Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Friday 20 April 2018

Warnings
Chance for convection/snow through this evening
Fog, ice and high humidity possible through the night

Cloud Cover and Precipitation Forecast
There is a moderate/high risk for fog, ice, high humidity, flurries and convection in the area mainly during the first half of the night. Banding high clouds will continue to pass overhead through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near freezing for today and into tonight. Winds will be from the north at 10-20 mph for today, switching to a more NE direction through the night. Seeing will likely exceed 1 arcsecond, while precipitable water will start out exceeding 4 mm, but will slip toward 3 mm through the second half of the night.

Graphical Summary

Discussion
Although the upper air mass is expected to rapidly dry out around sunset today, residual low-level moisture and instability may keep the inversion elevated and allow for more fog, ice, high humidity and perhaps flurries particularly during the first half of the night. The inversion is set to restrengthen near 7-8 thousand feet through tomorrow and ensure the summit remains dry/stable for the remainder of the forecast period. There is a possibility for isolated convection in the area and more snow for today, then daytime clouds will taper for tomorrow and become minimal and short-lived for Sunday and early part of next week.

Banding high clouds will continue to stream over the summit, contributing to mostly overcast skies for tonight. There is a chance that this stream will break up a bit, but extensive cloud cover will remain an issue until this band sags southward for Monday night. Unfortunately, this band may return northward, increasing cloud cover once again for Tuesday night.

Precipitable water will start out exceeding 4 mm, but will slip toward 3 mm through the second half of the night and will likely settle in near 2-3 mm for following 3 nights, then drop toward 1 mm for Tuesday night.

Strong turbulence in the free atmosphere, combined with lingering moisture/instability will result in bad seeing for tonight. While the air mass is set to stabilize/dry for subsequent nights, persistent turbulence in the free atmosphere will still contribute to poorer than average seeing for tomorrow night. There is a possibility that calmer skies will allow seeing to improve toward more average-like seeing for Sunday and Monday night, while laminar (albeit strengthening) flow aloft could help further improve seeing for Tuesday night.

An embedded short-wave trough in the large scale tropical upper-tropospheric trough is set to pass over the Big Island during the course of the day, then will shift eastward and weaken allowing the TUTT to retreat westward and a new mid-level ridge to build in from the west through the night and into tomorrow. Unfortunately, abundant deep moisture in the area will likely result in more fog, ice and snow at the summit, and there is still a possibility for isolated convection in the area through early this evening. The odds on convection and snow will rapidly drop as the short-wave shifts eastward through the night, but an elevated inversion may still allow for fog, ice and/or high humidity through the night. Eventually, large-scale subsidence associated with the ridge will help rebuild the inversion over the course of tomorrow and ensure the summit remains dry and stable for the remainder of the forecast period. However, the sub-tropical jet is expected to linger in the area and shuttle banding high clouds overhead through the weekend. While there is a possibility that the STJ will briefly shift southeastward, allowing skies to the north and perhaps open up for Monday night, the jet is set to expand westward and broaden/strengthen, bringing a fresh band of high clouds overhead for Tuesday night.

Astronomical Observing Quality Guidance
05 pm HST Friday 20 April (03 UTC Saturday 21 April)CN2
08 pm HST Friday 20 April (06 UTC Saturday 21 April)CN2
11 pm HST Friday 20 April (09 UTC Saturday 21 April)CN2
02 am HST Saturday 21 April (12 UTC Saturday 21 April)CN2
05 am HST Saturday 21 April (15 UTC Saturday 21 April)CN2
08 am HST Saturday 21 April (18 UTC Saturday 21 April)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Fri 20 Apr
08 pm HST
Fri 20 Apr
02 am HST
Sat 21 Apr
02 pm HST
Sat 21 Apr
08 pm HST
Sat 21 Apr
02 am HST
Sun 22 Apr
02 pm HST
Sun 22 Apr
08 pm HST
Sun 22 Apr
02 am HST
Mon 23 Apr
02 pm HST
Mon 23 Apr
02 am HST
Tue 24 Apr
02 pm HST
Tue 24 Apr
02 am HST
Wed 25 Apr
00 UTC
Sat 21 Apr
06 UTC
Sat 21 Apr
12 UTC
Sat 21 Apr
00 UTC
Sun 22 Apr
06 UTC
Sun 22 Apr
12 UTC
Sun 22 Apr
00 UTC
Mon 23 Apr
06 UTC
Mon 23 Apr
12 UTC
Mon 23 Apr
00 UTC
Tue 24 Apr
12 UTC
Tue 24 Apr
00 UTC
Wed 25 Apr
12 UTC
Wed 25 Apr
Cloud Cover (%) 80 to 100 80 to 100 80 to 100 80 to 100 70 to 90 70 to 90 60 to 80 60 to 80 60 to 80 20 to 40 10 to 30 20 to 40 50 to 70
Cloud Height (km) above sea level 4-10 8-10 9-10 9-10 9-10 8.5-10 8-10 8-10 8-10 8-9 8-9 9-10 9-10
Chance for Fog/Precip (%) 100/95 90/40 60/15 40/10 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 8 to 12 4 to 8 3 to 4 3 to 6 2 to 3 2 to 3 2 to 3 2 to 3 2.5 to 3.5 2 to 4 1.5 to 2.5 1 to 2 0.9 to 1.3
Mean Seeing (arcsecs) N/A 1.2 ± 0.4 1.0 ± 0.3 N/A 0.8 ± 0.2 0.75 ± 0.15 N/A 0.65 ± 0.1 0.6 ± 0.1 N/A 0.65 ± 0.15 N/A 0.55 ± 0.1
Summit Temp (°C) 0 -0.5 0 7 1.5 1.5 7.5 2.5 2 7 1.5 7 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)

N/10 to 20

NNE/10 to 20

NE/10 to 20

NE/10 to 20

NE/10 to 20

ENE/10 to 20

ENE/10 to 20

NE/10 to 20

NNE/10 to 20

NNE/10 to 20

N/10 to 20

N/10 to 20

N/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.Apr.20/Sat.Apr.21
18:51
19:58
4:43
5:50
N/A
23:53
33
6 45.7
20 04
Sat.Apr.21/Sun.Apr.22
18:51
19:58
4:42
5:50
N/A
0:51
44
7 46.1
19 21
Sun.Apr.22/Mon.Apr.23
18:51
19:59
4:41
5:49
N/A
1:44
55
8 45.4
17 25
Mon.Apr.23/Tue.Apr.24
18:52
19:59
4:41
5:48
N/A
2:34
66
9 42.8
14 24
Tue.Apr.24/Wed.Apr.25
18:52
20:00
4:40
5:47
N/A
3:19
76
10 38.1
10 34


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Friday 20 April (0300 UTC Saturday 21 April) 2018.
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