Current Conditions
Temp3.2 C
RH22 %
WindENE 11 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
5 PM HST Thursday 17 August (0300 UTC Friday 18 August) 2017
Chance for fog/high humidity later in the weekend
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 3 C this evening and 2.5 C tomorrow morning. Winds will be light and variable, with seeing near 0.35-0.4 arcseconds. Precipitable water is expected to be in the 1.25-1.75 mm range for the night.
The tradewind inversion will continue to cap low-level moisture at or below 8 thousand feet and ensure the summit remains dry and stable through at least Saturday afternoon. There is a possibility that the inversion will begin to weaken through Saturday night, perhaps lifting toward 15 thousand feet and/or breaking down entirely for the following 2 nights. Nonetheless, this will raise the stakes on fog, high humidity and rain at the summit during this time, particularly the latter two nights. Daytime clouds will be minimal and short-lived through Saturday, but could pick up for Sunday and early part of next week. There is also a chance for isolated afternoon convection along the slopes during those days.

Skies overhead will remain predominately clear throught at least Saturday night. There is a chance that patches of high clouds will drift in from the west and pass through the area early Sunday evening, while thicker clouds develop along the northern skies and possibly creep later that night. The latter set of clouds are slated to shift off toward the west, but high clouds may seep in from the south and pass along the eastern skies for Monday night.

Precipitable water is expected to increase subtly to 1.5-1.75 mm over the next 3 nights, then jump toward 4 mm for Sunday and Monday night.

Calm skies and deep subsidence should allow seeing to settle in near 0.35-0.4 arcseconds for tonight. A gradually increase in upper-level shear/turbulence will start to degrade seeing through Friday night, with seeing perhaps trending back toward more average-like values for the following night. Instability building into the area combined with this shear/turbulence will likely contribute to poor seeing for Sunday and Monday night.

Little change since the morning forecast...The mid/low-level ridge will continue to sit to the north of the state and promote strong/steady large-scale subsidence in the area until it weakens a bit in response to a rather deep upper-level low redeveloping and strengthening to the NNW over the weekend. Nonetheless, this subsidence will help maintain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass through at least Saturday afternoon. An upper-level ridge currently lingering overhead will not only enhance the subsidence overhead, but will promote calm/clear skies, which should translate to excellent seeing for tonight. This ridge is expected to shift off toward the west and weaken as the aformentioned upper-level low begins to restrengthen to the NNE through tomorrow. Initially, the position of the low (relative to the state) will have little impact on the stability of the atmosphere, but may start to increase shear/turbulence in the free atmosphere for tonight. However, the low is set to retrograde westward, which could begin to destabilize the air mass and thus weaken/lift the inversion as the weekend progresses. Somewhat unsurprisingly, the last few model runs have backed off on the development of the tropical storm/cyclone to SE over the next several days. However, the models still allow tropical moisture to be drawn toward this low and pass over/near the Big Island late in the weekend and for the early part of next week. This moisture combined with the large-scale instability associated with the low may further erode the inversion and thus increase the risk for periods of fog, high humidity and rain at the summit for Sunday and Monday night. It may also allow for the development of afternoon convection along the Big Island slopes during those days. The threat for moisture at the summit and convection at the slopes will depend primarily on the trajectory of the tropical moisture. Past runs had this moisture passing more to the north, but the last few are starting to track it through the Big Island for the early part of next week.

Please Note: There will be no forecasts tomorrow, August 18, in observance of Statehood Day. The normal forecast schedule will resume on Monday, August 21.
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)
Thu Aug 17 - 8 PM0-5Clear0 / 03SSE/0-100.25-0.451.5-2
Fri Aug 18 - 2 AM0-5Clear0 / 02.5WNW/0-100.3-0.51.5-2
2 PM0-20Clear0 / 08SSW/0-10NaN1.5-2.5
8 PM0-5Clear0 / 03SSW/0-100.35-0.551.5-2
Sat Aug 19 - 2 AM0-5Clear0 / 02.5SW/0-100.35-0.551.25-1.75
2 PM0-20Clear0 / 07.5S/5-15NaN1.5-2.5
8 PM0-5Clear10 / 02.5SSE/5-150.4-0.61.25-1.75
Sun Aug 20 - 2 AM0-5Clear15 / 02S/5-150.5-0.71.5-2
2 PM60-804-875 / 305SSE/0-10NaN4-8
Mon Aug 21 - 2 AM20-406-840 / 101.5SE/0-100.6-0.93-5
2 PM80-1004-1090 / 505SE/0-10NaN4-8
Tue Aug 22 - 2 AM20-405-875 / 251.5SSE/5-150.6-0.93-5
2 PM80-1004-1080 / 406ENE/5-15NaN4-8
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 Aug 19 - Sun Aug 20 18:56 20:03 4:47 5:55 4:52 17:32 2 8 47.7 15 58
Sun Aug 20 - Mon Aug 21 18:55 20:03 4:48 5:55 5:53 18:24 0 9 44.7 13 00
Mon Aug 21 - Tue Aug 22 18:55 20:02 4:48 5:55 6:52 19:12 1 10 39.2 9 18
Tue Aug 22 - Wed Aug 23 18:54 20:01 4:48 5:55 N/A 19:57 3 11 31.3 5 11
Wed Aug 23 - Thu Aug 24 18:53 20:00 4:49 5:56 N/A 20:38 8 12 21.3 0 53
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 18 August 2017.
Additional Information
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