Latest Forecast for Mauna Kea Observatories
800 AM HST (1800 UTC) Friday 21 July 2017

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a possibility for periods of fog, high humidity and perhaps summit-level clouds through the night; precipitation is unlikely.

Summary of Key Meteorological Variables
Summit temperatures will be near 5 C this afternoon, 2.5 C this evening and 2 C tomorrow morning. Winds will be from the SE at 5-15 mph, with seeing near 0.7-0.75 arcseconds. Precipitable water is expected to be in the 3-4 mm range for the night.

Graphical Summary

Discussion
The tradewind inversion will remain weak/elevated near 11-12 thousand feet, which may allow for periods of fog and high humidity through the weekend, particularly for tonight; precipitation is unlikely. The inversion is set to recover near 6-7 thousand feet, ensuring a dry/stable air mass for Monday and Tuesday night. Extensive daytime clouds and afternoon convection along the slopes are possible through the weekend, then will become minimal and short-lived for the early part of next week.

Skies will remain predominately clear of organized clouds throughout the forecast period. However, there is a possibility for summit-level clouds mainly for tonight. Small patches of cirrus may also move quickly in from the south and pass overhead for Saturday and Sunday night.

Precipitable water is expected to linger in the 3-4 mm range through Sunday evening, trend toward 2 mm for later that night, then settle in near 1-1.5 mm for the early part of next week.

An influx of low-level turbulence will contribute to poorer than average seeing for tonight. Seeing is set to gradually improve as this source of turbulence subsides through the weekend, and will eventually settle back in near 0.5 arcseconds as relatively calm skies prevail for at least Monday night. There is a chance that boundary layer turbulence will disrupt seeing again for Tuesday night.

Although Tropical Storm Fernanda is set to pass more than 300 km to the north of the state and along the SE flank of mid/low-level ridge to the NE, deep low-level moisture out surrounding (particularly to the east of) the storm will keep the inversion rather weak/elevated probably through the weekend. This could allow for periods of fog and high humidity at the summit during this time, tonight in particular. The bulk of this moisture is actually expected to shift just west of the Big Island by tomorrow evening, then slowly detrain into the surrounding air mass as the weekend progresses. This could help to decrease the risk for fog/high humidity at the summit for Saturday and especially Sunday night. Still, lingering moisture and minor instability may also allow for the development of afternoon convection along the slopes and extensive daytime clouds through Sunday. A drier more stable air mass is expected to return to the summit as the persistent ridge to the NE regains control of the atmosphere for the early part of next week. However, there is a possibility that easterly winds will strengthen toward 20 mph for the middle of the week. There is also a chance that the remnants of what looks to be Tropical Storm Greg could bring another round of moisture to the summit late next weekend.

Astronomical Observing Quality Guidance
05 pm HST Friday 21 July (03 UTC Saturday 22 July)CN2
08 pm HST Friday 21 July (06 UTC Saturday 22 July)CN2
11 pm HST Friday 21 July (09 UTC Saturday 22 July)CN2
02 am HST Saturday 22 July (12 UTC Saturday 22 July)CN2
05 am HST Saturday 22 July (15 UTC Saturday 22 July)CN2
08 am HST Saturday 22 July (18 UTC Saturday 22 July)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Fri 21 Jul
08 pm HST
Fri 21 Jul
02 am HST
Sat 22 Jul
02 pm HST
Sat 22 Jul
08 pm HST
Sat 22 Jul
02 am HST
Sun 23 Jul
02 pm HST
Sun 23 Jul
08 pm HST
Sun 23 Jul
02 am HST
Mon 24 Jul
02 pm HST
Mon 24 Jul
02 am HST
Tue 25 Jul
02 pm HST
Tue 25 Jul
02 am HST
Wed 26 Jul
00 UTC
Sat 22 Jul
06 UTC
Sat 22 Jul
12 UTC
Sat 22 Jul
00 UTC
Sun 23 Jul
06 UTC
Sun 23 Jul
12 UTC
Sun 23 Jul
00 UTC
Mon 24 Jul
06 UTC
Mon 24 Jul
12 UTC
Mon 24 Jul
00 UTC
Tue 25 Jul
12 UTC
Tue 25 Jul
00 UTC
Wed 26 Jul
12 UTC
Wed 26 Jul
Cloud Cover (%) 70 to 90 20 to 40 0 to 20 60 to 80 0 to 20 10 to 30 60 to 80 10 to 30 0 to 20 0 to 20 0 to 10 0 to 20 0 to 10
Cloud Height (km) above sea level 4-8 4-4.5 N/A 4-6 9.5-10 9.5-10 4-5 9.5-10 9.5-10 N/A N/A N/A N/A
Chance for Fog/Precip (%) 90/60 60/15 60/15 75/40 40/10 30/5 60/20 20/5 15/5 0/0 0/0 0/0 0/0
PW (mm, summit upward) 4 to 8 3 to 4 3 to 4 4 to 8 3 to 4 3 to 4 3 to 6 3 to 4 2 to 3 1.5 to 2.5 1 to 1.5 1 to 2 1 to 1.5
Mean Seeing (arcsecs) N/A 0.75 ± 0.2 0.7 ± 0.2 N/A 0.65 ± 0.15 0.65 ± 0.15 N/A 0.65 ± 0.15 0.6 ± 0.1 N/A 0.5 ± 0.1 N/A 0.7 ± 0.2
Summit Temp (°C) 5 2.5 2 6 2 2 6.5 3.5 3 8.5 3.5 8.5 3.5
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)

SE/5 to 15

SE/5 to 15

SE/5 to 15

SE/5 to 15

ESE/5 to 15

SE/5 to 15

S/5 to 15

SE/0 to 10

S/5 to 15

SSE/5 to 15

SE/10 to 20

E/10 to 20

ENE/15 to 25


 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
Fri.Jul.21/Sat.Jul.22
19:12
20:24
4:33
5:45
5:04
17:51
1
7 08.7
18 37
Sat.Jul.22/Sun.Jul.23
19:12
20:23
4:34
5:45
6:07
18:51
0
8 10.1
17 19
Sun.Jul.23/Mon.Jul.24
19:12
20:23
4:34
5:45
N/A
19:46
1
9 09.3
14 53
Mon.Jul.24/Tue.Jul.25
19:11
20:22
4:35
5:46
N/A
20:36
5
10 05.8
11 34
Tue.Jul.25/Wed.Jul.26
19:11
20:22
4:35
5:46
N/A
21:22
11
10 59.3
7 41


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Friday 21 July (0300 UTC Saturday 22 July) 2017.
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