Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Tuesday 02 January 2018

Warnings
Increasing winds

Cloud Cover and Precipitation Forecast
The summit will dry and stable, but there is a possibility for isolated to scattered cirrus passing along the northern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 7.5 C this afternoon, 1 C this evening and 0 C tomorrow morning. Winds will be from the NNW at 10-20 mph for today, increasing to 20-35 mph by the end of the night. Seeing will be near 0.8-1 arcsecond, while precipitable water is expected to be in the 0.7-0.9 mm range for the first half of the night and 0.8-1 mm range for the second half.

Graphical Summary

Discussion
A well-defined tradewind inversion will continue to cap the very limited supply of low-level moisture near 6 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 high clouds passing to the far north are expected to sag southward and scrape the Big Island over the next 5 nights. Still, cloud cover should not exceed 30% for any prolong period during this time.

Precipitable water is expected to start out near 0.7 mm for this evening, but will gradually increase toward 1 mm over the course of the night, 2-2.25 mm for Wednesday and Thursday night and probably 3-4 mm for the following 2 nights.

A mixture of boundary layer turbulence and mid-level turbulence will contribute to poor seeing probably throughout most of the forecast period. The possible exception could be as winds briefly taper, allowing seeing to improve toward more average-like values for Thursday night.

The eastern edge of a fairly deep mid/low-level ridge will continue to sit to the 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 tradewind inversion near 6-7 thousand feet and ensure a dry/stable summit-level air mass through at least the next 5 nights. Unfortunately, the position of the ridge relative to state will allow northerly winds to pick up at the summit, likely contributing to poor seeing over the next 2 nights. There is a chance that the ridge will retract northward a tad, allowing winds to taper and seeing to briefly improve for Thursday night. However, the ridge is set to shift back southward, increasing winds again and degrading seeing again for the following 2 nights. There is also a good chance that minor mid-level moisture will steadily flow out of the westerly jet to far north and pass through the area increasing PW, particularly after tonight. Fortunately, the bulk of the high clouds also spilling out of the jet will pass primarily along the northern skies through the next 4-5 nights.

Please Note: We will be working half days and available primarily during the morning hours (by phone, email anytime) between December 26 and January 5. Barring significant changes between the morning and afternoon model runs we will only be issuing morning forecasts during this time; there will be no forecasts on January 1. The regular forecast schedule will resume on Wednesday, January 8, 2018.

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


5-day Forecast
Summary

Graphical Trend
02 pm HST
Tue 02 Jan
08 pm HST
Tue 02 Jan
02 am HST
Wed 03 Jan
02 pm HST
Wed 03 Jan
08 pm HST
Wed 03 Jan
02 am HST
Thu 04 Jan
02 pm HST
Thu 04 Jan
08 pm HST
Thu 04 Jan
02 am HST
Fri 05 Jan
02 pm HST
Fri 05 Jan
02 am HST
Sat 06 Jan
02 pm HST
Sat 06 Jan
02 am HST
Sun 07 Jan
00 UTC
Wed 03 Jan
06 UTC
Wed 03 Jan
12 UTC
Wed 03 Jan
00 UTC
Thu 04 Jan
06 UTC
Thu 04 Jan
12 UTC
Thu 04 Jan
00 UTC
Fri 05 Jan
06 UTC
Fri 05 Jan
12 UTC
Fri 05 Jan
00 UTC
Sat 06 Jan
12 UTC
Sat 06 Jan
00 UTC
Sun 07 Jan
12 UTC
Sun 07 Jan
Cloud Cover (%) 0 to 20 0 to 20 10 to 20 10 to 30 10 to 30 10 to 30 0 to 20 0 to 20 0 to 20 0 to 20 10 to 30 0 to 20 0 to 20
Cloud Height (km) above sea level N/A 9.5-10 9.5-10 9-10 9-10 9-10 N/A N/A 9.5-10 9.5-10 9.5-10 9.5-10 N/A
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) 0.7 to 1 0.7 to 0.9 0.8 to 1 1 to 2 1.5 to 2 1.75 to 2.25 2 to 3 2 to 2.5 2 to 2.5 2 to 3 3 to 4 2.5 to 3.5 2.5 to 3.5
Mean Seeing (arcsecs) N/A 0.9 ± 0.2 1.0 ± 0.2 N/A 1.0 ± 0.2 0.8 ± 0.2 N/A 0.65 ± 0.15 0.6 ± 0.1 N/A 0.8 ± 0.2 N/A 1.0 ± 0.3
Summit Temp (°C) 7.5 1 0 5 -0.5 -0.5 5 0 -0.5 5.5 1 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)

NNW/10 to 20

NNW/15 to 30

NNW/20 to 35

N/20 to 35

NNE/20 to 35

NNE/10 to 20

NNE/5 to 15

NE/10 to 20

NNE/10 to 20

NE/15 to 30

NNE/15 to 30

NE/20 to 35

NE/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
Tue.Jan.02/Wed.Jan.03
18:05
19:13
5:39
6:48
18:55
N/A
97
8 16.3
18 32
Wed.Jan.03/Thu.Jan.04
18:05
19:14
5:39
6:48
20:00
N/A
92
9 18.8
15 51
Thu.Jan.04/Fri.Jan.05
18:06
19:15
5:40
6:48
21:03
N/A
84
10 17.7
12 13
Fri.Jan.05/Sat.Jan.06
18:07
19:15
5:40
6:49
22:03
N/A
75
11 12.8
7 59
Sat.Jan.06/Sun.Jan.07
18:07
19:16
5:40
6:49
22:59
N/A
65
12 04.8
3 30


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Tuesday 02 January (0300 UTC Wednesday 3 January) 2018.
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