Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 17 May (0300 UTC Friday 18 May) 2018

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a risk for fog and high humidity, mainly during the first half of the night; precipitation is unlikely. Scattered to broken high clouds will litter the eastern skies, particularly for this evening.

Summary of Key Meteorological Variables
Summit temperatures will be near 0.5 C, with light WNW winds. Seeing will be near 0.7-0.75 arcseconds, while precipitable water is expected to linger near or exceed 4 mm through the night.

Graphical Summary

Discussion
While the tradewind inversion will continue to restrengthen near 10 thousand feet over the next 12-18 hours, lingering low-level moisture may allow for periods of fog and high humidity, particularly for this evening; precipitation is unlikely. Dry and stable conditions will return to the summit once the inversion becomes firmly established at or below 9 thousand feet for the following 4 nights. Extensive daytime clouds are expected for today, then will become minimal and short-lived for the remainder of the forecast period.

Lingering patches of high clouds will continue to litter the eastern skies mainly during the first half of tonight, then will slowly shift eastward, opening up skies for at least tomorrow night. However, more scattered thin high clouds may begin to approach for the western skies through Saturday night and will likely drift overhead for Sunday and perhaps Monday night.

Precipitable water will likely linger in the 3-4 mm range for most of tonight, slip toward 2-3 mm for Friday night and perhaps 1-1.5 mm for Saturday night. It is then set to rebound back towards 4 mm for Sunday and Monday night.

An elevated inversion and lingering moisture may contribute to slightly poorer than average/variable seeing for tonight. Calmer skies and more stable atmosphere should allow seeing to improve toward 0.5-0.6 arcseconds for Friday and Saturday night. There is a possibility that shear/turbulence will increase in the free atmosphere, which could result in some degradation (toward more average-like values) for the following 2 nights.

Minimal change since the morning forecast...An upper-level short-wave trough currently sitting overhead is expected to shift gradually shift eastward, while a new mid-level ridge rebuilds to the north over the next 12-18 hours. Unfortunately, the persistent relatively deep low-level moisture to the east, is expected to get caught up in the building trades and shift through the Big Island over the course of the night. This will likely help keep the inversion rather elevated and may allow for periods of fog and high humidity at the summit, particularly during the first half of the night. The inversion is set to rebuild near 8 thousand feet once this moisture slips westward and the ridge to the north re-establishes strong/steady large-scale subsidence in the area just after sunrise tomorrow. The inversion may fluctuate in height thereafter, but will remain firmly established at or below 10 thousand feet and ensure the summit steers free of moisture for the remainder of the forecast period. Seeing will benefit from the subsidence and re-establishement of the inversion and improve toward better than average-values as relatively weak/calm flow prevails at and above the summit for Friday and probably Saturday night. However, the sub-tropical jet is set to gradually restrengthen, which may increase turbulence in the free atmosphere (as well as bring high clouds to the area) and degrade seeing a tad for the following 2 nights.

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


5-day Forecast
Summary

Graphical Trend
08 pm HST
Thu 17 May
02 am HST
Fri 18 May
02 pm HST
Fri 18 May
08 pm HST
Fri 18 May
02 am HST
Sat 19 May
02 pm HST
Sat 19 May
08 pm HST
Sat 19 May
02 am HST
Sun 20 May
02 pm HST
Sun 20 May
02 am HST
Mon 21 May
02 pm HST
Mon 21 May
02 am HST
Tue 22 May
02 pm HST
Tue 22 May
06 UTC
Fri 18 May
12 UTC
Fri 18 May
00 UTC
Sat 19 May
06 UTC
Sat 19 May
12 UTC
Sat 19 May
00 UTC
Sun 20 May
06 UTC
Sun 20 May
12 UTC
Sun 20 May
00 UTC
Mon 21 May
12 UTC
Mon 21 May
00 UTC
Tue 22 May
12 UTC
Tue 22 May
00 UTC
Wed 23 May
Cloud Cover (%) 20 to 40 10 to 30 0 to 20 0 to 10 0 to 10 0 to 20 10 to 30 20 to 40 20 to 40 30 to 50 20 to 40 10 to 30 0 to 20
Cloud Height (km) above sea level 8-10 8-9 8-8.5 N/A N/A N/A 9-10 9-10 9-10 8-10 8-9 8-9 8-8.5
Chance for Fog/Precip (%) 60/10 30/5 10/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 4 to 8 3 to 5 2 to 4 1.75 to 2.25 1.5 to 2 1 to 2 1 to 1.5 1 to 1.5 1.5 to 2.5 3 to 4 3 to 5 3 to 5 3 to 6
Mean Seeing (arcsecs) 0.75 ± 0.15 0.7 ± 0.1 N/A 0.55 ± 0.1 0.5 ± 0.1 N/A 0.5 ± 0.1 0.55 ± 0.1 N/A 0.6 ± 0.1 N/A 0.65 ± 0.15 N/A
Summit Temp (°C) 0.5 0.5 6 1.5 1.5 7 2.5 2.5 8 2.5 8 2.5 7
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/0 to 10

NE/5 to 15

NE/5 to 15

NE/10 to 20

E/10 to 20

E/5 to 15

ESE/5 to 15

SSW/10 to 20

SW/10 to 20

SW/10 to 20

W/10 to 20

W/15 to 30


 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.May.17/Fri.May.18
19:01
20:12
4:24
5:35
N/A
21:44
11
6 29.0
20 05
Fri.May.18/Sat.May.19
19:01
20:13
4:24
5:35
N/A
22:44
20
7 30.9
19 45
Sat.May.19/Sun.May.20
19:02
20:13
4:23
5:35
N/A
23:41
30
8 31.5
18 06
Sun.May.20/Mon.May.21
19:02
20:14
4:23
5:35
N/A
0:32
41
9 29.8
15 18
Mon.May.21/Tue.May.22
19:03
20:14
4:23
5:34
N/A
1:19
52
10 25.5
11 38


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Friday 18 May 2018.
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