Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Friday 20 November 2020

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
Skies will remain clear, but there is a possibility for periods of fog and high humidity, mainly toward the end of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 6 C this afternoon, 1 C this evening and 0.5 C tomorrow morning. Winds will be light and from the east for today, picking up a bit as the night progresses. Seeing will be near 0.65-0.7 arcseconds, while precipitable water is expected to be in the 3-4 mm range for the night.

Discussion
There is a possibility that the inversion will become rather indistinct, which could increase the risk for periods of fog, high humidity and ice at the summit later tonight and the early part of tomorrow night. The inversion is set to recover again on Sunday, allowing dry/stable conditions to return to the summit for that night, but may fall apart again as deep moisture and instability fills into the area, increasing the risk for more fog, high humidity, ice and perhaps light flurries for Monday and Tuesday night. Daytime clouds will be minimal and short-lived for today, then may pick up on Saturday, taper for Sunday, only to possibly turn extensive again for Monday and Tuesday.

Skies will remain predominately clear for tonight, but thin high clouds are set to spread in from the south through tomorrow night. These clouds are expected to thicken and become more widespread on Sunday, likely contributing to mostly overcast skies for the remainder of the forecast period.

Precipitable water is expected to linger in the 3-4 mm range for tonight, slip toward 2.5-3.5 mm for the next 2 nights, then jump back to 4+ mm for Monday and Tuesday night.

Persistent mid-level turbulence, combined with the possibility for an elevated/indistinct inversion will likely contribute mostly poorer than average seeing over the next 2-3 nights. An influx of moisture may further degrade seeing perhaps toward 1 arcsecond for Monday and especially Tuesday night.

While the mid-level ridge will remain rather indistinct and/or displaced further to the north, the surface ridge will persist to the NE, instilling subsidence in the area and allowing deep easterly trades to prevail probably well into next week. Unfortunately, passing upper-level trough are expected to negate some of this subsidence and perhaps destabilize the air mass mainly for much of tomorrow and again beginning Monday afternoon/evening. The former event may weaken the inversion a bit and coupled with the deep trades (low-level forcing) could allow for short periods of fog and high humidity for tomorrow (the odds are small). Although the inversion is set to quickly recover on Sunday ahead of the next trough, a broad sub-tropical jet is expected to develop overhead, bringing widespread broken to overcast clouds to the area between midnight Saturday and probably Tuesday night. Building more organized instability associated with the second trough will eventually to creep toward the Big Island, eroding the inversion late Monday. This will deepen the low-level cloud flow and combined with influx of moisture via the sub-tropical jet could allow the air mass to turn fairly saturated by Tuesday and into Wednesday. Regardless, this will significantly increase the risk for extensive fog, high humidity, ice and light flurries at the summit for Monday and Tuesday night.