Current Conditions
Temp7.9 C
RH14 %
WindWNW 12 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Monday 25 September 2017
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 8 C this afternoon and 3 C for the night. Winds will be from the NW at 10-20 mph, with seeing near 0.55-0.6 arcseconds. Precipitable water is expected to be in the 1-1.5 mm range for the first half of the night and 1.5-2 mm range for the second half.
Discussion
The tradewind inversion will continue to cap the fairly limited supply of low-level moisture at or below 7 thousand feet and ensure the summit remains dry and stable throughout the forecast period. Daytime clouds will be minimal and short-lived throughout the forecast period.

Skies overhead will remain predominately clear of organized clouds through Thursday night. However, there is a small possibility for patches of thin high clouds in the area, particularly to the north for Wednesday and Thursday night. Scattered high clouds may also begin to develop in the area and/or move in from the south on Friday night.

Precipitable water is expected to start out near 1 mm for this evening, but will trend toward 2 mm by the end of the night, ~3 mm for tomorrow night and will eventually top off around 3-4 mm for Wednesday night. It is set to gradually decrease over the following 2 nights, settling in near 1 mm by the end of Friday night.

Diminishing turbulence in the free atmosphere should allow seeing to improve toward 0.45-0.5 arcseconds for tonight and at least the opening half of tomorrow night. However, increasing boundary layer turbulence may begin to degrade seeing during the second half of the latter night and likely contribute to poor seeing for Wednesday and Thursday night. Calmer skies should allow for an improvement in seeing again through Friday night.

A well-defined rather robust mid/low-level ridge to the north of the state will will continue to promote steady/strong large-scale subsidence in the area probably through the week. This subsidence will easily maintain the tradewind inversion at or below 7 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. An upper-level ridge building in from the west will help shuttle the sub-tropical jet and a persistent upper-level low out of the area, and help calm skies over the summit over the next ~36 hours. While this should allow seeing to settle in near 0.45-0.5 arcseconds during this time, winds are set to increase as the upper-level ridge becomes vertically stacked with its low-level counterparts around the middle of the week. This will help increase boundary layer turbulence and probaby contribute to poor seeing for Wednesday and Thursday night. The upper-level ridge is set to rapidly fall apart as a large trough/low develops to the NW of the state begining Friday. This should help decrease summit-level winds, allowing calmer skies to build in aloft and improve seeing for Friday night and probably over the weekend. While this low/trough is not expected to affect the stability of the atmosphere during its tenure, southerly flow along its eastern flank could bring high clouds in from the tropics during that time.
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)
Mon Sep 25 - 2 PM0-20Clear0 / 08NW/10-20NaN1-1.5
8 PM0-5Clear0 / 03NW/10-200.4-0.61-1.5
Tue Sep 26 - 2 AM0-5Clear0 / 03NNW/10-200.35-0.551.5-2
2 PM0-20Clear0 / 09.5N/5-15NaN2-3
8 PM0-5Clear0 / 05NNE/5-150.4-0.61.5-2.5
Wed Sep 27 - 2 AM0-5Clear0 / 04.5NE/10-200.5-0.72.5-3.5
2 PM0-20Clear0 / 08ENE/20-35NaN3-4
8 PM0-20Clear0 / 03ENE/25-400.6-13-4
Thu Sep 28 - 2 AM0-20Clear0 / 03ENE/20-350.7-1.13-4
2 PM0-20Clear0 / 08ENE/20-35NaN2-3
Fri Sep 29 - 2 AM0-20Clear0 / 03ENE/15-300.6-0.91.5-2.5
2 PM10-309-100 / 09E/10-20NaN1-2
Sat Sep 30 - 2 AM20-409-100 / 04E/10-200.45-0.650.8-1.2
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
Mon Sep 25 - Tue Sep 26 18:24 19:28 4:59 6:03 N/A 22:37 34 16 53.6 -18 01
Tue Sep 26 - Wed Sep 27 18:23 19:27 4:59 6:03 N/A 23:22 43 17 43.1 -19 21
Wed Sep 27 - Thu Sep 28 18:22 19:26 4:59 6:03 N/A 0:09 52 18 33.3 -19 50
Thu Sep 28 - Fri Sep 29 18:21 19:25 4:59 6:04 N/A 0:58 61 19 24.0 -19 28
Fri Sep 29 - Sat Sep 30 18:20 19:24 5:00 6:04 N/A 1:49 71 20 15.0 -18 12
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Monday 25 September (0300 UTC Tuesday 26 September) 2017.
Additional Information
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