Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Friday 12 January 2018

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while patches of high clouds linger along the eastern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this afternoon, -1 C this evening and -0.5 C tomorrow morning. Winds will be from the WNW at 10-20 mph for today, switching to a more northerly direction and increasing to 15-30 mph through the night. Seeing will be near 1 arcsecond, while precipitable water is expected to be in the 2-3 mm range for the first half of the night and 1.75-2.25 mm range for the second half.

Graphical Summary

Discussion
The tradewind inversion will continue to rebuild near 7-8 thousand feet as very stable air builds in from the west through the day. This will help ensure the summit steers free of fog and precipitation and allow summit-level humidity to settle in near 20-40% range over the next 5 nights. Some afternoon clouds are possible for today, but will become minimal and short-lived for the remainder of the forecast period.

While skies to the west remain predominately clear throughout the forecast period, the high clouds to the east are expected to briefly retract westward and could contribute to periods of extensive cloud cover for tonight. These clouds will eventually shift off toward the south through tomorrow, leaving predominately clear skies for the remainder of the forecast period.

Precipitable water is expected to start out near 3 mm for this evening, briefly drop toward 2 mm through the night, increase back to 3-4 mm for the following 2 nights, then slip back to 1 mm or less for the early part of next week.

An increase in boundary layer and low-level turbulence will contribute to poor seeing for tonight and perhaps the opening part of tomorrow night. Calmer skies are set to build in during the latter night, allowing seeing to improve toward more average-like values as that night progresses. Seeing will eventually settle in near 0.5-0.6 arcseconds as deep subsidence builds in over the area for Sunday night and early part of next week.

A rather deep ridge building in from the WSW will continue to flatten out and shift the passing trough off toward the NE, allowing strong/steady large-scale subsidence to fill in to the area and persist well into next week. This subsidence will help re-establish a well-defined tradewind inversion at or below 8 thousand feet by this evening and ensure the summit steers free of moisture through the next 5 nights. Unfortunately, a rather tight northerly wind gradient between the departing trough and the building ridge will stir up boundary layer turbulence and contribute to poor seeing for tonight. There is also a possibility for low-level turbulence in the wake of the departing trough, which could linger into tomorrow evening. Winds are set to diminish as the ridge flattens out across most of the Pacific and lingers just north of the state over the remainder of the weekend and early part of next week. This should allow calmer skies to settle in at and above the summit, which will help improve seeing toward better than average values for Sunday night and especially the early part of next week. Lingering mid-level moisture may still keep PW near 3-4 mm for most of the next 3 nights, but PW should slip to 1 mm or less as the deep/strong subsidence associated with the ridge becomes better established in the area for the early part of next week>



Astronomical Observing Quality Guidance
05 pm HST Friday 12 January (03 UTC Saturday 13 January)CN2
08 pm HST Friday 12 January (06 UTC Saturday 13 January)CN2
11 pm HST Friday 12 January (09 UTC Saturday 13 January)CN2
02 am HST Saturday 13 January (12 UTC Saturday 13 January)CN2
05 am HST Saturday 13 January (15 UTC Saturday 13 January)CN2
08 am HST Saturday 13 January (18 UTC Saturday 13 January)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Fri 12 Jan
08 pm HST
Fri 12 Jan
02 am HST
Sat 13 Jan
02 pm HST
Sat 13 Jan
08 pm HST
Sat 13 Jan
02 am HST
Sun 14 Jan
02 pm HST
Sun 14 Jan
08 pm HST
Sun 14 Jan
02 am HST
Mon 15 Jan
02 pm HST
Mon 15 Jan
02 am HST
Tue 16 Jan
02 pm HST
Tue 16 Jan
02 am HST
Wed 17 Jan
00 UTC
Sat 13 Jan
06 UTC
Sat 13 Jan
12 UTC
Sat 13 Jan
00 UTC
Sun 14 Jan
06 UTC
Sun 14 Jan
12 UTC
Sun 14 Jan
00 UTC
Mon 15 Jan
06 UTC
Mon 15 Jan
12 UTC
Mon 15 Jan
00 UTC
Tue 16 Jan
12 UTC
Tue 16 Jan
00 UTC
Wed 17 Jan
12 UTC
Wed 17 Jan
Cloud Cover (%) 40 to 60 60 to 80 60 to 80 40 to 60 20 to 40 0 to 20 0 to 20 0 to 5 0 to 5 0 to 20 0 to 5 0 to 20 0 to 5
Cloud Height (km) above sea level 9-10 8-10 8-10 9-10 9.5-10 N/A N/A N/A N/A N/A N/A N/A N/A
Chance for Fog/Precip (%) 25/0 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 2 to 3 1.75 to 2.25 2 to 4 3 to 4 3 to 4 3 to 4 3 to 4 3 to 4 1 to 2 0.6 to 1 0.8 to 1.2 0.6 to 1
Mean Seeing (arcsecs) N/A 1.0 ± 0.3 1.0 ± 0.3 N/A 0.8 ± 0.2 0.65 ± 0.15 N/A 0.55 ± 0.1 0.55 ± 0.1 N/A 0.55 ± 0.1 N/A 0.5 ± 0.1
Summit Temp (°C) 4.5 -1 -0.5 6 2 1 7 2 1 6.5 1.5 6.5 1.5
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)

WNW/10 to 20

NW/10 to 20

N/15 to 30

NNE/10 to 20

NNE/10 to 20

ESE/5 to 15

ESE/0 to 10

NNE/5 to 15

N/10 to 20

N/10 to 20

NE/10 to 20

E/10 to 20

E/5 to 15


 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
Fri.Jan.12/Sat.Jan.13
18:11
19:19
5:42
6:50
4:07
N/A
11
16 52.3
-17 53
Sat.Jan.13/Sun.Jan.14
18:12
19:20
5:42
6:50
4:56
N/A
6
17 41.2
-19 22
Sun.Jan.14/Mon.Jan.15
18:12
19:20
5:42
6:50
5:45
16:38
3
18 30.7
-20 01
Mon.Jan.15/Tue.Jan.16
18:13
19:21
5:42
6:50
6:32
17:25
0
19 20.4
-19 49
Tue.Jan.16/Wed.Jan.17
18:14
19:21
5:42
6:50
7:18
18:15
0
20 09.9
-18 45


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Friday 12 January (0300 UTC Saturday 13 January) 2018.
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