Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 11 October (0300 UTC Thursday 12 October) 2017

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a risk for fog and high humidity mainly toward the end of the night; precipitation is unlikely. Scattered high clouds are set to move in from the west through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 3 C this evening and 2 C tomorrow morning. Winds will be from the ESE at 5-15 mph, with seeing near 0.5-0.55 arcseconds. Precipitable water is expected to linger near 4 mm for the night.

Graphical Summary

Discussion
The tradewind inversion is expected to breakdown during the second half of the night, allowing the air mass to become saturated and unstable for tomorrow night and especially the following 3 nights. Nonetheless, this will increase the risk for fog, high humidity and ice at the summit through tonight and particularly for the following 4 nights. Convection in the area, and a mix of snow/rain and extensive fog and at the summit are also possible mainly after tonight. Extensive daytime clouds are expected for the remainder of the week, then will taper for Monday.

Scattered high clouds are set to move in from the west increasing cloud cover through the night. Thicker more widespread clouds are expected to develop in the area, contributing to extensive cloud cover or even overcast skies for the remainder of the forecast period.

Precipitable water is expected to linger near or exceed 4 mm probably throughout the forecast period.

Relatively calm skies and uniform/light westerly flow in the free atmosphere should allow for better than average seeing for much of tonight (some degradation is possible toward the very end of the night). An increase in upper-level instability/turbulence as well as moisture will likely contribute to poor/bad seeing for the following 4 nights.

No change since the morning forecast....A patch of tropical moisture associated with a mid/low-level low passing to the south is expected to move in from the SE over the next 24 hours, eroding the inversion toward the end of the night. This moisture will eventually stall out in the area, while more unstable residual low-level moisture (from an old low) fills in from the NE with the low-level trades and persists through the weekend. This influx of moisture combined with instability associated with a persistent upper-level trough building to the NW will further erode the inversion and allow the air mass to become quite saturated between late Thursday afternoon and probably most of Sunday night. This will significantly increase the risk for overcast skies heavy fog, high humidity as well as ice and a mixture of snow/rain at the summit during that time. There is also a possibility for convection in the area (especially along the eastern slopes) mainly for Friday and Saturday night. The moisture and instability are set to shift off toward the south and SE, respectively as a rather broad ridge builds in to the north during the early part of next week. This should help to dry out the air mass and allow more normal summit-level conditions to return for Monday night.

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


5-day Forecast
Summary

Graphical Trend
08 pm HST
Wed 11 Oct
02 am HST
Thu 12 Oct
02 pm HST
Thu 12 Oct
08 pm HST
Thu 12 Oct
02 am HST
Fri 13 Oct
02 pm HST
Fri 13 Oct
08 pm HST
Fri 13 Oct
02 am HST
Sat 14 Oct
02 pm HST
Sat 14 Oct
02 am HST
Sun 15 Oct
02 pm HST
Sun 15 Oct
02 am HST
Mon 16 Oct
02 pm HST
Mon 16 Oct
06 UTC
Thu 12 Oct
12 UTC
Thu 12 Oct
00 UTC
Fri 13 Oct
06 UTC
Fri 13 Oct
12 UTC
Fri 13 Oct
00 UTC
Sat 14 Oct
06 UTC
Sat 14 Oct
12 UTC
Sat 14 Oct
00 UTC
Sun 15 Oct
12 UTC
Sun 15 Oct
00 UTC
Mon 16 Oct
12 UTC
Mon 16 Oct
00 UTC
Tue 17 Oct
Cloud Cover (%) 20 to 40 40 to 60 80 to 100 60 to 80 70 to 90 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 0 to 20
Cloud Height (km) above sea level 9-10 8-9 4-10 4-10 4-10 4-10 4-10 4-10 4-10 4-10 4-10 4-10 4-5
Chance for Fog/Precip (%) 10/0 40/10 75/25 80/40 90/60 95/90 95/90 95/90 95/90 95/90 95/90 90/75 50/10
PW (mm, summit upward) 3.5 to 4.5 3.5 to 4.5 4 to 8 6 to 10 6 to 10 10 to 15 8 to 12 8 to 12 10 to 15 8 to 12 10 to 15 8 to 12 2 to 4
Mean Seeing (arcsecs) 0.5 ± 0.1 0.55 ± 0.15 N/A 0.75 ± 0.15 0.9 ± 0.2 N/A 1.5 ± 0.4 1.5 ± 0.4 N/A 1.5 ± 0.4 N/A 1.2 ± 0.4 N/A
Summit Temp (°C) 3 2 5 0.5 0.5 3 -0.5 -0.5 2 -0.5 2 0 6
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)

ESE/5 to 15

ESE/5 to 15

SSE/10 to 20

SE/5 to 15

SSE/10 to 20

S/10 to 20

SSW/5 to 15

NW/0 to 10

ENE/0 to 10

W/0 to 10

NW/0 to 10

NW/0 to 10

NE/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
Wed.Oct.11/Thu.Oct.12
18:10
19:14
5:02
6:07
23:34
N/A
50
7 20.1
19 01
Thu.Oct.12/Fri.Oct.13
18:09
19:14
5:03
6:07
0:34
N/A
39
8 18.7
17 34
Fri.Oct.13/Sat.Oct.14
18:09
19:13
5:03
6:07
1:33
N/A
29
9 15.1
15 06
Sat.Oct.14/Sun.Oct.15
18:08
19:12
5:03
6:08
2:31
N/A
19
10 09.1
11 50
Sun.Oct.15/Mon.Oct.16
18:07
19:11
5:04
6:08
3:27
N/A
12
11 00.8
8 00


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Thursday 12 October 2017.
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