Latest Forecast for Mauna Kea Observatories
4:30 PM HST Friday 19 May (0230 UTC Saturday 20 May) 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 3.5 C this evening and 2.5 C tomorrow morning. Winds will be from the NNW at 5-15 mph, with seeing near 0.4-0.45 arcseconds. Precipitable water is expected to be in the 2-2.5 mm range for the night.

Graphical Summary

Discussion
A well established and stable tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure the summit remains dry and stable through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Skies overhead will remain predominately clear through at least Saturday evening. High clouds will be advected over from the south just after midnight on Saturday and will likely drift over or just scrape the southern tip of the Big Island through Monday evening. There is a possibility that these clouds will spread further northward contributing to extensive cloud cover during the second half of Monday night. These clouds will begin to shift eastward through Tuesday, opening up skies during that night.

Precipitable water is expected to linger in the 2-2.5 mm range tonight and linger close to 2mm on Saturday night, increase to 2.5-3.5 mm for Sunday and Monday night, then slip back into the 2-3 mm range for the following night.

Deep subsidence and calm skies are slated to fill in overhead, allowing seeing to improve toward 0.35-0.4 arcseconds through tonight and especially for tomorrow night. Upper-level westerly flow is set to strengthen a bit as the subsidence tapers a bit over the following 3 nights. Still, calm skies will prevail allowing seeing to linger close to 0.55 arcseconds for most of the remaining nights.

Not much is changed since this morning forecast... The persistent low-level ridge to the NE of the state will continue to promote strong/steady large-scale subsidence in the area, help maintain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass well into next week. In addition, a deep mid/upper-level ridge is expected to fill in from the west over the next 24 hours, then gradually dissipate in the area through the weekend. This ridge will not only enhance the subsidence in the area, but promote clear/calm skies, which should allow seeing to settle in near 0.35-0.4 mm for much of the next 2 nights. A trough passing far to the north will begin to breakdown the upper-portion of the ridge and gradually strengthen and drag the sub-tropical jet in from the south late in the weekend and for the early part of next week. While the trough is not expected to affect the stability of the atmosphere, the STJ will likely bring scattered/broken high clouds to the area mainly for Sunday and Monday night. There is a chance that light/moderate turbulence/shear associated with the jet will degrade seeing toward 0.5-0.55 arcseconds for the second half of the forecast period.

Astronomical Observing Quality Guidance
05 pm HST Friday 19 May (03 UTC Saturday 20 May)CN2
08 pm HST Friday 19 May (06 UTC Saturday 20 May)CN2
11 pm HST Friday 19 May (09 UTC Saturday 20 May)CN2
02 am HST Saturday 20 May (12 UTC Saturday 20 May)CN2
05 am HST Saturday 20 May (15 UTC Saturday 20 May)CN2
08 am HST Saturday 20 May (18 UTC Saturday 20 May)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Fri 19 May
02 am HST
Sat 20 May
02 pm HST
Sat 20 May
08 pm HST
Sat 20 May
02 am HST
Sun 21 May
02 pm HST
Sun 21 May
08 pm HST
Sun 21 May
02 am HST
Mon 22 May
02 pm HST
Mon 22 May
02 am HST
Tue 23 May
02 pm HST
Tue 23 May
02 am HST
Wed 24 May
02 pm HST
Wed 24 May
06 UTC
Sat 20 May
12 UTC
Sat 20 May
00 UTC
Sun 21 May
06 UTC
Sun 21 May
12 UTC
Sun 21 May
00 UTC
Mon 22 May
06 UTC
Mon 22 May
12 UTC
Mon 22 May
00 UTC
Tue 23 May
12 UTC
Tue 23 May
00 UTC
Wed 24 May
12 UTC
Wed 24 May
00 UTC
Thu 25 May
Cloud Cover (%) 0 to 10 0 to 10 0 to 20 0 to 20 10 to 30 20 to 40 30 to 50 20 to 40 10 to 30 10 to 30 0 to 20 0 to 20 0 to 20
Cloud Height (km) above sea level N/A N/A N/A N/A 8-9 7-8 7-8 6-7 7-8 8-9 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 to 2.5 2 to 2.5 2 to 3 1.75 to 2.25 1.75 to 2.25 2.5 to 3.5 2.5 to 3 2.5 to 3 3 to 4 2.5 to 3.5 2.5 to 3.5 2 to 2.5 2 to 2.5
Mean Seeing (arcsecs) 0.50 ± 0.15 0.45 ± 0.15 N/A 0.55 ± 0.15 0.50 ± 0.15 N/A 0.60 ± 0.15 0.60 ± 0.15 N/A 0.55 ± 0.15 N/A 0.55 ± 0.15 N/A
Summit Temp (°C) 3.5 2.5 10 4 3.5 10 4.5 3.5 9 3 9 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)

NNW/5 to 15

NNW/5 to 15

NNW/5 to 15

NW/5 to 15

NNW/5 to 15

NNW/5 to 15

NNW/5 to 10

NNE/5 to 15

NE/5 to 15

NE/5 to 15

ENE/5 to 15

ESE/5 to 15

NW/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
Fri.May.19/Sat.May.20
19:02
20:13
4:23
5:35
1:46
N/A
35
23 15.5
-7 04
Sat.May.20/Sun.May.21
19:02
20:14
4:23
5:34
2:28
N/A
25
0 06.7
-2 44
Sun.May.21/Mon.May.22
19:03
20:15
4:22
5:34
3:12
N/A
16
0 59.2
1 51
Mon.May.22/Tue.May.23
19:03
20:15
4:22
5:34
3:58
N/A
9
1 53.7
6 26
Tue.May.23/Wed.May.24
19:04
20:16
4:22
5:34
4:48
N/A
3
2 50.5
10 43


Forecast issued by: Tiziana Cherubini
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Next update at 10 AM HST (2000 UTC) Monday 22 May 2017.
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