Current Conditions
Temp3.0 C
WindE 8 mph
Mauna Kea Observatories Forecast
5 PM HST Wednesday 23 September (0300 UTC Thursday 24 September) 2020
Cloud Cover and Fog/Precipitation Forecast
There is a risk for periods of fog and high humidity, mainly during the first half of the night; precipitation is not expected and skies will remain predominately clear.
Summary of Key Meteorological Variables
Summit temperatures will be near 3.5 C this evening and 4 C tomorrow morning. Winds will be from the east at 10-20 mph for much of today, increasing to 15-25 mph through the night. Seeing will be near 0.6-0.7 arcseconds, while precipitable water is expected to be in the 3-4 mm range for the first half of the night and 2-3 mm range for the second half.
There is a risk for periods of fog and high humidity as the inversion briefly lifts toward 13 thousand feet through at least midnight. The inversion is set to quickly recover thereafter, diminshing the odds on fog/high humidity at the summit during the second half of the night and ensure a dry/stable air mass for the remainder of the forecast period. Daytime clouds will become minimal and short-lived for the remainder of the week.

Skies overhead will remain predominately clear of organized clouds throughout the forecast period, but there is a chance for patches of thin nearly transparent cirrus in the area for Saturday and Sunday night.

Precipitable water is expected to start out near 4 mm for this evening, but will slip toward 2 mm by the end of the night, then settle in near or just below 1 mm for the following 4 nights.

While relatively calm skies will prevail in the free atmosphere throughout the forecast period, a slight increase in boundary layer turbulence as well as a possibility for moisture will likely contribute to slightly poorer than average seeing for much of tonight. Dry and stable conditions, combined with lighter summit-level winds should allow seeing to improve through tomorrow night and settle in near 0.4 arcseconds over the weekend.

No change since the morning forecast...A patch of moisture filling in with the rather deep easterly trades will continue to interact with mesoscale instability associated with a weak upper-level low to the NW over the next 3-6 hours. This will keep the inversion rather elevated and may leave lingering periods of fog and high humidity at the summit for this evening. The inversion is set to recover as the low and moisture slip further to the NW and west, respectively, as the night progresses, allowing the mid/low-level ridge to the north to regain control of the air mass through widespread subsidence for the remainder of the week. This will not only ensure dry/stable conditions for the remainder of the forecast period, but an upper-level ridge persisting to the south will keep free atmospheric turbulence to a minimum. This, combined with weaker/laminar easterly flow at and above the summit should allow seeing to settle back in near 0.4-0.5 arcseconds after tonight (light boundary layer turbulence and moisture may negatively impact seeing for much of tonight). The enhanced subsidence associated with the upper-level ridge should also help to squash PW toward 1 mm or less during that period. However, there is a possibility that light, nearly transparent cirrus embedded in the ridge will slowly pass near the summit area late in the weekend.
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 Sep 23 - 8 PM0-5Clear60 / 03.5E/10-200.5-0.83-4
Thu Sep 24 - 2 AM0-5Clear20 / 04E/15-250.5-0.82-3
2 PM0-20Clear0 / 09.5E/10-20NaN1-2
8 PM0-5Clear0 / 04.5ENE/10-200.4-0.60.9-1.1
Fri Sep 25 - 2 AM0-5Clear0 / 04.5NE/5-150.35-0.550.9-1.1
2 PM0-20Clear0 / 010ENE/10-20NaN1-1.5
8 PM0-5Clear0 / 04.5ENE/10-200.35-0.550.8-1
Sat Sep 26 - 2 AM0-5Clear0 / 04ENE/5-150.3-0.50.8-1
2 PM0-20Clear0 / 09E/5-15NaN0.8-1.2
Sun Sep 27 - 2 AM0-209.75-100 / 04ESE/5-150.3-0.50.7-1.1
2 PM0-209.75-100 / 09SE/0-10NaN0.8-1.2
Mon Sep 28 - 2 AM0-209.75-100 / 04SSE/0-100.3-0.50.7-1.1
2 PM0-209.75-100 / 09S/0-10NaN1-1.5
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
Wed Sep 23 - Thu Sep 24 18:25 19:30 4:58 6:02 N/A 0:05 53 18 22.4 -24 42
Thu Sep 24 - Fri Sep 25 18:24 19:29 4:58 6:03 N/A 1:02 63 19 20.7 -24 37
Fri Sep 25 - Sat Sep 26 18:24 19:28 4:59 6:03 N/A 1:58 73 20 17.2 -23 13
Sat Sep 26 - Sun Sep 27 18:23 19:27 4:59 6:03 N/A 2:54 82 21 11.0 -20 40
Sun Sep 27 - Mon Sep 28 18:22 19:26 4:59 6:03 N/A 3:47 89 22 01.9 -17 11
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Thursday 24 September 2020.
Additional Information
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