Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
815 AM HST (1815 UTC) Wednesday 16 January 2019

Warning(s)
Increasing winds

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 8 C this afternoon, 3 C this evening and 2 C tomorrow morning. Winds will be from the west at 5-15 mph for today, switching to more southerly direction and increasing to 15-30 mph for the night. Seeing will start out near 0.5 arcseconds, but will drift toward 0.7-0.8 arcseconds as the night progresses. Precipitable water is expected to be in the 0.6-0.8 mm range for the night.

Discussion
A strong well-defined tradewind inversion will continue to cap low-level moisture near 5-6 thousand feet and ensure the summit remains dry and stable into Thursday afternoon. A band of mid/low-level moisture/instability is expected sweep through the area, which could weaken/erode the inversion and increase the risk for fog, high humidity, ice and perhaps light flurries at the summit late Thursday night and into Friday night. This band should slip eastward and dissipate during the latter night, allowing the inversion to rebuild and dry/stable conditions to return to the summit for Saturday and Sunday night. Daytime clouds will be minimal and short-lived through Thursday, but could turn extensive for Friday, then taper over the weekend.

Skies will remain predominately clear into tomorrow evening. There is a possibility that patches of mid-level clouds will sweep move in from the NW and pass mainly along the northern skies late Thursday night, then gradually dissipate and/or shift eastward through Friday night; clear skies will prevail for Saturday and Sunday night.

Precipitable water is expected to remain near 0.7 mm for tonight, increase toward 1-1.5 mm for tomorrow night and probably 2-3+ mm for most of Friday night. There is a good chance that it will dip back toward 1 mm or less for the following 2 nights.

While there is a good chance that relatively benign conditions/skies will allow seeing to start out near 0.5 arcseconds for this evening, an increase in boundary layer turbulence will likely degrade seeing as the night progresses and contribute to poor/bad seeing for Thursday, Friday and Saturday night. There is also a possibility for patches of mid/low-level turbulence and/or gravity waves passing through the area late Friday night and into Saturday night. A decrease in winds on Sunday should allow seeing to improve through that night.

A fairly deep ridge lingering overhead will begin to progress eastward as a westerly trough/low fills in from the NW later tonight. Nevertheless, the ridge will remain close enough to promote steady/strong large-scale subsidence in the area, which will help maintain a well-defined inversion near 5-6 thousand feet and ensure a dry/stable summit-level air mass through early tomorrow evening. Unfortunately, the incoming trough will start to tighten the wind gradient along the NW flank of the ridge, which will respond to an increase in summit-level winds over the course of the night. Winds will eventually hold fairly steady near 35-40 mph between late tomorrow morning and early Friday afternoon, then subside a touch for Saturday, tne slip back toward 10 mph for Sunday night. Nevertheless, this will result in another round of boundary layer turbulence which will likely contribute to poor/bad seeing between late tonight and early Sunday evening. In addition, the aforementioned trough/low is set to send a cold front through the state between early Thursday evening and late Saturday night. While this front is set to fairly rapidly dissipate as it progresses through the state, it will likely remain strong enough to breakdown the inversion, bring very cold air to the area, and thus increase the risk for fog, ice and flurries at the summit mainly between sunrise Friday and early Saturday morning. A new ridge is set to quickly fill in behind the wake of the front, which should help re-establish a well defined inversion near 5-6 thousand feet by sunrise Saturday and ensure the summit remains dry and stable for the remainder of the weekend.

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 Jan 16 - 2 PM	0-20	Clear	0 / 0	8	W/5-15	NaN	0.7-1
8 PM	0-5	Clear	0 / 0	3	SW/10-20	0.4-0.6	0.6-0.8
Thu Jan 17 - 2 AM	0-5	Clear	0 / 0	2	SSW/15-30	0.6-0.9	0.6-0.8
2 PM	0-20	Clear	0 / 0	4	SW/30-45	NaN	0.8-1.2
8 PM	0-5	Clear	10 / 0	-2	WSW/30-45	1-2	1-1.5
Fri Jan 18 - 2 AM	0-20	7-8	40 / 10	-3.5	WSW/30-45	1-2	1-1.5
2 PM	80-100	4-8	90 / 40	-1	WSW/25-40	NaN	4-8
8 PM	40-60	4.5-7	75 / 25	-6	W/20-35	1-2	3-6
Sat Jan 19 - 2 AM	20-40	6-7	40 / 10	-5	NW/20-35	1-2	2-3
2 PM	0-20	Clear	0 / 0	2	NNE/25-40	NaN	1-2
Sun Jan 20 - 2 AM	0-5	Clear	0 / 0	-1	ENE/20-35	0.8-1.6	0.6-1
2 PM	0-20	Clear	0 / 0	4.5	E/15-30	NaN	1-2
Mon Jan 21 - 2 AM	0-5	Clear	0 / 0	-1	SW/5-15	0.5-0.8	0.8-1.2

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 Jan 16 - Thu Jan 17	18:14	19:21	5:42	6:50	N/A	3:35	80	4 13.3	16 56
Thu Jan 17 - Fri Jan 18	18:14	19:22	5:42	6:50	N/A	4:36	89	5 12.3	19 44
Fri Jan 18 - Sat Jan 19	18:15	19:22	5:42	6:50	N/A	5:40	95	6 15.2	21 19
Sat Jan 19 - Sun Jan 20	18:16	19:23	5:43	6:50	16:45	6:42	99	7 20.4	21 25
Sun Jan 20 - Mon Jan 21	18:16	19:24	5:43	6:50	17:50	7:42	100	8 26.2	19 55

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Wednesday 16 January (0300 UTC Thursday 17 January) 2019.

Additional Information
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Current Conditions
Temp		3.8 C
RH		5 %
Wind		NNW 9 mph
Road		Open (4x4)