Current Conditions
Temp9.9 C
RH27 %
WindENE 11 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Tuesday 29 September 2020
Warning(s)
None
Cloud Cover and Fog/Precipitation Forecast
The summit will remain clear, dry and stable through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 10 C this afternoon and 5 C for the night. Winds will be from the ESE at 5-15 mph, with seeing near 0.4 arcseconds. Precipitable water is expected to be in the 2-3 mm range for the night.
Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture near 8-9 thousand feet and ensure a dry/stable summit-level air mass for at least tonight. There is a chance that the inversion will begin to weaken as tropical moisture fills into the area, increasing the risk for periods of fog and high humidity at the summit for Wednesday night and especially the following 2 nights; precipitation is unlikely. The inversion is set to quickly recover again on Saturday, allowing dry/stable conditions to return to the summit for that night. Minor daytime clouds are possible for today, and could turn extensive with the possibility of afternoon convection for Wednesday, and especially Thursday and Friday, only to taper again over the weekend.

Skies will remain predominately clear for tonight, but there is a chance that summit-level clouds will begin to develop in the area and/or slip in from the east between midnight Wednesday and Friday night. High clouds may also start to build in the area and drift in from the south late Thursday night and could contribute to extensive cloud cover for the following 2 nights.

Precipitable water is expected to linger in the 2-3 mm range for tonight, increase to 3-4 mm for tomorrow night, exceed 4 mm for Thursday and Friday night, then slip back to 2 mm for Saturday night.

Calm skies with light/laminar flow at and above the summit should allow seeing to settle in near 0.4 arcseconds for tonight and perhaps the opening part of tomorrow night. However, an influx of moisture may begin to erode the inversion and degrade seeing through the latter night and especially Thursday and Friday night. While conditions are set to improve on Saturday, an increase in boundary layer turbulence may prevent seeing from improving much for that night.

A fairly indistinct mid/low-level ridge will continue to sit almost directly over the state before rebounding northward and strengthening over the weekend. Initially, subsidence associated with the ridge will help maintain a well-defined inversion near 8-9 thousand feet and ensure a dry/stable air mass for tonight. Very light winds embedded in the ridge, combined with the absence of the sub-tropical jet overhead will allow for clear/calm skies and likely excellent seeing for tonight and perhaps even tomorrow evening. However, the ridge is expected to become more undefined, while tropical moisture begins to fill into the area through the latter night. This will result in an erosion of the inversion, which will increase the risk for fog and high humidity at the summit and contribute to degradation in seeing during that time, and especially for Thursday and Friday night. Daytime heating could also help spark afternoon convection along the Big Island slopes as early as tomorrow, but again especially for the Thursay and Friday. The inversion is set to quickly recover as the bulk of the moisture slides westward and the ridge rebounds northward around Saturday morning. While this should help to dry out the summit, stronger easterly flow along the southern flank of the ridge will stir up boundary layer turbulence and may prevent seeing from improving for that night.
WRF Astronomical Observing Quality Guidance
Cloud Cover and Precipitable Water Analyses
MK CN² Profiles
5 Day Forecast Summary (Graphical Trend)
HST Cloud Fog/Precip Temp Wind Seeing PW
Cover (%) Height (km) Probability (%) (Celcius) (Dir/MPH) (Arcseconds) (mm)
Tue Sep 29 - 2 PM20-404-4.525 / 010ESE/5-15NaN2-4
8 PM0-5Clear0 / 05ESE/5-150.3-0.52-3
Wed Sep 30 - 2 AM0-5Clear0 / 05ESE/5-150.3-0.52-3
2 PM60-804-550 / 108S/0-10NaN3-6
8 PM0-20Clear25 / 53.5ESE/0-100.3-0.63-4
Thu Oct 01 - 2 AM0-204-4.540 / 103SE/5-150.45-0.753-4
2 PM80-1004-690 / 606SSE/5-15NaN4-8
8 PM20-404-565 / 202.5ESE/0-100.5-0.96-10
Fri Oct 02 - 2 AM40-604-875 / 252E/0-100.7-1.16-10
2 PM80-1004-1090 / 655ESE/0-10NaN10-12
Sat Oct 03 - 2 AM60-804-1075 / 251NE/10-200.6-14-8
2 PM20-408-1025 / 58E/10-20NaN2-4
Sun Oct 04 - 2 AM60-808-100 / 04ESE/15-250.6-11.5-2.5
Rise and Set times for the Sun and Moon
Night (HST) Sun Set Twilight End Twilight Beg Sun Rise Moon Rise Moon Set Illumination (%) RA DEC
Tue Sep 29 - Wed Sep 30 18:20 19:24 5:00 6:04 17:14 5:28 98 23 36.4 -8 22
Wed Sep 30 - Thu Oct 01 18:19 19:23 5:00 6:04 17:48 6:17 100 0 21.1 -3 26
Thu Oct 01 - Fri Oct 02 18:18 19:22 5:00 6:04 18:21 7:05 100 1 05.1 1 34
Fri Oct 02 - Sat Oct 03 18:17 19:21 5:00 6:05 18:54 N/A 98 1 49.0 6 30
Sat Oct 03 - Sun Oct 04 18:16 19:21 5:01 6:05 19:28 N/A 94 2 33.7 11 09
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Tuesday 29 September (0300 UTC Wednesday 30 September) 2020.
Additional Information
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