Current Conditions
Temp6.7 C
RH18 %
WindNE 14 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Monday 23 April 2018
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 7 C this afternoon, 2 C this evening and 1 C tomorrow morning. Winds will be from the ENE at 10-20 mph for today, diminishing to 5-15 mph as the night progresses. Seeing will be near 0.55-0.6 arcseconds, while precipitable water is expected to be in the 2-3 mm range for the first half of the night and 1.75-2.25 mm range for the second half.
Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 10 thousand feet and ensure the summit remains dry and stable through at least Thursday evening. Building instability combined with moisture filling into the area could erode the inversion through Friday, and increase the risk for fog, high humidity, ice, rain/flurries and perhaps convection in the area for that night and over the weekend. Daytime clouds will be minimal and short-lived through Thursday, but could pick up for Friday and turn extensive over the weekend.

The high clouds will continue to clear out of the area, leaving predominately clear skies for tonight and probably the opening part of tomorrow night. However, a narrow stream of high clouds are set to build in from the south during the latter night and pass over the southern half of the Big Island by Wednesday night. This stream will eventually thicken and spread northward, contributing to extensive cloud cover or even overcast skies for Thursday and especially Friday night. Thicker mid-level clouds may also build in from the south and/or develop in the area for Friday night.

Precipitable water is expected to start out near 2.5 mm for tonight, but slip toward 2 mm or less as the night progress, and will eventually settle in near 1.5 mm for tomorrow night and into Wednesday night. It will begin to slowly climb again thereafter, likely reaching 4 mm by Friday morning and quickly jump toward 10 mm by that night.

Calmer skies should allow seeing to slip toward 0.55-0.6 arcseconds for tonight, but an increase in upper-level turbulence may contribute to average or slightly poorer seeing for the following 2 nights. An influx in boundary layer turbulence wiill likely result in poor seeing for Thursday night and a combination moisture/instability may further degrade seeing for Friday night.

The mid-level ridge to the north of the state will continue to promote relatively strong/steady large-scale subsidence in the area until its demise as a rather deep late-season trough develops to the northwest and deposits a Kona Low-like feature beginning early Friday. Nonetheless, this subsidence will help maintain a well-defined inversion at or below 10 thousand feet and ensure the summit remains dry and stable through at least Thursday evening. Flow aloft will also become more zonally oriented, while the sub-tropical jet briefly retreats southward and weakens for tonight. This should allow clear/calm skies to prevail, which should also translate to better than average seeing for tonight. However, the STJ is set to quickly restrengthen and build northward over the Big Island, bringing another stream of high clouds to the area and increase turbulence in the free atmosphere by tomorrow night. The turbulence will likely contribute to average or slightly poorer than average seeing for that night and Wednesday night The high clouds will eventually be relagated to the southern half of the Big Island into Wednesday, but will spread northward and thicken by Thursday, likely resulting in overcast skies for that night and into Friday night. There is also a good chance that the developing trough will strengthen summit-level winds beginning Thursday night, which will probably stir up boundary layer turbulence and contribute to poor/bad seeing during that time and into the weekend. This trough will start to destabilize the air mass early Friday morning and eventually drag deep tropical moisture northward by later that night. This combination could quickly erode the inversion and allow fog, ice, high humidity and light rain/flurries to plague the summit for much of the weekend, should this scenario pan out. There is also a possibility that deep convection will develop in the area particularly around Saturday.
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
Cover (%) Height (km) Probability (%) (Celcius) (Dir/MPH) (Arcseconds) (mm)
Mon Apr 23 - 2 PM0-209.5-100 / 07ENE/10-20NaN2-3
8 PM0-10Clear0 / 02ENE/10-200.5-0.72-3
Tue Apr 24 - 2 AM0-5Clear0 / 01ENE/5-150.45-0.651.75-2.25
2 PM0-20Clear0 / 06N/5-15NaN1.5-2.5
8 PM0-20Clear0 / 01N/5-150.5-0.81.25-1.75
Wed Apr 25 - 2 AM20-409.5-100 / 00.5NNW/5-150.55-0.851.25-1.75
2 PM40-609-100 / 05.5WNW/5-15NaN1.5-2.5
8 PM50-709-100 / 00.5WNW/10-200.5-0.81.25-1.75
Thu Apr 26 - 2 AM50-709-100 / 00.5WNW/10-200.5-0.81.75-2.25
2 PM60-808-100 / 05.5W/10-20NaN2-3
Fri Apr 27 - 2 AM80-1006-1010 / 00WSW/15-300.6-13-4
2 PM80-1004-1060 / 303WSW/20-35NaN4-8
Sat Apr 28 - 2 AM80-1004-1080 / 60-1SW/20-350.8-1.68-12
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
Mon Apr 23 - Tue Apr 24 18:52 19:59 4:41 5:48 N/A 2:34 66 9 42.8 14 24
Tue Apr 24 - Wed Apr 25 18:52 20:00 4:40 5:47 N/A 3:19 76 10 38.1 10 34
Wed Apr 25 - Thu Apr 26 18:52 20:00 4:39 5:47 N/A 4:02 85 11 31.4 6 09
Thu Apr 26 - Fri Apr 27 18:53 20:01 4:38 5:46 N/A 4:44 92 12 23.1 1 27
Fri Apr 27 - Sat Apr 28 18:53 20:01 4:37 5:45 N/A 5:24 97 13 14.0 -3 17
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Monday 23 April (0300 UTC Tuesday 24 April) 2018.
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