Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Wednesday 06 December 2017

Warnings
Moderate/strong winds

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while high clouds continue to pass along the northern skies and occasionally overhead for tonight.

Summary of Key Meteorological Variables
Summit temperatures will be near 0 C this afternoon, -4 C this evening and -3 C tomorrow morning. Winds will be from the WNW at 35-50 mph for today, switching to a more NNW direction and easing to 25-40 mph as the night progresses. Seeing will exceed 1 arcsecond, while precipitable water is expected to increase to the 3-4 mm range for the night.

Graphical Summary

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 6-7 thousand feet and ensure the summit remains dry and stable through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Scattered to broken high clouds passing along to the north will continue to drop down and quickly pass overhead and/or scrape the northern skies for tonight. There is also an outside chance for lenticular clouds along the Big Island slopes mainly through this evening. The high clouds are expected retreat further northward, opening up skies again for tomorrow night. However, there is a good chance that these clouds will sag southward on Friday, and periodically pass overhead during the next 3 nights.

Precipitable water is expected to increase to 3-4 mm for tonight and probably tomorrow evening, slip toward 2.5-3 mm for the much of Friday and probably 1 mm for Saturday night, then increase back to 2-3 mm for Sunday night.

Strong boundary layer turbulence will contribute to bad seeing for tonight. Winds are set to abruptly decline through tomorrow, allowing seeing to settle back in near 0.6 arcseconds for most of tomorrow night and into Friday evening. However, another round of boundary layer turbulence will start to degrade seeing during the latter night and contribute poor/bad seeing for Saturday and Sunday night.

The eastern edge of the mid/low-level ridge will continue to sit over or just north of the state and promote steady/strong large-scale subsidence in the area through the week. This subsidence will help maintain a well-defined inversion near 6-7 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Unfortunately, a tight wind gradient between the ridge to the NW and trough to the NNE will continue to stir up boundary layer turbulence and contribute to bad seeing for tonight. High clouds flowing out of the westerly jet far to the north and along the western flank of the trough will also continue to pass over and/or along the northern skies for tonight. Winds are set to rapidly diminish and the high clouds will retreat northward as the trough weakens and lifts off toward the NE through tomorrow. While that should allow seeing to improve and clear skies to prevail for most of that night and into Friday night, a new trough is expected to dig in to the north during the latter night. This trough is not expected to have much of an affect on the stability of the atmosphere, but could could bring more high clouds to the area and increase winds again for much of the weekend.

Astronomical Observing Quality Guidance
05 pm HST Wednesday 06 December (03 UTC Thursday 07 December)CN2
08 pm HST Wednesday 06 December (06 UTC Thursday 07 December)CN2
11 pm HST Wednesday 06 December (09 UTC Thursday 07 December)CN2
02 am HST Thursday 07 December (12 UTC Thursday 07 December)CN2
05 am HST Thursday 07 December (15 UTC Thursday 07 December)CN2
08 am HST Thursday 07 December (18 UTC Thursday 07 December)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Wed 06 Dec
08 pm HST
Wed 06 Dec
02 am HST
Thu 07 Dec
02 pm HST
Thu 07 Dec
08 pm HST
Thu 07 Dec
02 am HST
Fri 08 Dec
02 pm HST
Fri 08 Dec
08 pm HST
Fri 08 Dec
02 am HST
Sat 09 Dec
02 pm HST
Sat 09 Dec
02 am HST
Sun 10 Dec
02 pm HST
Sun 10 Dec
02 am HST
Mon 11 Dec
00 UTC
Thu 07 Dec
06 UTC
Thu 07 Dec
12 UTC
Thu 07 Dec
00 UTC
Fri 08 Dec
06 UTC
Fri 08 Dec
12 UTC
Fri 08 Dec
00 UTC
Sat 09 Dec
06 UTC
Sat 09 Dec
12 UTC
Sat 09 Dec
00 UTC
Sun 10 Dec
12 UTC
Sun 10 Dec
00 UTC
Mon 11 Dec
12 UTC
Mon 11 Dec
Cloud Cover (%) 60 to 80 40 to 60 50 to 70 30 to 50 10 to 30 0 to 20 0 to 20 10 to 30 30 to 50 20 to 40 30 to 50 10 to 30 20 to 40
Cloud Height (km) above sea level 6-10 7-9 7-10 9-10 9.5-10 N/A N/A 9.5-10 8-10 9-10 9-10 9-10 9-10
Chance for Fog/Precip (%) 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 2 to 4 3 to 4 3 to 4 4 to 6 3 to 4 2.5 to 3.5 2 to 3 2 to 3 2.5 to 3.5 1 to 2 0.8 to 1.2 1 to 2 2 to 4
Mean Seeing (arcsecs) N/A 1.5 ± 0.5 1.5 ± 0.5 N/A 0.65 ± 0.15 0.6 ± 0.1 N/A 0.55 ± 0.1 0.7 ± 0.2 N/A 1.2 ± 0.3 N/A 1.0 ± 0.3
Summit Temp (°C) 0 -4 -3 5 1 1 7 2 1.5 5 0 6 1
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)

WNW/35 to 50

NW/35 to 50

NNW/25 to 40

NNW/15 to 30

NNW/10 to 20

N/10 to 20

WSW/5 to 15

WNW/5 to 15

W/15 to 30

WNW/30 to 45

NW/25 to 40

WNW/20 to 35

WNW/20 to 35


 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
Wed.Dec.06/Thu.Dec.07
17:52
19:01
5:26
6:35
21:16
N/A
80
8 44.8
17 18
Thu.Dec.07/Fri.Dec.08
17:52
19:01
5:27
6:35
22:18
N/A
70
9 43.3
14 15
Fri.Dec.08/Sat.Dec.09
17:53
19:01
5:27
6:36
23:17
N/A
59
10 38.2
10 30
Sat.Dec.09/Sun.Dec.10
17:53
19:02
5:28
6:36
0:13
N/A
48
11 29.8
6 19
Sun.Dec.10/Mon.Dec.11
17:53
19:02
5:28
6:37
1:07
N/A
38
12 19.0
1 58


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Wednesday 06 December (0300 UTC Thursday 7 December) 2017.
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