Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 24 May (0300 UTC Saturday 25 May) 2013

Warning(s)

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, but thin high clouds will continue to dissipate along the southwestern skies and perhaps drift over the Big Island through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 1.5 C this evening and 1 C tomorrow morning. Winds will be from the north at 5-15 mph, with seeing around 0.65-0.7 arcseconds. Precipitable water is expected to be in the 3.5-4.5 mm range for the night.

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 at least Sunday night. There is a chance that building instability may weaken/lift the inversion and increase the risk for fog for Monday night; precipitation is unlikely. Daytime clouds will be minimal and short-lived through Sunday, but could pick for Monday and Tuesday.

Patches of light cirrus will continue to fill in from the southwest and dissipate over or just near the summit for one more night. These clouds are expected to retract southwand and/or dissipate through tomorrow, leaving mostly clear skies for that night and Sunday night. However, there is a chance that more high clouds will develop in the area and perhaps move in from the east late Monday night and especially for Tuesday night.

Precipitable water is expected to linger near 4 mm for tonight, then slip toward 1.25 mm for tomorrow night. It will probably increase back to 2-3 mm for Sunday night and 4+ mm for the early part of next week.

Lingering free atmospheric turbulence will likely contribute to poorer than average seeing through around Sunday evening. This turbulence is slated to pass, allowing seeing to improve back toward average to slightly better than average values for much of Monday. However, a new source of turbulence and instability will likely degrade seeing again on Tuesday.

A weak ridge will continue to linger over the Islands for another 24 hours, then will begin to strengthen before settling in near its typical climitological position to the north of the state late in the weekend and into the early part of next week. While this will be ensure strong large-scale subsidence prevails in the area and keep the summit-level air mass dry and stable through the weekend, there is a chance that building instability associated with the tropical upper-tropospheric trough (TUTT) will begin to negate the subsidence for the early part of next week. The TUTT has actually already established itself over the Islands during the last 24-48 hours, but the Islands remain situated in a col-like region sandwiched betweened two embedded lows (one to the west and another, which will start to further develop, to the east). Still, cirrus exhaust from the low to the west is expected to fill into the area for another 24 hours. Turbulence associated with the TUTT will also contribute to poorer than average seeing for much of the weekend. Models still insist that the developing low to the east will shift westward scraping the Big Island area and bringing moisture to the summit late Monday night and especially for Tuesday night. These lows are fairly typical for this time of the year, but the amount of moisture/instability will likely be dependent on the positioning of the low to the Big Island, rather than if this low actually develops. At any rate, given the fairly consistent model projection over the last 2-3 days, I have increased the risk for fog and precipitation at the summit (since yesterday) for the early part of next week.

Please Note: There will be no forecasts on Monday, May 27, in observance of Memorial Day. The regular forecast schedule will resume on Tuesday, May 28.

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 May 24 - 8 PM	40-60	9-10	0 / 0	1.5	NNE/5-15	0.55-0.85	3.5-4.5
Sat May 25 - 2 AM	20-40	9-10	0 / 0	1	N/5-15	0.6-0.8	3.5-4.5
2 PM	0-20	Clear	0 / 0	6	NNE/5-15	NaN	3-5
8 PM	0-10	Clear	0 / 0	1	NE/5-15	0.6-0.9	2-3
Sun May 26 - 2 AM	0-10	Clear	0 / 0	1	NNE/5-15	0.55-0.75	1-2
2 PM	0-20	Clear	0 / 0	6.5	N/5-15	NaN	1-2
8 PM	0-10	Clear	0 / 0	2	NE/5-15	0.6-0.8	2-3
Mon May 27 - 2 AM	0-10	Clear	0 / 0	1.5	NE/5-15	0.45-0.65	2-3
2 PM	20-40	4-5	50 / 20	4	NE/5-15	NaN	2-4
Tue May 28 - 2 AM	0-20	9.5-10	60 / 20	-1	S/5-15	0.4-0.8	3-5
2 PM	40-60	4-10	75 / 25	4	SSW/10-20	NaN	4-8
Wed May 29 - 2 AM	60-80	6-10	60 / 20	-1	SSW/5-15	0.5-1.1	4-8
2 PM	60-80	4-8	75 / 25	4	SE/5-15	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 May 25 - Sun May 26	19:04	20:17	4:21	5:33	19:46	N/A	98	17 30.9	-20 47
Sun May 26 - Mon May 27	19:05	20:17	4:21	5:33	20:47	N/A	92	18 35.8	-19 41
Mon May 27 - Tue May 28	19:05	20:18	4:20	5:33	21:44	N/A	85	19 38.2	-17 16
Tue May 28 - Wed May 29	19:06	20:18	4:20	5:33	22:36	N/A	75	20 37.3	-13 49
Wed May 29 - Thu May 30	19:06	20:19	4:20	5:33	23:23	N/A	64	21 32.8	-9 42

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 27 May 2013.

Additional Information
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Current Conditions
Temp		0.6 C
RH		98 %
Wind		ESE 9 mph
Road		Open