Current Conditions
Temp-1.9 C
RH70 %
WindWSW 7 mph
Mauna Kea Observatories Forecast
5 PM HST Friday 17 February (0300 UTC Saturday 18 February) 2017
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 2 C this evening and 1 C tomorrow morning. Winds will be from the SSE and increase to 15-30 mph, with seeing near 0.7 arcseconds. Precipitable water is expected to be in the 0.9-1.1 mm range for the night.
The tradewind inversion will continue to cap low-level moisture at or below 7 thousand feet and ensure the summit remains dry and stable through at least Saturday afternoon. There is a possibility that the inversion will briefly weaken as instability and moisture rolls through, increasing the risk for fog, ice and high humidity mainly for second half of Saturday night. While the inversion is set to recover by Sunday evening, patches of mid-level moisture/clouds may move in, which could still allow for short periods of fog, high humidity and ice for that night and perhaps the early part of next week; precipitation is unlikely throughout the forecast period. Daytime clouds will be minimal and short-lived through Saturday, but could turn extensive for Sunday, then taper a bit for Monday and Tuesday.

Skies will remain predominately clear for tonight and probably the opening half of tomorrow night. A band of clouds will start to organize along the northern skies during the latter night, and make a push southward on Sunday. The bulk of these clouds will dissipate prior to reaching the Big Island, but there is a possibility that patches of mid-level clouds will fill in overhead for Sunday night, then linger along the eastern and/or southern skies for the following 2 nights. There is also a chance that a batch of thick clouds will begin to reorganize along the southern skies through Tuesday night.

Precipitable water is expected to linger near 1 mm for tonight, increase to 2 mm for tomorrow night and probably 4 mm for Sunday night. There is a fairly good chance that it will slip back into (probably the lower half of the) 1-2 mm range for Monday night, only to increase toward 3 mm for the following night.

An increase in boundary layer turbulence will contribute to poorer than average seeing for the next 2 nights. Patches of upper-level turbulence is also set to build in overhead for Saturday night, with stronger pockets of mid-level turbulence passing through for Sunday night. This could further degrade seeing for those two nights. The mid-level turbulence is expected to dissipate for the early part of next week, but more uppper-level turbulence will build in overhead, limiting much improvement in seeing for those nights.

Little change since the morning forecast...A large trough building to the NW is expected to push the mid-level ridge off toward ESE, tightening the mid-level wind gradient in the area over the next 2 nights. Although this will likely result in a bump in summit-level winds, which could disrupt seeing during that time, subsidence associated with the ridge will still prevail ensuring a dry and stable summit-level air mass through at least tomorrow afternoon. Unfortunately, the trough will begin to destabilize the air mass over the following 24 hours, which could erode the inversion, and increase the risk for moisture at the summit through Saturday night. There is a good chance that the trough will send a decaying cold front through the Big Island on Sunday, but a strong trailing low-level ridge is expected to rapidly fill in from the west and re-establish the subsidence in the area by that evening. While this should help to quickly rebuild the inversion, patches/bands of mid-level moisture/clouds will also pass through, which could allow for more short periods of high humidity, fog and/or ice at the summit for at least Sunday night. There is a very good chance that the patches/band of moisture will decay and shift off toward the SE of the Big Island late Monday. The latest model runs still suggests that this band will reorganize, but keeps it further south of the Big Island as another upper-level trough passes to the north on Tuesday/Wednesday. It still difficult to say if this band will play a role in upcoming conditions for next week, I feel pretty confident that the presense of a strong westerly jet aloft and its associated turbulence will likely contribute to poor seeing for the second half of the forecast period as well.

Please Note: There will be no forecasts on Monday, February 20, in observance of Presidents' Day. The normal forecast schedule will resume on Tuesday, February 21.
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)
Fri Feb 17 - 8 PM0-5Clear0 / 02SSE/15-300.6-0.80.9-1.1
Sat Feb 18 - 2 AM0-5Clear0 / 01S/15-300.6-0.80.9-1.1
2 PM0-20Clear0 / 05SSW/15-30NaN1-2
8 PM0-10Clear25 / 0-1SW/15-300.6-0.91.75-2.25
Sun Feb 19 - 2 AM0-20Clear75 / 15-2SW/15-300.6-11.75-2.25
2 PM40-604-560 / 15-1WSW/10-30NaN3-6
8 PM20-406-740 / 10-4WSW/10-200.7-1.33-5
Mon Feb 20 - 2 AM30-505-730 / 10-5W/10-200.7-1.13-5
2 PM10-304-4.520 / 0-2WNW/5-15NaN2-4
Tue Feb 21 - 2 AM0-205.5-610 / 0-6E/5-150.6-11-2
2 PM10-304-520 / 0-1W/5-15NaN2-4
Wed Feb 22 - 2 AM10-305-725 / 5-6NNW/5-150.6-12-4
2 PM0-20Clear10 / 00NE/10-20NaN1-2
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
Sun Feb 19 - Mon Feb 20 18:32 19:37 5:34 6:39 1:57 N/A 34 17 18.9 -18 08
Mon Feb 20 - Tue Feb 21 18:33 19:38 5:33 6:38 2:47 N/A 26 18 09.0 -18 53
Tue Feb 21 - Wed Feb 22 18:33 19:38 5:33 6:37 3:36 N/A 18 19 00.2 -18 48
Wed Feb 22 - Thu Feb 23 18:34 19:38 5:32 6:37 4:24 N/A 11 19 52.0 -17 50
Thu Feb 23 - Fri Feb 24 18:34 19:39 5:31 6:36 5:12 N/A 5 20 44.2 -15 59
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 20 February 2017.
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