Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 18 January (0300 UTC Friday 19 January) 2018

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 0.5 C this evening and 0 C tomorrow morning. Winds will be from the ESE at 15-25 mph for the first half of the night, diminishing to 10-20 mph and switching to a more SE direction for the second half. Seeing may start out near 0.7 arcseconds, but should settle back in near 0.5-0.6 arcseconds as the night progresses. Precipitable water is expected to be in the 0.7-0.9 mm range for the night.

Graphical Summary

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture at or below 7 thousand feet and ensure a dry/stable summit-level air mass through at least Saturday night. There is a chance that building instability will erode the inversion and increase the risk for fog, ice, high humidity and even light flurries at the summit for Sunday and Monday night. There is also a possibility for isolated convection along the eastern skies during that time. Daytime clouds will be minimal and short-lived through Saturday, but could turn extensive for Sunday and early part of next week.

Skies overhead will remain predominately clear for tonight. A stream of high clouds is expected to quickly move in from the west and pass just north and south of the summit area for Friday night, then exit eastward on Saturday, leaving clear skies again for that night. However, there is a possibility that patches of summit/low-level clouds will begin to develop over the eastern end of the Big Island on Sunday and could persist into Monday night.

Precipitable water is expected to linger neaer 0.8-0.9 mm for the next 2 nights, increase toward 1-1.5 mm through Saturday night, then jump to 4+ mm for the following 2 nights.

Minor boundary layer turbulence may contribute to slightly poorer than average seeing for this evening, but seeing should settle in near 0.5-0.6 arcseconds as calmer skies prevail for the second half of the night through Saturday evening. Building instability and upper-level turbulence may begin to degrade seeing early Sunday morning and perhaps contribute to poor/bad seeing for the following 2 nights.

Little change since the morning forecast...The fairly deep ridge will continue to slowly slide off toward the east over the next few days, but will still maintain steady/strong large-scale subsidence in the area through at least sunrise Sunday. This will subsidence will help sustain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass over the next 3 nights. It should also help keep PW primarily at or below 1 mm through at least late Saturday evening. There is a possibility that winds will stir up minor boundary layer turbulence as the ridge begins to slide off toward the NE over the next 12 hours. However, winds should subside as the upper-level portion of the ridge weakens and slides off toward the SE as the night progresses. There is possibility that a rather trough developing to the west will help briefly re-establish the sub-tropical jet (STJ) over the state during the early part of the weekend. The STJ could bring high clouds in from the southwest, while the westerly jet shuttles high clouds along the northern skies for Friday night. While the STJ is set to quickly shift off toward the east, the developing trough may begin to build overhead and destabilize the air mass during the second half of the weekend. This instability, combined with a persistent/residual cold front lingering in the area could help erode the inversion and increase the risk for moisture at the summit for Sunday and Monday night. There is also a chance that isolated convection will develop within this band, particularly along the eastern slopes of the Big Island between late Sunday afternoon and just after sunrise on Monday.

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


5-day Forecast
Summary

Graphical Trend
08 pm HST
Thu 18 Jan
02 am HST
Fri 19 Jan
02 pm HST
Fri 19 Jan
08 pm HST
Fri 19 Jan
02 am HST
Sat 20 Jan
02 pm HST
Sat 20 Jan
08 pm HST
Sat 20 Jan
02 am HST
Sun 21 Jan
02 pm HST
Sun 21 Jan
02 am HST
Mon 22 Jan
02 pm HST
Mon 22 Jan
02 am HST
Tue 23 Jan
02 pm HST
Tue 23 Jan
06 UTC
Fri 19 Jan
12 UTC
Fri 19 Jan
00 UTC
Sat 20 Jan
06 UTC
Sat 20 Jan
12 UTC
Sat 20 Jan
00 UTC
Sun 21 Jan
06 UTC
Sun 21 Jan
12 UTC
Sun 21 Jan
00 UTC
Mon 22 Jan
12 UTC
Mon 22 Jan
00 UTC
Tue 23 Jan
12 UTC
Tue 23 Jan
00 UTC
Wed 24 Jan
Cloud Cover (%) 0 to 5 0 to 5 0 to 20 20 to 40 40 to 60 20 to 40 0 to 20 0 to 10 60 to 80 40 to 60 70 to 90 20 to 40 60 to 80
Cloud Height (km) above sea level N/A N/A N/A 9-10 9-10 9-10 N/A N/A 4-7 4-8 4-9 4-5.5 4-6
Chance for Fog/Precip (%) 0/0 0/0 0/0 0/0 0/0 0/0 0/0 0/0 75/25 80/40 80/20 60/15 60/15
PW (mm, summit upward) 0.7 to 0.9 0.7 to 0.9 0.8 to 1.2 0.8 to 1 0.8 to 1 0.8 to 1.2 0.8 to 1 1 to 1.5 4 to 8 4 to 8 6 to 10 4 to 8 4 to 8
Mean Seeing (arcsecs) 0.65 ± 0.1 0.55 ± 0.1 N/A 0.55 ± 0.1 0.55 ± 0.1 N/A 0.6 ± 0.1 0.65 ± 0.15 N/A 0.9 ± 0.2 N/A 0.8 ± 0.2 N/A
Summit Temp (°C) 0.5 0 4.5 -1 -1.5 4 -1 -2 1 -2.5 2 -2 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)

ESE/15 to 25

SE/10 to 20

SE/5 to 15

SE/5 to 15

SE/5 to 15

SE/0 to 10

E/0 to 10

SE/5 to 15

SSW/5 to 15

SSE/0 to 10

SSE/0 to 10

SSW/0 to 10

E/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
Thu.Jan.18/Fri.Jan.19
18:15
19:23
5:42
6:50
N/A
19:56
5
21 47.3
-14 15
Fri.Jan.19/Sat.Jan.20
18:16
19:23
5:43
6:50
N/A
20:47
10
22 35.1
-11 00
Sat.Jan.20/Sun.Jan.21
18:16
19:24
5:43
6:50
N/A
21:38
16
23 22.4
-7 14
Sun.Jan.21/Mon.Jan.22
18:17
19:24
5:43
6:50
N/A
22:29
24
0 09.8
-3 06
Mon.Jan.22/Tue.Jan.23
18:18
19:25
5:43
6:50
N/A
23:22
33
0 58.0
1 16


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Friday 19 January 2018.
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