Current Conditions
Temp2.5 C
RH7 %
WindNE 17 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
5 PM HST Tuesday 23 May (0300 UTC Wednesday 24 May) 2017
Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while there is a chance for scattered thin cirrus along the northern skies through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 2.5 C this evening and 1.5 C tomorrow morning. Winds will be from the east at 5-15 mph, switching to a more SE direction through the night. Seeing will be near 0.5-0.6 arcseconds, while precipitable water is expected to be in the 1.5-2 mm range for the first half of the night and 1.25-1.75 mm range for the second half.
The tradewind inversion will continue to cap low-level moisture near 7 thousand feet and ensure the summit remains dry and stable through at least Thursday night. There is a chance that building instability will weaken/erode the inversion and increase the risk for fog and high humidity at the summit for Friday and especially Saturday night; precipitation is unlikely. Daytime clouds will be minimal and short-lived through Thursday, but could pick up on Friday and over the weekend. There is also a possibility for isolated afternoon convection along the slopes over the weekend.

While the bulk of the high clouds will continue to shift eastward through the day, there is a chance for thin scattered cirrus along the northern skies for tonight. Thicker broken high clouds are set to briefly scrape the southern half of the Big Island for Wednesday night, and there is a chance for quick moving isolated patches of cirrus passing over/through the summit area for the following 2 nights. More organized widespread high clouds are slated to develop in the area, contributing to extensive cloud cover or even overcast skies for Saturday night.

Precipitable water is expected to linger near 1.5-1.75 mm for tonight, briefly increase to 3-4 mm for tomorrow night, slip back to 1.5-1.75 mm for Thursday night, then jump to 4+ mm for the following 2 nights.

Relatively persistent laminar/uniform westerly flow above the summit, combined with light/moderate turbulence in upper portion of the free atmosphere will likely allow seeing to settle in near 0.5-0.6 arcseconds for the following 3 nights. Flow aloft will remain, for the most part, unchanged, but there is a possibility that building turbulence/instability will degrade seeing toward more average/poor values for Friday and Saturday night.

No change since the morning forecast...The mid-level ridge will remain relatively weak/indistinct before a new one gradually spreads in from the east around the early/middle part of next week. Fortunately, the low-level ridge will continue to sit to the north and promote fairly steady large-scale subsidence in the area through this week. This subsidence will help maintain the inversion near 7-8 thousand feet and ensure a dry and stable summit-level air mass through at least Thursday night. However, a fairly deep/broad tropical upper-tropospheric trough (TUTT) redeveloping to the west is set to gradually shift eastward and allow instability to spread into the area as the week progresses. This instability may begin to erode the inversion and deepen the low-level cloud field on Friday and may even allow the air mass to briefly become saturated for parts of the weekend. I suspect the latter is a little overdone, but the erosion of the inversion combined with daytime heating and widespread (but weak) instability could allow for afternoon convection along the slopes for much of the weekend. There is also a possibility for short periods of high humidity and fog at the summit for Friday and Saturday night; nighttime precipitation is unlikely. In addition, the TUTT and especially its slowly strengthening southern counterpart, the sub-tropical jet (STJ), will continue to send (mostly) scattered high clouds through the area over much of the week. There is a good chance that more organized widespread clouds will develop over the state, contributing to extensive cloud cover during the weekend. Fortunately, the STJ will remain relatively deep (vertically), allowing laminar flow to persist above the summit, which should result in minimal turbulence in the free atmosphere and better than average seeing through Thursday night. That could change as the building instability/turbulence associated with the TUTT begins to negatively impact seeing thereafter.
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)
Tue May 23 - 8 PM0-209.5-100 / 02.5E/5-150.45-0.651.5-2
Wed May 24 - 2 AM0-209.5-100 / 01.5SE/5-150.45-0.651.25-1.75
2 PM20-409-100 / 06W/5-15NaN2-4
8 PM40-609-100 / 01W/5-150.4-0.63-4
Thu May 25 - 2 AM40-609-100 / 00WSW/10-200.45-0.653-4
2 PM0-20Clear0 / 05.5WNW/10-20NaN2-4
8 PM0-20Clear0 / 01WNW/10-200.45-0.651.5-2
Fri May 26 - 2 AM0-209.5-100 / 01WNW/10-200.5-0.71.25-1.75
2 PM40-604-540 / 104WNW/10-20NaN3-6
Sat May 27 - 2 AM0-209.5-1020 / 5-0.5W/10-200.5-0.84-6
2 PM60-804-1065 / 204W/5-15NaN4-8
Sun May 28 - 2 AM80-1006-1060 / 10-1SW/5-150.6-14-8
2 PM80-1004-975 / 253NNW/5-15NaN6-10
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 May 23 - Wed May 24 19:04 20:16 4:22 5:34 4:48 N/A 3 2 50.5 10 43
Wed May 24 - Thu May 25 19:04 20:16 4:21 5:33 5:41 18:14 1 3 49.9 14 23
Thu May 25 - Fri May 26 19:04 20:17 4:21 5:33 6:38 19:20 1 4 51.5 17 04
Fri May 26 - Sat May 27 19:05 20:17 4:21 5:33 N/A 20:26 4 5 54.3 18 33
Sat May 27 - Sun May 28 19:05 20:18 4:20 5:33 N/A 21:28 10 6 56.9 18 42
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Wednesday 24 May 2017.
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