Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 15 June (0300 UTC Saturday 16 June) 2018

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while there is a chance that stray patches of thin cirrus will occasionally drop down from the north and pass over/near the summit area through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this evening and 3.5 C tomorrow morning. Winds will be from the north at 5-15 mph, with seeing near 0.5-0.55 arcseconds. Precipitable water is expected to be in the 1.25-1.75 mm range for the first half of the night and 1.75-2.25 mm range for the second half.

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 7 thousand feet and ensure the summit steers free of fog and precipation through the next 5 nights. However, there is a possibility that summit-level humidity will jump into the 60-70% range for Sunday night and early part of next week. Daytime clouds will be minimal and short-lived through Sunday, but could pick up for the early part of next week.

There is a chance that stray patches of thin cirrus will occasionally drop down from the north and pass over/near the summit area over the next 4 nights. Still, cloud cover should not exceed 20% over the next 2 nights, but there is a possibility that it will briefly approach 50% for Sunday night. Clear skies are expected for Monday and Tuesday night.

Precipitable water is expected to climb toward 2 mm over the course of the night and will exceed 4 mm for the following 4 nights.

Better than average relatively steady seeing is expected as light northerly flow aloft prevails at and above the summit for tonight. However, an increase in boundary and mid-level turbulence will begin to degrade seeing through tomorrow night and likely contribute to poor/bad seeing for the following 3 nights.

No change since the morning forecast...The eastern edge of a deep ridge will continue to sit to the north of the Big Island and promote strong/steady large-scale subsidence in the area well into next week. This subsidence will help maintain a well-defined tradewind inversion at or below 7 thousand feet and ensure the summit steers free of fog and precipitation through the next 5 nights. While the upper-portion of the ridge will easily prevent the sub-tropical jet from developing in the area, northerly flow along its easterly flank may send stray thin high clouds over/near the summit through the next 4 nights. Still, with the possible exception of Sunday night, cloud cover shouldn't exceed 20% during this time. Initially, the northerly flow will remain quite light/laminar, which will likely minimize turbulence in the free atmosphere and probably allow for better than average seeing for tonight. However, a rather deep mid/upper-level low is set to develop to the east and gradually shift westward over the weekend. While this low is not expected to affect the stability of the atmosphere, it may tighten the wind gradient, increasing summit-level winds and thus boundary layer turbulence beyond tonight. There is a possibility mid-moisture bordering the ridge and the low (to the west and east, respectively) will make a push through the area, increasing PW and RH also after tonight (humidity will likely peak near 60-70% around Sunday and Monday 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)
Fri Jun 15 - 8 PM	0-20	9.5-10	0 / 0	4	N/5-15	0.4-0.6	1.25-1.75
Sat Jun 16 - 2 AM	0-20	9.5-10	0 / 0	3.5	N/5-15	0.45-0.65	1.75-2.25
2 PM	0-20	9.5-10	0 / 0	9	NNE/5-15	NaN	3-5
8 PM	0-20	9.5-10	0 / 0	4	NNE/10-20	0.5-0.8	4-6
Sun Jun 17 - 2 AM	0-20	9.5-10	0 / 0	4	NNE/10-20	0.55-0.85	4-8
2 PM	0-20	9.5-10	0 / 0	9	NNE/15-30	NaN	6-10
8 PM	20-40	9.5-10	0 / 0	3	NNE/20-35	0.7-1.1	8-12
Mon Jun 18 - 2 AM	40-60	9-10	0 / 0	2.5	NNE/25-40	0.8-1.2	8-12
2 PM	20-40	Clear	0 / 0	8	NNE/20-35	NaN	6-10
Tue Jun 19 - 2 AM	0-5	Clear	0 / 0	2	NE/15-30	0.6-1	4-8
2 PM	0-20	Clear	0 / 0	7	ENE/10-20	NaN	4-8
Wed Jun 20 - 2 AM	0-5	Clear	0 / 0	2	E/15-30	0.6-1.1	4-6
2 PM	0-20	Clear	0 / 0	8.5	ENE/10-20	NaN	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
Sun Jun 17 - Mon Jun 18	19:13	20:27	4:19	5:33	N/A	23:16	27	10 10.9	12 48
Mon Jun 18 - Tue Jun 19	19:13	20:27	4:19	5:34	N/A	0:02	38	11 05.6	8 36
Tue Jun 19 - Wed Jun 20	19:13	20:27	4:20	5:34	N/A	0:45	49	11 57.8	4 00
Wed Jun 20 - Thu Jun 21	19:13	20:27	4:20	5:34	N/A	1:25	60	12 48.1	-0 43
Thu Jun 21 - Fri Jun 22	19:14	20:28	4:20	5:34	N/A	2:05	70	13 37.3	-5 19

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 18 June 2018.

Additional Information
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Current Conditions
Temp		2.2 C
RH		88 %
Wind		N 27 mph
Road		Open (4x4)