Latest Forecast for Mauna Kea Observatories
5 PM HST Tuesday 15 May (0300 UTC Wednesday 16 May) 2018

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a risk for fog and high humidity mainly toward the end of the night; precipitation is not expected. Scattered strands of thin high clouds will continue to pass over the summit area throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 1.5 C this evening and 1 C tomorrow morning. Winds will be from the west at 5-15 mph, with seeing near 0.55 arcseconds during the first half of the night, deteriorating toward 0.65 arcseconds and becoming more variable during the second half. Precipitable water is expected to be in the 3-4 mm range for the night.

Discussion
The tradewind inversion is expected to lift/weaken toward 12-13 thousand feet over the course of the night and remain elevated before restrengthening again near 8 thousand feet for Friday and Saturday night. Consequently there is a moderate risk for periods of fog and high humidity at the summit, mainly between midnight and late Friday morning; precipitation is unlikely. Dry and stable conditions should return to the summit for Friday and especially Saturday night. Daytime clouds will be minimal and short-lived for today, but could turn extensive over the next 3 days, then taper again for the weekend.

More scattered high clouds will flow in from the west before shifting east of the Big Island early Thursday morning. While this may open up skies for that morning, there is a possibility for summit-level clouds for that afternoon/evening. These clouds will eventually dissipate on Friday, leaving clear skies for that night and especially Saturday night.

Precipitable water is expected to linger near 3-4 mm for the next 3 nights, slip toward 2 mm for Friday night and probably 1 mm for Saturday night.

While relatively light/calm flow will prevail in the free atmosphere probably throughout the forecast period, an elevated inversion and a risk for moisture at summit-level may result in some degradation and/or variability in seeing over the next 3 nights. Dry and stable conditions are expected to return to the summit, allowing seeing to settle in near 0.5-0.55 arcseconds for Friday and Saturday night.

No change since the morning forecast...A large trough passing to the far north will help decay the mid-level ridge and allow an area focused instability associated with strengthening sub-tropical jet (along its right jet entrance region) to pass over/near the summit area through the night. This instability may help weaken/lift the inversion toward 12-13 thousand feet and increase the risk for moisture at the summit as the night progresses. Scattered high clouds embedded in the jet will also continue to pass overhead until the jet shifts off toward the SE as a weak tropical upper-tropospheric trough (TUTT) develops just west of the state around tomorrow night. While this may open up skies for subsequent nights, the TUTT will maintain the instability in the area, keep the inversion elevated and perhaps allow for more rounds of fog/high humidity at the summit for Wednesday and especially as the TUTT passes directly overhead on Thursday night. In addition, the lifted inversion combined with the possible moisture at the summit, may contribute to poorer than average/variable seeing over the next 3 nights. The atmosphere is set to stabilize and quickly dry out as the TUTT shifts eastward and allows a rather broad mid/low-level ridge to develop to the north of the state on Friday. This should help rebuild the inversion near 6-8 thousand feet, ensure the summit steers free of moisture and dramatically improve seeing as relatively calm skies prevail aloft for Friday and Saturday night.