Latest Forecast for Mauna Kea Observatories
5 PM HST Monday 12 August (0300 UTC Tuesday 13 August) 2019

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while scattered high clouds flow in from the SW, passing over and especially along the northern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this evening and 3 C tomorrow morning. Winds will be from the east at 10-20 mph, with seeing near 0.6-0.7 arcseconds. Precipitable water is expected to be in the 2-3 mm range for hte night.

Graphical Summary

Discussion
The tradewind inversion will continue to cap low-level moisture near 6-7 thousand feet and ensure the summit remains dry and stable for at least the next 2 nights. There is a possibility that a patch of moisture passing to the south, combined with instability to the north will briefly weaken/lift the inversion and increase the risk for short-lived fog and high humidity at the summit for Wednesday night; precipitation is not expected. The inversion is set to recover by late Thursday morning, which will ensure the summit steers free of moisture for the following 2 nights. Minor daytime clouds and perhaps isolated afternoon convection along the western skies are possible through Thursday, then will become minimal and short-lived on Friday and over the weekend.

More scattered to broken high clouds are projected to drift in from the SW and pass mainly along the northern skies for tonight. There is a good chance that these clouds will become more widespread and slip directly over the summit, contributing to extensive cloud cover or even periods of overcast skies for Tuesday and Wednesday night. These clouds will begin to break up a bit on Thursday, but will still continue to stream over/near the summit into the weekend.

Precipitable water is expected to linger in the 2-3 mm range through Tuesday night, briefly slip toward 1 mm for Wednesday night, then increase to 1.5 mm for Thursday night and probably 2+ mm again for Friday night.

Despite relatively calm skies in the free atmosphere, a slight increase in boundary layer turblence will contribute to more average-like seeing for tonight. Boundary layer turbulence is expected to pick up further tomorrow, likely resulting in poor seeing for the remainder of the forecast period.

Subtle changes have been made to the PW forecast....The mid/low-level ridge will continue to sit over or just north of the Big Island for the next 3 days, then will expand off toward the north and west later in the week and into the weekend. Nevertheless, the ridge will promote steady/strong large-scale subsidence in the area, which will help maintain a fairly well-defined inversion near 6-7 thousand feet and ensure the summit remains dry and stable throughout most of the forecast period. The only possible exception for moisture at the summit is when a when an old tropical wave passes through or just south of the Big Island around Wednesday night. This wave may take advantage of widespread (albiet weak) instability associated with the quasi-steady tropical upper-tropospheric trough (TUTT) to the NW, which could elevate the inversion and incerase the risk for short-lived fog at the summit for that night. In addition, daytime heating (of the land) combined with this instability may allow for the development of afternoon convection along the Big Island slopes, particularly the west-side, mainly through Thursday. The TUTT and its southern counterpart, the sub-tropical jet (STJ), is expected to gain access to more widespread clouds and stream it over/near the Big Island probably through the next 5 nights. There is a chance that these clouds will break up a bit as the TUTT shifts subtly northward and loses access to some of the tropical high clouds during the latter part of the week. Finally, it appears that summit-level winds are set to pick up as the ridge to the NE broadens/strengthens over the next 24-48 hours. This will result in an increase in boundary layer turbulence, which will contribute to more average-like seeing for tonight and probably poor seeing for the following 4 nights.

Astronomical Observing Quality Guidance
Seeing/CN2 Images are not available today. Sorry for the inconvenience.
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Mon 12 Aug
02 am HST
Tue 13 Aug
02 pm HST
Tue 13 Aug
08 pm HST
Tue 13 Aug
02 am HST
Wed 14 Aug
02 pm HST
Wed 14 Aug
08 pm HST
Wed 14 Aug
02 am HST
Thu 15 Aug
02 pm HST
Thu 15 Aug
02 am HST
Fri 16 Aug
02 pm HST
Fri 16 Aug
02 am HST
Sat 17 Aug
02 pm HST
Sat 17 Aug
06 UTC
Tue 13 Aug
12 UTC
Tue 13 Aug
00 UTC
Wed 14 Aug
06 UTC
Wed 14 Aug
12 UTC
Wed 14 Aug
00 UTC
Thu 15 Aug
06 UTC
Thu 15 Aug
12 UTC
Thu 15 Aug
00 UTC
Fri 16 Aug
12 UTC
Fri 16 Aug
00 UTC
Sat 17 Aug
12 UTC
Sat 17 Aug
00 UTC
Sun 18 Aug
Cloud Cover (%) 40 to 60 30 to 50 50 to 70 60 to 80 70 to 90 70 to 90 60 to 80 60 to 80 40 to 60 20 to 40 30 to 50 20 to 40 0 to 20
Cloud Height (km) above sea level 9-10 9-10 8.5-10 8.5-10 8.5-10 8.5-10 9-10 9-10 9-10 9-10 9-10 9-10 9-10
Chance for Fog/Precip (%) 0/0 0/0 25/5 0/0 0/0 35/10 15/0 15/0 10/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 2 to 3 2 to 3 3 to 5 2 to 3 2 to 3 3 to 5 0.8 to 1.2 0.8 to 1.2 3 to 5 1 to 2 1 to 2 2 to 3 2 to 4
Mean Seeing (arcsecs) 0.65 ± 0.15 0.65 ± 0.15 N/A 0.8 ± 0.2 0.9 ± 0.2 N/A 0.9 ± 0.2 0.9 ± 0.2 N/A 0.9 ± 0.2 N/A 1.0 ± 0.3 N/A
Summit Temp (°C) 4 3 9 3 2 7 2 1 9 4.5 10 4 9
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)

E/10 to 20

E/10 to 20

ENE/15 to 30

ENE/15 to 30

ENE/15 to 30

SE/15 to 30

SE/15 to 30

SE/15 to 30

ESE/15 to 30

ESE/15 to 30

ENE/15 to 30

ENE/20 to 35

NE/15 to 30


 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
Mon.Aug.12/Tue.Aug.13
19:02
20:10
4:44
5:52
N/A
4:40
96
19 56.6
-22 11
Tue.Aug.13/Wed.Aug.14
19:01
20:09
4:44
5:53
17:50
5:31
99
20 48.5
-20 24
Wed.Aug.14/Thu.Aug.15
19:00
20:08
4:45
5:53
18:33
6:22
100
21 38.5
-17 43
Thu.Aug.15/Fri.Aug.16
18:59
20:07
4:45
5:53
19:13
N/A
99
22 26.6
-14 20
Fri.Aug.16/Sat.Aug.17
18:59
20:07
4:46
5:54
19:51
N/A
97
23 12.9
-10 24


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Tuesday 13 August 2019.
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