Current Conditions
Temp1.6 C
RH95 %
WindSSW 3 mph
Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Wednesday 20 September 2017
Chance for fog/high humidity
Cloud Cover and Fog/Precipitation Forecast
There is a small risk for short-periods of fog and high humidity throughout the night; precipitation is not expected. Broken mid-level clouds will continue to hang around the southern skies, occasionally passing over the summit area through 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 NW at 5-15 mph, with seeing near 0.6-0.65 arcseconds. Precipitable water is expected to be in the 2.5-3.5 range for the night.
Although the atmosphere is set to stabilize a bit, mid/low-level moisture hanging around to the south as well as pockets of moisture occasionally drifting in from the NE may periodically lift the inversion toward 12 thousand feet and allow for short-lived fog/high humidity over the next 3 nights. Still, the odds are small and precipitation is not expected. The air mass is projected to dry out over the weekend, allowing dry and stable conditions to prevail during those nights. Some afternoon clouds are possible over the next few days, then will become minimal and short-lived over the weekend.

Broken Mid-level clouds will continue to linger along the southern skies and occasionally pass over the summit area during the next 2-3 nights. These clouds will be joined by banding high clouds streaming out of the WSW, which is set to drift over the summit, contributing to periods of extensive cloud cover between late Thursday evening and early Saturday afternoon. Both sets of clouds are expected to quickly retreat southward on Saturday, leaving relatively clear skies for the remainder of the forecast period.

Precipitable water is expected to linger primarily near 2-3 mm for the next 3 nights, though there is a possibility for periods of variability during that time. There is a good chance that it will trend back toward 1 mm for Saturday and Sunday night.

While summit-level winds are set to subside throughout the day, an increase in upper-level turbulence will likely keep seeing near 0.6-0.65 arcseconds over the next 3 nights. There is a also a slight possibility for slight/brief increase in boundary layer turbulence mainly for Saturday evening. The upper-level turbulence is expected to gradually subside over the weekend, perhaps allowing seeing to improve toward 0.5 arcseconds as that period progresses.

Little change since the morning forecast....A rather deep mid/upper-level low will continue to develop and strengthen to the NE of the state over the next 48 hours, then remain quasi-stationary/steady before losing upper-level support from the westerly jet around the early part of next week. While fortunately, the state will remain on the more stable portion of the low, mid/upper-level moisture is expected to hang just south of the Big Island until the low subtly digs southward around Saturday. This moisture, combined with relatively unstable low-level clouds passing beneath the low and within the low-level trades could occasionally lift the inversion toward 12 thousand feet and allow for the development of short-lived fog and/or high humidity at the summit over the next 3 nights. Banding high clouds embedded in the strengthening sub-tropical jet along the southern flank of the low is also expected to contribute to extensive cloud cover between late Thursday evening and early Saturday afternoon. The presence of the STJ will also limit much improvement in seeing as moderate/strong turbulence persists in the free atmosphere over the next 3 nights. Fortunately, these clouds as well as the moisture to the south is expected to retreat toward the tropics as the low shifts/digs southward a tad over the weekend. There is also a good chance that the turbulence in the free atmosphere will subside as the STJ drifts off toward the SE, allowing to seeing to improve a bit during that time.
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)
Wed Sep 20 - 2 PM30-506-840 / 107NNW/5-15NaN3-5
8 PM20-406-730 / 02.5NNE/5-150.5-0.82-3
Thu Sep 21 - 2 AM20-406-725 / 02NNE/5-150.5-0.71.5-2.5
2 PM40-607-850 / 107.5W/0-10NaN3-5
8 PM40-608-1040 / 03NW/5-150.55-0.751.5-2.5
Fri Sep 22 - 2 AM60-808-1030 / 02.5NW/10-200.4-0.61.5-2.5
2 PM70-908-1050 / 108WNW/10-20NaN3-6
8 PM60-808-1025 / 03NW/15-250.55-0.751.5-2.5
Sat Sep 23 - 2 AM50-708-1020 / 02.5NW/10-200.45-0.651.5-2.5
2 PM20-409-100 / 08NW/10-20NaN1.5-2.5
Sun Sep 24 - 2 AM0-5Clear0 / 02NW/10-200.45-0.751-1.5
2 PM0-20Clear0 / 07.5NW/10-20NaN1-2
Mon Sep 25 - 2 AM0-5Clear0 / 02NNW/5-150.35-0.650.8-1.2
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
Wed Sep 20 - Thu Sep 21 18:28 19:33 4:57 6:02 N/A 19:13 2 12 53.2 -1 50
Thu Sep 21 - Fri Sep 22 18:27 19:32 4:58 6:02 N/A 19:52 5 13 41.2 -5 59
Fri Sep 22 - Sat Sep 23 18:26 19:31 4:58 6:02 N/A 20:32 10 14 28.9 -9 48
Sat Sep 23 - Sun Sep 24 18:26 19:30 4:58 6:02 N/A 21:13 17 15 16.6 -13 10
Sun Sep 24 - Mon Sep 25 18:25 19:29 4:58 6:03 N/A 21:54 25 16 04.8 -15 56
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Wednesday 20 September (0300 UTC Thursday 21 September) 2017.
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