Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 21 January (0300 UTC Friday 22 January) 2021

Warnings
Increasing winds

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while broken mid/upper-level clouds continue to pass over the summit through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -1.5 C this evening and -2.5 C tomorrow morning. Winds will be from the east at 10-35 mph for today, increasing to 25-40 mph through the night. Seeing and precipitable water are expected to linger near 1 arcsecond and 4 mm, respectively, through the night.

Graphical Summary

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and keep the mid/summit-level air mass dry and stable through at least Saturday morning. There is a possibility that an influx of deep moisture will begin to erode the inversion early Sunday morning and eventually allow the atmosphere to turn quite saturated for Sunday and Monday night. This will increase the risk for extensive fog, ice and flurries at the summit for Saturday night and especially the following 2 nights. There is an outside chance for convection in the area and perhaps brief periods of moderate snow mainly for the latter 2 nights. Daytime clouds will be minimal and short-lived through tomorrow, but could pick up a bit on Saturday and turn extensive for Sunday and early part of next week.

Broken mid/upper-level clouds will continue to linger overhead and contribute to extensive cloud cover over the next 2 nights. There is a possibility that these clouds will break up a bit and shift to the east, opening up more than half of the sky for Saturday night, but thick overcast skies are set to build over the summit and blanket summit skies for Sunday and Monday night.

Precipitable water is expected to linger near 4 mm through tomorrow evening, then will trend toward 3 mm through Saturday, only to jump back up to 4+ mm for Sunday and Monday night.

An increase in boundary layer turbulence will contribute to poor/bad seeing probably throughout most of the forecast period. There is also a possiblity that moisture and perhaps instability will further degrade seeing for Sunday and Monday night.

No change since the morning forecast...A tight mid/low-level ridge developing to the NE is set to expand a bit and instill widespread large-scale subsidence in the area, before shifting subtly eastward in response to a low developing to the NW over the weekend. Although the subsidence will help strengthen the inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass for at least the next 2 nights, a tight wind gradient along the SW flank of the ridge will also increase summit-level winds during this time. This will likely contribute to poor/bad seeing, while residual mid-level moisture and clouds will result in high PW and extensive cloud cover over the next 2 nights. There is a chance that cloud cover will improve and winds will diminish a tad as the ridge and clouds slips eastward on Saturday night. However, mid/low-level winds will veer toward the SE and will begin to advect deep moisture toward the low and over the summit area perhaps as early as Sunday morning. Eventually this moisture will erode the inversion and saturate the entire air mass, likely contributing to extensive fog/CC, ice and light flurries at the summit for Sunday and Monday night. There is a remote chance for convection in the area and perhaps moderate/heavy snow, but right now it looks like the bulk of the more organized convection will pass along the southern skies.

Astronomical Observing Quality Guidance
Seeing/CN2 Images are not available today. Sorry for the inconvenience.
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Thu 21 Jan
02 am HST
Fri 22 Jan
02 pm HST
Fri 22 Jan
08 pm HST
Fri 22 Jan
02 am HST
Sat 23 Jan
02 pm HST
Sat 23 Jan
08 pm HST
Sat 23 Jan
02 am HST
Sun 24 Jan
02 pm HST
Sun 24 Jan
02 am HST
Mon 25 Jan
02 pm HST
Mon 25 Jan
02 am HST
Tue 26 Jan
02 pm HST
Tue 26 Jan
06 UTC
Fri 22 Jan
12 UTC
Fri 22 Jan
00 UTC
Sat 23 Jan
06 UTC
Sat 23 Jan
12 UTC
Sat 23 Jan
00 UTC
Sun 24 Jan
06 UTC
Sun 24 Jan
12 UTC
Sun 24 Jan
00 UTC
Mon 25 Jan
12 UTC
Mon 25 Jan
00 UTC
Tue 26 Jan
12 UTC
Tue 26 Jan
00 UTC
Wed 27 Jan
Cloud Cover (%) 70 to 90 70 to 90 60 to 80 60 to 80 50 to 70 60 to 80 40 to 60 20 to 40 60 to 80 80 to 100 80 to 100 80 to 100 80 to 100
Cloud Height (km) above sea level 6.5-9 7-9 7-9 6.5-9 7-8.5 6-8 6-7 6-7 4-8 4-10 4-10 4-10 4-10
Chance for Fog/Precip (%) 0/0 0/0 0/0 0/0 0/0 20/5 25/5 35/10 75/40 90/60 95/75 95/80 95/75
PW (mm, summit upward) 3.5 to 4.5 3.5 to 4.5 4 to 6 3.5 to 4.5 3 to 4 4 to 6 2.5 to 3.5 2.5 to 3.5 3 to 6 6 to 10 6 to 10 8 to 12 8 to 12
Mean Seeing (arcsecs) 0.9 ± 0.2 1.1 ± 0.3 N/A 1.1 ± 0.3 1.0 ± 0.3 N/A 0.9 ± 0.2 0.8 ± 0.2 N/A 1.2 ± 0.3 N/A 1.3 ± 0.4 N/A
Summit Temp (°C) -1.5 -2.5 -0.5 -3.5 -3.5 1 -2 -2 0.5 -3.5 0 -3.5 -0.5
Wind Dir/Speed (mph)
    Summit (615 hPa)
    19,000 ft (500 hPa)
    24,500 ft (400 hPa)
    31,000 ft (300 hPa)
    35,000 ft (250 hPa)
    40,000 ft (200 hPa)
    46,000 ft (150 hPa)
    54,000 ft (100 hPa)

E/20 to 35

E/25 to 40

E/25 to 40

E/25 to 40

ESE/20 to 35

ESE/15 to 30

E/10 to 20

ESE/10 to 20

SE/15 to 30

SE/20 to 35

SE/20 to 35

SE/20 to 35

SSE/15 to 30


 Rise and Set times for the Sun and Moon
Night (HST)
Sun Set
Twilight End
Twilight Beg
Sun Rise
Moon Rise
Moon Set
Illum (%)
RA
Dec
Thu.Jan.21/Fri.Jan.22
18:17
19:24
5:43
6:50
N/A
1:59
64
3 06.8
14 50
Fri.Jan.22/Sat.Jan.23
18:18
19:25
5:43
6:50
N/A
2:51
73
3 54.3
18 40
Sat.Jan.23/Sun.Jan.24
18:18
19:25
5:43
6:49
N/A
3:44
81
4 44.6
21 46
Sun.Jan.24/Mon.Jan.25
18:19
19:26
5:42
6:49
N/A
4:39
88
5 37.8
23 56
Mon.Jan.25/Tue.Jan.26
18:20
19:26
5:42
6:49
N/A
5:33
94
6 33.6
24 55


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Friday 22 January 2021.
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