Current Conditions
Temp5.0 C
RH9 %
WindNE 0 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
5 PM HST Wednesday 19 July (0300 UTC Thursday 20 July) 2017
Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while scattered high clouds pass over the summit before shifting off toward the SE during the second half of the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 4.5 C for the night. Winds will be light and somewhat variable, with seeing near 0.4-0.45 arcseconds. Precipitable water is expected to be in the 3-4 mm range for the night.
The tradewind inversion will continue to cap low-level moisture at or below 9 thousand feet and ensure the summit remains dry and stable through at least Friday night. There is a small possibility that the inversion will weaken, which could increase the risk for periods of fog and high humidity at the summit for Saturday and Sunday night. Daytime clouds will be minimal and short-lived through Friday, but could pick up over the weekend and early part of next week. There is an outside chance for isolated convection along the slopes during the afternoon hours for Saturday and Sunday.

Scattered high clouds will pass over the summit area for the next 6 hours, then shift off toward the SE, leaving predominately clear skies for at least Thursday and Friday night. There is a possibility that patches of mid/upper-level clouds will stray in from the NE for the following 2 nights.

Precipitable water is expected to linger in the 3-4 mm range for tonight, slip toward (probably the lower half of) the 1.5-2 mm for Thursday and Friday night, then increase back to 2-3 mm for the following 2 nights.

Relatively calm skies will persist at and above the summit, which should allow seeing to settle in near 0.4-0.5 arcseconds through the next 3 nights. However, very light winds may contribute to periods of variability mainly during the second half of tonight and Thursday night. There is a moderate chance that an influx of mid-level turbulence will degrade seeing toward average/poor values over the weekend.

No change since the morning forecast...The quasi-stationary mid/low-level ridge to the north is set to shift subtly toward the east over the next few days, but will continue to promote strong/steady large-scale subsidence in the area well into next week. This subsidence will help maintain the tradewind inversion primarily at or below 9 thousand feet and ensure a dry/stable air mass probably throughout most of the forecast period (there is a possiblity for a couple exceptions, which I'll touch on in a bit). Scattered high clouds will continue to pass over the area via a weak/persistent sub-tropical jet for the next 6 hours, then shift off toward the SE with the jet during the second half of the night. Although the jet is set to decay a bit as it departs, it could aid in slight weakening of the approaching Hurricane Fernanda, which is expected to pass along the southeast fringe of the ridge to the north. Fortunately, the ridge (as alluded to previously) is set to shift off toward the east, which should help steer Fernanda off toward the WNW, allowing it to pass rather quietly 400-500 km to the north of the state over the weekend. Still, there is a chance that patches of clouds/moisture will stray through the area, which could aid in the brief weakening of the inversion and an increase risk for fog/high humidity at the summit and afternoon convection along the slopes during that time. Though even less likely, there is a possibility that pockets of turbulence surrounding the storm could also wander into the area and disrupt seeing for Saturday and Sunday night. Prior to that, very calm skies at and above the summit should allow seeing to continue to settle in near 0.4-0.5 arcseconds (though some variability is possible during periods of extended dead winds, especially during the second half of these nights).
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)
Wed Jul 19 - 8 PM20-408-100 / 04.5NNW/0-100.3-0.53-4
Thu Jul 20 - 2 AM0-208-90 / 04.5NNW/0-100.3-0.63-4
2 PM0-20Clear0 / 09.5NNE/0-10NaN2-3
8 PM0-10Clear0 / 04.5NE/5-150.3-0.51.5-2
Fri Jul 21 - 2 AM0-5Clear0 / 04E/0-100.35-0.651.5-2
2 PM0-20Clear0 / 08ESE/5-15NaN1.5-2.5
8 PM0-5Clear0 / 03.5SE/5-150.35-0.551.5-2
Sat Jul 22 - 2 AM0-5Clear0 / 03SSE/10-200.35-0.551.5-2
2 PM60-804-650 / 108SE/5-15NaN4-8
Sun Jul 23 - 2 AM20-408-925 / 54SSE/5-150.45-0.852-3
2 PM40-604-540 / 108SSW/5-15NaN3-6
Mon Jul 24 - 2 AM10-308-915 / 54S/5-150.45-0.752-3
2 PM40-604-525 / 59SW/5-15NaN2-4
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
Wed Jul 19 - Thu Jul 20 19:13 20:25 4:32 5:44 3:02 N/A 13 5 04.8 17 21
Thu Jul 20 - Fri Jul 21 19:13 20:24 4:33 5:44 4:02 N/A 6 6 06.4 18 38
Fri Jul 21 - Sat Jul 22 19:12 20:24 4:33 5:45 5:04 17:51 1 7 08.7 18 37
Sat Jul 22 - Sun Jul 23 19:12 20:23 4:34 5:45 6:07 18:51 0 8 10.1 17 19
Sun Jul 23 - Mon Jul 24 19:12 20:23 4:34 5:45 N/A 19:46 1 9 09.3 14 53
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Thursday 20 July 2017.
Additional Information
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