Mauna Kea Weather Center Forecast

Maunakea Observatories Forecast
8 AM HST (1800 UTC) Thursday 16 September 2021

Warning(s)
Chance for fog/high humidity

Cloud Cover and Fog/Precipitation Forecast
There is a possibility for periods of fog and high humidity, mainly for the early part of the night; precipitation is unlikely. Broken thick clouds will continue to pass overhead, contributing to extensive cloud cover through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 7.5 C this afternoon, 2.5 C this evening and 3 C tomorrow morning. Winds will be from the south at 10-20 mph for today, switching to a more easterly direction and easing to 5-15 mph through the night. Seeing will start out near 0.7 arcseconds, but will improve toward 0.5 arcseconds as the night progresses. Precipitable water is expected to exceed 4 mm for the night.

Discussion
Although the inversion is set to rebuild through the day, residual moisture lingering in the area may allow for periods of fog and high humidity mainly for this evening; precipitation is unlikely. A drier more stable air mass, combined with a well-defined inversion at or below 9 thousand feet will ensure the summit steers free of fog and precipitation for the remainder of the forecast period. Extensive daytime clouds and isolated afternoon convection are possible for today, then will become minimal and short-lived for the remainder of the week and Monday, only to pick-up again on Tuesday.

Broken thick clouds will continue to pass overhead, contribute to periods of extensive cloud cover for tonight. These clouds will begin to slowly break up and shift westward through tomorrow, but there is a chance that scattered high clouds will continue to linger in the area and/or fill in from the north for Friday night. While mostly clear skies will prevail for Saturday evening, a patch of dissipating cirrus may fill in from the east for much of Sunday. More organized cirrus could slip in from the SE for Monday night.

Precipitable water is expected to linger near or exceed 4 mm through Friday night, then slip toward or just below 2 mm for the remainder of the forecast period.

Residual moisture in the area may contribute to poorer than average seeing for this evening, but there is a good chance that seeing will improve and settle in near 0.5 arcseconds as the night progresses. There is good chance that a minor increase in boundary layer turbulence will degrade seeing toward 0.65-0.7 arcseconds for Friday night. Seeing will eventually settle back in around 0.5-0.6 arcseconds as mostly calm skies and deep easterly flow prevails for the following 3 nights.

The mid/upper-level low to the NW will continue to migrate westward, allowing the air mass to stabilize as a relatively deep ridge builds in the eastern Pacific through the day. While subsidence associated with the ridge will help restrengthen the inversion near 9 thousand feet over the next 24 hours, residual moisture trailing the low may still allow for periods of fog and high humidity at the summit, mainly through early this evening. A very distinct inversion near 8-9 thousand feet will dry out the air mass by late tomorrow morning and ensure this summit steers free of fog and precipitation for the remainder forecast period. Skies are also set to open up as the low slips further westward and an upper-level ridge fills in overhead for the early part of the weekend. While the presence of the upper ridge will minimize turbulence in the free atmosphere, winds are set to briefly pick up, enough to stir up boundary layer turbulence and perhaps contribute to poor seeing for tomorrow night. Seeing is set to settle back in near 0.5-0.55 arcseconds as winds taper in response to a zonal stretching of the ridge over 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 (%)	(Celsius)	(Dir/MPH)	(Arcseconds)	(mm)
Thu Sep 16 - 2 PM	60-80	4-10	75 / 25	7.5	S/10-20	NaN	4-8
8 PM	60-80	6-10	60 / 10	2.5	E/5-15	0.55-0.85	4-8
Fri Sep 17 - 2 AM	60-80	6-10	30 / 5	3	E/5-15	0.4-0.6	4-8
2 PM	60-80	8-10	5 / 0	9.5	ENE/15-25	NaN	4-8
8 PM	40-60	8-10	0 / 0	4.5	ENE/15-25	0.5-0.9	4-6
Sat Sep 18 - 2 AM	20-40	8-9	0 / 0	4.5	NE/10-20	0.5-0.8	3.5-4.5
2 PM	0-20	Clear	0 / 0	10	NE/5-15	NaN	2-4
8 PM	0-10	Clear	0 / 0	5	NE/5-15	0.45-0.65	1.5-2
Sun Sep 19 - 2 AM	10-30	8-9	0 / 0	4.5	NE/5-15	0.4-0.6	1.5-2
2 PM	20-40	8-9	0 / 0	8.5	E/10-20	NaN	2-4
Mon Sep 20 - 2 AM	10-30	8-9	0 / 0	3	SE/10-20	0.425-0.675	1.5-2
2 PM	20-40	8-10	0 / 0	7.5	SE/10-20	NaN	2-4
Tue Sep 21 - 2 AM	30-50	8-10	0 / 0	2	E/10-20	0.425-0.675	1.5-2.5

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 Sep 16 - Fri Sep 17	18:32	19:36	4:56	6:01	N/A	3:17	85	20 55.4	-22 35
Fri Sep 17 - Sat Sep 18	18:31	19:35	4:57	6:01	N/A	4:16	92	21 51.4	-18 42
Sat Sep 18 - Sun Sep 19	18:30	19:35	4:57	6:01	17:12	5:13	97	22 44.0	-13 55
Sun Sep 19 - Mon Sep 20	18:29	19:34	4:57	6:02	17:51	6:07	99	23 33.5	-8 34
Mon Sep 20 - Tue Sep 21	18:28	19:33	4:57	6:02	18:28	6:59	100	0 20.9	-2 57

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Thursday 16 September (0300 UTC Friday 17 September) 2021.

Additional Information
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Current Conditions
Temp		4.9 C
RH		84 %
Wind		SSW 4 mph
Road		Open (4x4)