textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Messages:

- A strong storm system moving into Prince William Sound this morning will continue to bring widespread rain and snow from the Western Chugach and Talkeetna Mountains east across the Copper River Basin through Tuesday night.

- Multiple Winter Weather Advisories remain in effect for snow from Hatcher Pass, Matanuska Valley, and Anchorage Bowl east across the Northern and Western Copper River Basin and Thompson Pass.

- Temperatures trend colder for the latter half of the week in the wake of this storm system.

Discussion:

A potent upper-level wave is driving east from Southwest Alaska into the northwestern Gulf of Alaska this morning along with a strong surface low entering western Prince William Sound. Location of the low is easily noted with surface observations showing gusty ESE winds from Cordova to Bligh Reef and a strengthening westerly wind from Whittier through Passage Canal. Given the faster and more easterly track of the low and upper-level wave first noted yesterday, there is already a clear cutoff in the precipitation from the far western Susitna Valley south across Cook Inlet as of 4 AM this morning.

Overall, impacts from this system look on track, with widespread precipitation, mainly in the form of snow, or a rain/snow mix at the lowest of elevations, from Anchorage and Talkeetna east across the Copper River Basin. The heaviest of the precipitation still looks to be from Prince William Sound into the Copper River Basin through late this evening before slowly tapering off through Wednesday morning. Farther west, the precipitation will continue until later this afternoon across the Anchorage Bowl, Hillside, and Mat Valley. The northwesterly flow aloft and weak cold air advection will favor higher snow amounts along the western foothills of the Chugach from Sutton south to Eagle River and the Anchorage Hillside. A light southwesterly wind at the surface later this morning across Cook Inlet will likely keep West Anchorage warm enough to allow for any snow early this morning to change back to rain before ending. Nonetheless, 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, especially after sunrise, 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, dipping into the teens and 20s in most spots for early Wednesday and Thursday morning, and perhaps even into the single digits for portions of the Copper River Valley if skies remain clear. Cool and dry conditions will persist into Thursday as a developing upper ridge begins to encroach onto Southcentral from the west.

-AS/TM

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.