Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Thursday 11 October 2018

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a moderate risk for periods of fog and high humidity, mainly toward the end of the night; precipitation is unlikely. Scattered high clouds may also begin to slip in from the west during the second half of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 6 C this afternoon and 1.5 C for the night. Winds will be from the west at 5-15 mph, increasing to 10-20 mph as the night progresses. Seeing will be near 0.55-0.6 arcseconds (with the possibility for some variability during the second half of the night), while precipitable water is expected to linger near or exceed 4 mm through the night.

Discussion
The tradewind inversion will remain weak/elevated as deep low-level moisture and/or upper-level instability persists in the area, which could allow for periods of fog, high humidity and perhaps ice through the weekend. There is also a possibility that deeper mid/upper-level moisture will make a brief push through the area, saturating the environment, further increasing this risk as well as the possibility for light flurries and convection in the area for Friday night. The inversion is set to recover near 8-9 thousand feet on Monday, likely ensuring a dry/stable air mass for that night. Extensive daytime clouds and isolated afternoon convection are also possible through Sunday (particuarly Friday and Saturday), then will taper for Monday.

Scattered high clouds are expected to spread in from the west, passing mainly along the northern skies as the night progresses. Thicker clouds are set to follow suit tomorrow, contributing to extensive cloud cover, if not overcast skies for much that night. These clouds will slowly clear eastward through Saturday, leaving mostly residual patches of mid-level clouds in the area for the early part of that night. Mostly clear skies will prevail for Sunday and Monday night.

Precipitable water is expected to linger near or exceed 4 mm through the next 3 nights, then slip toward 1.5-1.75 mm for Sunday and Monday night.

Relatively laminar westerly flow will likely allow seeing to linger near 0.55-0.6 arcseconds, despite a rather elevated inversion for most of tonight (some variability is possible due to boundary layer turbulence and perhaps moisture during the second half of the night). An increase in moisture, instability and boundary layer turbulence will contribute to bad seeing for Friday night and likely the early part of Saturday night. Improving conditions should help seeing to slip toward more average-like values during the second half of the latter night and eventually settle back in near 0.5-0.6 arcseconds as relatively calm skies prevail for Sunday and Monday night.

While the atmosphere is set to stabilize a tad as a weak upper-level trough shifts eastward in anticipation of another sharp/strong mid/upper-level trough digging in from the NW, a patch of relatively deep low-level moisture lingering in the area will keep the inversion rather elevated near 12 thousand feet for much of tonight. This may allow for short-periods of fog/high humidity at the summit and perhaps isolated afternoon convection, mainly along the eastern and interior slopes for today. The incoming larger trough is expected to drag deeper moisture in from the SW and destabilize the air mass, which will likely erode the inversion and and allow the atmosphere near the Big Island to become mostly saturated between late tomorrow morning and Saturday afternoon. This will probably lead to inoperable conditions, as extensive fog, ice, clouds and light flurries plague the summit for Friday night. There is also a good chance for more convection in the area, which will not be limited to aformentioned slopes/times, but could pop up at anywhere at anytime and drop heavier snow/rain at the summi during this period. The bulk of the moisture is set to slip east of the Big Island by early Saturday evening, but lingering instability may still keep the inversion rather elevated and allow for more periods of fog/high humidity until the air mass stabilizes in response to the slow eastward departure of the large trough late Sunday night.