Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Wednesday 16 August (0300 UTC Thursday 17 August) 2017

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 C this evening and 2.5 C tomorrow morning. Winds will be light and from the NE, with seeing near 0.35-0.4 arcseconds for the night. Precipitable water is expected to linger in the 0.8-1 mm range for the night.

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and ensure the summit remains dry and stable through at least Friday night. There is a chance that building upper-level instability coupled with an influx of tropical moisture will weaken/erode the inversion, which could raise the stakes on fog, high humidity and light rain at the summit for Saturday and Sunday night. Daytime clouds will be minimal and short-lived through Saturday, but could pick up for the following 2 days. There is also a chance that convection will build in from the east and/or develop along the slopes for Sunday and Monday.

Skies overhead will remain predominately clear for the next 3 nights, but there is a chance for thin cirrus along the western skies for Wednesday and Thursday night. There is a possibility that thick clouds will begin to fill in from the ESE and develop just north of the Big Island for Saturday and Sunday night. The bulk of these clouds should remain offshore at night, but there is a chance that isolated clouds will stray over/near the summit during that time.

Precipitable water is expected to linger near or just below 1 mm for the next 3 nights, then increase toward 2-4 mm and/or become more variable for Saturday and Sunday night.

While very calm skies and deep subsidence will allow seeing to settle in near 0.35-0.4 arcseconds over the next 2-3 night, there is a possibility for periods of variability mainly during the second half of these nights. An increase in upper-level shear may degrade seeing toward 0.5-0.6 arcseconds for Friday night. There is a possibility that building instability and low-level turbulence will further degrade seeing for the following 2 nights.

Little change since the morning forecast...The mid/low-level ridge will continue to sit to the north of the state through Saturday afternoon, while an upper-level ridge builds in from the south and settles in overhead for the next 2 nights. This will allow strong/steady large-scale subsidence to persist in the area, which will easily maintain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass through at least Friday night. The presence of the upper-level ridge will also allow calm, calm and dry skies to prevail, which should translate to excellent seeing and PW near 1 mm for the next 2-3 nights. An upper-level trough/low displaced (by the aforementioned ridge) 1000 km to the NNE is expected to become reinforced/rejuvenated by a westerly trough passing to the north late Thursday night. This will allow the low to strengthen, deepen and dig back southward, which will intially increase shear/turbulence and free atmosphere and degrade seeing a bit for Friday night. However, the low is set to migrate westward a tad, which may start to destabilize the air mass and weaken the inversion through Saturday night. Last several model runs also suggests that a weak tropical storm/cyclone will develop 800-900 km to the SE of the Big Island over the next 24-48 hours and get drawn toward this low over the weekend. While this storm will likely weaken (a result of strong shear associated with the southern flan of the low) as it nears the Big Island, its surrounding tropical moisture may take advantage of the instability associated with the low. This could result in periods of fog/high humidity and precipitation at the summit, as well as the development of the convection near/over the Big Island for the second half of the weekend and 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)
Wed Aug 16 - 8 PM	0-20	Clear	0 / 0	3	E/0-10	0.25-0.45	0.8-1
Thu Aug 17 - 2 AM	0-20	Clear	0 / 0	2.5	NE/0-10	0.25-0.55	0.8-1
2 PM	0-20	Clear	0 / 0	8.5	SSE/0-10	NaN	1-1.5
8 PM	0-20	Clear	0 / 0	3.5	NNE/0-10	0.25-0.45	0.8-1
Fri Aug 18 - 2 AM	0-20	Clear	0 / 0	3	NW/0-10	0.25-0.55	0.8-1
2 PM	0-20	Clear	0 / 0	8.5	SW/0-10	NaN	1-1.5
8 PM	0-5	Clear	0 / 0	3.5	SW/0-10	0.4-0.6	0.9-1.1
Sat Aug 19 - 2 AM	0-5	Clear	0 / 0	2.5	W/0-10	0.45-0.65	0.9-1.1
2 PM	0-20	Clear	0 / 0	7.5	SSE/5-15	NaN	1-2
Sun Aug 20 - 2 AM	20-40	4-5	40 / 10	2	SSW/5-15	0.6-0.9	1-2
2 PM	80-100	4-10	75 / 40	5.5	SSE/5-15	NaN	4-8
Mon Aug 21 - 2 AM	20-40	4-7	60 / 15	1.5	ESE/5-15	0.6-1	2-4
2 PM	80-100	4-10	80 / 60	4.5	ESE/10-20	NaN	8-12

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
Wed Aug 16 - Thu Aug 17	18:58	20:06	4:46	5:54	1:49	N/A	23	5 47.4	18 25
Thu Aug 17 - Fri Aug 18	18:58	20:05	4:46	5:54	2:48	N/A	14	6 48.0	18 49
Fri Aug 18 - Sat Aug 19	18:57	20:04	4:47	5:54	3:50	N/A	7	7 48.5	17 59
Sat Aug 19 - Sun Aug 20	18:56	20:03	4:47	5:55	4:52	17:32	2	8 47.7	15 58
Sun Aug 20 - Mon Aug 21	18:55	20:03	4:48	5:55	5:53	18:24	0	9 44.7	13 00

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Thursday 17 August 2017.

Additional Information
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Current Conditions
Temp		2.4 C
RH		14 %
Wind		NE 11 mph
Road		Open (4x4)