Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Tuesday 20 August (0300 UTC Wednesday 21 August) 2019

Warning(s)
Chance for afternoon convection
Possibility for fog/high humidity

Cloud Cover and Fog/Precipitation Forecast
There is a small risk for periods of fog and high humidity, mainly during the first half of the night; precipitation is not expected, but there is a possibility for isolated thin high clouds along the southern skies for 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 ESE at 5-15 mph, with seeing near 0.5-0.6 arcseconds, while precipitable water is expected to be in the 3-4 mm range for the night.

Discussion
Although the tradewind inversion will remain fairly well established near 8 thousand feet throughout most of the forecast period, patches of tropical moisture passing by may increase summit-level humidity and perhaps the risk for short-lived fog at the summit through this evening and again between midnight Thursday and early Saturday evening; precipitation is not expected over the next 5 nights and the summit will steer free of fog and precipitation for the remainder of the forecast period. Daytime clouds will be minimal and short-lived over the next 2 days, but there is a possibility for extensive afternoon clouds and convection on Friday and Saturday, then will taper on Sunday.

There is a chance that isolated thin high clouds will spread out along the southern skies for tonight. These clouds will quickly dissipate through tomorrow morning, leaving predominately clear skies probably throughout most of the forecast period. There is a slight possibility that isolated mid/upper-level clouds will appear along the southern skies for part of Friday night.

Precipitable water is expected to increase into the 3-4 mm range for tonight, dip into the (lower half of the) 2-3 mm range for Wednesday night and Thursday evening, then increase back to 4+ mm probably for the remainder of the forecast period.

Winds are set to taper, diminishing boundary layer turbulence and likely allowing seeing to slip back toward 0.5-0.6 arcseconds for the most part of the next 2 nights. However, another round of light/moderate boundary layer turbulence will probably contribute to poorer than average seeing for Thursday, Friday and Saturday night.

No change since the morning forecast...A deep ridge will continue to sit ot the north of the state before the upper-level portion is squeezed northward as an upper-level low passes to the north around Friday and Saturday. Still, persistence subsidence associated with at least the low-level ridge will help maintain a well-defined tradewind inversion near 8 thousand feet throughout the forecast period. Unfortunately, mesoscale patches of moisture associated with old tropical waves are expected to pass through the area over the next 12 hours and again between late Thursday night and Saturday evening. These patches may bring minor mid-level moisture to the summit, which could increase the risk for short-periods of fog and high humidity at the summit during those times. Daytime heating may also provide enough low-level instability to fire off afternoon convection along the slopes on Friday and Saturday, despite the large-scale subsidence provided by the ridge. Nevertheless, skies, for the most, will remain relatively clear (overhead), with just a few brief moments of high clouds along the southern skies, throughout most of the forecast period. Flow aloft will also remain relatively light and stable, minimizing turbulence in the free atmospehre, but there is a chance that boundary layer turbulence will contribute to poor seeing mainly after tomorrow 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)
Tue Aug 20 - 8 PM	0-20	9-10	25 / 0	3.5	ESE/5-15	0.45-0.75	3-4
Wed Aug 21 - 2 AM	0-20	9.5-10	10 / 0	3	ESE/5-15	0.4-0.6	3-4
2 PM	0-20	Clear	0 / 0	9	NE/5-15	NaN	2-4
8 PM	0-5	Clear	0 / 0	3	NE/5-15	0.45-0.65	2-3
Thu Aug 22 - 2 AM	0-5	Clear	0 / 0	3	NE/10-20	0.5-0.7	2-3
2 PM	0-20	Clear	0 / 0	9	ENE/15-25	NaN	2-3
8 PM	0-5	Clear	0 / 0	3	E/15-25	0.6-1	2-3
Fri Aug 23 - 2 AM	0-5	Clear	0 / 0	2	SE/15-25	0.7-1.1	3-5
2 PM	60-80	4-9	60 / 15	8	SE/15-25	NaN	4-8
Sat Aug 24 - 2 AM	0-20	9.5-10	25 / 0	2.5	ESE/15-25	0.7-1.1	4-6
2 PM	40-60	4-8	50 / 10	8	ESE/15-25	NaN	4-8
Sun Aug 25 - 2 AM	0-5	Clear	20 / 0	3	E/15-25	0.6-1	3-5
2 PM	0-20	Clear	0 / 0	9	ENE/15-25	NaN	2-4

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
Tue Aug 20 - Wed Aug 21	18:56	20:03	4:47	5:55	22:11	N/A	71	2 12.2	7 29
Wed Aug 21 - Thu Aug 22	18:55	20:02	4:48	5:55	22:48	N/A	61	2 59.2	11 43
Thu Aug 22 - Fri Aug 23	18:54	20:01	4:48	5:55	23:28	N/A	51	3 48.6	15 30
Fri Aug 23 - Sat Aug 24	18:53	20:00	4:49	5:55	0:13	N/A	41	4 41.0	18 37
Sat Aug 24 - Sun Aug 25	18:53	19:59	4:49	5:56	1:02	N/A	31	5 36.6	20 50

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Wednesday 21 August 2019.

Additional Information
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Current Conditions
Temp		3.0 C
RH		80 %
Wind		E 9 mph
Road		Closed