Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 22 May (0300 UTC Saturday 23 May) 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 4.5 C this evening and 4 C tomorrow morning. Winds will be from the NNE at 5-15 mph for this evening, easing and switching to a more ENE direction through the night. Seeing will be near 0.35-0.4 arcseconds, while precipitable water is expected to be in the 0.7-0.9 mm range for the night.

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture primarily near 7-8 thousand feet and ensure a dry and 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 remain predominately clear for the next 2 nights. Scattered high clouds are expected to fill the southern skies on Sunday and perhaps spread overhead for that night and Monday night. While there is a chance that these clouds will disband northward a bit on Tuesday, residual patches of cirrus may still linger in the area for that night.

Precipitable water is expected to slip toward 0.8 mm for tonight, increase toward 1-1.5 mm for the next 2 nights, and 3+ mm for Monday and Tuesday night.

Very calm skies with laminar/light easterly flow and a stable air mass should allow for excellent seeing for at least the next 2 nights. There is a chance that minor upper-level turbulence will build into the area as flow aloft switches to a more southerly direction, which could result in a very subtle degradation in seeing for Sunday, Monday and Tuesday night.

No change since the morning forecast....A fairly flat mid/low-level ridge will continue to sit to the north of the state and stretch across most of the Pacific, promoting strong/steady large scale subsidence in the area well into next week. This subsidence will easily maintain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass through at least the next 5 nights. The ridge will also aid in suppressing the tropical upper-tropospheric trough off toward the south, which will keep high clouds passing to the south and minimize turbulence in the free atmosphere through the next 2 nights. This should allow for excellent seeing as summit-level winds sit near 5-10 mph and probably PW near 1 mm during that time. There is a chance that the TUTT will expand a bit northward as a weak upper-level low develops to the SW of the state late in the weekend. Although the TUTT and the low are not expected to affect the stability of the atmosphere, it could bring minor upper-level turbulence to the area, which could subtly disrupt seeing for Sunday night and early part of next week. There is also a good chance that the eastern flank of the low will send scattered high clouds and perhaps mid-level moisture over the summit, which could drive PW toward 3-4 mm for Monday and Tuesday night.

Please Note: There will be no forecasts on Monday, May 25, in observance of Memorial Day. The normal forecast schedule will resume on Tuesday, May 26.

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 May 22 - 8 PM	0-5	Clear	0 / 0	4.5	NNE/5-15	0.3-0.5	0.7-0.9
Sat May 23 - 2 AM	0-5	Clear	0 / 0	4	ENE/0-10	0.25-0.45	0.7-0.9
2 PM	0-20	Clear	0 / 0	10	ENE/0-10	NaN	0.8-1.2
8 PM	0-5	Clear	0 / 0	5	ENE/5-15	0.25-0.45	1-1.2
Sun May 24 - 2 AM	0-5	Clear	0 / 0	4.5	ENE/5-15	0.3-0.5	1-1.2
2 PM	0-20	Clear	0 / 0	10	E/0-10	NaN	1-2
8 PM	10-30	9.5-10	0 / 0	4.5	ENE/5-15	0.35-0.55	1-1.5
Mon May 25 - 2 AM	20-40	9-10	0 / 0	4.5	E/0-10	0.4-0.6	1-1.5
2 PM	40-60	8-10	0 / 0	10	ESE/5-15	NaN	1.5-2.5
Tue May 26 - 2 AM	20-40	8-10	0 / 0	4.5	ESE/0-10	0.4-0.6	2.5-3.5
2 PM	20-40	8-9	0 / 0	10	SE/5-15	NaN	3-5
Wed May 27 - 2 AM	0-20	8-9	0 / 0	4	SE/10-20	0.35-0.55	3-5
2 PM	0-20	8-9	0 / 0	9.5	ESE/5-15	NaN	3-5

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
Sun May 24 - Mon May 25	19:04	20:16	4:21	5:33	N/A	21:13	7	6 23.0	23 21
Mon May 25 - Tue May 26	19:05	20:17	4:21	5:33	N/A	22:08	13	7 19.2	23 21
Tue May 26 - Wed May 27	19:05	20:17	4:21	5:33	N/A	23:01	21	8 15.9	22 03
Wed May 27 - Thu May 28	19:05	20:18	4:20	5:33	N/A	23:51	31	9 12.2	19 29
Thu May 28 - Fri May 29	19:06	20:19	4:20	5:33	N/A	0:37	41	10 07.5	15 45

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 25 May 2020.

Additional Information
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Current Conditions
Temp		6.0 C
RH		4 %
Wind		ENE 1 mph
Road		Open (4x4)