textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Messages:

- A large storm system will bring widespread rain and snow to most of Southcentral tonight through Tuesday night.

- Multiple Winter Weather Advisories are in effect for snow including for Broad Pass until 10 pm this evening; and Thompson Pass, Western and North Copper River Basin until 4 am Wednesday morning.

- A new Winter Weather Advisory is now in effect for the Anchorage Hillside from 10 pm this evening through 10 pm Tuesday.

Discussion:

A complex and active weather pattern continues to unfold across the Gulf and to the north across Southcentral this afternoon. A large plume of moisture is spreading northeast into Southcentral along a belt of stout southwest flow situated between a deep trough out west and a strong ridge over the Northeast Pacific. A shortwave trough that pushed into the region from the southwest earlier today is beginning to move off towards the northern Alaska Range, while an associate surface front approaches the north Gulf Coast. A fierce barrier jet moving along and ahead of the front has been producing easterly wind gusts close to storm force near Hinchinbrook Entrance and just offshore from Cordova. Widespread precipitation also continues to overspread much of the region, with mostly rain now falling at sea level along the coast. Widespread higher elevation and mountain snow is continuing across nearly all of the interior parts of Southcentral, where a number of Winter Weather Advisories are either in effect or set to go into effect shortly. Much of the Susitna Valley is also still seeing mainly snow, where temperatures have been cooler compared to areas farther southeast.

Looking forward towards tonight onwards, there are a number of key forecast changes and details that have shifted compared to indications from the past day or so. Models have struggled considerably to hone in the the track and forward speed of a new low and shortwave trough now taking shape south of the AKPen, but we are finally seeing solutions come into better alignment today. It now looks like the new low developing near the Gulf will move northeast towards Prince William Sound much faster compared to prior indications, likely reaching the Sound by early Tuesday morning instead of Tuesday afternoon. This shift has a number of implications for expected timing, type and duration of additional precipitation expected near and out ahead of this secondary approaching system. The most notable change to the forecast was to clear heavier rain/snow out of the Kenai Peninsula and Susitna Valley much earlier in the day on Tuesday due to the faster progression of the Gulf low. The low also will not have as much time to deepen out over open water before it begins to interact with steep terrain along the coast, so marine winds will not only peak earlier in the day tomorrow, but also remain on the weaker side compared to earlier expectations.

In terms of the impacts from anticipated heavy precipitation and snowfall, timing for the heaviest rates has also shifted earlier. It now looks like the heaviest precipitation will fall during the morning hours on Tuesday as the incoming trough and attendant low reach the coastline, including just about everywhere aside from the western Susitna Vally, where things will dry out by this evening.

There is increasing concern that the "wet bulb effect" could play a role in precipitation type at lower elevations overnight tonight into Tuesday morning. As precipitation intensity picks up, melting snow falling into the column could help low level temperatures cool enough for cold rain to switch over to wet snow, including across parts of the Mat Valley, western Kenai Peninsula and Anchorage Bowl. Snowfall amounts along this corridor remain highly uncertain, with amounts depending on when rain transitions to snow overnight tonight and how long precipitation stays snow through Tuesday evening. Parts of the Anchorage Bowl and Mat Valley could see several inches of slushy accumulation, mainly on elevated and grassy surfaces. Snow will probably struggle to consistently stick on roadways close to sea level, mainly due to the still seasonably warm ground temperatures. At higher elevations and where temperatures will remain colder, much higher storm total snowfall amounts remain on track. An additional 6-12" is expected across much of the Anchorage Hillside and along the Glenn Highway from Sutton east to Tolsona. Even higher additional amounts on the order of 12" or more are expected near the top of Thompson Pass and along northern parts of the Richardson Highway near Paxson.

By late Tuesday into Wednesday, snow will let up as a lingering upper level trough drifts slowly southeast towards the Copper Basin and northern Gulf and as a cooler air mass filters in from the northwest. Gusty northwest gap winds will linger across the western Gulf through Wednesday as the then weakening low drifts east across the Gulf while northwest flow continues to pull cool air across the AKPen. Meanwhile, overnight lows across the bulk of Southcentral will be much colder Tuesday night and Wednesday night, dipping into the 20s to low 30s in most spots, perhaps even colder for areas that end up seeing the highest snowfall totals through Tuesday evening. Cool and dry conditions will persist into Thursday as a developing upper ridge begins to encroach onto Southcentral from the west.

-AS

LONG TERM FORECAST (Days 4 through 7: Friday through Monday)

Key Messages:

* A series of strong storms move across the Bering Sea and into Alaska from Friday through the weekend, bringing strong winds and heavy precipitation.

* Though confidence is low in exact impacts at this time, strong onshore winds and elevated coastal water levels are a concern for Southwest Alaska, mainly Friday and Saturday and again on Monday.

Discussion:

The biggest story in the long-term forecast will be a strong low transiting from the Kamchatka Peninsula to Alaska. Ensembles are in relatively good agreement with this storm, with seemingly higher-than-average forecast confidence given that this feature is 4 to 7 days out. Good run-to-run consistency among deterministic global models is also contributing to higher-than-average forecast confidence. The areas of highest agreement with this feature are for gale to storm force sustained winds along the front as it moves across the Aleutian Chain, the Southwest Alaska coastline, and the Alaska Peninsula. These onshore winds are raising concerns for coastal impacts (including high surf and perhaps even coastal flooding) for south-facing portions of the Western coast. It's still too early to say for now what projected water levels will be, but we will be closely monitoring this storm. Southcentral should be spared the worst of the winds with this storm, as the front will very likely be weakening by the time it exits Southwest Alaska.

In addition to winds, there will also be plenty of moisture along the front, which will bring periods of moderate to heavy precipitation to the Aleutians, Alaska Peninsula, and Southwest Alaska. Precipitation amounts are uncertain at this time, as overall amounts will be dependent on where the front may slow down and linger as upper waves move through. Given the strong winds associated with the front, there will likely also be areas of downsloping that will reduce precipitation totals. While warm air advection will likely keep precipitation mostly rain for the majority of Southwest Alaska, colder air lingering in Southcentral could mean that lower elevation areas may see snow.

AVIATION

PANC...Not a straightforward forecast with a series of low pressure systems moving across the region. Generally MVFR cigs expected through the TAF period except for a drop to IFR during the overnight. Have carried LLWS in the TAF this past afternoon (Monday) but will leave out from this evening onward with winds from 2-5kft diminishing. Winds at the surface will remain under 10kts through tonight, becoming somewhat gusty by around 14z. Precip type is also tricky. Expect mainly rain for the PANC terminal with a mix or change over to snow late tonight into tomorrow morning. Also expecting periodic changes between rain- snow throughout Tuesday. Little if any accumulation is expected at the airport.