Latest Forecast for Mauna Kea Observatories
800 AM HST (1800 UTC) Friday 21 July 2017
Warnings
Chance for fog/high humidity
Cloud Cover and Precipitation Forecast
There is a possibility for periods of fog, high humidity and perhaps summit-level clouds through the night; precipitation is unlikely.
Summary of Key Meteorological Variables
Summit temperatures will be near 5 C this afternoon, 2.5 C this evening and 2 C tomorrow morning. Winds will be from the SE at 5-15 mph, with seeing near 0.7-0.75 arcseconds. Precipitable water is expected to be in the 3-4 mm range for the night.
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Graphical Summary
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Discussion
The tradewind inversion will remain weak/elevated near 11-12 thousand feet, which may allow for periods of fog and high humidity through the weekend, particularly for tonight; precipitation is unlikely. The inversion is set to recover near 6-7 thousand feet, ensuring a dry/stable air mass for Monday and Tuesday night. Extensive daytime clouds and afternoon convection along the slopes are possible through the weekend, then will become minimal and short-lived for the early part of next week.
Skies will remain predominately clear of organized clouds throughout the forecast period. However, there is a possibility for summit-level clouds mainly for tonight. Small patches of cirrus may also move quickly in from the south and pass overhead for Saturday and Sunday night.
Precipitable water is expected to linger in the 3-4 mm range through Sunday evening, trend toward 2 mm for later that night, then settle in near 1-1.5 mm for the early part of next week.
An influx of low-level turbulence will contribute to poorer than average seeing for tonight. Seeing is set to gradually improve as this source of turbulence subsides through the weekend, and will eventually settle back in near 0.5 arcseconds as relatively calm skies prevail for at least Monday night. There is a chance that boundary layer turbulence will disrupt seeing again for Tuesday night.
Although Tropical Storm Fernanda is set to pass more than 300 km to the north of the state and along the SE flank of mid/low-level ridge to the NE, deep low-level moisture out surrounding (particularly to the east of) the storm will keep the inversion rather weak/elevated probably through the weekend. This could allow for periods of fog and high humidity at the summit during this time, tonight in particular. The bulk of this moisture is actually expected to shift just west of the Big Island by tomorrow evening, then slowly detrain into the surrounding air mass as the weekend progresses. This could help to decrease the risk for fog/high humidity at the summit for Saturday and especially Sunday night. Still, lingering moisture and minor instability may also allow for the development of afternoon convection along the slopes and extensive daytime clouds through Sunday. A drier more stable air mass is expected to return to the summit as the persistent ridge to the NE regains control of the atmosphere for the early part of next week. However, there is a possibility that easterly winds will strengthen toward 20 mph for the middle of the week. There is also a chance that the remnants of what looks to be Tropical Storm Greg could bring another round of moisture to the summit late next weekend.
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