Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Friday 24 January 2020

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 6.5 C this afternoon, 1.5 C this evening and 1 C tomorrow morning. Winds will be from the WSW at 5-15 mph for today, easing through the night. Seeing will be near 0.6-0.65 arcseconds, while precipitable water is expected to be in the 0.6-0.8 mm range through the night.

Discussion
A strong well-defined inversion will continue to cap low-level moisture near 5-6 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Skies will remain predominately clear through the next 5 nights.

Precipitable water is expected to settle in primarily near 0.65-0.7 mm for the next 5 night.

Although relatively laminar NW to westerly flow will prevail in the free atmosphere throughout most of the forecast period, an increase in horizontal upper-level shear could degrade toward more average-like values and/or contribute to variability in seeing into Sunday night. Seeing may improve as this shear subsides through Sunday night, but another round of horizontal shear could impact seeing for the following night, only to improve again as more laminar/even westerly builds in aloft for Monday night.

A deep mid/low-level ridge will continue to sit just north the Big Island and stretch across most of the Pacific, promoting strong/steady subsidence in the area probably well into next week. This subsidence will help maintain a well-defined inversion near 5-6 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. The westerly jet will maintain a zonal orientation off to the north, allowing clear/calm skies and very dry air to prevail above the summit during this time. The dry air and deep subsidence will also allow PW to settle in near 0.65-0.7 mm throughout the forecast period. However, an expansion of the westerly jet may increase horizontal shear in the free atmosphere, perhaps resulting in minor degradation/variability in seeing through Sunday evening. The jet will expand further southward allowing more uniform flow to prevail and seeing to improve for Sunday night. However, stronger winds toward the core of the jet could move into the area, producing another round of horizontal shear and result in more degradation/variability in seeing for Monday night. The core of this jet will eventually settle in overhead, allowing more laminar/even westerly flow to prevail in the free atmosphere for the following 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 Jan 24 - 2 PM	0-20	Clear	0 / 0	6.5	WSW/5-15	NaN	0.6-1
8 PM	0-5	Clear	0 / 0	1.5	WSW/5-15	0.5-0.8	0.6-0.8
Sat Jan 25 - 2 AM	0-5	Clear	0 / 0	1	WSW/0-10	0.45-0.75	0.6-0.8
2 PM	0-20	Clear	0 / 0	6	WSW/5-15	NaN	0.6-1
8 PM	0-5	Clear	0 / 0	0.5	WSW/0-10	0.45-0.75	0.55-0.75
Sun Jan 26 - 2 AM	0-5	Clear	0 / 0	0	WSW/0-10	0.5-0.8	0.55-0.75
2 PM	0-20	Clear	0 / 0	5	NNW/0-10	NaN	0.6-1
8 PM	0-5	Clear	0 / 0	0	NE/0-10	0.45-0.75	0.6-1
Mon Jan 27 - 2 AM	0-5	Clear	0 / 0	0	NE/0-10	0.4-0.6	0.55-0.75
2 PM	0-20	Clear	0 / 0	6.5	E/5-15	NaN	0.6-1
Tue Jan 28 - 2 AM	0-5	Clear	0 / 0	2	ENE/0-10	0.4-0.7	0.6-0.8
2 PM	0-20	Clear	0 / 0	7.5	ESE/5-15	NaN	0.6-1
Wed Jan 29 - 2 AM	0-5	Clear	0 / 0	2.5	E/0-10	0.3-0.6	0.55-0.75

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 Jan 24 - Sat Jan 25	18:18	19:26	5:42	6:49	7:36	18:28	0	20 55.1	-20 14
Sat Jan 25 - Sun Jan 26	18:19	19:26	5:42	6:49	N/A	19:21	2	21 45.0	-17 22
Sun Jan 26 - Mon Jan 27	18:20	19:27	5:42	6:49	N/A	20:13	6	22 32.5	-13 48
Mon Jan 27 - Tue Jan 28	18:20	19:27	5:42	6:49	N/A	21:03	11	23 18.0	-9 44
Tue Jan 28 - Wed Jan 29	18:21	19:28	5:42	6:49	N/A	21:51	18	0 02.0	-5 20

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Friday 24 January (0300 UTC Saturday 25 January) 2020.

Additional Information
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Current Conditions
Temp		3.9 C
RH		18 %
Wind		S 9 mph
Road		Open (4x4)