Latest Forecast for Mauna Kea Observatories
5 PM HST Tuesday 10 September (0300 UTC Wednesday 11 September) 2019

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while high clouds pass along the far northern and southern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 3 C this evening and 2.5 C tomorrow morning. Winds will be from the WNW at 5-15 mph, with seeing near 0.5-0.6 arcseconds. Precipitable water is expected to be in the 2.5-3.5 mm range for the night.

Graphical Summary

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 9 thousand feet and ensure a dry/stable summit-level air mass through at least Thursday night. Building instability to the north, combined with a possible influx of moisture from the south could begin to weaken/lift the inversion and increase the risk for periods of fog and light rain at the summit for Friday and especially Saturday night. Some minor daytime clouds are possible over the next 2 days, then could turn extensive with a chance for afternoon thunderstorms for Friday and over the weekend.

Skies overhead will remain predominately clear as high clouds pass along the far northern and southern skies for tonight. The latter set is set to creep northward through tomorrow and could scrape the southern portion of the Big Island for Wednesday and Thursday night. There is a good chance that these clouds will sag back southward on Friday, but patches of mid/summit-level clouds may begin to develop in the area and/or drift in from the NE for that night and especially Saturday night.

Precipitable water is expected to linger near 3 mm over the next 2 nights, then increase to 4+ mm for Thursday, Friday and Saturday night.

Predominately calm/stable skies as laminar westerly flow prevails in the free atmosphere, likely allowing for better than average seeing over the next 3 nights. However, there is a possibility for periods of variability due to horizontal shear mainly for Thursday night. A possible influx of moisture and instability, combined with more rounds of horizontal shear in the free atmosphere will likely contribute to poor seeing for Friday and Saturday night.

No change since the morning forecast...Although the mid-level ridge will remain rather indistinct and displaced further northward in response to a relatively deep low to the NE, the persistent low-level ridge to the NE will continue to promote large-scale subsidence in the area throughout the forecast period. In addition, weak subsidence associated with the western part of the low will prevail over the state through the next 2-3 days. Consequently, the subsidence will help maintain a well-defined inversion at or below 9 thousand feet and ensure a dry/stable summit-level air mass for at least the next 3 nights. Skies overhead will remain predominately calm as the sub-tropical jet along the southern flank of the low, sits just north of the Big Island for during this time. This should allow for better than average seeing, though bouts of horizontal shear associated with the southern flank of the jet may pass overhead, causing minor disruption in seeing, particularly later in the week. The last few model runs suggest that the upper-level low will eventually wobble westward, which could begin to destabilize the air mass and perhaps even drag tropical moisture over the Big Island beginning late Friday night. This combination may erode/lift the inversion and thus increase the risk for fog and rain at the summit over the weekend. There is also a possibility that this could lead to the development of afternoon convection over the Big Island slopes during that time.

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
Tue 10 Sep
02 am HST
Wed 11 Sep
02 pm HST
Wed 11 Sep
08 pm HST
Wed 11 Sep
02 am HST
Thu 12 Sep
02 pm HST
Thu 12 Sep
08 pm HST
Thu 12 Sep
02 am HST
Fri 13 Sep
02 pm HST
Fri 13 Sep
02 am HST
Sat 14 Sep
02 pm HST
Sat 14 Sep
02 am HST
Sun 15 Sep
02 pm HST
Sun 15 Sep
06 UTC
Wed 11 Sep
12 UTC
Wed 11 Sep
00 UTC
Thu 12 Sep
06 UTC
Thu 12 Sep
12 UTC
Thu 12 Sep
00 UTC
Fri 13 Sep
06 UTC
Fri 13 Sep
12 UTC
Fri 13 Sep
00 UTC
Sat 14 Sep
12 UTC
Sat 14 Sep
00 UTC
Sun 15 Sep
12 UTC
Sun 15 Sep
00 UTC
Mon 16 Sep
Cloud Cover (%) 0 to 10 0 to 10 40 to 60 0 to 20 10 to 30 20 to 40 10 to 30 0 to 20 60 to 80 0 to 20 70 to 90 20 to 40 0 to 20
Cloud Height (km) above sea level N/A N/A 4-5 9-10 9-10 4-4.5 9-10 9.5-10 4-5.5 6-8 4-8 5-8 4-9
Chance for Fog/Precip (%) 0/0 0/0 30/5 0/0 0/0 20/5 0/0 0/0 60/20 50/10 75/30 80/40 90/60
PW (mm, summit upward) 2.5 to 3.5 2.5 to 3.5 3 to 6 2.5 to 3.5 2.5 to 3.5 3 to 5 4 to 6 4 to 6 4 to 8 4 to 6 6 to 10 6 to 10 8 to 12
Mean Seeing (arcsecs) 0.55 ± 0.1 0.55 ± 0.1 N/A 0.5 ± 0.1 0.55 ± 0.1 N/A 0.55 ± 0.1 0.6 ± 0.15 N/A 0.75 ± 0.15 N/A 0.9 ± 0.2 N/A
Summit Temp (°C) 3 2.5 8 3 2.5 7.5 2.5 2 5.5 1 4.5 0.5 4
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)

WNW/5 to 15

WNW/5 to 15

WNW/0 to 10

NE/0 to 10

ESE/0 to 10

NE/0 to 10

NE/0 to 10

W/5 to 15

SSW/5 to 15

SW/5 to 15

SW/10 to 20

SSW/5 to 15

SW/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.Sep.10/Wed.Sep.11
18:38
19:43
4:55
6:00
N/A
4:18
93
21 24.7
-18 29
Wed.Sep.11/Thu.Sep.12
18:37
19:42
4:55
6:00
17:13
5:09
97
22 13.0
-15 16
Thu.Sep.12/Fri.Sep.13
18:36
19:41
4:55
6:00
17:51
5:58
99
22 59.7
-11 26
Fri.Sep.13/Sat.Sep.14
18:35
19:40
4:55
6:00
18:27
6:46
100
23 45.2
-7 10
Sat.Sep.14/Sun.Sep.15
18:34
19:39
4:56
6:00
19:02
N/A
98
0 29.9
-2 38


Forecast issued by: Ryan Lyman
Contact Us - (808) 932-2323
Hours: Mon-Fri 8 AM - 5 PM HST (1800-0300 UTC)

For public road conditions and snow report message please call (808) 935-6268.
This message is also available at the MKWC road conditions page.

Next update at 10 AM HST (2000 UTC) Wednesday 11 September 2019.
Please call or email to be placed on the Mauna Kea Weather Bulletin email list.