textproduct: Juneau

UPDATE

After 06z TAF issuance. No surprises as the forecast running on track. Most ceilings above 3000 feet, with a few areas of 2500 to 3000 feet late this evening, that may linger to morning.

PREV DISCUSSION......ISSUED AT 814 pm Aug 11

UPDATE...The glacier dammed lake in Suicide Basin has begun its annual release. See the updated hydrology section for more details.

SYNOPSIS...

Key Messages:

- Suicide Basin has started to drop today and we continue to monitor levels in the basin as well as the Mendenhall River.

- Warming trend for the week with highs in the 70s. Lower 80s will be possible, especially across interior locations across the south and far northern interior for mid to late week.

- The warm and dry conditions will bring elevated threat for fire starts across much of SE AK. A few small fires have been observed already over the past week.

SHORT TERM.../Through Thursday Night/... A high pressure ridge will remain over the Gulf of Alaska through the rest of the week, the placement of the ridge axis will be the determining factor on the overall weather for SE AK.

The main forecast story for for the area will be the warm temperatures for most of panhandle. As the ridge builds over the Gulf, marine stratus will be mostly confined to the coastal regions, as a result skies will continue to clear over the panhandle. This coupled with some downsloping winds will result in high temperatures in the low to mid 70s for much of the Panhandle. The exception to this is the area around Skagway, Haines and Klukwan where temperatures away from the water are expected to be in the low 80s. Temperatures will continue to increase across the Panhandle and by Thursday highs are expected to be in the mid to upper 70s for the central and most of the northern Panhandle. For the NE portions (Skagway, Haines and Klukwan) as well as for the Southern Panhandle highs are expected to be in the low 80s with some extremely isolated and elevated areas in the mid 80s.

Additionally, models have been suggesting the potential for thunderstorms for along the Coastal Range ridge line as well as for the SE corner of Panhandle. However, even according to the model guidance these are unlikely. The reason being that while there is instability, there is not a lot of low/mid level moisture that would be needed. That said, some of the ingredients are there and we will have to monitor conditions.

Looking away from the Panhandle, the ridge of high pressure is resulting in a slightly different weather regime. Moisture is wrapping around the ridge and creating a broken marine layer that has been moving in and out of the eastern Gulf Coast and has been more persistent for the NE sections. This marine layer will keep temperatures lower compared to the rest of the region. With highs in the mid to upper 60s expected over the next few days.

LONG TERM.../Friday through Monday/...Ensemble models agreeing on an upper level ridge dissipating and being replaced by a weak trough over the southern panhandle by Friday, with the ridge rebuilding early next week. This shift will bring positive vorticity advection and some lifting, while moisture advection arrives via an easterly wave in the 700mb level. Additionally, a surface maritime ridge forming in the eastern gulf will generate northwesterly winds across the panhandle which could work in conjunction with diurnally driven sea breezes to promote weak onshore flow toward the end of the week.

Regarding severe weather potential for end of week/early weekend, guidance is keying in on seasonably high CAPE values in parts of the central and southern regions, though forecaster confidence remains low due to drier conditions in many of the atmospheric pressure levels and a lack of support for sustaining storms moving in from Canada. Cumulus fields are more likely than storms, as a lack of a cap and significant moisture inhibits potential convection. While westerly flow on Friday might drive some localized convection through upsloping, marine influences will likely stabilize the surface layer and choke off development. Meanwhile, light winds and clearing skies are expected to boost temperatures, with Friday and Saturday continuing this warm spell as temperatures reach the upper 70s to low 80s, before cooling returns Sunday into Monday as the surface high pressure weakens. There is still a lot of spread in the track of a low pressure center moving into the central gulf next week, and if lingering high pressure will block precipitation from moving in. We will continue to monitor this next system and associated rainfall accumulation.

AVIATION.../through Wednesday afternoon/... High pressure will maintain its presence over the Gulf, allowing a marine layer to develop during the overnight period. Some lower cigs under 3kft could drop PAKW, PAKT and PAWG to MVFR or even IFR conditions for several hours. With another warm day on tap for SEAK, the marine layer will lift, and with it CIGS across the region, leading to VFR conditions for a majority of the day Wednesday. Winds will pick up modestly with sea breeze development, largely remaining 5-10 kts for all TAF sites, although PAGY will be 15G25kt this evening and become 10-15kt late tomorrow afternoon. Precipitation and LLWS not expected to pose any concerns over the next 24 hour TAF valid period.

MARINE... Outside: Marine forecast remains on track. High pressure over the Gulf will strengthen and shift eastward, tightening the gradient over the eastern Gulf. Gentle to moderate northwest breezes will become fresh for the outer coastal waters from near Cape Spencer on down to Dixon Entrance. Wave heights of 3 to 4 ft will increase to 4 to 6 ft by midweek and continue through the end of the week. Winds will diminish by the weekend as the high weakens. Main marine concern for outer and coastal waters will be nightly return of marine layer, with fog and low stratus impacting visibility.

Inside: High pressure out in the Gulf is expected to linger through the week, with winds moderate to fresh for the inner channels. Localized wind shifts due to sea breezes are possible, but overall wind pattern is expected to be northwest. Strongest inner channel winds up to 15 to 20 kts are expected for the afternoon and evening hours as sea breezes increase in strength before diminishing during the overnight hours.

FIRE WEATHER...High pressure continues to sit over the area with no real signal of a pattern change likely until the upcoming weekend. With the persistent weather, we have started to see temperatures rise into the 70s with some pockets of 80s for the inland areas with warm temperatures expected to continue through the week. Looking at the past few days, relative humidities have fallen down to the 30s to 40s for some areas during the daytime hours with rebounding during the overnight hours into the 90s. With the continued lack of rain, fine fuels such as grasses will continue to dry. Models continue to suggest the potential for isolated thunderstorms along the Panhandle border with Canada as well as the SE panhandle on Friday. However, due to the lack of low/mid level moisture the chance of thunderstorms is less that 15%.

Overall, expecting the dry and warm conditions to continue through the week with fire weather conditions continuing to develop, the main variable to watch for will be daytime winds. As of now, expecting the worst fire weather conditions to be Thursday and Friday afternoon, however given the persistent pattern can't rule out this extending further into the weekend.

HYDROLOGY...The glacier dammed lake in Suicide Basin has begun its annual release. Water levels in the basin have shown a distinct and exponential decline since Tuesday morning while gages along the Mendenhall River and Lake have started showing distinct rises in water level. It usually takes around 2 days from when the release starts to see the highest water levels. This year based on the estimated volume of water in the basin and when the release started, the crest is expected some time late Wednesday night or early Thursday morning with an expected height of around 16.1 to 16.6 ft which is over 2 ft above major flood stage of 14.0 ft. Compared to previous years this is slightly below last year's high water mark, and above the high water marks of 2023 and 2024. Folks should continue to monitor weather.gov/ajk/SuicideBasin and social media for the latest updates.

AJK WATCHES/WARNINGS/ADVISORIES

PUBLIC...Heat Advisory from noon Wednesday to 7 PM AKDT Thursday for AKZ318-319. MARINE...None.