Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Tuesday 22 May 2018

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while there is a possibility for thin cirrus along the northern and southern skies during the first and second half, respectively, of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 9 C this afternoon, 4 C this evening and 3.5 C tomorrow morning. Winds will be from the WSW at 5-15 mph for today, switching to a more NE direction and increasing to 10-20 mph for the night. Seeing will be near 0.55-0.6 arcseconds, while precipitable water is expected to be in the 3.5-4.5 mm range for the night.

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure the summit remains dry and stable through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

There is a possibility for thin cirrus along the northern half of the sky for this evening, while more high clouds rapidly build in from the south near sunrise tomorrow. This latter set will spread northward through tomorrow, but should remain contained to the western half of the sky for that night. These clouds to the west may spill eastward on Thursday, contributing to periods of extensive cloud cover for that night. There is a chance that these clouds will breakdown a bit for Friday night, but a fresh band of high clouds may spread in from the SW for Saturday night.

Precipitable water is expected to linger near 3.5-4 mm probably through the next 5 nights.

Relatively calm skies will allow for better than average seeing for tonight and probably the first half of tomorrow night. However, increasing turbulence in the free atmosphere could begin to degrade seeing through the latter night and contribute to poor seeing for Thursday night. This source of turbulence is set to subside, allowing seeing to return toward more average-like values for Friday and especially Saturday night.

A relatively deep mid/low-level ridge will continue to sit to the north of the state and promote steady/strong large-scale subsidence in the area into next week. This subsidence will help maintain a well-defined tradewind inverison near 8 thousand feet and ensure the summit remains dry/stable and quite warm throughout the forecast period. An upper-level low/trough is expected to remain quasi-stationary well west of the state (near the dateline) and will have no affect on the stability of the atmosphere. However, it will help restrengthen the sub-tropical jet overhead, which will send mainly scattered high clouds into the area over the next 2 nights. There is a chance that more organized high clouds will pass through as the low digs southward and taps into more widespread cirrus flowing out of the tropics around Thursday night and perhaps even for the following 2 nights. In addition, turbulence along the southern flank of the jet is set to pass overhead, which could negatively impact seeing mainly for Thursday night. Laminar flow within the jet should allow seeing to settle in near 0.55-0.6 arcseconds prior to this night and the aforementioned source of turbulence is expected to slip northward, allowing seeing to improve again for Friday and Saturday night.