Mauna Kea Weather Center Forecast

Maunakea Observatories Forecast
10 AM HST (2000 UTC) Friday 18 June 2021

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
There is a small risk for periods of fog and high humidity throughout the night; precipitation is not expected. A band of broken high clouds will continue to stream overhead contributing to extensive cloud cover through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this afternoon and 0.5 C for the night. Winds will be from the west at 10-20 mph for today, easing to 5-15 mph for the night. Seeing will be near 0.7-0.8 arcseconds, while precipitable water is expected to exceed 4 mm for the night.

Discussion
Although the inversion is expected to recover a bit today, patches of mid-level moisture and upper-level instability in the area may allow for periods of fog and high humidity at the summit over the next 2 nights. This risk will quickly subside as the air mass dries/stabilizes and the inversion restrengthens on Sunday, which will help for more normal summit like conditions to return for that night and early part of next week. Extensive afternoon clouds and isolated afternoon convection along the Big Island slopes are possible through Saturday, then will taper for Sunday and early part of next week.

A band of broken clouds will continue to stream overhead, contributing to extensive cloud cover for tonight. This band will shift eastward tomorrow, opening up most of the sky for at least that evening. However, more patches of thin high clouds could fill in from the SW just before sunrise on Sunday. There is a chance that another narrow band will develop over or along the SE of the Big Island for the early part of next week.

Precipitable water is expected to linger or exceed 4 mm for the next 2 nights, briefly slip toward 3 mm for Sunday evening, then increase back to 4+ mm for the early part of next week.

A mixture of low-level turbulence and upper-level shear could contribute to poor seeing for tonight. There is a good chance that seeing will improve as both subside through tomorrow night, but an increase in boundary/low-level turbulence may degrade seeing through Sunday night. Seeing is set to settle back in near 0.5-0.6 arcseconds as light/laminar westerly flow and a stable air mass prevails above the summit for Monday and Tuesday night.

The mid-level ridge will remain weak/indistinct as the tropical upper-tropospheric trough (TUTT) persists to the west of the state well into next week and an embedded midupper-level to the north gradually slips eastward over the next 2 nights. While fortunately, the low-level ridge will continue to sit to the north of the state and promote relatively steady large-scale subsidence in the area and partially negate the affects of the low/TUTT throughout the forecast period. However, pockets of instability associated with the low could still enhance the low-level cloud field and allow for the periods of fog/high humidity at the summit over the next 2 nights. In addition, the low may enhance/deepen the sub-tropical jet overhead, which will bring broken high clouds and turbulence to the area mainly for tonight. The turbulence and clouds are set to shift off toward the east with the low, which should help to improve seeing and clear summit skies, respectively, through tomorrow night. The mid-level ridge is also set to slowly build in from the west, helping to enhance subsidence in the area and rebuilding a more defined inversion near 6-7 thousand feet by that afternoon/evening. This, coupled with the eastward departure of the low will eliminate the risk for fog/high humidity allow dry/stable conditions to return to the summit for Sunday night and early part of next week. The STJ and TUTT area also expected to weaken, but may persist in the area, and the former may still stream high clouds out of the tropics and over/near the summit during that time

WRF Astronomical Observing Quality Guidance
Cloud Cover and Precipitable Water Analyses
MK CN² Profiles

5 Day Forecast Summary (Graphical Trend)

HST	Cloud		Fog/Precip	Temp	Wind	Seeing	PW
HST	Cover (%)	Height (km)	Probability (%)	(Celsius)	(Dir/MPH)	(Arcseconds)	(mm)
Fri Jun 18 - 2 PM	40-60	4-8	60 / 15	4	W/10-20	NaN	4-8
8 PM	60-80	7-9	30 / 5	0.5	W/5-15	0-1.6	4-6
Sat Jun 19 - 2 AM	60-80	7-10	25 / 5	0.5	W/5-15	0.55-0.85	4-6
2 PM	20-40	4-8	60 / 15	5	WSW/5-15	NaN	4-8
8 PM	0-20	Clear	35 / 5	0	WSW/5-15	0.5-0.8	4-6
Sun Jun 20 - 2 AM	10-30	9.5-10	30 / 5	-0.5	SW/5-15	0.45-0.65	4-6
2 PM	20-40	9-10	20 / 5	7	SSW/15-25	NaN	3-5
8 PM	20-40	9-10	0 / 0	2.5	SW/15-25	0.5-0.8	2.5-3.5
Mon Jun 21 - 2 AM	20-40	9-10	0 / 0	2	SW/15-25	0.55-0.85	3-4
2 PM	40-60	9-10	0 / 0	8	SSW/10-20	NaN	4-6
Tue Jun 22 - 2 AM	30-50	9-10	0 / 0	4	SSW/5-15	0.4-0.7	4-6
2 PM	20-40	9-10	0 / 0	10	W/5-15	NaN	4-8
Wed Jun 23 - 2 AM	10-30	8-9	0 / 0	4.5	N/5-15	0.4-0.7	4-6

Rise and Set times for the Sun and Moon

Night (HST)	Sun Set	Twilight End	Twilight Beg	Sun Rise	Moon Rise	Moon Set	Illumination (%)	RA	DEC
Fri Jun 18 - Sat Jun 19	19:13	20:27	4:20	5:34	N/A	1:39	63	12 57.0	-2 03
Sat Jun 19 - Sun Jun 20	19:13	20:27	4:20	5:34	N/A	2:20	74	13 49.1	-8 10
Sun Jun 20 - Mon Jun 21	19:13	20:28	4:20	5:34	N/A	3:03	84	14 44.1	-14 03
Mon Jun 21 - Tue Jun 22	19:14	20:28	4:20	5:34	N/A	3:52	92	15 42.9	-19 14
Tue Jun 22 - Wed Jun 23	19:14	20:28	4:20	5:35	N/A	4:47	97	16 45.7	-23 16

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Friday 18 June (0300 UTC Saturday 19 June) 2021.

Additional Information
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Current Conditions
Temp		1.2 C
RH		18 %
Wind		W 0 mph
Road		Open (4x4)