Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 18 May (0300 UTC Friday 19 May) 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 2 C this evening and 1.5 C tomorrow morning. Winds will be from the NW at 10-20 mph, with seeing near 0.6-0.7 arcseconds. Precipitable water is expected to be in the 1.75-2.25 mm range for the night.

Graphical Summary

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 9 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 overhead will remain predominately clear for at least the next 2 nights. However, banding high clouds are expected to scrape the southern tip of the Big Island beginning early Sunday morning and may drift overhead for that evening. After briefly retracting southward on Monday morning, more widespread cirrus may drift in from the SW for that night.

Precipitable water is expected to increase toward 2 mm for tonight, 3 mm for Friday and Saturday night and 4 mm for the following 2 nights.

Despite relatively laminar flow aloft, minor boundary layer and/or mid-level turbulence will likely keep seeing near 0.6-0.7 arcseconds for tonight. Summit-level winds are set to diminish, allowing calm skies and deep subsidence to prevail in the free atmosphere and seeing to settle in near 0.4-0.45 arcseconds for at least Friday and Saturday night. There is a possibility for a slight increase in upper-level turbulence, which may degrade seeing toward 0.5-0.55 arcseconds for the following 2 nights.

Little change since the morning forecast...A deep mid/upper-level ridge advancing in from the west, combined with the persistent low-level ridge to the NE, will promote steady/strong large-scale subsidence in the area well into the middle part of next week. This subsidence will easily maintain a well-defined inversion primarily near 8-9 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. Unfortunately, minor mid-level moisture (associated with a long-gone dissipating front) along the east and southern flank of this building ridge will push PW toward 2 mm for tonight and probably 3-4 mm for the following 4 nights. Initially, seeing may also not benefit from this ridge, as summit-level NW winds are set to increase for tonight. However, winds will taper as the center of the ridge fills in overhead, enhancing subsidence and allowing calm skies to prevail in the area for at least the next 2 nights. This should help to improve seeing toward more excellent-like values during that time. However, the upper-ridge is slated to decay as the sub-tropical jet is drawn northward, toward a trough passing far to the north late in the weekend and for the early part of next week. This may increase upper-level turbulence/shear, which may degrade seeing a tad for Sunday and Monday night. There is also a possibility that the STJ will bring banding high clouds to the area during that time.

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


5-day Forecast
Summary

Graphical Trend
08 pm HST
Thu 18 May
02 am HST
Fri 19 May
02 pm HST
Fri 19 May
08 pm HST
Fri 19 May
02 am HST
Sat 20 May
02 pm HST
Sat 20 May
08 pm HST
Sat 20 May
02 am HST
Sun 21 May
02 pm HST
Sun 21 May
02 am HST
Mon 22 May
02 pm HST
Mon 22 May
02 am HST
Tue 23 May
02 pm HST
Tue 23 May
06 UTC
Fri 19 May
12 UTC
Fri 19 May
00 UTC
Sat 20 May
06 UTC
Sat 20 May
12 UTC
Sat 20 May
00 UTC
Sun 21 May
06 UTC
Sun 21 May
12 UTC
Sun 21 May
00 UTC
Mon 22 May
12 UTC
Mon 22 May
00 UTC
Tue 23 May
12 UTC
Tue 23 May
00 UTC
Wed 24 May
Cloud Cover (%) 0 to 5 0 to 5 0 to 20 0 to 5 0 to 5 0 to 20 0 to 20 20 to 40 60 to 80 20 to 40 40 to 60 50 to 70 40 to 60
Cloud Height (km) above sea level N/A N/A N/A N/A N/A N/A N/A 9-10 9-10 9-10 9.5-10 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) 1.75 to 2.25 1.75 to 2.25 2 to 3 2.5 to 3.5 2.5 to 3.5 3 to 4 3 to 4 3 to 4 3 to 5 3.5 to 4.5 3 to 5 3 to 4 2 to 4
Mean Seeing (arcsecs) 0.65 ± 0.15 0.65 ± 0.15 N/A 0.45 ± 0.1 0.4 ± 0.1 N/A 0.4 ± 0.1 0.45 ± 0.1 N/A 0.55 ± 0.15 N/A 0.55 ± 0.15 N/A
Summit Temp (°C) 2 1.5 8 3 3 9 4.5 4 9 3.5 7.5 2 7.5
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

NNW/5 to 15

NNW/5 to 15

NNW/5 to 15

N/0 to 10

NNW/0 to 10

NW/0 to 10

NNW/0 to 10

NNE/5 to 15

NE/5 to 15

NE/5 to 15

NE/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
Thu.May.18/Fri.May.19
19:02
20:13
4:24
5:35
1:03
N/A
45
22 24.9
-10 57
Fri.May.19/Sat.May.20
19:02
20:13
4:23
5:35
1:46
N/A
35
23 15.5
-7 04
Sat.May.20/Sun.May.21
19:02
20:14
4:23
5:34
2:28
N/A
25
0 06.7
-2 44
Sun.May.21/Mon.May.22
19:03
20:15
4:22
5:34
3:12
N/A
16
0 59.2
1 51
Mon.May.22/Tue.May.23
19:03
20:15
4:22
5:34
3:58
N/A
9
1 53.7
6 26


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Friday 19 May 2017.
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