Current Conditions
Temp4.4 C
RH81 %
WindE 13 mph
RoadOpen (4x4)
Maunakea Observatories Forecast
5 PM HST Friday 24 September (0300 UTC Saturday 25 September) 2021
Moderate/strong winds
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 5.5 C this evening and 5 C tomorrow morning. Winds will be from the ENE at 25-40 mph, while seeing will exceed 1 arcsecond for the night. Precipitable water is expected to be in the 0.6-0.8 mm range for the night.
A very well-defined inversion will continue to cap low-level moisture near 9-10 thousand feet and ensure a dry/stable summit-level air mass through at least late Saturday morning. Deep low-level moisture and widespread (weak) instability are set to pass through the area, eroding the inversion and increasing the risk for fog, high humidity and light rain at the summit between Saturday afternoon and late Sunday night. There is also an outside chance for isolated convection in the area during that time. The inversion is expected to recover near 8-10 thousand feet on Monday and will ensure the summit steers free of fog/precipitation for that night and Tuesday night. Daytime clouds will will turn extensive for the weekend, only to taper again for the early part of next week.

Skies will remain predominately clear of organized high clouds throughout the forecast period. However, there is a chance for patches of low/summit-level clouds passing through, particularly along the southern skies for Saturday and Sunday night. Thin/light cirrus may also begin to develop in the area late Monday night and into Tuesday night.

Precipitable water is expected to slip toward 0.7 mm for tonight, increase to briefly 4+ mm for tomorrow night and perhaps Sunday evening. It will then trend back toward 1 mm through the latter night and drift toward 0.8-0.9 mm for Monday and Tuesday night.

An increase in boundary layer turbulence will contribute to poor/bad seeing through Monday night. There is also a good chance that an moisture, instability and an erosion of the inversion will further degrade seeing mainly for Saturday night. Calmer winds and a more stable air mass should allow seeing to slip back toward 0.55-0.6 arceconds through Tuesday night.

No change since the morning forecast...The mid/surface ridge will continue to sit to the north of the state and promote steady/strong large-scale subsidence in the area throughout most of the forecast period. Initially, this will be enough to maintain a well-defined inversion near 9-10 thousand feet and ensure a dry/stable summit-level air mass through at least late Saturday morning. However, a tighter wind gradient will continue to build over the area as a new ridge fills in from the west and a tropical wave/mid-level low/trough drifts in from the ESE over the next 24 hours. This will result in an increase in summit-level winds, which will stir up boundary layer turbulence and contribute to poor/bad seeing until the trough/low passes to the south and shifts well west of the Big Island early next week. In addition, moisture/instability along the northern flank of this low/wave is now set to make a push over the Big Island between late Saturday afternoon and Monday morning. This will likely contribute to an erosion of the inversion, which will increase the risk for periods of fog, high humidity and light rain at the summit mainly for Saturday night and early Sunday evening. There is also an outside chance that isolated convection will develop along the eastern slopes of the Big Island and within its wake to the west. The moisture will slide westward thereafter by sunrise Monday morning, allowing the inversion to re-strengthen near 9 thousand feet and virtually ensuring dry/stable conditions for that night and Tuesday night. Summit-level winds will begin to dip below 20 mph by Tuesday evening, which should also allow seeing to improve as that night progresses.
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 (%) (Celsius) (Dir/MPH) (Arcseconds) (mm)
Fri Sep 24 - 8 PM0-5Clear0 / 05.5E/25-400.8-1.40.6-0.8
Sat Sep 25 - 2 AM0-5Clear0 / 05E/25-400.8-1.60.6-0.8
2 PM60-804-575 / 258ESE/20-35NaN2-4
8 PM50-704-590 / 601ESE/20-350.8-1.84-8
Sun Sep 26 - 2 AM50-704-695 / 650.5ESE/20-350.8-1.84-8
2 PM60-804-890 / 607.5ENE/15-30NaN4-8
8 PM20-404-560 / 152.5ENE/20-350.7-1.32-4
Mon Sep 27 - 2 AM0-20Clear30 / 52NE/15-300.7-1.11-2
2 PM0-20Clear10 / 07.5ENE/15-30NaN1-2
Tue Sep 28 - 2 AM0-209.5-100 / 02E/15-300.6-10.8-1.2
2 PM0-209.5-100 / 07E/15-30NaN1-1.5
Wed Sep 29 - 2 AM0-209.5-100 / 01E/10-200.475-0.7250.7-1.1
2 PM0-20Clear0 / 06.5ESE/5-15NaN1-2
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
Sat Sep 25 - Sun Sep 26 18:24 19:28 4:59 6:03 21:25 N/A 74 4 13.9 21 01
Sun Sep 26 - Mon Sep 27 18:23 19:27 4:59 6:03 22:07 N/A 65 5 04.1 23 42
Mon Sep 27 - Tue Sep 28 18:22 19:26 4:59 6:03 22:53 N/A 56 5 56.0 25 19
Tue Sep 28 - Wed Sep 29 18:21 19:25 4:59 6:04 23:43 N/A 46 6 49.1 25 46
Wed Sep 29 - Thu Sep 30 18:20 19:24 5:00 6:04 0:36 N/A 36 7 42.8 24 59
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 27 September 2021.
Additional Information
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