Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Wednesday 11 December 2019

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 6.5 C this afternoon, 1.5 C this evening and 1 C tomorrow morning. Winds will be from the SE at 5-15 mph, with seeing near 0.6 arcseconds. Precipitable water is expected to be in the 2.5-3.5 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture near 6-7 thousand feet and ensure the summit remains dry and stable through tonight. There is a chance that the inversion will breifly weaken/lift toward 12-13 thousand feet and increase the risk for periods of fog, high humidity and possibily ice for tomorrow night. The inversion is set to recover on Friday, but lingering mid-level moisture may still allow for short periods of fog/high humidity at the summit through Saturday night. Precipitation is unlikely throughout the forecast period and dry/stable conditions will return to the summit for Sunday night. Daytime clouds will be minimal and short-lived for today, but could pick up a bit for the remainder of the week, only to back off again for the early part of next week.

Skies will remain predominately clear for tonight, but high clouds will begin to fill the western high clouds through tomorrow and will eventually pass overhead contributing to extensive cloud cover by the second half of that night. Thicker clouds may follow suit from the SW on Friday, perhaps blanketing skies for much of that night and into Saturday evening. These clouds will begin to break up and shift off toward the SE on Sunday, leaving predominately clear skies by later that night.

Precipitable water is expected to linger near 3 mm into early tomorrow evening, then will increase to 4+ mm for the remainder of the forecast period.

Deep southerly flow switching to a more SW direction will prevail in the free atmosphere, resulting in light/moderate shear and probably slightly better than average seeing for tonight and into tomorrow evening. An elevated inversion, increase in free atmospheric turbulence and/or mid-level moisture will likely contribute to poorer than average seeing for the second half of that night and the following 2 nights. There is a possibility that an increase in boundary layer turbulence will prohibit seeing from improving for Sunday night.

The mid/low-level ridge will linger just over the Big Island before shifting off toward the east in response to a trough building in from the west later tomorrow. Nevetheless, subsidence associated to the ridge will help maintain a well-defined inversion near 6-7 thousand feet and ensure a dry/stable summit-level air mass for tonight. The building trough will begin to destabilize the air mass, enhance the incoming low-level flow, which could weak/elevate the inversion and increase the risk for fog/high humidity and possibily ice at the summit for tonight, particularly the second half. Fortunately, the trough is slated to quickly flatten out, while depositing an upper-level low to the west of the state early Friday morning. The low will remain to far off to the west and is not expected to impact the stability of the atmosphere near the Big Island during its tenure, instead allowing the low-level to the north to retake control. However, the low will begin to draw deep mid-level moisture/clodus in from the tropics and advect it over the summit, contributing to high PW, extensive cloud cover or even overcast skies, and perhaps short periods of fog/high humidity for Friday and Saturday night. The bulk of the moisture and clouds will eventually slide off toward the east as the low falls apart, opening up skies again late in the weekend.

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)
Wed Dec 11 - 2 PM	0-20	Clear	0 / 0	6.5	SE/5-15	NaN	3-5
8 PM	0-5	Clear	0 / 0	1.5	SE/5-15	0.5-0.7	2.5-3.5
Thu Dec 12 - 2 AM	0-20	Clear	0 / 0	1	SE/5-15	0.5-0.7	2.5-3.5
2 PM	20-40	9.5-10	10 / 0	6	SE/5-15	NaN	3-5
8 PM	40-60	9-10	25 / 5	1	ESE/5-15	0.45-0.75	2.5-3.5
Fri Dec 13 - 2 AM	60-80	8-10	60 / 15	0	SE/5-15	0.6-1	3.5-4.5
2 PM	80-100	4-10	40 / 10	5	NW/5-15	NaN	4-8
8 PM	80-100	5-10	30 / 5	1	N/5-15	0.7-1.1	4-8
Sat Dec 14 - 2 AM	80-100	5-10	30 / 5	1	N/5-15	0.6-1	4-8
2 PM	80-100	4-9	25 / 5	6	E/5-15	NaN	6-10
Sun Dec 15 - 2 AM	60-80	5-8	10 / 0	2	WNW/5-15	0.6-0.9	6-10
2 PM	20-40	4.5-6	0 / 0	6	WNW/15-30	NaN	6-10
Mon Dec 16 - 2 AM	0-5	Clear	0 / 0	1	NNE/20-35	0.7-1.1	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
Wed Dec 11 - Thu Dec 12	17:54	19:02	5:29	6:37	17:31	7:26	100	5 28.3	21 49
Thu Dec 12 - Fri Dec 13	17:54	19:03	5:29	6:38	18:24	N/A	98	6 27.4	23 07
Fri Dec 13 - Sat Dec 14	17:54	19:03	5:30	6:38	19:21	N/A	94	7 28.0	23 01
Sat Dec 14 - Sun Dec 15	17:55	19:03	5:30	6:39	20:22	N/A	88	8 28.6	21 28
Sun Dec 15 - Mon Dec 16	17:55	19:04	5:31	6:40	21:24	N/A	79	9 27.6	18 34

Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Wednesday 11 December (0300 UTC Thursday 12 December) 2019.

Additional Information
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Current Conditions
Temp		6.9 C
RH		20 %
Wind		ENE 7 mph
Road		Closed