Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Thursday 17 August 2017

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain clear, dry and stable through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 8 C this afternoon, 3 C this evening and 2.5 C tomorrow morning. Winds will be light and variable, with seeing near 0.35-0.4 arcseconds. Precipitable water is expected to be in the 1.25-1.75 mm range for the night.

Graphical Summary

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure the summit remains dry and stable through at least Saturday afternoon. There is a possibility that the inversion will begin to weaken through Saturday night, perhaps lifting toward 15 thousand feet and/or breaking down entirely for the following 2 nights. Nonetheless, this will raise the stakes on fog, high humidity and rain at the summit during this time, particularly the latter two nights. Daytime clouds will be minimal and short-lived through Saturday, but could pick up for Sunday and early part of next week. There is also a chance for isolated afternoon convection along the slopes during those days.

Skies overhead will remain predominately clear throught at least Saturday night. There is a chance that patches of high clouds will drift in from the west and pass through the area early Sunday evening, while thicker clouds develop along the northern skies and possibly creep later that night. The latter set of clouds are slated to shift off toward the west, but high clouds may seep in from the south and pass along the eastern skies for Monday night.

Precipitable water is expected to increase subtly to 1.25-1.5 mm over the next 3 nights, then jump to 2-4 mm for Sunday and Monday night.

Calm skies and deep subsidence should allow seeing to settle in near 0.35-0.4 arcseconds for tonight. A gradually increase in upper-level shear/turbulence will start to degrade seeing through Friday night, with seeing perhaps trending back toward more average-like values for the following night. Instability building into the area combined with this shear/turbulence will likely contribute to poor seeing for Sunday and Monday night.

The mid/low-level ridge will continue to sit to the north of the state and promote strong/steady large-scale subsidence in the area until it weakens a bit in response to a rather deep upper-level low redeveloping and strengthening to the NNW over the weekend. Nonetheless, this subsidence will help maintain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass through at least Saturday afternoon. An upper-level ridge currently lingering overhead will not only enhance the subsidence overhead, but will promote calm/clear skies, which should translate to excellent seeing for tonight. This ridge is expected to shift off toward the west and weaken as the aformentioned upper-level low begins to restrengthen to the NNE through tomorrow. Initially, the position of the low (relative to the state) will have little impact on the stability of the atmosphere, but may start to increase shear/turbulence in the free atmosphere for tonight. However, the low is set to retrograde westward, which could begin to destabilize the air mass and thus weaken/lift the inversion as the weekend progresses. Somewhat unsurprisingly, the last few model runs have backed off on the development of the tropical storm/cyclone to SE over the next several days. However, the models still allow tropical moisture to be drawn toward this low and pass over/near the Big Island late in the weekend and for the early part of next week. This moisture combined with the large-scale instability associated with the low may further erode the inversion and thus increase the risk for periods of fog, high humidity and rain at the summit for Sunday and Monday night. It may also allow for the development of afternoon convection along the Big Island slopes during those days.

Astronomical Observing Quality Guidance
05 pm HST Thursday 17 August (03 UTC Friday 18 August)CN2
08 pm HST Thursday 17 August (06 UTC Friday 18 August)CN2
11 pm HST Thursday 17 August (09 UTC Friday 18 August)CN2
02 am HST Friday 18 August (12 UTC Friday 18 August)CN2
05 am HST Friday 18 August (15 UTC Friday 18 August)CN2
08 am HST Friday 18 August (18 UTC Friday 18 August)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Thu 17 Aug
08 pm HST
Thu 17 Aug
02 am HST
Fri 18 Aug
02 pm HST
Fri 18 Aug
08 pm HST
Fri 18 Aug
02 am HST
Sat 19 Aug
02 pm HST
Sat 19 Aug
08 pm HST
Sat 19 Aug
02 am HST
Sun 20 Aug
02 pm HST
Sun 20 Aug
02 am HST
Mon 21 Aug
02 pm HST
Mon 21 Aug
02 am HST
Tue 22 Aug
00 UTC
Fri 18 Aug
06 UTC
Fri 18 Aug
12 UTC
Fri 18 Aug
00 UTC
Sat 19 Aug
06 UTC
Sat 19 Aug
12 UTC
Sat 19 Aug
00 UTC
Sun 20 Aug
06 UTC
Sun 20 Aug
12 UTC
Sun 20 Aug
00 UTC
Mon 21 Aug
12 UTC
Mon 21 Aug
00 UTC
Tue 22 Aug
12 UTC
Tue 22 Aug
Cloud Cover (%) 0 to 20 0 to 5 0 to 5 0 to 20 0 to 5 0 to 5 0 to 20 0 to 5 0 to 5 60 to 80 20 to 40 60 to 80 20 to 40
Cloud Height (km) above sea level N/A N/A N/A N/A N/A N/A N/A N/A N/A 4-8 6-8 4-8 5-8
Chance for Fog/Precip (%) 0/0 0/0 0/0 0/0 0/0 0/0 0/0 10/0 15/0 75/30 40/10 80/50 60/20
PW (mm, summit upward) 1 to 2 1.25 to 1.75 1.25 to 1.75 1 to 2 1 to 1.5 1 to 1.5 1 to 2 1.25 to 1.75 1.25 to 1.75 3 to 6 2 to 4 4 to 8 4 to 8
Mean Seeing (arcsecs) N/A 0.35 ± 0.1 0.4 ± 0.1 N/A 0.45 ± 0.1 0.45 ± 0.1 N/A 0.5 ± 0.1 0.6 ± 0.1 N/A 0.75 ± 0.15 N/A 0.75 ± 0.15
Summit Temp (°C) 8 3 2.5 8 3 2.5 7.5 2.5 2 5 1.5 5 1.5
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)

SSE/0 to 10

E/0 to 10

NNW/0 to 10

SSW/0 to 10

SW/0 to 10

WNW/0 to 10

S/0 to 10

S/5 to 15

SSW/5 to 15

S/0 to 10

SE/0 to 10

ESE/5 to 15

ESE/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.Aug.17/Fri.Aug.18
18:58
20:05
4:46
5:54
2:48
N/A
14
6 48.0
18 49
Fri.Aug.18/Sat.Aug.19
18:57
20:04
4:47
5:54
3:50
N/A
7
7 48.5
17 59
Sat.Aug.19/Sun.Aug.20
18:56
20:03
4:47
5:55
4:52
17:32
2
8 47.7
15 58
Sun.Aug.20/Mon.Aug.21
18:55
20:03
4:48
5:55
5:53
18:24
0
9 44.7
13 00
Mon.Aug.21/Tue.Aug.22
18:55
20:02
4:48
5:55
6:52
19:12
1
10 39.2
9 18


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Thursday 17 August (0300 UTC Friday 18 August) 2017.
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