-20 mph for the night. Seeing will start out near 0.7 arcseconds, but could improve toward 0.55 arcseconds as the night progresses. Precipitable water is expected to exceed 4 mm through the night.

Discussion
Although the air mass is expected to stabilize as the inversion rebuilds (particularly along the eastern half of the Island), lingering instability/moisture may still contribute to the development of afternoon convection along the western slopes, which may allow for periods of fog, high humidity and perhaps light rain at the summit, mainly for the early hours of the night. The inversion will continue to restrengthen near 6-7 thousand feet through the night, and will eventually ensure a dry/stable summit-level air mass for the following 3 nights. There is a possibility that the inversion will lift back toward 12 thousand feet again, increasing the risk for moisture at the summit for Monday night. There is a possibility for extensive afternoon clouds/convection in the area for tonight, then daytime clouds will become minimal and short-lived for Friday and over the weekend, only to possibily pick up again for the early part of next week.

There is a possibility that patches of mid/upper-level clouds will pass through/near the summit area (particularly along the SE skies) throughout the night. More high clouds may begin to gradually spread in from the SW through tomorrow and may contribute to periods of extensive cloud cover, mainly during the second half of that night and perhaps into early Saturday evening. There is a good chance that these clouds will break up a bit through the latter night, opening up skies for much of Sunday and into Monday. However, more small patches of high clouds may drift in from the south and there is a small possibility for patches of summit-level clouds in the area for Monday night.

Precipitable water is expected to linger near or exceed 4 mm through the next 5 nights.

While there is a possibility for poor/variable seeing for the early part of the night, a drying/stabilizing trend should help to dramatically improve seeing as the night progresses. Relatively calm skies at and above the summit combined with deep subsidence the in the free atmosphere should allow seeing to settle in near 0.45-0.55 arcseconds for Friday, Saturday and Sunday night. Another influx of moisture, instability and perhaps boundary layer turbulence may degrade seeing for the early part of next week.

An upper-level ridge building in from the south, combined with a mid-level ridge just east of the Big Island and the low-level ridge to the NE will begin to reinstill strong/steady large-scale subsidence in the area, rebuilding the inversion near 6-7 thousand feet over the next 24 hours. Unfortunately, residual moisture/instability associated with a passing tropical wave, combined with daytime heating may help fire off afternoon convection, particularly along the western and interior slopes of the Big Island, which could allow for the development of evening fog at the summit. The odds on fog will rapidly decline as the sunsets, downslope flow develops and the inversion rebuilds through the night, ensuring dry and stable conditions for at least the following 3 nights. The presence of a relatively deep ridge in the area, should also contribute to relatively calm skies in the free atmosphere and likely allow seeing to settle in near 0.5 arcseconds (occasionally better) over most of the next 4 nights. However, there is a possibility that a fairly deep mid/upper-level low/trough to the west will send patches of high clouds into the area mainly between late Friday evening and Saturday night. Models suggest that this trough/low will slowly progress eastward, drawing tropical moisture into the area around the early part of next week. This could help weaken/erode the inversion and increase the risk for moisture at the summit for that night, but I suspect this is a bit overdone, considering the presence of steady stability provided by the deep ridge. Nonetheless, I did increase the risk for fog/precipitation at the summit for Monday night.