Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Thursday 20 April 2017

Warnings
None

Cloud Cover and Precipitation Forecast
Skies will remain clear, but there is an outside chance for short-lived fog/high humidity throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 7 C this afternoon, 2 C this evening and 1.5 C tomorrow morning. Winds will be from the west at 5-15 mph for today, becoming light and somewhat variable for the night. Seeing will start out near 0.55 arcseconds, but may drift toward 0.7 arcseconds through the night. Precipitable water is expected to be in the 1.75-2.25 mm range for the night.

Graphical Summary

Discussion
Although the inversion will remain rather indistinct before strengthening on Sunday, lack of moisture in the area should limit the risk for fog/high humidity over the next 2 nights. There is a possibility that an influx of low-level moisture will raise this risk a bit for Saturday night; precipitation is not expected and dry/stable conditions are expected for Sunday and Monday night. Some minor afternoon clouds are possible through tomorrow, and could pick up a tad on Saturday, then become minimal and short-lived for Sunday and early part of next week.

Skies will remain predominately clear for tonight, but scattered high clouds are set to spread in from the west passing over and especially along the northern skies for tomorrow night. There is a chance that more widespread high clouds will follow suit from the SW on Saturday, contributing to periods of extensive cloud cover for the remainder of the weekend. These clouds will breakdown a bit and shift to the northern skies early Monday morning, but more streamin high clouds may spread in from the south by sunrise on Tuesday.

Precipitable water is expected to linger near 1.75-2 mm for the next 3 nights, then increase to 2-3 mm for Sunday and Monday night.

A relatively calm skies should allow for better than average seeing for early this evening, but increasing mid-level turbulence may degrade seeing (toward poor values) for the second half of tonight and especially tomorrow night. Seeing should improve through Saturday night, eventually allowing seeing to settle in near 0.5 arcseconds for Sunday and Monday night.

While the mid/low-level ridge (and thus the inversion) will remain rather weak/indistinct until later in the weekend, the air mass near the Big Island will remain quite dry as the bulk of the moisture is drawn northward to a cold front that is set to stall out near the northern/central half of the state over the next 2 nights. This should limit the risk for fog/high humidity at the summit, but turbulence out ahead of the front could disrupt seeing during much of this time. There is a chance that the bulk of the turbulence will shift off toward the SE, allowing seeing to improve through Saturday night. However, remnant moisture associated with the front may get caught up in the building low-level trades and make a push through the area between early Saturday afternoon and Sunday morning. There is a small chance that some of this moisture will make it past the unassuming inversion and allow for periods of fog/high humidity at the summit for that night. Fortunately, the ridge filling in from the west will reinstill strong/steady large-scale subsidence in the area, which will help rebuild the inversion near 6-7 thousand feet and ensure a dry/stable summit-level air mass for Sunday night and at least the early part of next week. A rather broad but relatively weak sub-tropical jet will also allow laminar flow to persist in the free atmosphere, which could make for better than average seeing during that time.

Astronomical Observing Quality Guidance
05 pm HST Thursday 20 April (03 UTC Friday 21 April)CN2
08 pm HST Thursday 20 April (06 UTC Friday 21 April)CN2
11 pm HST Thursday 20 April (09 UTC Friday 21 April)CN2
02 am HST Friday 21 April (12 UTC Friday 21 April)CN2
05 am HST Friday 21 April (15 UTC Friday 21 April)CN2
08 am HST Friday 21 April (18 UTC Friday 21 April)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
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
08 pm HST
Sat 22 Apr
02 am HST
Sun 23 Apr
02 pm HST
Sun 23 Apr
02 am HST
Mon 24 Apr
02 pm HST
Mon 24 Apr
02 am HST
Tue 25 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
06 UTC
Sun 23 Apr
12 UTC
Sun 23 Apr
00 UTC
Mon 24 Apr
12 UTC
Mon 24 Apr
00 UTC
Tue 25 Apr
12 UTC
Tue 25 Apr
Cloud Cover (%) 20 to 40 0 to 5 0 to 5 20 to 40 20 to 40 40 to 60 60 to 80 50 to 70 60 to 80 60 to 80 40 to 60 20 to 40 40 to 60
Cloud Height (km) above sea level 4-4.5 N/A N/A 4-4.5 9.5-10 9-10 4-10 8.5-10 8-10 8-10 9-10 9-10 9-10
Chance for Fog/Precip (%) 20/0 10/0 10/0 20/0 10/0 15/0 60/10 40/0 30/0 5/0 0/0 0/0 0/0
PW (mm, summit upward) 2 to 4 1.75 to 2.25 1.75 to 2.25 2 to 4 1.5 to 2 1.5 to 2 3 to 6 1.75 to 2.25 1.75 to 2.25 2 to 3 2 to 3 2 to 3 2 to 3
Mean Seeing (arcsecs) N/A 0.55 ± 0.1 0.65 ± 0.1 N/A 0.75 ± 0.15 0.75 ± 0.15 N/A 0.65 ± 0.15 0.55 ± 0.15 N/A 0.5 ± 0.1 N/A 0.45 ± 0.1
Summit Temp (°C) 7 2 1.5 7 2 1 5 1 0.5 6 0.5 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)

W/5 to 15

WNW/0 to 10

SSE/0 to 10

SE/0 to 10

SE/0 to 10

SSE/5 to 15

SE/5 to 15

SE/10 to 20

SE/10 to 20

SE/10 to 20

SE/10 to 20

ESE/10 to 20

E/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.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
Mon.Apr.24/Tue.Apr.25
18:52
20:00
4:40
5:47
5:23
17:26
2
1 20.7
3 56


Forecast issued by: Ryan Lyman
Contact Us - (808) 932-2323
Hours: Mon-Fri 8 AM - 5 PM HST (1800-0300 UTC)

For public road conditions and snow report message please call (808) 935-6268.
This message is also available at the MKWC road conditions page.

Next update at 5 PM HST Thursday 20 April (0300 UTC Friday 21 April) 2017.
Please call or email to be placed on the Mauna Kea Weather Bulletin email list.