textproduct: Juneau

UPDATE

After 06z TAF issuance..VFR Conditions continue through tonight. Light snow wills spread through the coastal area of the panhandle Thursday.

PREV DISCUSSION...ISSUED AT 343 pm Wednesday 24 December

SYNOPSIS...

Key Messages:

- A low pressure will bring light snow accumulations to the outside coast for Christmas Day.

- The next impactful system moves in Friday bringing heavy rain and snow to the panhandle for the weekend.

- Temperatures warm up with the next incoming low bringing an end to the low teens and near zero temperatures.

SHORT TERM...Outflow conditions continue to weaken this afternoon as the pressure gradient between the panhandle and the Yukon continues to weaken. This brief weakening will be short lived though as a surface low moves to the south and east from Prince William Sound will start to increase the pressure gradient across the panhandle. This low will bring some snow to the outer coast for Christmas Day with 1-3" of snow possible for Sitka down to the western PoW Island starting Christmas morning before dissipating tomorrow evening. This low will also bring clouds to other parts of the panhandle which will limit how much we cool later tonight. By Friday morning, this low will have moved out of the area and a shortwave ridge of high pressure will move into the panhandle allowing for a nice break before the next system moves into the area starting Friday.

LONG TERM...We continue to watch the upcoming shift in the weather pattern that would give the next round of significant snow to Southeast Alaska this weekend.

The next major system will begin to move into the Gulf of Alaska Friday night from the west. In terms of winds, the northern and easterly offshore flow will increase again ahead of the next low moving in from the west. Winds begin to pick up across the entire panhandle Friday night into the weekend as the stronger low approaches, and a gale force front begins to move in from the southwest into Saturday morning. These southerly winds moving into the panhandle will allow for some warming across the panhandle alongside bringing precipitation through from W to E with this first front and the following wave moving through by Sunday.

As far as temperature relating to snow potential, the ensembles and deterministic models have come into greater agreement over low tracks and 500mb flow. The 500 mb flow continues to have the deep trough further west, keeping a southwesterly flow into the panhandle and allowing for deep moisture and warmer air to be moved in from lower latitudes with the deep trough position, with the arctic trough and 500 mb cutoff low over the Pacific to the southwest connecting over the Kenai Peninsula down the west Gulf. Some of the deterministic models still show the arctic trough separating from this lower latitude troughing as it becomes a cutoff again and moves southward, leaving a southwesterly flow into the southern panhandle but the arctic trough allowing for colder arctic air to move across the northern Gulf into the northern panhandle, bringing the potential for cooling the north much sooner than the southern panhandle into Monday. This only is beginning to show up on 30% of the clusters with this separation and having an arctic flow into the northern panhandle, while the rest of the 70% of the ensembles keeping the deep connected troughing with SW flow move into the whole panhandle with resulting warmer 850 mb temperatures. This means that the timing of the cold air coming back into the northern panhandle is still a bit uncertain for the period into early next week, and seeming a bit more likely into mid week based on when the ensembles agree on colder arctic air bringing down temperatures again across the north.

Looking at the EFI tables, as well as looking at AR and IVT tools for this heavier precipitation coming up this weekend, QPF amounts have been increased over the northern panhandle (particularly Yakutat and in areas closer to the coastal mountain range) as the first wave hits Saturday with a bit less hitting the southern panhandle directly until Sunday. Adjusted to keep the southern panhandle at between 2 and 3 inches of liquid precipitation every 24 hours Saturday and between 3 and 4 inches in 24 hours Sunday. This will down first as snow Saturday morning before quickly transitioning to rain from the SW coastline inwards as the warm front pushes through, with the switch to rain expected during the morning hours for PoW and Sitka, into midday for areas like Ketchikan and Wrangell that are a bit further inland. Overall the heavy rain will begin for the southern panhandle Saturday, but the next wave of precipitation pushing through will bring another round of heavier rain on Sunday that has been primed by the first warm front pushing through the day prior. The warm temperatures in the high 30s to low 40s will continue into the early week as onshore flow continues to bring in southerly warmer flow into the southern half of the panhandle.

For the rest of the panhandle, which includes everywhere north of Wrangell and Sitka, the snow will last into Saturday as that first front moves in over the cold airmass in place, as the warmer temperatures will not bring temperatures up into the mid to high 30s until Sunday. This will allow for a switch to more of a mix potentially becoming rain by Sunday afternoon/evening in the central panhandle up to Icy Strait corridor. These areas will see between 12 and 18 inches in 24 hours Saturday into Sunday morning before transitioning to rain/snow mix to potentially rain. Snow will start off lighter and fluffier Saturday as snow liquid ratios remain high, but as the area warms Saturday evening into Sunday, ratios will decrease to become a heavier and wetter snow. Areas to the north of Icy Strait corridor will remain snow the entire time through both fronts moving through, giving a significant amount of snowfall to the Skagway and Haines areas with between 12 and 18 inches in 24 hours both Saturday and Sunday with more expected along the Klondike Highway, and moderate to heavy snow lasting into Monday. However these amounts for all of the panhandle depend largely on how much of the precipitation actually reaches parts of the panhandle, particularly in these northern areas and especially the Haines Highway and Skagway areas. There remains some uncertainty on exactly how much snowfall there will be during this system and the exact timing of a changeover to rain for the central parts of the panhandle up to Icy Strait corridor on Sunday.

Overall this will start as a heavy snow event for the entire panhandle Saturday morning, before becoming a heavy rain event for the southern panhandle from Sitka and Wrangell southwards later Saturday and lasting through Sunday with the next heavier round of precipitation. Expecting between 3 and 7 inches of snow accumulation for Sitka, Annette Island, and PoW Saturday morning before transitioning to rain in the late morning to midday. For Petersburg, Wrangell, and Ketchikan an expected 8 to 12 inches of snow will accumulate Saturday before they too begin to transition to rain by the evening hours. This timing could change for Wrangell and Petersburg depending on when the warmer air pushes in and for how long they hold onto snow into Saturday evening.

AVIATION.../Until 00Z Friday/...VFR CIGs & VISs are anticipated through the 24-hour TAF period. Light snow showers will occur during the day tomorrow for Outer Coast areas, including PASI, which will potentially lower conditions down to the MVFR category. Low-level turbulence & wind shear values up to around 30KT will also last through this evening for the PAJN & PAWG areas, which are areas that are prone to gap winds. The LLWS will start diminishing around the mid-afternoon through the evening timeframe. Strong northerly outflow winds of up to around 30G50KT continue for the PAGY & northern Lynn Canal area through the early evening hours, diminishing to around 20G35KT starting late this evening & lasting through the end of the TAF timeframe.

MARINE... Inside (Inner Channels): Wednesday afternoon, we continue to see gale force conditions in Lynn Canal, Glacier Bay, and major river inlets; however, winds are starting to slowly trend down. Expecting winds to briefly diminish to moderate breezes tonight into Thursday morning for Lynn Canal as a gale force system slides along the northern coast, bringing a more west-to- east pressure gradient over the Panhandle. Main message is that winds will subside in Lynn Canal but they will increase back to gale force by late Thursday, so dont get fooled. Over the weekend a storm force system will move into the gulf, increasing winds across the region to strong-breeze to gale force. For areas south of Icy Strait, expecting a southerly regime, with easterly winds in Icy Strait. We will continue to maintain northerly gale force winds in Lynn Canal through the weekend, building to strong gales by Sunday.

Outside (Coast and Gulf): Current sea state along the coast shows 2-4ft significant wave heights, dominated by a westerly swell at 7 to 10 seconds, masking an underlying northeasterly outflow wind chop. Cross Sound is seeing NE sustained near-gale force winds bringing fully developed seas near 10 ft at 6 to 8 seconds. In the central gulf we are seeing higher northerly seas from outflow along the northern coast with more organization and energy at 10 seconds. Overnight Wednesday a gale force low will develop near Cape St. Elias, transiting east along the northern coast. Expecting northwesterly winds to increase through the central gulf to near-gales, with easterly-northerly outflow winds strengthening to gale force for major bays/inlets along the northern coast. Westerly swell continues but will be masked by northwesterly fresh seas of 12 to 15ft, with the main wave energy focused from the central gulf toward Haida Gwaii.

Seas briefly diminish Friday afternoon before a storm force low approaches the gulf, driving 25 ft ESE fresh seas along the northern coast Friday night into Saturday, becoming 15 to 20ft from the SW by Sunday into the start of next week. Storm force winds are forecasted to be coming out of Cross Sound/Palma Bay by Saturday, creating large and confused seas through the Fairweather Grounds.

AJK WATCHES/WARNINGS/ADVISORIES

PUBLIC...Winter Storm Watch from Friday afternoon through Sunday afternoon for AKZ317>322-325-331. Cold Weather Advisory until 5 PM AKST Thursday for AKZ318. Cold Weather Advisory from midnight tonight to 3 AM AKST Thursday for AKZ320-325. Winter Storm Watch from Friday evening through Sunday morning for AKZ323-324-326>330-332. Extreme Cold Warning until 9 PM AKST this evening for AKZ325. Cold Weather Advisory from 3 PM to 6 PM AKST Thursday for AKZ325. Cold Weather Advisory until 3 AM AKST Thursday for AKZ329. MARINE...Heavy Freezing Spray Warning for PKZ011. Heavy Freezing Spray Warning for PKZ012. Heavy Freezing Spray Warning for PKZ031. Heavy Freezing Spray Warning for PKZ053. Gale Warning for PKZ012-053-643-644-651-652-663-664-671. Small Craft Advisory for PKZ011-013-021-022-031-032-036-661-662- 672.