Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Wednesday 19 April 2017

Warnings
None

Cloud Cover and Precipitation Forecast
There is a small risk for periods of fog and high humidity throughout the night; precipitation is not expected. Small patches of high clouds are expected to pass over and especially along the northern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 6.5 C this afternoon and 1.5 C for the night. Winds will be from the WSW at 5-15 mph, with seeing near 0.5-0.55 arcseconds. Precipitable water is expected to be in the 3.5-4.5 mm range for the night.

Graphical Summary

Discussion
Although the tradewind inversion will remain rather weak/indistinct before finally strengthening early Sunday morning, the lack of moisture in the area and a relatively stable air mass should limit the risk for fog/high humidity at the summit (to less than 25%) for at least the next 3 nights. There is a chance that moisture will creep into the area, which could raise the risk for fog/high humidity at the summit for Saturday night; precipitation is not expected throughout the forecast period and dry/stable conditions will prevail for Sunday night. Some afternoon clouds are possible through Saturday, but will become minimal and short-lived for Sunday.

Small patches of high clouds are epxected to pass over and especially along the northern skies for tonight, but will shift eastward, leaving clear skies for tomorrow night. More widespread scattered high clouds are slated to fill in from the west for Friday night, while banding high clouds begin to stream in from the SW and pass over or just south of the Big Island for the following 2 nights. There is a possibility for periods of extensive cloud cover through the weekend.

Precipitable water is expected to linger primarily near 3-4 mm throughout the forecast period.

Relatively calm skies should allow for better than average seeing for tonight and at least the opening half of tomorrow night. There is a possibility that buiding mid-level turbulence will degrade seeing toward average/poor values between early Friday morning and Saturday evening. Laminar flow and a relatively stable air mass should allow seeing to improve once again for much of Sunday.

A large trough developing to the far north will keep the ridge supressed to the south of the Island until the ridge has a chance to redevelop to the north and spread out across most of the Pacific late in the weekend. Fortunately, organized moisture will be fairly hard to come by near the Big Island, which should limit the risk for fog/high humidity at the summi over the next 3 nights. There is a chance that a developing cold front to the NW of the state will shift southward, stall over the northern/central part of the state during this time, then subtly shift southward as a new low-level ridge fills in from the west on Saturday. This could raise the stakes on fog/moisture a tad for that night, but subsidence associated with this building ridge as well as the developing zonal mid-level ridge to the north should help re-establish strong/steady large-scale subsidence in the area thereafter. This will allow a well-defined inversion to strengthen near 6-7 thousand feet and ensure the summit remains dry and stable for Sunday night and early part of next week. While relatively calm skies should prevail (allowing for better than average seeing for the next 36 hours), turbulence associated with this developing front may make a push through the Big Island area disrupting seeing for much of Friday and into Saturday. Seeing is set to impove once again as the atmosphere stabilizes and a rather broad but fairly weak sub-tropical jet develops over the state for the second half of the weekend.

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


5-day Forecast
Summary

Graphical Trend
02 pm HST
Wed 19 Apr
08 pm HST
Wed 19 Apr
02 am HST
Thu 20 Apr
02 pm HST
Thu 20 Apr
08 pm HST
Thu 20 Apr
02 am HST
Fri 21 Apr
02 pm HST
Fri 21 Apr
08 pm HST
Fri 21 Apr
02 am HST
Sat 22 Apr
02 pm HST
Sat 22 Apr
02 am HST
Sun 23 Apr
02 pm HST
Sun 23 Apr
02 am HST
Mon 24 Apr
00 UTC
Thu 20 Apr
06 UTC
Thu 20 Apr
12 UTC
Thu 20 Apr
00 UTC
Fri 21 Apr
06 UTC
Fri 21 Apr
12 UTC
Fri 21 Apr
00 UTC
Sat 22 Apr
06 UTC
Sat 22 Apr
12 UTC
Sat 22 Apr
00 UTC
Sun 23 Apr
12 UTC
Sun 23 Apr
00 UTC
Mon 24 Apr
12 UTC
Mon 24 Apr
Cloud Cover (%) 40 to 60 20 to 40 20 to 40 20 to 40 0 to 10 0 to 10 20 to 40 40 to 60 60 to 80 60 to 80 50 to 70 60 to 80 60 to 80
Cloud Height (km) above sea level 4-5 9-10 9-10 4-4.5 N/A N/A 4-4.5 9-10 8-10 4-10 8-10 7-10 8-10
Chance for Fog/Precip (%) 30/0 20/0 15/0 25/0 15/0 10/0 30/0 15/0 25/0 50/10 30/0 10/0 0/0
PW (mm, summit upward) 4 to 6 3.5 to 4.5 3.5 to 4.5 4 to 6 3.5 to 4.5 3.5 to 4.5 4 to 8 3 to 4 3 to 4 4 to 8 3 to 4 3 to 6 3 to 4
Mean Seeing (arcsecs) N/A 0.55 ± 0.1 0.5 ± 0.05 N/A 0.5 ± 0.1 0.65 ± 0.1 N/A 0.75 ± 0.15 0.7 ± 0.15 N/A 0.6 ± 0.1 N/A 0.55 ± 0.1
Summit Temp (°C) 6.5 1.5 1.5 7 2 1 6 1.5 1 5.5 0.5 5.5 0.5
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)

WSW/5 to 15

WSW/5 to 15

WSW/5 to 15

WSW/5 to 15

WNW/5 to 15

S/0 to 10

S/0 to 10

SSE/0 to 10

SSE/0 to 10

SE/5 to 15

SE/10 to 20

ESE/10 to 20

ESE/10 to 20


 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.Apr.19/Thu.Apr.20
18:50
19:57
4:44
5:51
1:40
N/A
39
20 59.2
-15 46
Thu.Apr.20/Fri.Apr.21
18:51
19:58
4:43
5:50
2:25
N/A
30
21 50.3
-12 54
Fri.Apr.21/Sat.Apr.22
18:51
19:58
4:42
5:49
3:09
N/A
21
22 41.7
-9 21
Sat.Apr.22/Sun.Apr.23
18:51
19:59
4:41
5:49
3:53
N/A
13
23 33.6
-5 13
Sun.Apr.23/Mon.Apr.24
18:52
19:59
4:40
5:48
4:37
N/A
6
0 26.4
-0 43


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Wednesday 19 April (0300 UTC Thursday 20 April) 2017.
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