Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 25 July (0300 UTC Saturday 26 July) 2014

Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable for this evening, but there is slight chance for fog and high humidity mainly toward the end of the night; skies will remain predominately clear through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 5.5 C this evening and 4.5 C tomorrow morning. Winds will be from the ENE at 5-15 mph, with seeing around 0.45 arcseconds. Precipitable water is expected to be in the 1.75-2.25 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture near 6-7 thousand feet and keep the mid/summit-level air mass dry and stable through at least midnight. However, there is a possibility that the inversion will lift toward 11-12 thousand feet, which could increase the risk for periods of fog and high humidity toward the end of the night and perhaps tomorrow night; precipitation is not expected. The inversion is slated to recover again by late Sunday afternoon, ensuring a dry/stable air mass for the remainder of the forecast period. Daytime clouds will be minimal and short-lived for today, but could pick up over the weekend, only to taper again for next week.

Skies will remain predominately clear for tonight, but a band of high clouds is slated to set up to the north later in the night and creep toward the summit over the weekend. This band will likely be visible from the summit by late tomorrow evening and could block half of the sky as that night progresses. These clouds are expected to breakdown and retract northward through Sunday, while more widespread high clouds fill in from the south during that night, perhaps contributing to extensive cloud cover for much of Monday. This patch of clouds are expected to shift eastward early Tuesday morning, leaving relatively clear skies for that night.

Precipitable water is expected to linger near 2-2.5 mm probably throughout the forecast period.

Relatively calm skies should allow for better than average seeing over the next 2 nights. However, building turbulence in the free atmosphere will likely degrade seeing to slightly poorer than average seeing for Sunday and Monday night. This turbulence is expected to subside on Tuesday, allowing seeing to subtly improve for that night.

No change since the morning forecast...The mid-level ridge is set to weaken as an upper-level trough develops to the north of the state over the next 12 hours and lingers in the area well into next week. Nonetheless, the low-level ridge will remain intact to the north and will instill relatively strong/steady large-scale subsidence in the area well into next week. This will help maintain the tradewind inversion near 6-7 thousand feet and ensure a predominately dry and stable summit-level air mass throughout most of the forecast period. The only exception could be as patch of low-level tropical moisture strays towards the Big Island and may briefly weaken/lift the inversion toward 12 thousand feet between tomorrow morning and late Sunday afternoon. I suspect this is a little overdone, and the past few runs have continously downsized the patch of this moisture, but I still included a small risk for periodic fog and high humidity during that time. Nonetheless, the trough aloft will not impact summit skies for tonight, but it could bring high clouds to the area for subsequent nights. It also may increase turbulence in the free atmosphere as the jet drifts closer to the area mainly for Sunday and Monday night. On the other hand, the jet and trough will easily dismantle TS Genevieve well before it reaches the Big Island vicinity later next week. There is a possibility that remnant moisture could become an issue at the summit around Thursday through Saturday, but I suspect the bulk (if not all) of the moisture will slip by to the south.

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 Jul 25 - 8 PM	0-5	Clear	0 / 0	5.5	ENE/5-15	0.4-0.5	1.75-2.25
Sat Jul 26 - 2 AM	0-5	Clear	10 / 0	4.5	E/5-15	0.45-0.55	1.75-2.25
2 PM	40-60	4-5	40 / 0	9	E/5-15	NaN	3-5
8 PM	20-40	9-10	15 / 0	4	ENE/5-15	0.45-0.55	2-3
Sun Jul 27 - 2 AM	40-60	9-10	10 / 0	4	ESE/5-15	0.45-0.55	1.75-2.25
2 PM	40-60	9-10	15 / 0	8.5	SE/5-15	NaN	2-4
8 PM	20-40	9-10	0 / 0	4	ESE/0-10	0.55-0.65	1.75-2.25
Mon Jul 28 - 2 AM	40-60	9-10	0 / 0	3.5	SSE/0-10	0.55-0.75	1.75-2.25
2 PM	60-80	8.5-10	0 / 0	8	SE/0-10	NaN	2-3
Tue Jul 29 - 2 AM	40-60	9-10	0 / 0	3	S/0-10	0.5-0.7	2-3
2 PM	20-40	9-10	0 / 0	8	SW/0-10	NaN	2-3
Wed Jul 30 - 2 AM	0-20	Clear	0 / 0	3	S/0-10	0.45-0.65	2-3
2 PM	0-20	Clear	0 / 0	7.5	SW/0-10	NaN	2-3

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 Jul 27 - Mon Jul 28	19:10	20:21	4:36	5:47	N/A	19:44	2	9 27.8	9 48
Mon Jul 28 - Tue Jul 29	19:10	20:20	4:37	5:47	N/A	20:22	5	10 13.0	6 25
Tue Jul 29 - Wed Jul 30	19:09	20:20	4:37	5:48	N/A	20:59	10	10 57.8	2 46
Wed Jul 30 - Thu Jul 31	19:09	20:19	4:38	5:48	N/A	21:35	17	11 42.6	-1 03
Thu Jul 31 - Fri Aug 01	19:08	20:18	4:38	5:48	N/A	22:12	24	12 28.1	-4 53

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 28 July 2014.

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Current Conditions
Temp		1.3 C
RH		9 %
Wind		ESE 8 mph
Road		Open