Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Wednesday 13 September 2017

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 9 C this afternoon, 4 C this evening and 3.5 C tomorrow morning. Winds will be light and from the NNW, with seeing near 0.5-0.55 arcseconds. Precipitable water is expected to be in the 2.5-3.5 mm range for the night.

Graphical Summary

Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 10 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.

Skies will remain predominately clear for tonight, but strands of high clouds will begin to drift in from the south just after sunrise tomorrow and become more widespread through the day. These clouds will likely contribute to periods of extensive cloud cover for tomorrow night, then will rapidly shift off toward the NW through Friday, opening up skies for that night and leaving clear skies for the following 2 nights.

Precipitable water is expected to linger near 3-3.5 mm for the next 2 nights, slip toward 1.5 mm for Friday night, then settle in near 1-1.5 mm for the following 2 nights.

Despite relatively calm skies will prevail in the free atmosphere likely allowing for better than average seeing for the next 3 nights, there is a possibility for ground-level turbulence (induced by very weak summit-level winds), which could contribute to large variability particularly during the second half of the next 2 nights (there is a good chance seeing will briefly dip toward more excellent-like values for Friday night). Summit-level winds are set to increase, which will likely stir up boundary layer turbulence and contribute to average/poor seeing for Saturday night and most of Sunday night.

Although the ridge will remain somewhat displaced to the east in response to a fairly deep low to the north over the next 36-48 hours, organized moisture will be fairly hard to come by during this period. In addition, the ridge will remain close enough to promote large-scale subsidence in the area and maintain a fairly well-defined inversion near 9-10 thousand feet, ensuring a dry/stable summit-level air mass. However, there is a good chance that the low will draw tropical high clouds over the summit area, which could contribute to periods of extensive cloud cover for tomorrow night. These clouds will shift off toward the NW as an upper-level ridge builds in from the SE through Friday. This will not only help open up summit skies, but augment subsidence in the free atmosphere and perhaps allow seeing to slip near/below 0.4 arcseconds for Friday night. Unfortunately, the upper-level ridge will eventually help restrengthen the mid/low-level ridge, tightening the mid-level wind gradient over the weekend. This could result in an increase in summit-level winds, which will likely stir up boundary layer turbulence and contribute to poorer than average seeing for Saturday and Sunday night.

Astronomical Observing Quality Guidance
05 pm HST Wednesday 13 September (03 UTC Thursday 14 September)CN2
08 pm HST Wednesday 13 September (06 UTC Thursday 14 September)CN2
11 pm HST Wednesday 13 September (09 UTC Thursday 14 September)CN2
02 am HST Thursday 14 September (12 UTC Thursday 14 September)CN2
05 am HST Thursday 14 September (15 UTC Thursday 14 September)CN2
08 am HST Thursday 14 September (18 UTC Thursday 14 September)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Wed 13 Sep
08 pm HST
Wed 13 Sep
02 am HST
Thu 14 Sep
02 pm HST
Thu 14 Sep
08 pm HST
Thu 14 Sep
02 am HST
Fri 15 Sep
02 pm HST
Fri 15 Sep
08 pm HST
Fri 15 Sep
02 am HST
Sat 16 Sep
02 pm HST
Sat 16 Sep
02 am HST
Sun 17 Sep
02 pm HST
Sun 17 Sep
02 am HST
Mon 18 Sep
00 UTC
Thu 14 Sep
06 UTC
Thu 14 Sep
12 UTC
Thu 14 Sep
00 UTC
Fri 15 Sep
06 UTC
Fri 15 Sep
12 UTC
Fri 15 Sep
00 UTC
Sat 16 Sep
06 UTC
Sat 16 Sep
12 UTC
Sat 16 Sep
00 UTC
Sun 17 Sep
12 UTC
Sun 17 Sep
00 UTC
Mon 18 Sep
12 UTC
Mon 18 Sep
Cloud Cover (%) 0 to 20 0 to 5 0 to 20 20 to 40 60 to 80 60 to 80 40 to 60 20 to 40 0 to 20 0 to 20 0 to 5 0 to 20 0 to 5
Cloud Height (km) above sea level N/A N/A N/A 9-10 9-10 9-10 9-10 9-10 9.5-10 N/A N/A N/A 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) 3 to 5 2.5 to 3.5 2.5 to 3.5 3 to 5 3 to 4 3 to 4 2 to 4 1.25 to 1.75 1.25 to 1.75 1 to 2 1 to 1.5 1 to 2 1 to 1.5
Mean Seeing (arcsecs) N/A 0.5 ± 0.1 0.55 ± 0.15 N/A 0.45 ± 0.1 0.5 ± 0.15 N/A 0.45 ± 0.1 0.4 ± 0.1 N/A 0.7 ± 0.2 N/A 0.65 ± 0.15
Summit Temp (°C) 9 4 3.5 8.5 3.5 3.5 10 5.5 5 11 5.5 10 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)

NNW/0 to 10

NNW/0 to 10

NW/0 to 10

NNE/0 to 10

ENE/0 to 10

E/0 to 10

ESE/5 to 15

E/5 to 15

ENE/5 to 15

ENE/10 to 20

NE/15 to 30

NE/15 to 30

NE/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
Wed.Sep.13/Thu.Sep.14
18:35
19:39
4:56
6:00
0:41
N/A
36
6 33.2
19 01
Thu.Sep.14/Fri.Sep.15
18:34
19:38
4:56
6:01
1:40
N/A
26
7 32.7
18 29
Fri.Sep.15/Sat.Sep.16
18:33
19:37
4:56
6:01
2:40
N/A
16
8 31.0
16 49
Sat.Sep.16/Sun.Sep.17
18:32
19:36
4:56
6:01
3:40
N/A
9
9 27.5
14 09
Sun.Sep.17/Mon.Sep.18
18:31
19:36
4:57
6:01
4:39
17:04
4
10 21.8
10 42


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Wednesday 13 September (0300 UTC Thursday 14 September) 2017.
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