Maunakea Weather Center

Maunakea Observatories Forecast
5 PM HST Thursday 21 September (0300 UTC Friday 22 September) 2023
Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while broken thin high clouds fill in from the west, contributing to periods of extensive cloud cover through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4.5 C, with winds from the ENE at 15-25 mph for this evening, easing to 5-15 mph through the night. Seeing will start out near 0.7 arcseconds, but could drop toward 0.5-0.6 arcseconds as the night progresses. Precipitable water is expected to be in the 1.25-1.75 mm range through the night.

Discussion
A well-defined tradewind inversion will continue to cap low-level moisture at or below 9 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

While more broken high clouds will fill in from the west and pass predominately along the northern skies over the next 12-18 hours, there is a good chance for periods of extensive cloud cover as more cirrus builds in from the south through the night. These clouds are set to merge over or along the southern half of the Big Island for the next 2 nights, then shift off toward the south around sunrise Sunday. This could allow for mostly clear skies for that night, but more high clouds may build along the southern skies by sunrise Monday and may stream over the summit area for that night.

Precipitable water is expected to slip toward 1.5 mm for tonight, increase toward 3 mm for the next 2 nights, then slip into the 2-3 mm range Sunday and Monday night.

A bout of light boundary layer turbulence may contribute to poorer than average seeing for this evening, but seeing should improve as winds subside as the night progresses. Despite a gradually increase in upper-level shear, seeing will likely settle in around 0.5-0.55 arcseconds as a predominately stable air mass prevail for much of the weekend. There is a chance that seeing will improve further as upper-level winds tapers for the early part of next week.

Subtle changes have been made mainly to the PW forecast...The mid/surface ridge will continue to sit to the north of the state and promote steady/strong large-scale subsidence in the area well into next week. This subsidence will easily maintain a well-defined inversion at or below 9 thousand feet, ensure a dry and stable summit-level air mass and easily negate the affects of the tropical upper-tropospheric trough (TUTT) lingering to the north of the Islands during this time. The TUTT continues to show signs of flattening out and weakening as the northern hemisphere transitions toward a more fall pattern with a more active westerly jet in the North Pacific. The sub-tropical jet (STJ) will still persist just north of the Big Island and will continue to shuttle cirrus exhaust from an upper-level low near the Dateline over the summit area for the next 3 nights. While the STJ will also promote relatively moderate shear in the free atmosphere, better than average seeing should still prevail, particularly as summit-level winds settle (to > 10 mph) later this evening. There is a possibility for a break in the high clouds as the low to the west dissipates and the STJ shifts subtly to the SE late in the weekend. This may actually allow for an improvement in seeing, with the reduction of free atmospheric dynamic turbulence, but could also permit high clouds to flow out of the tropics and through the summit area for the early part of next week.
5 Day Forecast Summary (Graphical Trend)
HST Cloud Fog/Precip Temp Wind Seeing PW
Cover (%) Height (km) Probability (%) (Celsius) (Dir/MPH) (Arcseconds) (mm)
Thu Sep 21 - 8 PM40-609-100 / 04.5ENE/15-250.55-0.851.25-1.75
Fri Sep 22 - 2 AM60-809-100 / 04.5ENE/10-200.45-0.651.25-1.75
2 PM60-807.5-100 / 09NE/5-15NaN2-4
8 PM50-707.5-100 / 04.5NNE/5-150.45-0.652.5-3.5
Sat Sep 23 - 2 AM50-708-100 / 04N/0-100.45-0.652.5-3.5
2 PM40-608-100 / 09N/5-15NaN3-5
8 PM40-608-90 / 04N/0-100.4-0.62.5-3.5
Sun Sep 24 - 2 AM20-408-90 / 03.5W/0-100.4-0.62.5-3.5
2 PM0-20Clear0 / 010NW/5-15NaN3-5
Mon Sep 25 - 2 AM0-10Clear0 / 05SW/0-100.45-0.652-3
2 PM20-408-90 / 011NW/0-10NaN2-4
Tue Sep 26 - 2 AM30-508-90 / 06N/0-100.35-0.552-3
2 PM10-308-90 / 012NE/5-15NaN2-4
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
Fri Sep 22 - Sat Sep 23 18:27 19:31 4:58 6:02 N/A 0:15 56 18 31.4 -28 35
Sat Sep 23 - Sun Sep 24 18:26 19:30 4:58 6:02 N/A 1:19 67 19 34.8 -27 18
Sun Sep 24 - Mon Sep 25 18:25 19:29 4:58 6:03 N/A 2:26 77 20 37.5 -24 13
Mon Sep 25 - Tue Sep 26 18:24 19:28 4:59 6:03 N/A 3:32 87 21 38.1 -19 30
Tue Sep 26 - Wed Sep 27 18:23 19:27 4:59 6:03 N/A 4:36 94 22 35.9 -13 30
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 22 September 2023.
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