Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Wednesday 21 August (0300 UTC Thursday 22 August) 2019

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain clear, dry and stable through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 3.5 C this evening and 3 C tomorrow morning. Winds will be from the NE at 5-15 mph, with seeing near 0.55-0.6 arcseconds. Precipitable water is expected to be in the 2-3 mm range for the first half of the night and 1.5-2 mm range for the second half.

Discussion
The tradewind inversion will remain well-established near 7-8 thousand feet through out the forecast period, and will ensure the summit steers free of fog and precipitation through at least tomorrow night. However, there is a chance that an influx of mid-level moisture will contribute to spike in summit-level humidity toward 80-90% and perhaps increase the risk for fog/rain at the summit between late Friday evening and early Sunday morning. The air mass is set to dry out through Sunday, allowing the summit to steer clear of fog/precipitation for that night. Daytime clouds will be minimal and short-lived through tomorrow, but could pick up on Friday and especially Saturday, only to taper again for Sunday and early part of next week.

Skies overhead will remain predominately clear through the next 3 nights, but there is a chance for scattered high clouds along the northern skies for tomorrow night. Patches of thick mid/upper-level clouds may drift in from the SE around Saturday morning, perhaps contributing to brief periods of extensive cloud cover for that night. The bulk of these clouds will retreat southward and break up a bit early Sunday morning, leaving only residual isolated patches of clouds in the area for that night.

Precipitable water is expected to start out near 3 mm for early this evening, but slip toward 1.5 mm by the end of the night. There is a good chance that it will trend back toward 3-4 mm through tomorrow night and linger in that range (or slightly higher) through Saturday night, then drift back to 2 mm through Sunday night.

Relatively calm skies at and above the summit should allow seeing to remain near 0.5-0.6 arcseconds for much of tonight, but an increase in boundary layer turbulence and occasionally low-level moisture will likely contribute to poor seeing for at least the following 3 nights. There is a possibility that seeing will settle back toward more average-like values as the atmosphere dries and the boundary layer turbulence subsides through Sunday night.

The upper portion of a fairly deep ridge to the north is expected to get squeezed toward Alaska as an upper-level low drops in from the NNE over the next 12-24 hours, then retreats westward over the following 2-3 days. Fortunately, the low will pass too far to the north to impact the stability of the atmosphere provided by the quasi-stationary mid/low-level ridge to the NE through the week. In fact, subsidence associated with the ridge will help maintain a well-defined inversion near 7-8 thousand feet probably well into next week. While the inversion will ensure the summit steers free of moisture over the next 2 nights, another tropical wave passing through is expected to increase humidity toward 80% as well as the risk for fog/precipitation at the summit between Friday afternoon and early Sunday morning. Though the odds are slim, daytime heating may contribute to development of afternoon convection along the slopes on Saturday. Aside from that, minimal turbulence will prevail in the free atmosphere, and with calm summit-level winds, should allow for another night of better than average seeing. However, an increase in summit-level winds toward 20 mph will likely increase boundary layer turbulence and moisture may contribute to poor seeing for the following 3 nights. There is a good possibility that calm summit winds combined with a dry air mass will allow seeing to improve again through 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 (%)	(Celcius)	(Dir/MPH)	(Arcseconds)	(mm)
Wed Aug 21 - 8 PM	0-5	Clear	0 / 0	3.5	NE/5-15	0.45-0.65	2-3
Thu Aug 22 - 2 AM	0-5	Clear	0 / 0	3	NE/10-20	0.5-0.7	1.5-2
2 PM	0-20	Clear	0 / 0	8.5	ENE/15-25	NaN	1.5-2.5
8 PM	0-20	9.5-10	0 / 0	3	E/15-25	0.6-1	2-3
Fri Aug 23 - 2 AM	0-20	9.5-10	0 / 0	1	SE/20-35	0.7-1.1	3-4
2 PM	40-60	4-5	40 / 0	7.5	SE/15-25	NaN	3-5
8 PM	0-5	Clear	20 / 0	2.5	ESE/15-25	0.6-1	2-4
Sat Aug 24 - 2 AM	0-20	Clear	60 / 0	2	ESE/15-25	0.6-1.1	3-5
2 PM	60-80	4-8	75 / 0	6	ESE/15-25	NaN	4-8
Sun Aug 25 - 2 AM	40-60	6-8	30 / 0	1.5	E/15-25	0.6-1.1	3-5
2 PM	0-20	Clear	0 / 0	9	ENE/10-20	NaN	3-6
Mon Aug 26 - 2 AM	0-5	Clear	0 / 0	4	ENE/10-20	0.5-0.8	2-3
2 PM	0-20	Clear	0 / 0	9	E/10-20	NaN	2-4

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 21 - Thu Aug 22	18:55	20:02	4:48	5:55	22:48	N/A	61	2 59.2	11 43
Thu Aug 22 - Fri Aug 23	18:54	20:01	4:48	5:55	23:28	N/A	51	3 48.6	15 30
Fri Aug 23 - Sat Aug 24	18:53	20:00	4:49	5:55	0:13	N/A	41	4 41.0	18 37
Sat Aug 24 - Sun Aug 25	18:53	19:59	4:49	5:56	1:02	N/A	31	5 36.6	20 50
Sun Aug 25 - Mon Aug 26	18:52	19:58	4:49	5:56	1:58	N/A	21	6 35.3	21 52

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Thursday 22 August 2019.

Additional Information
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Current Conditions
Temp		0.8 C
RH		45 %
Wind		NE 18 mph
Road		Closed