Mauna Kea Weather Center Forecast

Maunakea Observatories Forecast
5 PM HST Wednesday 17 August (0300 UTC Thursday 18 August) 2022

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
There is a small chance for periods of fog and high humidity throughout the night; precipitation is not expected. Isolated thin high clouds are expected to fill in from the south and pass over the summit area, mainly during the second half of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 1.5 C this evening and 1 C for tomorrow morning. Winds will be from the east at 5-15 mph, with seeing near 0.6-0.65 arcseconds. Precipitable water is expected to linger near 4 mm for the night.

Discussion
Weak instability and minor moisture passing through will keep the inversion rather indistinct, which could allow for short-lived fog and high humidity for tonight. The inversion is set to restrengthen and become quite well-defined at or below 9 thousand feet, ensuring a dry/stable summit-level air mass for the following 4 nights. Daytime clouds will taper for tomorrow, then will be minimal and short-lived for the remainder of the forecast period and early part of next week.

Isolated thin high clouds are set to develop to the south and pass over the summit area through early Friday morning. These clouds will shift to the western skies thereafter, leaving predominately clear skies for the weekend.

Precipitable water is expected to linger near 4 mm for tonight, trend back toward 2 mm through tomorrow night and to 1 mm through Friday night, then settle in around 0.8-1 mm for the following 2 nights.

Despite relatively weak flow aloft, minor low-level moisture and instability may contribute to more average-like and/or variable seeing for tonight. Calm skies and dry/stable conditions should allow seeing to settle in near 0.5-0.55 arcseconds for Thursday night. However, there is a possibility for light boundary layer turbulence which may result in slightly poorer than average seeing for Friday and Saturday night. Seeing is expected to improve toward 0.5 arcseconds as winds summit-level winds taper and deep easterly flow prevails in the free atmosphere for Sunday night.

Little change since the morning forecast...While the surface ridge will persist to the north of the state and promote steady large-scale subsidence in the area through the week, the mid-level ridge will remain rather indistinct and displaced toward the west coast for another 18 hours. A rather broad upper-level low to the west will provide weak instability in the area and draw patches of organized low-level moisture up from the tropics during this time. This may help elevate the inversion and could allow for short-lived fog and high humidity at the summit for tonight, though the odds are low. More likely the moisture/instability will contribute to high PW and perhaps slightly poorer than average seeing. The inversion is set to quickly restrengthen as the mid-level ridge spread in from the east through tomorrow, which will help ensure a dry/stable summit-level air mass for the remainder of the week. PW will also respond to the drying of the mid/upper-level air mass through tomorrow night and probably settle in near or just below 1 mm over the weekend. Seeing should also initially improve with the strengthening of the inversion for Thursday night, but there is a possibility for a modest increase in summit-level winds as a weak tropical wave passes to the south for the following 2 nights. This could stir up minor boundary layer turbulence and contribute to poorer than average seeing during that time, but seeing is set to quickly settle back in near 0.5 arcseconds as deep/light easterly winds prevail at and above the summit for Sunday night.

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)
Wed Aug 17 - 8 PM	0-20	9-10	20 / 0	1.5	E/5-15	0.525-0.775	3.5-4.5
Thu Aug 18 - 2 AM	0-20	9-10	20 / 0	1	E/5-15	0.475-0.725	3.5-4.5
2 PM	10-30	9-10	10 / 0	6	E/5-15	NaN	4-6
8 PM	0-20	9-10	0 / 0	1	E/5-15	0.45-0.65	3-4
Fri Aug 19 - 2 AM	0-20	Clear	0 / 0	0.5	E/5-15	0.4-0.6	2-3
2 PM	0-20	Clear	0 / 0	7	NE/10-20	NaN	1.5-2.5
8 PM	0-10	Clear	0 / 0	2	NE/15-25	0.55-0.85	1-1.5
Sat Aug 20 - 2 AM	0-10	Clear	0 / 0	3	ENE/10-20	0.525-0.775	0.7-1.1
2 PM	0-20	Clear	0 / 0	9	ENE/10-20	NaN	1-1.5
Sun Aug 21 - 2 AM	0-5	Clear	0 / 0	5	ENE/10-20	0.525-0.775	0.7-1.1
2 PM	0-20	Clear	0 / 0	10.5	NE/5-15	NaN	1-1.5
Mon Aug 22 - 2 AM	0-5	Clear	0 / 0	6	NE/5-15	0.4-0.6	0.7-1.1
2 PM	0-20	Clear	0 / 0	12	N/0-10	NaN	1-1.5

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
Wed Aug 17 - Thu Aug 18	18:58	20:05	4:46	5:54	22:56	N/A	57	3 00.5	16 30
Thu Aug 18 - Fri Aug 19	18:57	20:05	4:47	5:54	23:35	N/A	47	3 49.6	20 37
Fri Aug 19 - Sat Aug 20	18:56	20:04	4:47	5:54	0:17	N/A	38	4 40.0	23 44
Sat Aug 20 - Sun Aug 21	18:56	20:03	4:48	5:55	1:03	N/A	29	5 31.6	25 44
Sun Aug 21 - Mon Aug 22	18:55	20:02	4:48	5:55	1:52	N/A	21	6 23.9	26 33

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Thursday 18 August 2022.

Additional Information
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This message is also available at the MKWC road conditions page.
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Current Conditions
Temp		2.4 C
RH		30 %
Wind		ENE 0 mph
Road		Open (4x4)