Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Thursday 31 July (0300 UTC Friday 1 August) 2014

Warning(s)
Increasing humidity
Chance for fog, rain and convection

Cloud Cover and Fog/Precipitation Forecast
There is a moderate/high chance for fog, high humidity and perhaps rain, particularly as the night progresses. Broken mid/upper-level clouds will also block most, if not all of the sky mainly during the second half of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 2.5 C this evening and 2 C tomorrow morning. Winds will be relatively light and from the SSE, while seeing will start out near 0.5 arcseconds, but increase toward 0.7 arcseconds through the night. Precipitable water is expected to exceed 4 mm through the night.

Discussion
The tradewind inversion is expected to weaken considerably as deep moisture and minor instability fills in through the night, likely leading to a fairly saturated atmosphere between tomorrow morning and early Saturday morning. This will raise the stakes on fog, high humidity and rain at the summit over the next 2 nights; isolated convection is also a possibility mainly for tomorrow morning and afternoon. The air mass is expected to stabilize as the inversion re-strengthens through Saturday. While this will diminish the risk for fog and rain during that time, humidity may linger near 60% through the remainder of the weekend. Normal dry/stable conditions will prevail for next week. Some afternoon clouds are possible through the weekend (and will likely be extensive for tomorrow), but should taper for next week.

Broken to overcast thick clouds are slated to develop over the summit area over the next 12 hours and will likely block most, if not all of the sky for much of otmorrow. These clouds are slated to breakdown through Saturday, opening up skies as that night progresses, and probably leaving relatively clear skies for Sunday and especially Monday night.

Precipitable water is expected to exceed 4 mm through Saturday night, but will likely dip back toward 3 mm (perhaps less) for Sunday and Monday night.

Increasing turbulence in the free atmosphere, combined with an influx of mid/low-level moisture/instability will likely degrade seeing from 0.5 arcseconds for this evening to 0.7-0.8 arcseconds through the night. Poor/bad conditions/skies and seeing are expected for tomorrow night and there is a good chance that low-level/boundary layer turbulence will limit much improvement in seeing for Saturday and Sunday night. There is a possibility that seeing will dip toward 0.4-0.5 arcseconds as this turbulence subsides for the early part of next week.

Aside from this evening CC forecast, little change since the morning forecast...Deep tropical moisture and minor instability will continue to fill in from the southeast, while the upper-level trough to the west sends mid/upper-level moisture in from the southwest. This will help to weaken the inversion, significantly increase cloud cover through the night, and perhaps lead to a fairly saturated atmosphere between tomorrow morning and early Saturday morning. Consequently, there is a high risk for fog, high humidity and light rain at the summit over the next 2 nights. There is also a possibility for isolated convection in the area mainly around sunrise and sunset tomorrow. The bulk of the moisture and clouds are slated to slide off toward the west as an upper-level ridge fills in from the southeast and the mid/upper-level ridge strengthens to the north starting late Saturday morning. Although this will help to re-strengthen the inversion near 6-7 thousand feet, stabilize the environment and reduce the risk for fog/precipitation, lingering pockets of mid-level moisture may allow humidity and PW to remain quite high probably through the weekend. In addition, there is possibility that boundary layer/low-level turbulence will increase with summit/mid-level winds, which won't help seeing during that time. There is a fairly good chance that this source of turbulence will subside and allow seeing to improve again for the early 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)
Thu Jul 31 - 8 PM	40-60	6-8	25 / 5	2.5	SSE/5-15	0.4-0.6	4-8
Fri Aug 01 - 2 AM	60-80	5-8	75 / 50	2	SSE/5-15	0.5-0.9	6-10
2 PM	80-100	4-10	95 / 75	6	SE/5-15	NaN	8-12
8 PM	80-100	4-10	90 / 50	3	ESE/10-20	0.8-1.6	8-12
Sat Aug 02 - 2 AM	80-100	6-8	75 / 25	3	SE/10-20	0.7-1.3	4-8
2 PM	70-90	4-8	50 / 10	7	SE/15-30	NaN	4-8
8 PM	60-80	6-8	25 / 0	3	E/10-20	0.6-1.2	4-6
Sun Aug 03 - 2 AM	40-60	7-8	10 / 0	3	E/15-30	0.6-1.2	4-6
2 PM	20-40	4-5	25 / 5	8.5	E/15-30	NaN	4-8
Mon Aug 04 - 2 AM	20-40	Clear	0 / 0	4	ENE/15-30	0.5-1.1	2-4
2 PM	0-20	Clear	0 / 0	10	ENE/10-20	NaN	2-4
Tue Aug 05 - 2 AM	0-10	Clear	0 / 0	4.5	NNE/5-15	0.3-0.7	2-4
2 PM	0-20	Clear	0 / 0	10	NE/10-20	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
Thu Jul 31 - Fri Aug 01	19:08	20:18	4:38	5:48	N/A	22:12	24	12 28.1	-4 53
Fri Aug 01 - Sat Aug 02	19:08	20:18	4:39	5:49	N/A	22:51	33	13 14.9	-8 36
Sat Aug 02 - Sun Aug 03	19:07	20:17	4:39	5:49	N/A	23:32	43	14 03.7	-12 04
Sun Aug 03 - Mon Aug 04	19:07	20:16	4:40	5:49	N/A	0:17	53	14 55.2	-15 06
Mon Aug 04 - Tue Aug 05	19:06	20:16	4:40	5:50	N/A	1:05	64	15 49.7	-17 28

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 1 August 2014.

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Current Conditions
Temp		0.8 C
RH		78 %
Wind		SSE 15 mph
Road		Open