Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Monday 20 May 2019

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while the scattered high clouds shift off toward the east early this evening, but there is a possibility for patches of mid-level clouds in the area 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 south at 15-25 mph, with seeing near 0.8-0.9 arcseconds. Precipitable water is expected to exceed 4 mm for the night.

Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived for today, but could pick up tomorrow, then taper for Wednesday and the remainder of the week.

While the scattered high clouds are expected to shift off toward the east early this evening, there is a possibility for patches of mid-level clouds in the area for tonight and perhaps tomorrow night. Predominately clear skies will prevail overhead for the remainder of the week, but there is a chance that scattered high clouds will approach and dissipate along the western skies on Wednesday night.

Precipitable water is expected to linger near or exceed 4 mm for the next 2 nights, then slip toward 2-3 mm through Wednesday night and likely settle in near 2 mm for the following 2 nights.

A brief increase in boundary layer turbulence will likely contribute to poor seeing for tonight. Weaker summit winds, combined with a stable air mass and light/uniform flow aloft should allow seeing to linger near 0.5 arcseconds for the next 2-3 nights. However, another round of light/moderate boundary layer turbulence could degrade seeing again for Friday night.

A large trough brewing to the NW will continue to supress the ridge over the state before lifting off toward the north and allowing the ridge to rebound back toward its usual place (to the north of the Islands) later in the week. Nevertheless, the ridge will still promote steady/strong large-scale subsidence in the area, which will easily maintain a fairly well-defined tradewind inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. Southerly flow along the SW tip of the ridge will advect mid-level moisture/clouds into the area over the next 2 nights. There is also a chance that a tighter wind gradient (between the ridge and trough) will briefly move over the area, increasing summit-level winds and boundary layer turbulence, and likely contributing to poorer than average seeing for tonight. Winds are set to subside as the ridge shifts subtly to the north, diminishing boundary layer turbulence for the next 2-3 nights. This should allow seeing to improve and eventually settle in near 0.5 arcseconds as very light/uniform prevails in the free atmosphere during that time. However, winds may pick up again when the ridge rebounds further northward and expands/strengthen in response to the departure of the trough to the NW over 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)
Mon May 20 - 2 PM	30-50	8-9	0 / 0	8	S/15-25	NaN	4-8
8 PM	10-30	6-8	0 / 0	3	SSW/15-25	0.65-1.05	4-8
Tue May 21 - 2 AM	10-30	6-8	0 / 0	2	S/15-25	0.65-1.05	4-8
2 PM	40-60	4-8	0 / 0	8	SSE/10-20	NaN	4-8
8 PM	0-20	6-7	0 / 0	2.5	SE/5-15	0.5-0.7	4-6
Wed May 22 - 2 AM	0-20	6-7	0 / 0	2.5	SE/5-15	0.4-0.6	3.5-4.5
2 PM	0-20	Clear	0 / 0	9	ESE/5-15	NaN	3-5
8 PM	0-20	Clear	0 / 0	4	ESE/5-15	0.35-0.55	2.5-3.5
Thu May 23 - 2 AM	0-20	Clear	0 / 0	4	ESE/5-15	0.35-0.55	2-3
2 PM	0-20	Clear	0 / 0	10	ENE/10-20	NaN	2-4
Fri May 24 - 2 AM	0-10	Clear	0 / 0	4.5	E/10-20	0.45-0.65	1.5-2.5
2 PM	0-20	Clear	0 / 0	9	E/10-20	NaN	2-4
Sat May 25 - 2 AM	0-10	Clear	0 / 0	3	ENE/15-25	0.6-1	1.5-2.5

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
Mon May 20 - Tue May 21	19:02	20:14	4:23	5:35	20:49	N/A	93	18 07.1	-22 27
Tue May 21 - Wed May 22	19:03	20:14	4:23	5:34	21:42	N/A	86	19 02.4	-22 48
Wed May 22 - Thu May 23	19:03	20:15	4:22	5:34	22:32	N/A	79	19 56.0	-22 02
Thu May 23 - Fri May 24	19:03	20:15	4:22	5:34	23:19	N/A	70	20 47.7	-20 16
Fri May 24 - Sat May 25	19:04	20:16	4:21	5:34	0:01	N/A	61	21 37.0	-17 40

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Monday 20 May (0300 UTC Tuesday 21 May) 2019.

Additional Information
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Current Conditions
Temp		12.9 C
RH		20 %
Wind		SE 6 mph
Road		Open (4x4)