Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Thursday 24 July (0300 UTC Friday 25 July) 2014

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, with winds from the ENE at 10-20 mph for this evening, easing a bit as night progresses.Precipitable water is expected to be in the 2-3 mm range, with seeing around 0.5 arcseconds.

Discussion
The tradewind inversion will continue to cap low-level moisture near 7 thousand feet and keep the mid/summit-level air mass dry and stable through at least midnight Friday. However, a patch of low-level moisture passing through the area combined with minor upper-level instability could lift the inversion toward 12-13 thousand feet and increase the risk for periodic fog and high humidity at the summit between early Saturday morning and Sunday afternoon. The inversion is slated to recover near 6-8 thousand feet through Sunday night and will ensure a dry/stable air mass by Monday night. Daytime clouds will be minimal and short-lived for tomorrow, but could pick up over the weekend, only to taper again for next week.

Skies will remain predominately clear for tonight, but a narrow band of (mostly dissipating) cirrus will start to stream along the northern skies later in the night and drift closer to the summit area through tomorrow afternoon/evening. This band will likely continue to stream along the northern tip of the Big Island through Saturday night. More high clouds are slated to spread in from the south as this band breaks down on Sunday, but these clouds are not expected to drift directly over the summit for that night as well as Monday night.

Precipitable water is expected to linger in the 2-3 mm range for the next 2 nights, increase to 4+ mm for Saturday and perhaps Sunday night, then slip back toward 2-3 mm for Monday night.

Relatively calm skies should allow for better than average seeing over the next 2 nights. However, an influx of upper-level instability, turbulence and/or moisture will likely contribute to poorer than average seeing probably for the remainder of the forecast period.

Little change since the morning forecast...The mid/low-level ridge will continue to sit to the north of the Islands with an upper-level ridge lingering to the south probably for another 24-30 hours. This will allow strong/steady large-scale subsidence to prevail in the area, which will ensure a dry/stable, calm and relatively warm summit-level air mass through at least midnight Friday. An upper-level trough is slated to build to the north of the state and erode the mid-level ridge over the weekend. In addition, a patch of tropical moisture will likely be drawn to the trough and pass through Big Island around that time. These two features may help weaken/lift the inversion toward 12-13 thousand feet and increase the risk for fog and high humidity at the summit between early Saturday morning and Sunday afternoon/evening. This patch of moisture is slated to shift westward as the mid-level ridge rebuilds to the north of the state late Sunday night. This should help restore order to the area, rebuild the inversion and ensure a dry/stable summit-level air mass for the early part of next week. However, the upper-level trough may still produce some turbulence in the free atmosphere, which could contribute to poor seeing for Saturday, Sunday and Monday night.

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)
Thu Jul 24 - 8 PM	0-5	Clear	0 / 0	4.5	ENE/10-20	0.45-0.55	2-3
Fri Jul 25 - 2 AM	0-10	Clear	0 / 0	4.5	ENE/5-15	0.45-0.55	2-3
2 PM	0-20	Clear	0 / 0	10	ENE/5-15	NaN	2-3
8 PM	10-30	9-10	0 / 0	5	ENE/5-15	0.4-0.5	2-3
Sat Jul 26 - 2 AM	20-40	9-10	25 / 5	4.5	ENE/0-10	0.45-0.55	2-3
2 PM	60-80	4-10	75 / 25	8	ESE/0-10	NaN	4-8
8 PM	20-40	9-10	75 / 10	3.5	N/0-10	0.5-0.9	4-6
Sun Jul 27 - 2 AM	20-40	9-10	60 / 10	3	S/0-10	0.5-0.8	4-6
2 PM	40-60	4-10	60 / 10	8	WNW/0-10	NaN	4-8
Mon Jul 28 - 2 AM	10-30	9-10	15 / 0	3	SE/0-10	0.5-0.7	3-5
2 PM	10-30	9-10	0 / 0	8	S/0-10	NaN	2-4
Tue Jul 29 - 2 AM	10-30	9-10	0 / 0	3	SW/0-10	0.5-0.8	2-3
2 PM	10-30	9-10	0 / 0	8	S/0-10	NaN	2-4

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 Jul 24 - Fri Jul 25	19:11	20:23	4:34	5:46	4:49	17:39	2	7 06.0	17 04
Fri Jul 25 - Sat Jul 26	19:11	20:22	4:35	5:46	5:39	18:23	0	7 54.5	15 14
Sat Jul 26 - Sun Jul 27	19:11	20:21	4:36	5:46	6:29	19:04	0	8 41.7	12 47
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

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 25 July 2014.

Additional Information
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Current Conditions
Temp		2.8 C
RH		23 %
Wind		ESE 25 mph
Road		Open