Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Thursday 17 April (0300 UTC Friday 18 April) 2014

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, but broken band of high clouds will continue to fill in from the south contributing in extensive cloud cover or even periods of overcast skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 7.5 C this afternoon and 2.5 C for the night. WInds will be from the east at 15-30 mph, with seeing around 0.8 arcseconds. Precipitable water is expected to be in the 1.5-2 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 summit-level air mass dry and stable through at tomorrow night. While the inversion will remain in tact well into next week, mid-level moisture is expected to fill into the area, which could increase summit-level humidity toward 60-80% and perhaps the risk for fog, ice and light flurries at the summit for Saturday, Sunday and Monday night (the greatest risk appears centered around early Monday morning). Daytime clouds will be minimal and short-lived for tomorrow, but could pick up over the weekend and into next week.

Broken high clouds will continue to fill in from the south, resulting in extensive cloud cover and even periods of overcast skies for the next 3 nights. These high clouds are expected to shift eastward again early Sunday morning, but there is a possibility that patches of mid-level clouds will fill in from the south late Saturday afternoon and linger in the area through Monday night. These clouds may also contribute to periods of extensive cloud cover mainly between Saturday evening and early Monday morning.

Precipitable water is expected to increase toward 1.5-2 mm for tonight, 2-3 mm for tomorrow night, then jump toward 4-8 mm for Saturday, Sunday and Monday night.

A mixture of light/moderate boundary layer turbulence, low-level turbulence and/or free atmospheric turbulence will contribute to poor seeing probably throughout the forecast period.

Little change since the morning forecast, though I did lower the odds on fog for the second half of the forecast period...The mid/low-level ridge will continue to sit to the north of the state, promote strong/steady large-scale in the area and maintain a fairly well-defined tradewind inversion at or below 9 thousand feet well into next week. This will be enough to ensure dry/stable summit-level conditions prevails through Saturday morning, but a relatively deep tropical upper tropospheric trough (TUTT) currently setting up over the Islands is expected to pull mid-level moisture/clouds up from the tropics later that afternoon. This moisture will likely increase humidity toward 60-80%, and may allow for periods of fog, ice and light flurries at the summit thereafter (though the odds are quite small). The TUTT and its sub-tropical jet will also bring lots of banding high clouds over the area, which will contribute to extensive cloud cover and perhaps periods of overcast skies during the next 3 nights. While the high clouds are expected shift eastward as the TUTT flattens out and the sub-tropical jet weakens on Sunday, patches of mid-level clouds will likely linger in the area into the early part of next week.

Please Note: There will be no forecasts tomorrow, Apr 18, in observance of Good Friday. I will issue a brief update if conditions drastically deviate from the above early Saturday afternoon. The normal forecast schedule will resume on Monday, April 21.

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)
Thu Apr 17 - 8 PM	80-100	8-10	0 / 0	2.5	E/15-30	0.6-1	1.5-2
Fri Apr 18 - 2 AM	70-90	8-10	0 / 0	2.5	E/15-30	0.6-1	1.5-2
2 PM	80-100	8-10	0 / 0	8	ESE/15-30	NaN	2-3
8 PM	80-100	8-10	0 / 0	3	ESE/15-30	0.7-1.1	2-3
Sat Apr 19 - 2 AM	80-100	7-10	0 / 0	2	ESE/15-30	0.7-1.1	2.5-3.5
2 PM	80-100	6-10	20 / 15	5	ESE/15-30	NaN	3-5
8 PM	80-100	5-10	20 / 15	-1	ESE/15-30	0.65-1.05	4-6
Sun Apr 20 - 2 AM	60-80	5-9	20 / 10	-1	ESE/10-20	0.6-1	4-6
2 PM	60-80	4-8	20 / 15	-4	ENE/10-20	NaN	4-8
Mon Apr 21 - 2 AM	70-90	4.5-8	30 / 25	-1	E/10-20	0.5-1	4-6
2 PM	60-80	4-6	30 / 20	4	SE/10-20	NaN	4-8
Tue Apr 22 - 2 AM	20-40	5-6	20 / 10	-2	SE/5-15	0.4-1	4-6
2 PM	20-40	4-5	25 / 5	5	S/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
Fri Apr 18 - Sat Apr 19	18:50	19:57	4:45	5:52	22:09	N/A	80	17 25.2	-19 24
Sat Apr 19 - Sun Apr 20	18:50	19:57	4:44	5:51	23:06	N/A	71	18 24.8	-18 54
Sun Apr 20 - Mon Apr 21	18:51	19:58	4:43	5:50	0:01	N/A	60	19 23.8	-17 14
Mon Apr 21 - Tue Apr 22	18:51	19:58	4:42	5:50	0:52	N/A	48	20 21.5	-14 33
Tue Apr 22 - Wed Apr 23	18:51	19:59	4:42	5:49	1:41	N/A	37	21 17.5	-11 03

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 18 April 2014.

Additional Information
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Current Conditions
Temp		0.5 C
RH		6 %
Wind		SE 24 mph
Road		Open