Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Monday 28 September (0300 UTC Tuesday 29 September) 2020

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 3.5 C this evening and 4 C tomorrow morning. Winds will be light and from the ENE, with seeing near 0.4 arcseconds. Precipitable water is expected to be in the 1-1.5 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture near 8 thousand feet and ensure a dry/stable summit-level air mass through at least Wednesday night. There is a possibility that an influx of tropical moisture will erode the inversion and increase the risk for fog, high humdity and even ice on Thursday and Friday; precipitation is unlikely. Daytime clouds will be minimal and short-lived through Wednesday, but could turn extensive for Thursday, Friday and Saturday.

Skies will remain predominately clear for the next 3 nights, but scattered mid/upper-level clouds will begin to fill in from the east and could turn extensive for Thursday and especially Friday night.

Precipitable water is expected to linger 1-1.5 mm range for tonight, then increase to 2 mm for tomorrow night, 3 mm for Wednesday night and probably 4+ mm for the following 2 nights.

Very calm skies will prevail as light/laminar flow persists at and above the summit, allowing seeing to settle in near 0.35-0.4 arcseconds through Wednesday night. While this flow will likely continue, an injection of mid-level moisture could disrupt seeing for Thursday and especially Friday night.

Little change since the morning forecast...The mid/low-level ridge will continue to sit just north of the state and promote steady/strong subsidence in the area through at least Wednesday night. This subsidence will help maintain a fairly well-defined inversion primarily near 7-8 thousand feet, keep low-level moisture at bay and ensure a dry/stable summit-level air mass during this time. In addition, very light/laminar flow will prevail in the free atmosphere, with the southward displacement of the mid-level ridge and the absence of the tropical upper-tropospheric trough and its associated sub-tropical jet during this time. This should allow for calm/clear skies and excellent seeing over the next 3 nights. After that, there is a possibility that an influx of tropical moisture from the east will erode the inversion, which will incrase the risk for fog, high humidity and ice at the summit for Thursday and especially Friday night. This moisture could also have a negative impact in seeing, despite the continuation of light/laminar flow at and above the summit for those nights. There is a good chance that this moisture will quickly slip out of the area as the ridge rebounds northward and strengthens, which will diminish the risk for poor conditions/skies at the summit beginning late Saturday morning.

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)
Mon Sep 28 - 8 PM	0-5	Clear	0 / 0	3.5	ENE/0-10	0.3-0.5	1-1.5
Tue Sep 29 - 2 AM	0-5	Clear	0 / 0	4	ESE/0-10	0.3-0.5	1-1.5
2 PM	0-20	Clear	0 / 0	9.5	ESE/0-10	NaN	1-2
8 PM	0-5	Clear	0 / 0	4.5	ENE/5-15	0.3-0.5	1.75-2.25
Wed Sep 30 - 2 AM	0-5	Clear	0 / 0	4.5	ESE/0-10	0.25-0.45	1.75-2.25
2 PM	0-20	Clear	0 / 0	9	S/0-10	NaN	2-4
8 PM	0-5	Clear	0 / 0	4	ESE/0-10	0.3-0.5	2.5-3.5
Thu Oct 01 - 2 AM	0-5	Clear	0 / 0	3.5	ESE/0-10	0.25-0.55	2.5-3.5
2 PM	60-80	4-7	40 / 10	6	SE/5-15	NaN	4-8
Fri Oct 02 - 2 AM	20-40	6-10	65 / 20	2	SSE/5-15	0.5-0.9	6-10
2 PM	80-100	4-10	80 / 80	5	SE/5-15	NaN	8-12
Sat Oct 03 - 2 AM	60-80	4-10	75 / 25	0	ENE/5-15	0.7-1.1	6-10
2 PM	20-40	4-5	25 / 5	8	ENE/10-20	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
Mon Sep 28 - Tue Sep 29	18:21	19:25	4:59	6:04	N/A	4:39	94	22 50.2	-13 01
Tue Sep 29 - Wed Sep 30	18:20	19:24	5:00	6:04	17:14	5:28	98	23 36.4	-8 22
Wed Sep 30 - Thu Oct 01	18:19	19:23	5:00	6:04	17:48	6:17	100	0 21.1	-3 26
Thu Oct 01 - Fri Oct 02	18:18	19:22	5:00	6:04	18:21	7:05	100	1 05.1	1 34
Fri Oct 02 - Sat Oct 03	18:17	19:21	5:00	6:05	18:54	N/A	98	1 49.0	6 30

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Tuesday 29 September 2020.

Additional Information
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Current Conditions
Temp		4.8 C
RH		9 %
Wind		NE 0 mph
Road		Open (4x4)