Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 19 April (0300 UTC Friday 20 April) 2018

Warnings
Fog, ice and high humidity
Chance for convection and snow

Cloud Cover and Precipitation Forecast
Fog, ice, high humidity, thick clouds and flurries are expected to plague the summit through the night. There is also a high probability for convection in the area throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -2 C, with winds from the NW at 5-15 mph for this evening, switching to a more SW direction through the night. Seeing and precipitable water are expected to exceed 1 arcsecond and 4 mm, respectively, through the night.

Graphical Summary

Discussion
Building instability will continue to erode the inversion over and allow the atmosphere to become saturated through late tomorrow evening. This will result in inoperable conditions, as extensive fog, ice, high humidity and/or flurries plague the summit for tonight. There is also a high probability for convection in the area, which could deposit heavier snow at the summit during this period. There is a good chance that fog, ice and high humidity will continue to be an issue through Friday night, but a strengthening inversion should help diminish this risk through Saturday and ensure the summit remains dry and stable for the following 2 nights. Extensive daytime clouds are expected tomorrow, then will begin to taper on Saturday and become minimal and short-lived for Sunday and early part of next week.

Overcast thick clouds will continue to stream over the summit through early tomorrow evening. There is a very good chance that the mid-level clouds will shift off toward the east, but banding high clouds will continue to pass over the summit, contributing to extensive cloud cover, if not overcast skies through Monday morning. This band will shift toward the southern skies thereafter, opening up skies overhead and especially to the north for that night.

Precipitable water is expected to exceed 4 mm for the next 2 nights, then slip toward and settle in near 2 mm for the following 3 nights.

An influx of abundant moisture, instability and turblence will likely contribute to bad seeing over the next 2 nights. There is a chance that seeing could return toward more average-like values as the atmosphere dries and moderate turbulence prevails in the free atmosphere for Saturday and especially the following 2 nights.

No change since the morning forecast...The absence of a mid-level ridge, combined with the return of the tropical upper-tropospheric trough and a steady supply of moisture via the low-level trades will allow the atmosphere near the Big Island to quickly become saturated and unstable over the next several hours. These conditions are set to persist until the TUTT shifts off toward the east and weakens, while a new ridge builds in from the west early Saturday morning. Consequently, extensive fog, high humidity, ice, thick clouds and flurries are expected to plague the summit for tonight and into tomorrow evening. There is a very good chance for convection in the area during this period, which could periodically deposit heavier snow at the summit during this time. Subsidence associated with the building ridge is expected to quickly stabilize and dry out the air mass and rebuild the inversion near 8-9 thousand feet through Saturday. This should virtually ensure the summit steers free of moisture for that night and especially the following 2 nights. However, while the sub-tropical jet is expected to weaken with the decay of the TUTT, it will continue to stream high clouds out of the tropics, contributing to extensive cloud cover if not overcast skies until its final demise around the early part of next week. Free atmospheric turbulence associated with the jet will also keep seeing near average-like values for Saturday, Sunday and Monday night (though a very gradual modest improvement in seeing is possible during those nights).

Astronomical Observing Quality Guidance
05 pm HST Thursday 19 April (03 UTC Friday 20 April)CN2
08 pm HST Thursday 19 April (06 UTC Friday 20 April)CN2
11 pm HST Thursday 19 April (09 UTC Friday 20 April)CN2
02 am HST Friday 20 April (12 UTC Friday 20 April)CN2
05 am HST Friday 20 April (15 UTC Friday 20 April)CN2
08 am HST Friday 20 April (18 UTC Friday 20 April)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Thu 19 Apr
02 am HST
Fri 20 Apr
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
02 am HST
Mon 23 Apr
02 pm HST
Mon 23 Apr
02 am HST
Tue 24 Apr
02 pm HST
Tue 24 Apr
06 UTC
Fri 20 Apr
12 UTC
Fri 20 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
12 UTC
Mon 23 Apr
00 UTC
Tue 24 Apr
12 UTC
Tue 24 Apr
00 UTC
Wed 25 Apr
Cloud Cover (%) 80 to 100 80 to 100 80 to 100 80 to 100 70 to 90 80 to 100 80 to 100 80 to 100 80 to 100 70 to 90 50 to 70 30 to 50 10 to 30
Cloud Height (km) above sea level 4-10 4-10 4-10 4-10 9-10 9-10 8-10 8-10 8-10 8-10 8-9 8-9 9-10
Chance for Fog/Precip (%) 100/95 100/95 100/95 95/60 80/20 50/10 25/5 10/0 0/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 10 to 15 10 to 15 10 to 15 8 to 12 4 to 8 3 to 6 1.5 to 2.5 1.5 to 2.5 1.5 to 2.5 1.5 to 2.5 1.5 to 2.5 1.5 to 2.5 1.5 to 2.5
Mean Seeing (arcsecs) 1.5 ± 0.5 1.5 ± 0.5 N/A 1.2 ± 0.4 1.0 ± 0.3 N/A 0.7 ± 0.2 0.65 ± 0.15 N/A 0.65 ± 0.15 N/A 0.6 ± 0.1 N/A
Summit Temp (°C) -2 -2 1 -1 -0.5 5 1 1 7 2 7 2 6.5
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)

NW/5 to 15

SW/5 to 15

N/5 to 15

NE/5 to 15

NE/10 to 20

ENE/10 to 20

ENE/10 to 20

ENE/10 to 20

E/5 to 15

NE/5 to 15

NNE/5 to 15

NNE/5 to 15

NNE/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.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
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


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Friday 20 April 2018.
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