Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 22 January (0300 UTC Saturday 23 January) 2021

Warning(s)
Moderate/strong winds
Increasing risk for moisture late in the weekend

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while more scattered mid-level clouds return from the south for the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -5 C, with winds from the east at 30-45 mph for this evening, decreasing to 25-40 mph through the night. Seeing will exceed 1 arcsecond, while precipitable water is expected to be in the 3-4 mm range for the night.

Discussion
A well-defined inversion will continue to cap low-level moisture near 7 thousand feet and ensure a dry/stable summit-level air mass for tonight. The inversion will begin to weaken/lift as deep moisture scrapes by to the south, increasing the risk for fog, ice and high humidity through Saturday night. This risk will further increase, as well as the possibility for flurries as the atmosphere turns saturated for the following 3 nights. There is also a possibility for convection in the area as well as periods of moderate/heavy snow at the summit mainly for Monday and Tuesday night. Some afternoon clouds will likely pick up tomorrow and turn extensive for the remainder of the forecast period.

More scattered mid-level clouds are set to slowly spread back in from the south for tonight and into tomorrow night. A band of broken to overcast thick clouds will eventually quickly fill in from the south and develop overhead, blanketing summit skies for Sunday, Monday and Tuesday night.

Precipitable water is expected to slip into the 3-4 mm range for tonight, increase back to 4 mm for Saturday night, then jump to 8+ mm for the following 3 nights.

A mixture of boundary layer turbulence and/or instability/moisture will contribute to mostly bad seeing throughout the forecast period.

No change since the morning forecast...A tight mid/low-level ridge will continue to promote strong large-scale subsidence in the area, which will help maintain a well-defined inversion near 7 thousand feet and ensure a dry/stable summit-level air mass through at least late tomorrow morning. Unfortunately, a tight wind gradient along the southern flank of the ridge will also result in stronger than normal easterly flow, which will stir up boundary layer turbulence and contribute to bad seeing for tonight. Winds may subside a bit as the ridge weakens slightly and retracts northward in response to a trough building to the west of the state beginning tomorrow. Although initially the instability associated with the trough will remain off toward the SW, the displacement of the ridge will allow a band of deep moisture to fill in back into the area and will begin to scrape the southern portion of the Big Island by tomorrow evening. This band will start to erode the inversion, increasing the risk for fog, ice and high humidity and even cloud cover as that night progresses. Eventually, the air mass will turn completely saturated as this band of moisture further deepens and spreads across the Big Island by late Sunday afternoon and lingers overhead well into next week. This will likely contribute prolong period of inoperable conditions for Sunday, Monday and at least Tuesday night. In addition, there is a chance that convection will begin to develop in the area, which could deposit moderate/heavy snow as the trough shifts eastward and destabilizes the air mass between early Monday morning and Tuesday night. Winds may also pick up again as the trough closes in and tightens the wind gradient on Monday night, but should quickly subside as the ridge to the far NE weakens on Tuesday and into Wednesday.

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)
Fri Jan 22 - 8 PM	10-30	6-8.5	0 / 0	-5	E/30-45	0.9-1.7	3-4
Sat Jan 23 - 2 AM	20-40	6-8	0 / 0	-5	ESE/25-40	0.8-1.6	3-4
2 PM	40-60	4-7	60 / 15	0	ESE/20-35	NaN	4-8
8 PM	30-50	5-7	40 / 10	-3.5	E/15-30	0.7-1.1	3.5-4.5
Sun Jan 24 - 2 AM	40-60	5-7	50 / 15	-4	SE/15-30	0.7-1.1	3.5-4.5
2 PM	60-80	4-8	90 / 65	-1	SSE/10-20	NaN	4-8
8 PM	80-100	4-10	90 / 60	-3	SE/10-20	0.7-1.3	8-12
Mon Jan 25 - 2 AM	80-100	4-10	95 / 75	-3	SSE/10-20	0.8-1.6	8-12
2 PM	80-100	4-10	95 / 90	-1	SE/20-35	NaN	8-12
Tue Jan 26 - 2 AM	80-100	4-10	95 / 90	-3.5	SE/25-40	1-2	8-12
2 PM	80-100	4-10	95 / 75	-1	SSE/15-30	NaN	8-12
Wed Jan 27 - 2 AM	80-100	4-10	95 / 75	-3	E/10-20	0.8-1.8	8-12
2 PM	80-100	4-9	90 / 60	1.5	NE/10-20	NaN	6-10

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
Sat Jan 23 - Sun Jan 24	18:18	19:25	5:43	6:49	N/A	3:44	81	4 44.6	21 46
Sun Jan 24 - Mon Jan 25	18:19	19:26	5:42	6:49	N/A	4:39	88	5 37.8	23 56
Mon Jan 25 - Tue Jan 26	18:20	19:26	5:42	6:49	N/A	5:33	94	6 33.6	24 55
Tue Jan 26 - Wed Jan 27	18:20	19:27	5:42	6:49	16:21	6:27	98	7 31.1	24 35
Wed Jan 27 - Thu Jan 28	18:21	19:27	5:42	6:49	17:19	7:18	100	8 29.0	22 50

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 25 January 2021.

Additional Information
For public road conditions and snow report message please call (808) 935-6268.
This message is also available at the MKWC road conditions page.
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Honolulu National Weather Service Data and Products

Current Conditions
Temp		-3.9 C
RH		19 %
Wind		ESE 4 mph
Road		Open (4x4)