Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Monday 23 November (0300 UTC Tuesday 24 November) 2009

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 3.5 C this evening and 3 C tomorrow morning.Winds will be from the ENE at 10-20 mph, with seeing around 0.5 arcseconds.Precipitable water is expected to be in the 0.6-0.8 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture at or below 10-11 thousand feet and keep the mid/summit-level air mass dry and stable through the next 5 nights. However, minor instability in the area may result in periods of extensive afternoon clouds perhaps for Tuesday, but more likely Wednesday and Thursday. Daytime clouds will taper for Friday and the weekend.

Skies will remain clear for tonight and probably for most of tomorrow night. However, scattered to broken cirrus is expected to approach from the west during the latter night and will likely spread overhead near sunrise Wednesday morning. These clouds are expected to thicken in the area, perhaps resulting in extensive cloud cover for Wednesday night. The bulk of these clouds are expected to shift eastward through Thursday night, leaving clear skies for Friday night.

Precipitable water will likely linger near 0.6-0.7 mm for one more night, then will increase toward 2 mm for Tuesday and Wednesday night. It should slip back to 1 mm or less for Thursday and especially Friday night.

Light westerly flow aloft should result in minimal shear/turbulence in the free atmosphere and likely seeing near 0.5 arcseconds for tonight. Upper-level flow is slated to gradually strengthen starting late tomorrow morning, which will increase shear in the free atmosphere and degrade seeing toward 0.7 arcseconds for Tuesday night and probably 0.8-0.9 arcseconds for Wednesday night and Thursday evening. Seeing is expected to improve as upper-level flow tapers and takes on a more northerly component during the latter night and especially Friday night.

Not much change since the morning forecast...Although it will remain rather weak, the mid-level ridge will continue to sit to the north of the Islands and spread out across most of the Pacific throughout the forecast period. In the process, it will promote strong large-scale subsidence in the area, which will help maintain the tradewind inversion at or below 10 thousand feet during that time. A weak upper-level ridge passing overhead will also augment the subsidence aloft, likely translating to PW near 0.6-0.7 mm and seeing around 0.5 arcseconds for tonight. Unfortunately, that will be short-lived as a trough brewing to the northwest will start to dig toward the Islands through tomorrow night. This will help increase upper-level flow and thus shear in the free atmosphere during that time, and especially as the trough passes through the area on Wednesday and Thursday night. The trough will also bring lots of high clouds and some mid-level moisture into the area, which will increase cloud cover and PW. The trough is not expected to affect summit-level conditions much, just perhaps a slight increase in afternoon clouds mainly for Wednesday and Thursday. Skies are expected to improve as this trough shifts eastward briefly allowing another upper-level ridge to build back into the area for Friday 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)
Mon Nov 23 - 8 PM	0-5	Clear	0 / 0	3.5	ENE/10-20	0.4-0.6	0.6-0.8
Tue Nov 24 - 2 AM	0-5	Clear	0 / 0	3	ENE/10-20	0.4-0.6	0.6-0.8
2 PM	20-40	4-5	10 / 0	8	NE/5-15	NaN	1-2
8 PM	0-5	Clear	0 / 0	3	NNE/5-15	0.5-0.8	1-1.5
Wed Nov 25 - 2 AM	0-20	9-10	0 / 0	1.5	WSW/5-15	0.5-0.9	1.5-2
2 PM	60-80	4-10	50 / 10	5	W/5-15	NaN	3-6
8 PM	70-90	8-10	0 / 0	0	S/5-15	0.6-1	2-3
Thu Nov 26 - 2 AM	70-90	8-10	0 / 0	1	WSW/5-15	0.6-1.1	2-3
2 PM	60-80	8-10	25 / 0	6	N/10-20	NaN	3-6
Fri Nov 27 - 2 AM	20-40	4-6	0 / 0	1	NNE/10-20	0.5-1	1-2
2 PM	0-20	Clear	0 / 0	6	NNE/10-20	NaN	1-2
Sat Nov 28 - 2 AM	0-20	Clear	0 / 0	2	NE/10-20	0.4-0.8	0.8-1.2
2 PM	0-20	Clear	0 / 0	8	NNW/5-15	NaN	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
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
Thu Nov 26 - Fri Nov 27	17:51	18:59	5:21	6:28	N/A	2:23	73	0 02.3	5 38
Fri Nov 27 - Sat Nov 28	17:51	18:59	5:21	6:29	N/A	3:17	82	0 49.8	10 59

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Tuesday 24 November 2009.

Additional Information
For public road conditions and snow report message please call (808) 935-6268.
This message is also available at the MKWC road conditions page.
NWS Hawaiian Islands Synoptic Discussion and Guidance
Honolulu National Weather Service Data and Products

Current Conditions
Temp		0.9 C
RH		7 %
Wind		ENE 18 mph
Road		Open