Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 12 July (0300 UTC Friday 13 July) 2018

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while thin strands of high clouds fill in from the west throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this evening and 3 C tomorrow morning. Winds will be light and from the NNE, with seeing near 0.5-0.55 arcseconds. Precipitable water is expected to exceed 4 mm through the night.

Graphical Summary

Discussion
The tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and ensure a dry/stable summit-level air mass through at least tonight. There is a chance that an influx of mid/low-level moisture combined with building weak instability will weaken/lift the inversion through Friday night and into Saturday, perhaps allowing the atmosphere to turn fairly saturated for much of Sunday. This could increase the risk for fog, high humidity and light rain at the summit for much of the weekend. The inversion is set to recover on Monday, decreasing this risk for that night. Daytime clouds will be minimal and short-lived today and tomorrow, then could pick up over the weekend and early part of next week.

Thin strands of high clouds will continue to fill in from the west over the next 3 nights, then will shift northward on Sunday. However, there is a possibility that patches of thick clouds will develop in the area and/or drift in from the east and pass through the area for Saturday and Sunday night. More patches of mid/upper-level clouds may fill the western skies for the early part of next week.

Precipitable water is expected to linger near or exceed 4 mm through the next 5 nights.

While relatively calm skies will prevail, likely allowing for better than average seeing over the next 2 nights, there is a chance for variability in seeing if/when winds cease for prolong periods during this time. A possible increase in moisture and instability will likely degrade seeing toward 0.9-1 arcsecond for Saturday and Sunday night. Drier conditions and light winds should allow seeing to improve again for Monday night.

No change since the morning forecast....The mid/low-level ridge to the north, combined with an upper-level ridge to the SW, will promote steady/strong large-scale subsidence, which will help maintain a well-defined inversion near 6-7 thousand feet and ensure a dry/stable summit-level air mass through at least tomorrow afternoon. Calm/stable skies associated with the upper-level ridge should also allow for good/excellent seeing mainly for tonight. This ridge is slated to retreat southward as the tropical upper-tropospheric trough (TUTT) begins to strengthen to the NW of the state through tomorrow and over the weekend. The TUTT and embedded upper-level lows are expected to promote widespread/weak instability in the area, which could erode the inversion and deepen the incoming low-level cloud flow through Friday night, perhaps eventually leading a fairly saturated air mass between late Saturday evening and early Monday morning. This will raise the stakes on fog, high humidity, light rain and extensive cloud cover at the summit as early as Friday night and especially for the following 2 nights. Models suggest that the TUTT will subtly shift northward, allowing the atmosphere to stabilize a bit, which could rebuild the inversion and reduce the risk for moisture at the summit for Monday night. However, long term projections show a patch of unstable/deep low-level clouds moving in from the east as the TUTT digs in to the SW around the middle part of next week. This could result in another round of moisture at the summit during that time, should this pan out.

Astronomical Observing Quality Guidance
05 pm HST Thursday 12 July (03 UTC Friday 13 July)CN2
08 pm HST Thursday 12 July (06 UTC Friday 13 July)CN2
11 pm HST Thursday 12 July (09 UTC Friday 13 July)CN2
02 am HST Friday 13 July (12 UTC Friday 13 July)CN2
05 am HST Friday 13 July (15 UTC Friday 13 July)CN2
08 am HST Friday 13 July (18 UTC Friday 13 July)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Thu 12 Jul
02 am HST
Fri 13 Jul
02 pm HST
Fri 13 Jul
08 pm HST
Fri 13 Jul
02 am HST
Sat 14 Jul
02 pm HST
Sat 14 Jul
08 pm HST
Sat 14 Jul
02 am HST
Sun 15 Jul
02 pm HST
Sun 15 Jul
02 am HST
Mon 16 Jul
02 pm HST
Mon 16 Jul
02 am HST
Tue 17 Jul
02 pm HST
Tue 17 Jul
06 UTC
Fri 13 Jul
12 UTC
Fri 13 Jul
00 UTC
Sat 14 Jul
06 UTC
Sat 14 Jul
12 UTC
Sat 14 Jul
00 UTC
Sun 15 Jul
06 UTC
Sun 15 Jul
12 UTC
Sun 15 Jul
00 UTC
Mon 16 Jul
12 UTC
Mon 16 Jul
00 UTC
Tue 17 Jul
12 UTC
Tue 17 Jul
00 UTC
Wed 18 Jul
Cloud Cover (%) 30 to 50 20 to 40 10 to 30 20 to 40 20 to 40 60 to 80 60 to 80 60 to 80 80 to 100 50 to 70 60 to 80 20 to 40 60 to 80
Cloud Height (km) above sea level 9-10 9.5-10 9.5-10 9-10 9-10 4-8 4-9 4-9 4-9 4.5-8 4-6 9-10 4-7
Chance for Fog/Precip (%) 0/0 0/0 0/0 10/0 25/5 40/10 60/20 80/50 90/60 60/20 50/10 20/5 60/15
PW (mm, summit upward) 4 to 6 4 to 6 4 to 6 4 to 8 4 to 8 6 to 10 8 to 12 8 to 12 8 to 12 6 to 10 6 to 10 4 to 8 4 to 8
Mean Seeing (arcsecs) 0.5 ± 0.15 0.55 ± 0.15 N/A 0.55 ± 0.1 0.6 ± 0.15 N/A 0.9 ± 0.2 1.0 ± 0.3 N/A 0.9 ± 0.2 N/A 0.6 ± 0.2 N/A
Summit Temp (°C) 4 3 9 3.5 2.5 6 1 1 4 1 6 2 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)

NNE/0 to 10

NNE/0 to 10

NNE/0 to 10

N/0 to 10

W/0 to 10

NNW/0 to 10

E/0 to 10

SE/0 to 10

ESE/5 to 15

E/5 to 15

ENE/5 to 15

ENE/10 to 20

E/10 to 20


 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.Jul.12/Fri.Jul.13
19:14
20:27
4:28
5:41
6:23
19:12
0
7 47.3
19 29
Fri.Jul.13/Sat.Jul.14
19:14
20:27
4:29
5:41
N/A
20:12
3
8 50.1
17 27
Sat.Jul.14/Sun.Jul.15
19:14
20:27
4:29
5:42
N/A
21:06
8
9 50.5
14 13
Sun.Jul.15/Mon.Jul.16
19:14
20:26
4:30
5:42
N/A
21:56
15
10 47.8
10 07
Mon.Jul.16/Tue.Jul.17
19:14
20:26
4:30
5:43
N/A
22:41
24
11 42.1
5 31


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Friday 13 July 2018.
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