Latest Forecast for Mauna Kea Observatories
5 PM HST Tuesday 08 October (0300 UTC Wednesday 9 October) 2019

Warnings
None

Cloud Cover and Precipitation Forecast
There is a slight risk for fog toward the end of the night; precipitation is not expected and skies will remain clear.

Summary of Key Meteorological Variables
Summit temperatures will be near 9 C this afternoon, 4.5 C this evening and 3.5 C tomorrow morning. Winds will be light and from the south, increasing to 10-20 mph and switching to a more SE direction through the night. Seeing will start out near 0.4 arcseconds, but will likely degrade toward 0.6 arcseconds toward the end of the night. Precipitable water will also start out near 1 mm, then trend to 2+ mm as the night progresses.

Discussion
While a well-defined tradewind inversion will ensure the summit remains dry and stable for most of tonight, deep moisture will begin to fill into the area and may allow for the development of fog toward the end of the night. Advection of more widespread moisture, combined with minor instability building to the NW will erode the inversion and contribute to extensive fog, ice and light rain/flurries at the summit for Wednesday and probably most of Thursday night. The inversion is set to restrengthen as the moisture slips out of the area through Friday, allowing dry/stable conditions to return to the summit for that night and over the weekend. Extensive daytime clouds are expected over the next 2 days for Wednesday and Thursday, then will become minimal and short-lived for Friday and over the weekend.

Skies will remain predominately clear for tonight, but thick patches of clouds are set to spread in from the SE, contributing to extensive cloud cover for tomorrow night and probably the opening half of Thursday night. There is also a possibility for isolated convection in the area mainly around Thursday afternoon/evening. These clouds will shift off toward the NW through Thursday night, leaving residual very thin high clouds in the area for Friday and Saturday.

Precipitable water is expected to slip toward 1 mm for early this evening, but trend toward 2-3 mm as the night progresses and eventually top 10 mm for the next 2 nights. It will briefly dip back to 1 mm for part of Friday night, then settle in the 2-3 mm range for much of the weekend.

Seeing is expected to start out near 0.4 mm for this evening, but will likely degrade toward 0.6 arcseconds as boundary layer turbulence begins to pick up and moisture fills into the area toward the end of the night. The poor/windy conditions will contribute to bad seeing for Wednesday and Thursday night. Conditions are set to dry out on Friday, but boundary layer turbulence may still contribute to poor/variable seeing for that night and over the weekend.

No change since the morning forecast....A fairly flat but deep ridge will continue to sit over the state, promote large-scale subsidence in the area and help maintain a well-defined inversion near 7-8 thousand feet for another 12 hours. While this will ensure the summit remains dry and stable for most of tonight, a new ridge will quickly form to the NE, while a low develops the NW. The low will remain too far away from the Big Island to really have a big impact on the stability of the atmosphere, but deep southerly flow between the low and the ridge will advect an old tropical wave with widespread moisture into the area between late tomorrow morning and sunrise Friday. This will likely allow the atmosphere to turn quite saturated, contributing to extensive fog, ice and light rain/flurries at the summit for Wednesday and probably most of Thursday night. In addition, a fairly tight wind gradient between the low and ridge will result in an increase in summit winds toward 20-30 mph during that period. The moisture will eventually slip off toward the NW as it is drawn toward the low on Friday, allowing the inversion to rapidly rebuild near 7 thousand feet and dry out the summit air mass for the weekend. However, there is a good chance that winds will remain near 20 mph, as tight wind gradient persists in the area during that time.