Latest Forecast for Mauna Kea Observatories
5 PM HST Tuesday 17 April (0300 UTC Wednesday 18 April) 2018

Warnings
Chance for fog/high humidity

Cloud Cover and Precipitation Forecast
There is a small risk for fog and high humidity, mainly during the first half of the night; precipitation is not expected. Broken to overcast mid/upper-level clouds will continue to fill in from the WSW 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 light and from the north, with seeing near 0.7-0.75 arcseconds. Precipitable water is expected to exceed 4 mm through the night.

Graphical Summary

Discussion
While the tradewind inversion will continue to restrengthen, persistent mid-level moisture may still permit for periods of high humidity and perhaps short-lived fog mainly for this evening; precipitation is not expected. Unfortunately, building instability will help erode the inversion again through tomorrow night, allowing the atmosphere to become saturated early Thursday morning and late Friday night. This will significantly increase the risk for fog, ice and flurries at the summit as well as isolated convection in the area during that period. There is a good chance that the atmosphere will begin to stabilize while the inversion rebuilds near 10-12 thousand feet, which should help to reduce the risk for moisture at the summit as the weekend progresses. Some afternoon clouds are possible for today, then could pick up tomorrow, turn extensive for Thursday and Friday, then slowly taper over the weekend.

Broken to overcast high clouds will continue to pour in from the SW, contributing to extensive cloud cover, if not overcast skies, probably through the next 5 nights. There is also a possibility for thicker/summit-level clouds in the area mainly between late Wednesday night and early Saturday morning.

Precipitable water will continue to linger near or exceed 4 mm through at least Friday night, but could slip toward 3 mm for Saturday night.

Persistent mid-level moisture combined with light/moderate turblence in the free atmosphere will contribute to poorer than average seeing for much of tonight. An increase in moisture, instability and turblence will likely further degrade seeing for the following 3 nights. There is a chance that seeing could return toward more average-like values for as the atmosphere dries and moderate turbulence prevails in the free atmosphere for Saturday night.

No change since the morning forecast...Although the mid-level ridge will remain weak/indistinct into the weekend, the tropical upper-tropospheric trough will continue to shift westward for another 12-18 hours, allowing the low-level ridge to reinstate large-scale subsidence in the area for tonight. This should help rebuild the inversion near 7-8 thousand feet and decrease summit-level humidity as well as the risk for fog/ice at the summit as the night progresses. Unfortunately, a very persistent sub-tropical jet along the SE flank of the TUTT will linger over/near the Big Island and will continue to shuttle high clouds over the summit area probably throughout the forecast period. Furthermore, the TUTT is set to shift back eastward through tomorrow, which will likely destabilize the air mass, erode the inversion and allow the atmosphere to become mostly saturated between late Wednesday night and early Saturday morning. This will likely contribute to inoperable conditions, with extensive fog, ice, high humidity and light flurries plaguing the summit during that period (Thursday night in particular). Isolated convection is also possible in the area, especially along the slopes during the afternoon hours and just offshore near sunrise. The air mass is expected to restabilize again once the TUTT flattens out and a mid-level ridge begins to slowly fill in from the west over the weekend. Still, lingering instability associated with the TUTT may limit the inversion from strengthening much until Sunday, while more high clouds fill out of the SW late in the weekend and for the early part of next week.

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


5-day Forecast
Summary

Graphical Trend
08 pm HST
Tue 17 Apr
02 am HST
Wed 18 Apr
02 pm HST
Wed 18 Apr
08 pm HST
Wed 18 Apr
02 am HST
Thu 19 Apr
02 pm HST
Thu 19 Apr
08 pm HST
Thu 19 Apr
02 am HST
Fri 20 Apr
02 pm HST
Fri 20 Apr
02 am HST
Sat 21 Apr
02 pm HST
Sat 21 Apr
02 am HST
Sun 22 Apr
02 pm HST
Sun 22 Apr
06 UTC
Wed 18 Apr
12 UTC
Wed 18 Apr
00 UTC
Thu 19 Apr
06 UTC
Thu 19 Apr
12 UTC
Thu 19 Apr
00 UTC
Fri 20 Apr
06 UTC
Fri 20 Apr
12 UTC
Fri 20 Apr
00 UTC
Sat 21 Apr
12 UTC
Sat 21 Apr
00 UTC
Sun 22 Apr
12 UTC
Sun 22 Apr
00 UTC
Mon 23 Apr
Cloud Cover (%) 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 80 to 100 60 to 80 80 to 100
Cloud Height (km) above sea level 6.5-10 7-10 6-10 5-10 4-10 4-10 4-10 4-10 4-10 4-10 4-8 8-10 6-10
Chance for Fog/Precip (%) 25/0 10/0 60/20 75/40 90/60 100/95 100/95 100/95 95/75 90/30 75/15 25/5 10/0
PW (mm, summit upward) 4 to 6 4 to 6 4 to 8 6 to 10 8 to 12 10 to 15 10 to 15 10 to 15 10 to 15 6 to 10 4 to 8 2 to 4 3 to 5
Mean Seeing (arcsecs) 0.75 ± 0.15 0.7 ± 0.1 N/A 0.8 ± 0.2 1.1 ± 0.3 N/A 1.5 ± 0.5 1.5 ± 0.5 N/A 1.2 ± 0.4 N/A 0.7 ± 0.2 N/A
Summit Temp (°C) -0.5 0 4.5 0 -1.5 1 -1.5 -2 1.5 0 3 0.5 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)

N/0 to 10

N/0 to 10

NNW/0 to 10

WNW/0 to 10

SSE/0 to 10

W/5 to 15

NNE/0 to 10

SSW/0 to 10

NNE/0 to 10

ENE/5 to 15

E/10 to 20

ENE/5 to 15

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
Tue.Apr.17/Wed.Apr.18
18:50
19:56
4:46
5:53
N/A
20:50
7
3 48.4
14 41
Wed.Apr.18/Thu.Apr.19
18:50
19:57
4:45
5:52
N/A
21:51
14
4 45.8
17 37
Thu.Apr.19/Fri.Apr.20
18:50
19:57
4:44
5:51
N/A
22:53
22
5 45.3
19 27
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


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