Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 20 November (0300 UTC Saturday 21 November) 2009

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable through the night. There is a good chance that isolated cirrus will be visible as it passes along the northern skies and perhaps overhead for tonight.

Summary of Key Meteorological Variables
Summit temperatures will be near 0 C this evening and -1 C tomorrow morning.Winds will be from the north at 5-15 mph, with seeing around 0.8 arcseconds.Precipitable water is expected to be in the 1.5-2 mm range for the first half of the night and 2-3 mm range for the second half.

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 9 thousand feet and keep the mid/summit-level air mass dry and stable through the next 5 nights. Daytime clouds will be minimal and short-lived through Tuesday, but could turn extensive for Wednesday.

There is a good chance that isolated cirrus will be visible as it passes along the northern skies and perhaps overhead for tonight. These clouds are slated to shift eastward through tomorrow, leaving clear skies probably for the remainder of the forecast period. There is a possibility that thick clouds will begin to the develop in the area late Wednesday morning.

Precipitable water is expected to increase toward 2-3 mm for tonight and perhaps the early part of tomorrow night, but should slip toward 0.8-1 mm for the remainder of the forecast period. There is a good chance that it will dip below 0.8 mm mainly for Sunday night and Monday evening.

Strong shear/turbulence in the free atmosphere will likely result in poor seeing for the next 2 nights. Seeing is expected to dramatically improve as upper-level flow takes on a more northerly component and tapers for Sunday night and become very light for Monday night. This could drop seeing toward 0.4-0.5 arcseconds particularly for the latter night. A slight increase in westerly flow aloft may strengthen shear a bit and degrade seeing toward 0.6 arcseconds for Tuesday night.

Subtle changes have been made to the CC and PW forecast...Although the mid/low-level ridge will remain rather weak, it will continue to stretch across most of the Pacific and linger to north or over the state throughout the forecast period. This will invoke strong large-scale subsidence in the area, which will help maintain the tradewind inversion near 9 thousand feet and ensure the summit steers free of any moisture during that time. Unfortunately, a weak upper-level trough and its counterpart the sub-tropical jet will persist in the area, promoting moderate/strong shear in the free atmosphere and likely poor seeing over the next 2 nights. It also will transport a patch of high clouds through the area particularly during the latter half of tonight. The trough will not affect the stability of the atmosphere during its tenure and is expected to flatten out and shift eastward through Saturday night. This will not only leave clear skies in the area for the remainder of the forecast period, but significantly diminsh flow and thus shear in the free atmosphere and improve seeing toward 0.4-0.5 arcseconds for Sunday and especially Monday night. Flow aloft should remain relatively weak for Tuesday night, but may take on a more westerly component as a trough brews to the northwest. This trough may deepen near the Islands and could negatively impact summit skies/conditions for the latter part of next week.

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 20 - 8 PM	10-30	9-10	0 / 0	0	NNE/5-15	0.6-1	1.5-2
Sat Nov 21 - 2 AM	20-40	7.5-10	0 / 0	-1	NNW/5-15	0.6-1	2-3
2 PM	20-40	8.5-10	0 / 0	5	WSW/5-15	NaN	2-4
8 PM	0-20	9.5-10	0 / 0	0.5	WSW/5-15	0.5-1.1	2-3
Sun Nov 22 - 2 AM	0-5	Clear	0 / 0	0.5	W/5-15	0.5-1	2-3
2 PM	0-10	Clear	0 / 0	6	NE/5-15	NaN	1-2
8 PM	0-5	Clear	0 / 0	1	ENE/5-15	0.5-0.8	0.7-0.9
Mon Nov 23 - 2 AM	0-5	Clear	0 / 0	2	NE/10-20	0.45-0.65	0.6-0.8
2 PM	0-5	Clear	0 / 0	8	E/10-20	NaN	0.6-0.8
Tue Nov 24 - 2 AM	0-5	Clear	0 / 0	3	ESE/5-15	0.4-0.6	0.7-0.9
2 PM	0-10	Clear	0 / 0	7	S/10-20	NaN	1-2
Wed Nov 25 - 2 AM	0-5	Clear	0 / 0	2	SW/10-20	0.4-0.8	1-2
2 PM	40-60	4-8	50 / 25	4	WNW/15-25	NaN	4-8

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 21 - Sun Nov 22	17:51	18:58	5:18	6:25	N/A	22:15	27	20 18.3	-18 38
Sun Nov 22 - Mon Nov 23	17:51	18:58	5:18	6:26	N/A	23:06	35	21 04.5	-14 38
Mon Nov 23 - Tue Nov 24	17:51	18:58	5:19	6:27	N/A	23:55	45	21 49.2	-10 04
Tue Nov 24 - Wed Nov 25	17:51	18:58	5:19	6:27	N/A	0:44	54	22 33.1	-5 04
Wed Nov 25 - Thu Nov 26	17:51	18:58	5:20	6:28	N/A	1:33	64	23 17.1	0 13

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 23 November 2009.

Additional Information
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Current Conditions
Temp		0.0 C
RH		9 %
Wind		NE 3 mph
Road		Open