Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 16 May (0300 UTC Thursday 17 May) 2018

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a moderate risk for fog as humidity jumps into the 60-80% range for the night; precipitation is unlikely. A band of scattered/broken high clouds will continue to pass overhead and especially to the south of the Big Island throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 0 C, with winds from the SW at 5-15 mph and seeing near 0.7-0.8 arcseconds. Precipitable water is expected to linger near or exceed 4 mm for the night.

Discussion
Weak instability building passing through the area is expected to erode the inversion and lift the low-level cloud flow toward 12-13 thousand feet, which may increase summit-level humidity toward 60-80% and raise the risk for periods of fog at the summit over the next 2 nights; precipitation is unlikely. The atmosphere is set to quickly stabilize and dry out, allowing the inversion to restrengthen near 8 thousand feet through Friday, ensuring the summit steers free of fog for the weekend. Extensive daytime clouds are possible over the next 2 days, then will taper for Friday and over the weekend.

A band of scattered/broken high clouds will continue to pass overhead and especially along the southern half of the Big Island through the night. This band will eventually shift eastward through tomorrow, leaving isolated patches of high clouds in the area for tomorrow night. Clear skies will prevail for Friday night, but there is a possibility that thin strands of high clouds will fill in from the west for Saturday and Sunday night.

Precipitable is expected to linger near or exceed 4 mm for the next 2 nights, briefly slip toward 1-1.5 mm for Friday night, then increase back to 2-3 mm for the following 2 nights.

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 2 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 night and over the weekend.

Little change since the morning forecast...The mid-level ridge will continue to breakdown, while a trough passes to the far north and a patch of relatively deep low-level moisture fills into the area over the next 36-48 hours. While instability associated with the trough will remain off to the north, a short-wave trough spawned by this larger trough may briefly pass over the area during this period. This short-wave combined with the patch of moisture may help weaken/erode the inversion and allow for periods of high humidity and fog over the next 2 nights; precipitation is unlikely. High clouds flowing in from the west via a rather broad (but relatively weak) sub-tropical jet, may also contribute to periods of extensive cloud cover mainly for tonight. The bulk of these clouds will slide off toward the east with the STJ, opening up skies for that night and especially Friday night. The atmosphere is also set to stabilize as the short-wave slips eastward and a new mid-level ridge builds in from the west on Friday. The ridge will help reinstill large-scale subsidence in the area, which will rebuild a well-defined inversion near 8 thousand feet and ensure the summit steers free of fog/precipitation over the weekend.