Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 09 August (0300 UTC Friday 10 August) 2018
Chance for fog/high humidity
Possibility for afternoon convection
Cloud Cover and Precipitation Forecast
There is a moderate/high risk for high humidity and fog through the night; precipitation is unlikely. Thin high clouds will continue to fill in from the SW, contributing to extensive cloud cover through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 3 C this evening and 2.5 C tomorrow morning. Winds will be from the SE at 5-15 ph for today, switching to a more ENE direction for the night. Seeing will be near 0.8-0.9 arcseconds, while precipitable water is expected to exceed 4 mm for the night.
Humidity is expected to linger in the 60-80% range as the inversion remains rather weak/indistinct, which could allow for periods of fog at the summit mainly for tonight. The inversion is set to restrengthen near 6-7 thousand feet, diminishing this risk through tomorrow and likely ensuring dry/stable conditions at the summit for at Friday, Saturday and most of Sunday night. However, there is a possibility that inversion will begin to fall apart as deep moisture and instability fills into the area early Monday morning, increasing the risk for fog, high humidity, ice and light rain/flurries at the summit for that night. Extensive daytime clouds and possibily isolated afternoon convection are expected for today, then will taper for tomorrow and especially over the weekend, only to pick up again for the early part of next week.
Thin strands of cirrus exhaust will continue to drift in from the SW, contributing to periods of extensive cloud cover for tonight. These clouds will shift off toward the west, allowing predominately clear skies to prevail for Friday and Saturday night. However, more widespread high clouds are set to slip in from the SE through Sunday, eventually thickening on Monday, likely resulting in overcast skies for much of that night.
Precipitable water is expected to exceed 4 mm for tonight, slip into the 2-3 mm range for tonight, Saturday night and most of Sunday night, then increase back to 4+ mm for Monday night.
Residual mid/low-level moisture will contribute to poor seeing for tonight. A dry/stable and relatively calm air mass should allow seeing to improve and eventually settle in near 0.5-0.6 arcseconds for the following 3 nights. Another round of moisture, combined with a possibility of boundary layer and/or upper-level turbulence may degrade seeing once again for the early part of next week.
Little change since the morning forecast...Residual moisture flowing in along the eastern flank of departing Hector will keep the inversion from rebuilding and may allow for humidity to linger near 60-80% and for the development of more fog mainly through midnight. The air mass is expected to rapidly dry out and stabilize as Hector shifts further westward and the mid/low-level ridge to the NE regains control and instills strong large-scale subsidence in the area through tomorrow. This subsidence and the presence of a strong inversion near 6-7 thousand feet will ensure the summit steers free of fog/precipitation and high humidity for much of the weekend. However, the tropical upper-tropospheric trough and an embedded low are expected to the east and slowly retrograde westward during this time. Initially, neither will have an impact on the stability of the atmosphere, and may actually promote clear/calm skies for Friday and Saturday night. That could begin to change as the trough's axis moves west of the Big Island, switching upper-level flow to a more SSW direction, which could bring high clouds in from the tropics through Sunday. The TUTT may also start to destabilize the air mass later that night and may also allow low-level winds to switch to a more SE direction and bring deeper tropical moisture into the area for the early part of next week. This combination of widespread instability and deep tropical moisture may erode the inversion and increase the risk for fog/high humidity, ice and rain at the summit during that time.