Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 16 September (0300 UTC Thursday 17 September) 2020

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while isolated to scattered high clouds pass over and especially along the northern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 8.5 C for today and 3.5 C for the night. Winds will be from the south at 5-15 mph, with seeing near 0.4-0.45 arcseconds. Precipitable water is expected to start out near 1 mm, but increase toward 3 mm by the end of the night.

Graphical Summary

Discussion
The tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Isolated to scattered high clouds will continue to fill in from the SW, passing over and particularly along the northern skies for the next 3 nights. There is also a chance that a patch of slightly deeper clouds will spread in from the south and pass through the area on Friday. These sets of clouds are expected to shift off toward the NW on Saturday, opening up skies as the weekend progresses.

Precipitable water is expected to start out near 1 mm for this evening, but is expected to increase toward 3 mm by the end of the night and settle in near that value for tomorrow night. It will then slip below 2 mm for Friday night and drop back to 1 mm for the following 2 nights.

Mostly calm skies will prevail in the free atmosphere as weak/laminar SW flow switch to a more southerly direction as the forecast period progresses. This, combined with light summit-level winds and a stable air mass should allow seeing to linger primarily near 0.4 arcseconds over the next 3 nights. There is a chance for a slight increase in summit-level winds, which could contribute to light boundary layer turbulence and a minor degradation in seeing (toward 0.5 arcseconds) over the weekend.

Little change since the morning forecast...Although the mid/low-level ridge remain a bit weak/indistinct to the NE before strengthening over the weekend, an equally weak tropical upper-tropospheric trough will remain displaced to the NW while an upper-level ridge lingers off to the SE. This will allow predominately steady large-scale subsidence to prevail in the area, which will easily negate the minor affects of the TUTT, maintain a well-defined inversion near 7-8 thousand and ensure a dry/stable summit-level air mass throughout the forecast period. There is a chance that the STJ along the SE flank of the TUTT will persist over the next northern half of the state and gain access to mostly scattered high clouds coming out of the south over the next 3 nights. These clouds will disperse and lift off toward the north as the aforementioned upper-level ridge tries to build into the area over the weekend. The NW displacement and relatively weakness of the STJ will also minimize turbulence in the free atmosphere as light SW flow back toward more a southerly direction over the next 3-5 nights. Initially, this combined with a stable air mass and weak summit-level winds should allow seeing to settle in near 0.4 arcseconds over much of the next 3 nights. There is a chance that mid-level flow will begin to increase as the mid-level portion of the ridge strengthens and the upper portion builds in from the SE over the weekend. This could result in a minor increase in boundary layer turbulence, which may contribute to a slight degradation in seeing for Saturday and Sunday night.

Astronomical Observing Quality Guidance
Seeing/CN2 Images are not available today. Sorry for the inconvenience.
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Wed 16 Sep
02 am HST
Thu 17 Sep
02 pm HST
Thu 17 Sep
08 pm HST
Thu 17 Sep
02 am HST
Fri 18 Sep
02 pm HST
Fri 18 Sep
08 pm HST
Fri 18 Sep
02 am HST
Sat 19 Sep
02 pm HST
Sat 19 Sep
02 am HST
Sun 20 Sep
02 pm HST
Sun 20 Sep
02 am HST
Mon 21 Sep
02 pm HST
Mon 21 Sep
06 UTC
Thu 17 Sep
12 UTC
Thu 17 Sep
00 UTC
Fri 18 Sep
06 UTC
Fri 18 Sep
12 UTC
Fri 18 Sep
00 UTC
Sat 19 Sep
06 UTC
Sat 19 Sep
12 UTC
Sat 19 Sep
00 UTC
Sun 20 Sep
12 UTC
Sun 20 Sep
00 UTC
Mon 21 Sep
12 UTC
Mon 21 Sep
00 UTC
Tue 22 Sep
Cloud Cover (%) 0 to 20 10 to 30 10 to 30 20 to 40 30 to 50 30 to 50 20 to 40 10 to 30 0 to 20 0 to 5 0 to 20 0 to 5 0 to 20
Cloud Height (km) above sea level 9.5-10 9.5-10 9.5-10 9-10 8-10 7-9 7-9 8-10 9-10 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) 1 to 2 2 to 3 3 to 5 2.5 to 3.5 2.5 to 3.5 2 to 4 1.5 to 2 1.5 to 2 1 to 2 0.8 to 1.2 1 to 1.5 0.8 to 1.2 1 to 1.5
Mean Seeing (arcsecs) 0.4 ± 0.1 0.45 ± 0.1 N/A 0.4 ± 0.1 0.35 ± 0.1 N/A 0.4 ± 0.1 0.45 ± 0.1 N/A 0.5 ± 0.1 N/A 0.5 ± 0.1 N/A
Summit Temp (°C) 3.5 3 8.5 3.5 3.5 9 3.5 3.5 10 4.5 10.5 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)

S/5 to 15

S/5 to 15

S/0 to 10

E/5 to 15

E/5 to 15

ESE/5 to 15

ENE/5 to 15

ENE/5 to 15

ENE/10 to 20

E/10 to 20

ESE/10 to 20

E/10 to 20

E/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
Wed.Sep.16/Thu.Sep.17
18:32
19:36
4:56
6:01
6:20
18:30
0
11 47.0
6 22
Thu.Sep.17/Fri.Sep.18
18:31
19:35
4:57
6:01
N/A
19:14
2
12 41.4
0 19
Fri.Sep.18/Sat.Sep.19
18:30
19:34
4:57
6:01
N/A
19:57
6
13 35.7
-5 47
Sat.Sep.19/Sun.Sep.20
18:29
19:33
4:57
6:02
N/A
20:42
12
14 30.7
-11 33
Sun.Sep.20/Mon.Sep.21
18:28
19:32
4:57
6:02
N/A
21:28
21
15 27.0
-16 36


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Thursday 17 September 2020.
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