Mauna Kea Weather Center Forecast

Mauna Kea Observatories Forecast
5 PM HST Tuesday 16 October (0300 UTC Wednesday 17 October) 2018

Warning(s)
Chance for fog/high humidity
Possibility for afternoon convection

Cloud Cover and Fog/Precipitation Forecast
There is a small risk for periods of fog and high humidity throughout the night; precipitation is not expected. Strands of high clouds will continue to pass over and especially along the eastern skies for this evening, then will break up as the night progresses.

Summary of Key Meteorological Variables
Summit temperatures will be near 2 C, with light north winds and seeing around 0.6-0.65 arcseconds. Precipitable water is expected to linger near 4 mm for the night.

Discussion
The tradewind inversion will remain rather weak/indistinct near 10 thousand feet, which could allow for short periods of fog and high humidity at the summit until the inversion rebuilds near 6-7 thousand feet, ensuring dry/stable conditions for Friday and Saturday. Still, the odds are quite low due to the lack of low-level forcing and precipitation is not expected. Extensive afternoon clouds are possible and there is a chance for isolated afternoon convection along the slopes through Thursday; daytime clouds will taper on Friday and become minimal and short-lived over the weekend.

Strands of high clouds will continue to pass over and especially along the eastern skies for this evening, then will break up as the night progresses. However, more strands may fill in from the south and/or develop overhead later tomorrow, which may contribute to very brief periods of extensive cloud for the next 2 nights. There is a good chance that a more organized band of high clouds will begin to fill out of the south and pass over the summit on Friday, but should shift just east of the Big Island, opening up skies to the NW by later that night and likely for Saturday night.

Precipitable water is expected to linger near or exceed 4 mm through Thursday night, then slip toward 3 mm for Friday night and around 2 mm for Saturday night.

Persistent shear (both vertically and horizonally) in the free atmosphere will likely keep seeing near more average-like values (or perhaps slightly better) probably through the next 4 nights. However, there is a possibility for some variability should summit-level winds die off completely for any prolong periods mainly for tonight. An improvement in seeing is possible mainly as winds aloft weakens and takes on a more more laminar westerly flow for Saturday night.

No change since the morning forecast...The mid-level ridge will remain rather indistinct, allowing a broad upper-level trough lingering overhead to promote weak/widespread instability in the area, could keep the inversion rather elevated until a new ridge builds in from the west around Friday. Fortunately, lack of low-level forcing will keep most of the moisture capped near the inversion height (of 10-12 thousand feet) and the odds on fog/high humidity at the summit relatively low over the next 3 nights. Still, daytime heating may help fire off afternoon convection along the western and interior slopes of the Big Island through Thursday. A persistent sub-tropical jet along the SE flank of the large-scale trough will also continue to shuttle high clouds over or just east of the summit, periodically contributing to periods of extensive cloud cover mainly over the next 3 nights. The STJ and the trough are set to shift off toward the east as the new ridge builds in from the west on Friday. This will not only help promote stronger large-scale subsidence in the area and rebuild the inversion, virtually ensuring dry/stable conditions, but also push the high clouds off toward the eastern skies, opening up skies over the weekend. The departure of the jet will also minimize free atmospheric turbulence and with dry/stable conditions could allow for more optimal seeing particuarly for Saturday night (more average-like seeing will likely prevail prior to that 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)
Tue Oct 16 - 8 PM	20-40	8-9	25 / 0	2	N/0-15	0.5-0.8	3.5-4.5
Wed Oct 17 - 2 AM	0-20	8.5-9	20 / 0	2	NNE/0-10	0.45-0.75	3.5-4.5
2 PM	60-80	4-8	65 / 20	6	W/5-15	NaN	4-8
8 PM	20-40	8-9	25 / 0	1.5	W/5-15	0.55-0.75	4-6
Thu Oct 18 - 2 AM	40-60	8-10	25 / 0	1	W/5-15	0.55-0.75	4-6
2 PM	60-80	4-10	65 / 20	5	WSW/10-20	NaN	4-8
8 PM	40-60	8-10	30 / 0	0	W/10-20	0.55-0.75	4-6
Fri Oct 19 - 2 AM	40-60	8-10	20 / 0	-0.5	WSW/10-20	0.5-0.7	4-6
2 PM	40-60	9-10	10 / 0	5	W/10-20	NaN	3-6
Sat Oct 20 - 2 AM	20-40	9.5-10	0 / 0	0	W/10-20	0.5-0.7	2.5-3.5
2 PM	10-30	9.5-10	0 / 0	7	NNW/10-20	NaN	1.5-2.5
Sun Oct 21 - 2 AM	0-20	9.5-10	0 / 0	3	NNE/10-20	0.4-0.6	1.5-2.5
2 PM	0-20	Clear	0 / 0	9	ENE/5-15	NaN	1.5-2.5

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
Tue Oct 16 - Wed Oct 17	18:06	19:11	5:04	6:08	N/A	0:32	56	20 10.0	-20 09
Wed Oct 17 - Thu Oct 18	18:06	19:10	5:04	6:09	N/A	1:23	65	20 59.7	-18 13
Thu Oct 18 - Fri Oct 19	18:05	19:10	5:04	6:09	N/A	2:13	74	21 48.3	-15 30
Fri Oct 19 - Sat Oct 20	18:04	19:09	5:05	6:09	N/A	3:03	82	22 36.3	-12 05
Sat Oct 20 - Sun Oct 21	18:04	19:08	5:05	6:10	N/A	3:54	89	23 23.8	-8 07

Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Wednesday 17 October 2018.

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Current Conditions
Temp		5.7 C
RH		48 %
Wind		NNE 1 mph
Road		Open (4x4)