Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 16 November (0300 UTC Saturday 17 November) 2018

Warning(s)
Chance for fog, high humidity and ice through the weekend
Possibility for convection and flurries mainly for Saturday and Sunday

Cloud Cover and Fog/Precipitation Forecast
There is a small risk for fog, high humidity and ice, mainly toward the end of the night. Broken mid/upper-level clouds will continue to pass over the summit contributing to periods of overcast skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -0.5 C, with light and variable winds for the night. Seeing will be near 0.9-1 arcsecond, while precipitable water is expected to exceed 4 mm through the night.

Discussion
The tradewind inversion is expected to breakdown as moisture and instability fill into the area over the course of the night, likely allowing the atmosphere to turn fairly saturated between early tomorrow evening and Monday morning. This will likely result in periods of fog, high humidity and ice over the next 3 nights, Saturday and Sunday night, in particular. Isolated convection in the area and flurries at the summit are also possible mainly between late tomorrow afternoon and midnight Sunday. The air mass is expected to quickly stabilize and dry as the inversion rebuilds near 7-8 thousand feet by late Monday afternoon, reducing the risk for moisture at the summit for that night and especially Tuesday night. Daytime clouds will turn extensive over the weekend and perhaps Monday, then become minimal and short-lived for Tuesday and Wednesday.

Broken to overcast thick clouds will continue to spread in from the west and SW, contributing to periods of mostly overcast skies probably through the weekend. There is a good chance that the bulk of these clouds will shift off toward the east on Monday, leaving predominately clear skies for Monday and Tuesday night.

Precipitable water is expected to exceed 4 mm through the weekend, briefly slip toward 1 mm late Monday night, then rebound toward 2-3 mm again for Tuesday night.

Deep moisture and instability building into the area, combined with strong turbulence in the free atmosphere will contribute poor/bad seeing over the next 3 nights. A drier more stable air mass should allow seeing to improve toward more average-like values (or slightly better) as moderate turbulence prevails in the free atmosphere for Monday and Tuesday night.

No change since the morning forecast....A large trough brewing to the west is expected to become reinforced by a short-wave trough digging in from the NW over the next 24 hours, which will eventually pass through the area between early tomorrow evening and late Monday morning. Building instability, combined with an influx of deep moisture from the west and SW will begin to erode the inversion through the night and likely allow most of the atmosphere to turn quite saturated during the trough's passage. As a result, there is a moderate/high risk for fog, high humidity and ice at the summit over the next 3 nights, particularly after midnight tonight. Isolated convection in the area as well as snow at the summit are also possible as early as tomorrow afternoon and could remain an issue mainly through midnight Sunday. Broken thick clouds flowing over the summit will also contibute to periods of overcast skies during the next 3 nights. The clouds and moisture are expected to abruptly shift off toward the east as the trough axis passes over the Big Island around Monday morning. This will allow a rather deep low-level ridge to fill in from the west and reinstill large-scale subsidence in the area for the early part of next week. This subsidence should easily rebuild a well-defined inversion near 7-8 thousand feet and virtually ensure a dry/stable and relatively calm/clear air mass for Monday and Tuesday 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 Nov 16 - 8 PM	80-100	6-10	40 / 10	-0.5	ENE/0-10	0.7-1.1	6-10
Sat Nov 17 - 2 AM	80-100	6-10	75 / 15	-0.5	S/0-10	0.7-1.3	6-10
2 PM	80-100	4-10	90 / 60	3	N/0-10	NaN	8-12
8 PM	80-100	4-10	90 / 55	-1	E/5-15	0.8-1.6	8-12
Sun Nov 18 - 2 AM	60-80	4-9	95 / 65	-2	E/0-10	0.8-1.6	8-12
2 PM	80-100	4-9	95 / 90	2	SSE/0-10	NaN	8-12
8 PM	60-80	4-8	95 / 90	-2	ENE/5-15	1-1.8	8-12
Mon Nov 19 - 2 AM	40-60	4-7	90 / 30	-1.5	E/5-15	0.8-1.6	8-12
2 PM	40-60	4-5	50 / 10	4	NE/5-15	NaN	4-8
Tue Nov 20 - 2 AM	0-20	Clear	10 / 0	0	NNE/5-15	0.5-0.8	1-2
2 PM	0-20	Clear	0 / 0	6	ENE/5-15	NaN	1.5-2.5
Wed Nov 21 - 2 AM	0-20	Clear	0 / 0	1	ESE/5-15	0.45-0.75	2-3
2 PM	0-20	Clear	0 / 0	6	ESE/0-10	NaN	2-3

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
Sat Nov 17 - Sun Nov 18	17:51	18:58	5:16	6:23	N/A	2:34	75	23 52.8	-5 32
Sun Nov 18 - Mon Nov 19	17:51	18:58	5:16	6:24	N/A	3:25	83	0 40.1	-0 58
Mon Nov 19 - Tue Nov 20	17:51	18:58	5:17	6:24	N/A	4:19	90	1 28.7	3 47
Tue Nov 20 - Wed Nov 21	17:51	18:58	5:17	6:25	N/A	5:14	96	2 19.3	8 29
Wed Nov 21 - Thu Nov 22	17:51	18:58	5:18	6:25	16:49	6:12	99	3 12.7	12 54

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 19 November 2018.

Additional Information
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Current Conditions
Temp		1.9 C
RH		26 %
Wind		E 3 mph
Road		Closed