Mauna Kea Weather Center Forecast

Maunakea Observatories Forecast
10 AM HST (2000 UTC) Thursday 26 May 2022

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while patches of mid-level clouds build in from the south and east mainly during the second half of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 9.5 C this afternoon and 4.5 C for the night. Winds will be from the east at 10-20 mph for today, easing to 5-15 mph for the night. Seeing will be around 0.5-0.6 arcseconds, while precipitable water is expected to start out near 1 mm, but will increase toward 3 mm as the night progresses.

Discussion
A distinct tradewind inversion will continue to cap low-level moisture primarily near 7 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Skies overhead will start out clear for this evening, but patches of mid-level clouds will encroach from the east and south as the night progresses. More widespread broken/thick clouds will quickly follow suit near sunrise tomorrow, contributing to extensive cloud cover for at least tomorrow evening. There is a chance that these clouds will shift off toward the western skies by Saturday morning, but will spread back eastward, likely resulting in another round of extensive cloud cover for the remainder of the weekend. These clouds will breakdown early Monday morning, leaving mostly isolated residual patches of high clouds in the area for that night.

Precipitable water is expected to start out near 1 mm for this evening, but will trend toward 3 mm by the end of the night, jump toward 4+ mm for Friday night, then settle back in the 3-4 mm range for the following 3 nights.

While relatively deep SW winds are expected to strengthen a bit over the next several days, predominately calm skies will prevail at and above the summit, dynamically, throughout the forecast period. However, there is a chance that a patch of colder mid/upper-level air passes through and generates light/moderate free atmospheric turbulence between early Friday morning and early Saturday morning. This could contribute to more average-like seeing during that time, but good/excellent seeing should prevail for the remainder of the forecast period.

A well-defined mid/surface ridge will continue to sit to the north of the state and promote strong steady large-scale subsidence in the area well into next week. This subsidence will easily maintain a well-defined inversion near 7 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. A rather broad upper-level low will continue to develop to the WSW and remain quasi-stationary to the WSW of the state, before folding up late in the weekend. Initially, the low will gain access to tropical mid/upper-level clouds/moisture and stream it along the southern and eastern skies. However, this stream of clouds/moisture will retract westward with the low as the night progresses and will likely contribute to a sharp increase in CC and PW just after sunrise tomorrow. There is also a possibility that a pool of cold upper-level air rotating around the low will briefly pass through, generating an increase in free atmospheric turbulence for much of tomorrow (calm skies should generally allow good/excellent seeing for the remainder of the forecast period). The moisture/clouds are expected to retract further westward early Saturday morning, perhaps allowing for a partial clearing around that time. However, the broken high clouds will likely return to the area as the low begins to weaken, drift northward and get caught up in the westerly flow as the weekend progresses. These clouds should start to break up through Sunday night, perhaps opening up skies again for Monday 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 (%)	(Celsius)	(Dir/MPH)	(Arcseconds)	(mm)
Thu May 26 - 2 PM	0-20	Clear	0 / 0	9.5	E/10-20	NaN	1-1.5
8 PM	0-10	Clear	0 / 0	4.5	E/5-15	0.4-0.6	1-2
Fri May 27 - 2 AM	10-30	6-7	0 / 0	4.5	E/5-15	0.475-0.725	2-3
2 PM	60-80	6-10	0 / 0	10	S/0-10	NaN	4-8
8 PM	80-100	6-9	0 / 0	5	NNW/0-10	0.475-0.725	4-6
Sat May 28 - 2 AM	60-80	6-8	0 / 0	5	NE/5-15	0.4-0.6	4-6
2 PM	20-40	7-9	0 / 0	10	E/5-15	NaN	4-8
8 PM	40-60	8-10	0 / 0	5	SE/5-15	0.35-0.55	3-4
Sun May 29 - 2 AM	60-80	8-10	0 / 0	4.5	SSW/5-15	0.3-0.5	3-4
2 PM	70-90	8-10	0 / 0	9.5	W/0-10	NaN	3-5
Mon May 30 - 2 AM	50-70	8-10	0 / 0	4.5	N/0-10	0.35-0.55	3-4
2 PM	30-50	8-9	0 / 0	10	NNE/0-10	NaN	3-5
Tue May 31 - 2 AM	10-30	8-9	0 / 0	4	NE/5-15	0.35-0.55	3-4

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
Thu May 26 - Fri May 27	19:05	20:17	4:21	5:33	3:45	N/A	9	2 03.2	9 56
Fri May 27 - Sat May 28	19:05	20:18	4:20	5:33	4:21	N/A	4	2 48.9	14 51
Sat May 28 - Sun May 29	19:06	20:18	4:20	5:33	4:59	17:56	1	3 36.1	19 06
Sun May 29 - Mon May 30	19:06	20:19	4:20	5:33	5:41	18:49	0	4 25.0	22 30
Mon May 30 - Tue May 31	19:06	20:19	4:20	5:33	6:26	19:43	1	5 15.6	24 54

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Thursday 26 May (0300 UTC Friday 27 May) 2022.

Additional Information
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Current Conditions
Temp		7.8 C
RH		7 %
Wind		N 16 mph
Road		Open (4x4)