Latest Forecast for Mauna Kea Observatories
5 PM HST Friday 26 May (0300 UTC Saturday 27 May) 2017

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will be mostly dry and stable through the first part of the night. Moisture will increase in time during the second half. Isolated high clouds might be visible along the southern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -0.5 C this evening and -1.5 C tonight and tomorrow mornign. Winds from the West will be rather sustained at 15-30 mph with seeing variable and exceeding 6 arcseconds. Precipitable water is expected to range between 2 and 3 mm during the first half of the night and slowly increase through the second half, possibly reaching 4 mm.

Graphical Summary

Discussion
The tradewind inversion will start showing signs of weakening from later this evening (Lihue sounding already shows the weakening) as instability moves into the area. Moisture is therefore expected to increase through the night and possibly reach 60%-80% during the second half of tonight. There are risk for periodic fog and possibly light ice at the summit through early Monday morning. Precipitation is unlikely. The inversion is expected to rebuild from late Monday and through the rest of the forecast period. More stable conditions are likely then. Daytime clouds are very likely throughout the long weekend, then taper on Tuesday.

High clouds might be visible tonight and tomorrow along the southern horizon. Mid clouds might also develop from tomorrow and through Monday and block a good portion of the sky. They are expected to shift eastward by late Monday and clear skies will prevail afterward.

Precipitable water is expected to range between 2 and 3 mm during the first half of tonight, to then slowly increase and possibly reach 4 mm by the end of the night. It will exceed the 4 mm threshold through Saturday and part of Sunday to then slowly decrease and range between 3 and 4 mm and possibly stay in this range for the remaining nights.

Increased summit-level winds will increase boundary layer turbulence, which combined with building upper level instability will likely result in less than average seeing for tonight and poor seeing for tomorrow night. A more stable air mass aloft, combined with relatively calm skies should allow for an improvement in seeing for late Sunday and through the rest of this forecast period.

Not much is changed since this morning forecast if not minor touches-up to PW... A mid/upper-level tropical upper-trospheric trough is expected to shift eastward and pass north of the State through this long weekend. Winds at the summit are in fact increasing as gradients increase, and as a result, boundary layer turbulence is strenghtening. From tomorrow, the advected instability will erode most of the tradewind inversion and summit air mass might become nearly saturated, with chances for episodes of fog and possibly light nightime ice through part of Monday. High clouds passing along the southern fringe of the TUTT and within the sub-tropical jet, could also be visible along the southern skies for tonight, Saturday and Sunday night. There is also a chance that thick clouds will develop within the TUTT's axis as it digs southward tomorrow night. The TUTT is set to shift east of the Big Island as a relatively deep ridge fills in from the west and passes to the north during the early part of the week. This should help re-establish large-scale subsidence in the area, strengthen the inversion and decrease the risk for moisture at the summit for Monday night. Seeing will dip toward more average-good/like values as calm skies prevail during that time.

Astronomical Observing Quality Guidance
05 pm HST Friday 26 May (03 UTC Saturday 27 May)CN2
08 pm HST Friday 26 May (06 UTC Saturday 27 May)CN2
11 pm HST Friday 26 May (09 UTC Saturday 27 May)CN2
02 am HST Saturday 27 May (12 UTC Saturday 27 May)CN2
05 am HST Saturday 27 May (15 UTC Saturday 27 May)CN2
08 am HST Saturday 27 May (18 UTC Saturday 27 May)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Fri 26 May
02 am HST
Sat 27 May
02 pm HST
Sat 27 May
08 pm HST
Sat 27 May
02 am HST
Sun 28 May
02 pm HST
Sun 28 May
08 pm HST
Sun 28 May
02 am HST
Mon 29 May
02 pm HST
Mon 29 May
02 am HST
Tue 30 May
02 pm HST
Tue 30 May
02 am HST
Wed 31 May
02 pm HST
Wed 31 May
06 UTC
Sat 27 May
12 UTC
Sat 27 May
00 UTC
Sun 28 May
06 UTC
Sun 28 May
12 UTC
Sun 28 May
00 UTC
Mon 29 May
06 UTC
Mon 29 May
12 UTC
Mon 29 May
00 UTC
Tue 30 May
12 UTC
Tue 30 May
00 UTC
Wed 31 May
12 UTC
Wed 31 May
00 UTC
Thu 01 Jun
Cloud Cover (%) 0 to 20 0 to 20 30 to 50 40 to 60 30 to 50 30 to 50 20 to 40 10 to 30 0 to 20 0 to 20 0 to 20 0 to 20 10 to 30
Cloud Height (km) above sea level N/A N/A 4-6 4-8 4-8 4-6 6-7 7-8 N/A N/A N/A N/A 7-8
Chance for Fog/Precip (%) 20/0 60/10 70/20 80/20 60/10 50/5 30/0 10/0 0/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 2 to 3 3.5 to 4.5 4 to 8 4 to 8 4 to 8 4 to 8 3.5 to 4.5 3 to 4 3.5 to 4.5 3 to 4 3 to 4 3 to 4 3.5 to 4.5
Mean Seeing (arcsecs) 0.60 ± 0.20 0.75 ± 0.20 N/A 0.9 ± 0.2 1.0 ± 0.2 N/A 0.7 ± 0.2 0.7 ± 0.2 N/A 0.6 ± 0.2 N/A 0.5 ± 0.2 N/A
Summit Temp (°C) -0.5 -1.5 2.5 -1.55 -1 -2 3.5 1 0.5 5 2 6 2.5
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)

W/15 to 25

W/20 to 30

W/15 to 25

W/15 to 25

W/10 to 20

WNW/10 to 20

NW/5 to 25

WNW/5 to 25

NW/5 to 25

WNW/5 to 25

N/5 to 25

S/5 to 25

SW/5 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
Fri.May.26/Sat.May.27
19:05
20:17
4:21
5:33
N/A
20:26
4
5 54.3
18 33
Sat.May.27/Sun.May.28
19:05
20:18
4:20
5:33
N/A
21:28
10
6 56.9
18 42
Sun.May.28/Mon.May.29
19:06
20:18
4:20
5:33
N/A
22:27
18
7 57.8
17 34
Mon.May.29/Tue.May.30
19:06
20:19
4:20
5:33
N/A
23:20
27
8 56.0
15 20
Tue.May.30/Wed.May.31
19:07
20:19
4:20
5:33
N/A
0:08
38
9 51.1
12 15


Forecast issued by: Tiziana Cherubini
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Next update at 10 AM HST (2000 UTC) Monday 29 May 2017.
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