Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Friday 19 January 2018

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain clear, dry and stable through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4.5 C this afternoon, -1 C this evening and -1.5 C tomorrow morning. Winds will be from the SE at 5-15 mph, with seeing near 0.6 arcseconds. Precipitable water is expected to be in the 0.8-1 mm range for the night.

Graphical Summary

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure a dry/stable summit-level air mass through at least early Saturday evening. There is a chance that building instability combined with persistent low-level moisture will erode the inversion and increase the risk for fog, ice, high humidity and even light flurries at the summit between midnight Saturday and late Wednesday morning. There is also a possibility for isolated convection along the eastern skies during that time. Daytime clouds will be minimal and short-lived through Saturday, but could turn extensive for Sunday and early part of next week.

There is a possibility that patches of high clouds will pass along the southern and northern skies through early Saturday evening. These clouds will shift eastward thereafter, but there is a chance that patches of summit/low-level clouds will begin to develop over the eastern end of the Big Island on Sunday and could persist through Tuesday night.

Precipitable water is expected to linger near 1 mm for tonight, and perhaps early tomorrow evening, trend toward 2 mm through the latter night, and likely exceed 4 mm for Sunday night and early part of next week.

Slightly better than average seeing is expected through tomorrow evening, despite building/moderate turbulence in the free atmosphere. An increase in upper-level instability and eventually low-level moisture will likely degrade seeing through Saturday night and probably contribute to poor/bad seeing for the following 3 nights.

The mid/low-level ridge will continue to slowly slide off toward the NE and weaken as a rather large trough builds in from the west over the next 48 hours. Fortunately, the ridge will still promote fairly strong large-scale subsidence, which will help maintain a well-defined inversion near 8 thousand feet and ensure a dry/stable summit-level air mass through at least early tomorrow evening. However, a residual low-level frontal band currently lingering over/near the Big Island is expected to become enhanced by the building instability associated with the trough after midnight Saturday. This combination will help erode the inversion through Sunday and significantly increase the risk for fog, ice, high humidity and even light flurries at the summit particularly for that night. There is also a possibility for isolated convection along the eastern skies between sunrise Sunday and Monday afternoon. There is a chance that the trough will help push the bulk of this moisture eastward through Monday night, but persistent instability will prohibit the inversion from restrengthening thereafter. In fact, there is also a possibility that this moisture will eventually return westward with the developing trades around the middle/later part of next week. Regardless, an elevated/indistinct inversion will likely allow for more periods of fog, ice and high humidity at the summit for the early part of next week.

Astronomical Observing Quality Guidance
05 pm HST Friday 19 January (03 UTC Saturday 20 January)CN2
08 pm HST Friday 19 January (06 UTC Saturday 20 January)CN2
11 pm HST Friday 19 January (09 UTC Saturday 20 January)CN2
02 am HST Saturday 20 January (12 UTC Saturday 20 January)CN2
05 am HST Saturday 20 January (15 UTC Saturday 20 January)CN2
08 am HST Saturday 20 January (18 UTC Saturday 20 January)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Fri 19 Jan
08 pm HST
Fri 19 Jan
02 am HST
Sat 20 Jan
02 pm HST
Sat 20 Jan
08 pm HST
Sat 20 Jan
02 am HST
Sun 21 Jan
02 pm HST
Sun 21 Jan
08 pm HST
Sun 21 Jan
02 am HST
Mon 22 Jan
02 pm HST
Mon 22 Jan
02 am HST
Tue 23 Jan
02 pm HST
Tue 23 Jan
02 am HST
Wed 24 Jan
00 UTC
Sat 20 Jan
06 UTC
Sat 20 Jan
12 UTC
Sat 20 Jan
00 UTC
Sun 21 Jan
06 UTC
Sun 21 Jan
12 UTC
Sun 21 Jan
00 UTC
Mon 22 Jan
06 UTC
Mon 22 Jan
12 UTC
Mon 22 Jan
00 UTC
Tue 23 Jan
12 UTC
Tue 23 Jan
00 UTC
Wed 24 Jan
12 UTC
Wed 24 Jan
Cloud Cover (%) 10 to 30 20 to 40 20 to 40 20 to 40 0 to 20 0 to 20 60 to 80 20 to 40 20 to 40 60 to 80 10 to 30 60 to 80 20 to 40
Cloud Height (km) above sea level 9.5-10 9-10 8-10 8-9 8-9 N/A 4-8 4-6 4-6 4-8 4-5 4-8 4-6
Chance for Fog/Precip (%) 0/0 0/0 0/0 0/0 0/0 25/5 75/30 75/25 75/25 75/25 50/10 75/25 60/15
PW (mm, summit upward) 0.8 to 1.2 0.9 to 1.1 0.9 to 1.1 1 to 2 1 to 1.5 1.5 to 2.5 4 to 8 4 to 6 4 to 6 4 to 8 4 to 6 4 to 8 4 to 6
Mean Seeing (arcsecs) N/A 0.6 ± 0.1 0.6 ± 0.1 N/A 0.6 ± 0.1 0.7 ± 0.15 N/A 0.9 ± 0.2 0.9 ± 0.2 N/A 0.8 ± 0.2 N/A 0.85 ± 0.2
Summit Temp (°C) 4.5 -1 -1.5 3.5 -1.5 -2 2 -2 -2 2 -2 2.5 -2
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)

SE/5 to 15

SE/5 to 15

SE/5 to 15

SE/5 to 15

ESE/5 to 15

SE/5 to 15

S/5 to 15

SE/0 to 10

SSW/0 to 10

SW/0 to 10

SSW/0 to 10

N/0 to 15

ESE/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.19/Sat.Jan.20
18:16
19:23
5:43
6:50
N/A
20:47
10
22 35.1
-11 00
Sat.Jan.20/Sun.Jan.21
18:16
19:24
5:43
6:50
N/A
21:38
16
23 22.4
-7 14
Sun.Jan.21/Mon.Jan.22
18:17
19:24
5:43
6:50
N/A
22:29
24
0 09.8
-3 06
Mon.Jan.22/Tue.Jan.23
18:18
19:25
5:43
6:50
N/A
23:22
33
0 58.0
1 16
Tue.Jan.23/Wed.Jan.24
18:18
19:25
5:43
6:50
N/A
0:16
44
1 47.5
5 42


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Friday 19 January (0300 UTC Saturday 20 January) 2018.
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