Latest Forecast for Mauna Kea Observatories
5 PM HST Tuesday 21 May (0300 UTC Wednesday 22 May) 2019

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 4 C this evening and 3.5 C tomorrow morning. Winds will be from the SE at 5-15 mph, with seeing near 0.4-0.5 arcseconds. Precipitable water is expected to be in the 2.5-3 mm range for the night.

Graphical Summary

Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 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.

There is a possibility for patches of mid/upper-level clouds in the area for tonight. These clouds will dissipate and lift off toward the north through tomorrow, leaving predominately clear skies for the remainder of the forecast period.

Precipitable water is expected to gradually decline through the 2-3 mm range over the next 3 nights, then settle in near 1.5-2 mm for Friday and Saturday night.

Calm skies, a stable air mass and light winds at the summit should allow seeing to linger primarily near 0.4-0.5 arcseconds over the next 2 nights (some variability is possible should winds die off completely for a prolong period, mainly for tomorrow night). An increase in boundary layer turbulence could start to degrade seeing through Thursday night and contribute to poor seeing for the following 2 nights.

No change since the morning forecast...The mid/low-level ridge will remain squashed over the state by a rather large trough brewing to the NW, then will lift northward as the trough weakens beginning late Thursday. Nevertheless, the ridge will still promote steady/strong large-scale subsidence in the area, which will easily maintain a well-defined inversion near 8 thousand feet and ensure a dry/stable summit-level air mass through the week; the trough will have little, if any, direct impact on summit conditions. The southward supression of the ridge will keep summit winds relatively light, and combined with the absence of the sub-tropical jet aloft, should allow for calm skies and good/excellent seeing over the next 2 nights. However, summit winds are set to pick up again as the ridge rebounds northward and easterly flow along the southern flank of the ridge builds in overhead late Thursday and into the weekend. This could result in an increase in boundary layer turbulence, which will probably contribute to poorer than average seeing perhaps as early as Thursday night, but more likely the following 2 nights.

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
Tue 21 May
02 am HST
Wed 22 May
02 pm HST
Wed 22 May
08 pm HST
Wed 22 May
02 am HST
Thu 23 May
02 pm HST
Thu 23 May
08 pm HST
Thu 23 May
02 am HST
Fri 24 May
02 pm HST
Fri 24 May
02 am HST
Sat 25 May
02 pm HST
Sat 25 May
02 am HST
Sun 26 May
02 pm HST
Sun 26 May
06 UTC
Wed 22 May
12 UTC
Wed 22 May
00 UTC
Thu 23 May
06 UTC
Thu 23 May
12 UTC
Thu 23 May
00 UTC
Fri 24 May
06 UTC
Fri 24 May
12 UTC
Fri 24 May
00 UTC
Sat 25 May
12 UTC
Sat 25 May
00 UTC
Sun 26 May
12 UTC
Sun 26 May
00 UTC
Mon 27 May
Cloud Cover (%) 10 to 30 0 to 20 0 to 20 0 to 10 0 to 5 0 to 20 0 to 5 0 to 5 0 to 20 0 to 5 0 to 20 0 to 5 0 to 20
Cloud Height (km) above sea level 7-8 7-8 7-8 N/A N/A N/A N/A N/A N/A 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) 2.5 to 3 2.5 to 3 2 to 4 2.25 to 2.75 2.25 to 2.75 2 to 4 2 to 2.5 2 to 2.5 1.5 to 2.5 1.5 to 2 1.5 to 2.5 1.5 to 2 1 to 2
Mean Seeing (arcsecs) 0.5 ± 0.1 0.45 ± 0.1 N/A 0.4 ± 0.1 0.45 ± 0.15 N/A 0.6 ± 0.1 0.7 ± 0.2 N/A 0.9 ± 0.2 N/A 0.9 ± 0.2 N/A
Summit Temp (°C) 4 3.5 9.5 4.5 4.5 10 5 4.5 9 3 7.5 2.5 8
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

ESE/0 to 10

E/0 to 10

E/10 to 20

ENE/10 to 20

ENE/15 to 25

E/15 to 25

ENE/20 to 35

NE/20 to 35

ENE/20 to 35

E/20 to 35


 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
Tue.May.21/Wed.May.22
19:03
20:14
4:23
5:34
21:42
N/A
86
19 02.4
-22 48
Wed.May.22/Thu.May.23
19:03
20:15
4:22
5:34
22:32
N/A
79
19 56.0
-22 02
Thu.May.23/Fri.May.24
19:03
20:15
4:22
5:34
23:19
N/A
70
20 47.7
-20 16
Fri.May.24/Sat.May.25
19:04
20:16
4:21
5:34
0:01
N/A
61
21 37.0
-17 40
Sat.May.25/Sun.May.26
19:04
20:17
4:21
5:33
0:41
N/A
52
22 24.4
-14 23


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Wednesday 22 May 2019.
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