Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 23 October (0300 UTC Thursday 24 October) 2019

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while scattered high clouds spread in from the west and pass mainly along the southern half of the Big Island through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this evening and 3.5 C tomorrow morning. Winds will light and variable, with seeing near 0.4 arcseconds. Precipitable water is expected to be in the 2-3 mm range for the night.

Graphical Summary

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and ensure the summit remains dry and stable through at least Thursday night. A possible influx of moisture and instability into the area may weaken/lift the inversion toward 10 thousand feet and could increase the risk for periods of fog and high humidity at the summit for Friday night. The inversion is set to rebuild again near 7-8 thousand feet as the bulk of the moisture passes, leaving dry/stable conditions at the summit for the following 2 nights. Daytime clouds will be minimal and short-lived through tomorrow, could briefly pick up for Friday, then taper again over the weekend. There is also an outside chance for isolated afternoon convection along the slopes for Friday and perhaps on Saturday (mainly to the west for the latter).

Scattered high clouds are expected to spread in from the west and pass over and especially along the southern skies through the night. These clouds will shift eastward tomorrow, opening up skies for that evening, but widespread banding high clouds are set to fill in from the SW as that night progresses and could contribute to overcast skies for much of Friday. There is a chance that this band will become more narrow and shift northward a tad on Saturday, then will shift back overhead for that night. This band will eventually dissipate and sag off toward the SE, leaving clear skies for Sunday night.

Precipitable water is expected to linger in the 2-3 mm range for tonight, increase toward 3-4 mm for the next 3 nights (and perhaps could peak at 4+ mm for Saturday night), then slip back into the 2-3 mm range for Sunday night.

While calm/stable skies will prevail, allowing seeing to linger near 0.4 arcseconds over the next 2 nights, some variability is possible as winds will remain quite light, which could contribute to ground layer turbulence, particularly during the second half of the night. An influx of moisture/instability and an elevated inversion may degrade seeing toward more average-like values for Friday night. The air mass is set to stabilize/dry on Saturday, but a possible increase in free atmospheric turbulence could prohibit seeing from improving until Sunday night.

No change since the morning forecast...The mid/low-level ridge will continue to sit over the state before shifting off toward the east as a weak mid/upper-level trough passes to the north around Friday. Nevertheless, the ridge will promote strong/steady large scale subsidence in the area, which will maintain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable air mass for at least the next 2 nights. Flow aloft will also remain relatively weak, minimizing turbulence in the free atmosphere and likely allow seeing to settle in near 0.4-0.45 arcseconds during this time. There is a possibility for some variability due to very light summit-level winds, which could allow for ground layer turbulence (via thermal bubbles and lack of mixing), particularly during the second half of these nights. The incoming aforementioned trough may destabilize the air mass a tad, but is also expected to veer low-level winds toward a more SE direction on Friday. This flow could advect deep low-level tropical moisture into the area, and together with the minor instability, weaken/lift the inversion toward 10-11 thousand feet and increase the risk for fog/high humidity at the summit for that night. The bulk of this moisture is set to slide back off toward the SW as the trough lifts off toward the north and the ridge expands westward, allowing winds to switch back to more easterly direction over the weekend. This should help reinstill large-scale subsidence in the area, rebuild the inversion near 7-8 thousand feet and ensure the summit steers free of moisture during that time.

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 23 Oct
02 am HST
Thu 24 Oct
02 pm HST
Thu 24 Oct
08 pm HST
Thu 24 Oct
02 am HST
Fri 25 Oct
02 pm HST
Fri 25 Oct
08 pm HST
Fri 25 Oct
02 am HST
Sat 26 Oct
02 pm HST
Sat 26 Oct
02 am HST
Sun 27 Oct
02 pm HST
Sun 27 Oct
02 am HST
Mon 28 Oct
02 pm HST
Mon 28 Oct
06 UTC
Thu 24 Oct
12 UTC
Thu 24 Oct
00 UTC
Fri 25 Oct
06 UTC
Fri 25 Oct
12 UTC
Fri 25 Oct
00 UTC
Sat 26 Oct
06 UTC
Sat 26 Oct
12 UTC
Sat 26 Oct
00 UTC
Sun 27 Oct
12 UTC
Sun 27 Oct
00 UTC
Mon 28 Oct
12 UTC
Mon 28 Oct
00 UTC
Tue 29 Oct
Cloud Cover (%) 20 to 40 10 to 30 0 to 20 0 to 20 20 to 40 60 to 80 80 to 100 60 to 80 40 to 60 60 to 80 0 to 20 0 to 5 0 to 20
Cloud Height (km) above sea level 9-10 9.5-10 N/A N/A 9-10 4-10 7-10 7-10 6-8 9-10 N/A N/A N/A
Chance for Fog/Precip (%) 0/0 0/0 0/0 0/0 0/0 40/10 30/5 30/5 10/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 2 to 3 2 to 3 3 to 5 3 to 4 3 to 4 3 to 5 3 to 4 3 to 4 4 to 8 3 to 5 2 to 4 2 to 3 2 to 4
Mean Seeing (arcsecs) 0.4 ± 0.1 0.4 ± 0.15 N/A 0.45 ± 0.1 0.4 ± 0.15 N/A 0.55 ± 0.1 0.6 ± 0.15 N/A 0.65 ± 0.15 N/A 0.5 ± 0.15 N/A
Summit Temp (°C) 4 3.5 9 3.5 3 8 3 2.5 7.5 2 8 3 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)

ESE/0 to 10

WSW/0 to 10

WSW/0 to 10

NW/0 to 10

NNE/0 to 10

NNE/0 to 10

N/0 to 10

NNE/5 to 15

NNW/5 to 15

N/5 to 15

N/10 to 20

NNE/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
Wed.Oct.23/Thu.Oct.24
18:02
19:07
5:06
6:11
2:40
N/A
19
10 50.9
11 45
Thu.Oct.24/Fri.Oct.25
18:01
19:06
5:06
6:11
3:43
N/A
10
11 46.1
6 32
Fri.Oct.25/Sat.Oct.26
18:01
19:06
5:06
6:11
4:45
16:40
4
12 40.5
0 53
Sat.Oct.26/Sun.Oct.27
18:00
19:05
5:07
6:12
5:47
17:24
1
13 34.7
-4 49
Sun.Oct.27/Mon.Oct.28
17:59
19:05
5:07
6:12
6:49
18:09
0
14 29.2
-10 13


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Thursday 24 October 2019.
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