textproduct: Juneau

UPDATE

Update after 06z TAF package and the Long term gridded package done.

PREV DISCUSSION......ISSUED AT 343 pm Mon Sep 28

SYNOPSIS...

Key Messages:

- Rain showers likely Monday night and through the day Tuesday. Drying possible at times along the northern gulf coast on Tuesday.

- Winds likely to remain gusty at times through early in the week.

- Temperatures mainly seasonable with highs averaging in the 50s.

SHORT TERM...A large upper level circulation over the gulf continues to bring onshore flow Monday with showers scattered across the entirety of the panhandle. A weak surface circulation is expected to move into the northern gulf overnight into Tuesday before gradually weakening and pushing inland in the vicinity of Glacier Bay. This will lead to a weakening of the gradient over the northern gulf and a brief transition to weak offshore flow over the northeastern gulf coast. As this circulation progresses into the northeast gulf on Tuesday, onshore flow and instability aloft will become more conducive for thunderstorm potential to shift to the southeastern gulf, to the coastal waters from Dixon Entrance to southern Baranof Island. The primary impacts from these would be erratic gusts and localized heavy rain quickly reducing visibility.

Overall the pattern in the gulf remains unsettled through midweek, as the low deteriorates and lifts inland, before another forms near Cook Inlet and brings a fresh batch of moisture towards the northern half of the panhandle. Temperatures will remain relatively unchanged through the early week, though any areas that see more clearing could see more overnight cooling, particularly along the northeastern coast Tuesday night. With continued saturation in the lower levels and persistent onshore flow, patchy fog development is not out of the question tonight, though there is higher forecast confidence for the northeast coast Tuesday night.

LONG TERM.../Thursday through Sunday/...A series of small disturbances and onshore flow will bring steady rain to the region through the end of the week. A weak surface high building over the southeastern Gulf will reinforce onshore flow and could potentially tighten pressure gradients across the panhandle as the next system approaches. A low pressure system originating near Russia will move east, tracking south of the Aleutian Islands and into the Gulf of Alaska by early weekend. There is a growing likelihood of tropical moisture entraining into the system, supported by an 850mb southerly jet, increased precipitable water, and seasonably warmer temperatures aloft. Though this will likely not affect the surface much due to consistent widespread cloud cover and small diurnal trends.

Model spread is still significant, but guidance favors landfall along the north-central coast late weekend. The system could bring rounds of moderate to heavy rain across most of the panhandle, with the highest totals currently expected along the northeast Gulf coast, though this is dependent on the system's exact track and strength. There is also increasing potential for stronger winds and building seas along the outer coast and northern inner channels ahead of this low. While some guidance suggests a more aggressive evolution, the system generally lacks upper level support beyond a weak, negatively tilted trough and jet setup. We will continue to monitor developments through the week.

AVIATION...After 06z TAF issuance.

Shower mode across nearly all of the panhandle area with areas of the Southeast portion of the panhandle having the fewest showers. The highest and strongest showers are moving into Baranof and Chichagof Islands and into the NE Gulf Southeast of Yakutat. A low over the northern central gulf is drifting east slowly, and is supporting a large area of enhanced looking showers from satellite imagery.

Overall most of the panhandle is classified VFR but may drop to the MVFR in the showers as they move through the area. A potential concern near the showers may be gusty outflow winds, which could have random directional shifts and speed shifts resulting in potential wind shear at times.

MARINE... Outside (Gulf and Coastal Waters): A 992mb low in the northern Gulf of Alaska will shift eastward through Monday night, bringing a swath of westerly fresh to strong breezes (17 to 27 kts) to the southern gulf and easterly fresh to strong breezes (17 to 27 kts) through the coastal waters of the northern gulf. Onshore flow will continue to push developing showers into the northern panhandle, spreading southward to envelop the rest of the panhandle by tonight. A slight chance for thunderstorm development remains today for the northeastern gulf coast, shifting southeastward along the coast Tuesday to follow the showers.

The low center will continue to shift eastward through Tuesday before falling apart as it makes landfall along the northeastern Gulf Coast Wednesday. Winds in the northern gulf will become relatively light and variable Tuesday before starting to increase again from the southeast Wednesday. Winds in the southern gulf will shift southwesterly and decrease to moderate to fresh breezes (11 to 21 kts) through Wednesday. The gulf is currently experiencing 8 to 10 ft wave heights with a 5 to 6 ft WSW swell at a period of 10 to 12 seconds, which will decrease to around 5 to 6 ft with a 2 to 3 ft WSW swell at a period of 6 to 8 seconds through Wednesday following the winds.

Inside (Inner Channels): Onshore flow from a system in the northern gulf is bringing southerly moderate to fresh breezes (11 to 21 kts) through the inner channels and widespread shower potential through Tuesday. Currently the strongest winds are being observed through the northern and parts of the central inner channels, but this swath will shift southeastward into the southern panhandle as the system tracks eastward over the next 24 hours. Northern Lynn Canal is expected to continue blowing up to 20 kts through the rest of Monday before becoming light Tuesday morning. Overnight, the central panhandle will feel the brunt of the winds, with 15 to 20 kt southeasterly winds increasing through Stephens Passage, Frederick Sound, and Southern Chatham Strait. Through Tuesday morning, the strongest winds will be increasing through Sumner and Clarence Straits. Winds will relatively decrease Tuesday night before another system approaches from the northern gulf, increasing winds back to fresh breezes (17 to 21 kts) through the N/S oriented inner channels up to 20 kts in Northern Lynn Canal Wednesday. Wave heights of 3 to 4 ft are expected in the areas of stronger winds through Tuesday, decreasing to a widespread 1 to 2 ft overnight before increasing to 2 to 3 ft again in windy areas Wednesday.

AJK WATCHES/WARNINGS/ADVISORIES

PUBLIC...None. MARINE...Small Craft Advisory for PKZ641>644-651-652-661>664-671-672.