textproduct: Juneau

UPDATE

Update to the aviation section to include the 18z set of TAFs.

PREV DISCUSSION......ISSUED AT 609 AM AKST Sun Dec 14

SYNOPSIS...

Key Messages:

- Multiple Winter Weather Advisories and Warnings are in effect for Sunday across the central Panhandle and the Icy Strait Corridor.

- Cold temperatures continue, with very cold wind chills near White Pass.

- Gusty outflow winds down Lynn Canal, Stephens Passage, near Point Couverden, and Downtown Juneau continue tonight.

- Moderate to heavy snow rates expected for the central panhandle and Icy Strait on Sunday, with precipitation continuing into early next week.

SHORT TERM...Next several waves of precipitation is currently moving up the panhandle at time of writing. The Snowfall Rate Product has been tracking this incoming line of precipitation well, with the band of snow near Petersburg and Wrangell over to Port Alexander. Snow rates of 0.4 to 0.7 inch per hour are being record, and the band is expected to stall around Frederick Sound. The secondary wave of precipitation will move up late this morning and into the afternoon, moving the main area of precipitation to near the Icy Strait corridor.

Couple of changes regarding this upcoming storm, mainly adding uncertainty rather than changing the message. The first being the strength of the WAA aloft associated with the second wave. Areas near Wrangell and Petersburg have the highest uncertainty, with 850 mb temperatures ranging from -3 to 2 degrees C. Naturally, this means that precipitation type is a concern here, with both wintry mix and rain increasing in likelihood during the afternoon hours today. Stuck mostly with the inherited forecast as currently expecting rates to hold precipitation type as snow, but did trim down snow totals slightly to account for increased compaction from dense snow. If precipitation type does transition to wintry mix or rain, then existing snow on the ground will quickly become dense and difficult to move.

Secondly, the second wave moving up from the south looks to set up near Icy Strait. As the low pressure to the south has a positive trough axis, expecting to see the trowal axis from around Juneau to Hoonah, which is where the heavier rates are expected to be. For downtown Juneau though, expecting to see some mountain wave activity coinciding with the heaviest rates late this afternoon into late tonight. For the mountain wave, limiting factor will be the critical layer. The shortwave trough moving through during this period has vorticity advection aloft over the area, shifting winds somewhat southerly. However, cross barrier flow is expected to exceed 45 knots, which could lead to an induced mountain wave. Furthermore, downsloping winds have a drying and warming effect, which may limit precipitation during this timeframe. This all being said, if a mountain wave does develop with sustained winds of 35 knots or greater, temperatures hovering around 25 degrees F, snow rates exceeding 0.5 inches per hour, and fresh snow of 2 inches or greater (expected with snowfall earlier in the day), Juneau could see blizzard conditions. Current confidence for half mile visibility is greater than 80%, with blizzard conditions around 50-60%. Will need to watch how the moisture moving up later today will behave to get better confidence on any blizzard conditions.

LONG TERM.../Monday through Wednesday/...

The active weather pattern continues in full force, with changes to the forecast significantly increasing chances of additional accumulating snowfall for the central panhandle and Icy Strait Corridor through Tuesday, before the primary band of precipitation moves further and southward through the remainder of the week and departs the area entirely on Friday.

The big change to the forecast has been the track of a strong low in the gulf Monday/Tuesday. This low was previously forecast to remain largely offshore and out over the central gulf, which would have encouraged weaker WAA across the southern panhandle alongside the better chances of PoPs in the same area. That situation has changed. Operational guidance, led by the Euro/Canadian models, have started falling in line behind a solution which would see this low moving NE, up into the panhandle directly on Tuesday. Ensembles have also begun to favor this scenario more heavily. This result would bring significantly more precipitation into the central panhandle Monday through Tuesday, with the greatest amounts between the Icy Strait Corridor and Petersburg/Wrangell. While confidence is growing in the potential for additional significant snowfall amounts across the Icy Strait Corridor through much of Tuesday as a result, the precipitation type from Kake southward is very much in flux. The low's much closer positioning to the panhandle means that WAA will be stronger than previously anticipated and could result in rain mixing in or even supplanting the snow entirely for these areas through parts of Monday and lasting through Tuesday. The final precipitation type for these areas will be dependent on the final trajectory of the low, with a track that takes it into the more northern section of the panhandle pushing the rain/snow line further north, while a track which would take it into the southern panhandle (as some ensemble runs are suggesting) would see the rain/snow line remain further to the south. The southern trajectory would increase chances of continued snow accumulations in places like Kake and Petersburg. Ketchikan, Annette Island, and southern PoW Island at least look set to largely remain rain during this time frame.

By Wednesday, precipitation will have largely ended across the Icy Strait Corridor, as the upper level trough over the Gulf shifts eastward, and northerly flow begins to win out and shift further southward. Chances of precipitation will continue in the southern half of the panhandle as the advance of the northerly outflow will finds itself eventually stalling as a low approaching the Dixon Entrance keeps chances of PoPs in the forecast through Thursday. Chances of PoPs finally depart through early Friday, as drier weather and northerly outflow prevail across the entire area, and we find ourselves on the W flank of the upper level trough through early next week.

Winds will remain elevated as well, with strong land-based winds especially prevalent across the northern panhandle as the strong northerly gradient accelerates cross barrier flow, and keeps the potential for elevated winds in Skagway and Haines, alongside the possibility for a mountain wave (Taku) wind event for Downtown Juneau, Douglas, and Thane. Temperatures begin falling from North to South once more through the latter half of the week.

AVIATION.../through Monday morning/...Flying conditions continue to deteriorate across the panhandle as the band of precipitation continues to move northward. South of Sumner Strait, the precip remains all rain while to the north, the snow is working its way up to the Icy Strait corridor. Places seeing the rain are expected to remain MVFR but could drop to IFR during the heavier periods of rain. Meanwhile, places seeing the snow are seeing conditions ranging from VFR down to LIFR. The trend is conditions will continue to stay near MVFR to IFR with places from the central panhandle up to Icy Strait seeing conditions drop to LIFR. With the system moving in, AAWU continues to highlight concerns for icing and turbulence which is reflected in PIREPs as well. Another concern for the panhandle for this TAF period will be the continued LLWS.

MARINE... Outside Waters: Northeasterly outflow gap wind speeds remain elevated tonight. Increased winds out of interior passes up to storm force as the strong low pressure gets closer to the coastline. Areas near the upper arms of Yakutat Bay and near Dangerous River look to be the highlights, with violent storm force winds and gusts up to hurricane force. Wave heights in the outer waters as a result of these winds look to exceed 20 ft from Yakutat Bay down to Cape Edgecombe.

Inside: Mostly unchanged forecast for the inner channels, with mild changes in Stephens Passage in response to current conditions. Lynn Canal is holding steady at a strong gale with Point Couverden's storm force tip jet. Fully developed seas exceeding 12 ft in both Lynn Canal and Stephens Passage are expected to continue well through the week, with nothing slowing down the strong winds. Freezing spray possibilities remain for Stephens Passage, Northern Chatham, Icy Strait Corridor, and Lynn Canal through, progressively getting heavier as we go into next week.

AJK WATCHES/WARNINGS/ADVISORIES

PUBLIC...Cold Weather Advisory until 9 AM AKST Monday for AKZ318. Winter Weather Advisory until 6 AM AKST Monday for AKZ320-322- 323. Winter Storm Warning until 6 AM AKST Monday for AKZ321-325. Winter Storm Warning until 6 AM AKST Monday for AKZ324-326-327- 331. Winter Weather Advisory until 3 AM AKST Monday for AKZ329. MARINE...Heavy Freezing Spray Warning for PKZ011-053. Heavy Freezing Spray Warning for PKZ012. Storm Warning for PKZ053-643-651-663-664-671. Gale Warning for PKZ011>013-022-031-036-641-642-644-661-662-672. Small Craft Advisory for PKZ021-032>035-652.