Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Friday 30 September (0300 UTC Saturday 1 October) 2016

Warning(s)
Fog and high humidity
Chance for convection and rain/flurries

Cloud Cover and Fog/Precipitation Forecast
There is a moderate/high risk for fog, high humidity and perhaps light rain/flurries and convection in the area, particularly as the night progresses. Scattered high clouds will continue to stream over and especially along the southern skies, and there is a possibility for summit/mid-level clouds drifting in from the east toward the end of the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 2.5 C this evening and 2 C tomorrow morning. Winds will be from the ENE at 5-15 mph, while seeing will start out near 0.6 arcseconds, but will trend toward 0.8 arcseconds through the night. Precipitable water is expected to begin the night in the 1-1.5 mm range, but will increase toward 4 mm by the end of the night.

Discussion
The tradewind inversion is expected to rapidly fall apart as tropical moisture spreads in from the east and instability builds to the west through the night, allowing the atmosphere to turn fairly saturated for much of tomorrow. While there is a slight possiblity that the mid-portion atmosphere will dry out a bit for Sunday night, the inversion will remain rather indistinct as instability persists. In addition, more deep tropical moisture is expected to build back into the area for the early/middle part of next week. Consequently, there is a moderate/high risk for fog, high humidity and ice through the next 5 nights. Flurries are also possible at the summit and there is a chance for convection in the area particularly for Saturday night and again early next week. Extensive daytime clouds are expected through the next 5 days, at least.

Thin scattered high clouds will continue to stream just over the Big Island and especially along the southern skies for tonight. There is also a possibility for summit/mid-level clouds drifting in from the east toward the end of the night. More widespread rather thick clouds are set to stream in from the west and/or develop in the area, likely blanketing skies probably for the remainder of the forecast period.

Precipitable water may start out near 1 mm for early this evening, but will trend toward 4 mm through the night and likely exceed that value probably for most of the next 4 nights (there is a slight chance it occasionally will dip toward 3 mm for Sunday night).

Building turbulence and instability in the free atmosphere, coupled with an influx in low-level moisture will degrade seeing as the night progresses and will likely contribute to poor/bad seeing for the next 4 nights.

Very little change since the morning forecast....The tropical upper tropospheric trough (TUTT) will continue to develop to the northwest of the state, while the remnants of tropical storm Ulika begin to fill in from the east overnight. These two features, combined with a pair of upper-level lows (embedded in the TUTT) expected to develop to the NW and NE of the state, will help erode the inversion and saturate the atmosphere near the Big Island between early tomorrow morning and at least Sunday morning. There is a slight possibility that the tropical moisture associated with Ulika will sag a bit southward, which may help to dry out the mid-level portion of the atmosphere for Sunday night. However, instability will still persist in the area, limiting any strengthening of the inversion during that time. In addition, the TUTT and the embedded low/trough to the NW may start to gain access to more organized deep tropical moisture to the SW and stream it back over the Big Island area for the early/middle part of next week. This could help re-saturate the entire air mass once again during that time. At any rate, there is a moderate/high risk for fog, high humidity and perhaps ice and precipitation through the next 5 nights. There is also a possibility for convection in the area and flurries at the summit at virtually any time during this period, but particularly for Saturday night and again for Tuesday.

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 Sep 30 - 8 PM	20-40	9-10	25 / 5	2.5	ENE/5-15	0.5-0.7	1-1.5
Sat Oct 01 - 2 AM	40-60	4-10	75 / 40	2	ESE/5-15	0.6-1	2-4
2 PM	80-100	4-10	100 / 90	3	SE/5-15	NaN	8-12
8 PM	80-100	4-10	100 / 90	0.5	ENE/0-10	1-2	8-12
Sun Oct 02 - 2 AM	80-100	4-10	95 / 80	0	NNE/0-10	1-2	8-12
2 PM	70-90	4-10	95 / 60	3	NE/5-15	NaN	8-12
8 PM	80-100	7-10	75 / 25	0.5	ENE/5-15	0.7-1.3	4-8
Mon Oct 03 - 2 AM	80-100	7-10	75 / 25	0.5	NE/0-10	0.6-1.2	4-8
2 PM	80-100	4-10	75 / 25	3	NE/0-10	NaN	8-12
Tue Oct 04 - 2 AM	80-100	4-10	90 / 50	-0.5	S/0-10	0.8-1.6	10-15
2 PM	80-100	4-10	95 / 80	3	W/5-15	NaN	10-15
Wed Oct 05 - 2 AM	80-100	4-10	90 / 75	0	SSW/0-10	1-2	10-15
2 PM	0-20	4-10	95 / 80	3.5	E/0-10	NaN	10-15

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 Oct 01 - Sun Oct 02	18:18	19:22	5:00	6:04	N/A	19:06	2	13 37.4	-6 39
Sun Oct 02 - Mon Oct 03	18:17	19:21	5:00	6:05	N/A	19:44	5	14 22.9	-10 01
Mon Oct 03 - Tue Oct 04	18:16	19:21	5:01	6:05	N/A	20:23	10	15 09.1	-13 00
Tue Oct 04 - Wed Oct 05	18:16	19:20	5:01	6:05	N/A	21:04	16	15 56.2	-15 28
Wed Oct 05 - Thu Oct 06	18:15	19:19	5:01	6:05	N/A	21:47	24	16 44.5	-17 19

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 3 October 2016.

Additional Information
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Current Conditions
Temp		3.5 C
RH		93 %
Wind		ENE 16 mph
Road		Open (4x4)