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.

Discussion:

Widespread precipitation will continue to spread across the Kenai Peninsula, Susitna Valley, and Prince William Sound through Monday morning with 0.15" to 0.45" of precipitation already falling across parts of the Kenai Peninsula and Anchorage Bowl. Although temperatures are hovering in the mid 30s for most locations with rain falling, snow has mixed in at times as heavier bands of precipitation helped to cool the near surface just enough for a few snowflakes to reach lower elevations. Farther north into the Susitna and Northern Copper River Valleys, temperatures in the upper 20s to lower 30s have allowed the precipitation to begin as snow.

As noted in the previous forecast discussion, precipitation type continues to be a forecast challenge as it will be highly dependent on morning temperatures. A rain/snow mix will be possible at times across the western Kenai, Anchorage, and the Mat Valley before quickly changing over to rain as warmer southeasterly flow erodes the cooler airmass currently in place. Higher elevations (>2000 ft) will see snow with a possible changeover to a rain/mix for coastal areas. As temperatures drop back down towards freezing Monday night, should see precipitation change back. Snowfall is expected to be slower to change over with rain in the Susitna Valley Monday morning through Monday afternoon while the Northern Copper Basin remains cold enough to see all snow. Winter Weather Advisories are in effect for these areas for accumulating snowfall.

By Tuesday morning, an additional, stronger low pressure system lifts north in the Gulf and tracks towards Prince William Sound by Tuesday evening, leading to an additional round of precipitation Tuesday into Wednesday. Total two-day snowfall in the Susitna Valley is expected to range from 1" to 3" with 4" to 8" through Broad Pass. The Northern Copper River Basin along the Richardson Highway north of Gulkana could see around 4" to 10" of snow with amounts as high as 10" to 20" from Paxson north. For areas along the Glenn Highway west of Mendeltna, 6" to 12" of snow is likely. These areas will be the most likely areas to see temperatures hovering near to below freezing allowing for snow to accumulate. Thompson Pass will also see mostly snow, though there is a chance it could swing from snow to a mix of precipitation at times. However, it should remain cold enough for modest snowfall rates of 1" to 1.5" per hour with the potential to see accumulations above 10 inches from Monday through Tuesday night. The coastal areas along the Gulf and Prince William Sound will see the bulk of precipitation from the system. The lower-elevations along the coast could see rainfall amounts of several inches, from 1" to 3" of rain across the eastern Kenai to 2" to 4" from Whittier to Valdez and up to 6" of rain for Cordova. Upwards of an inch or more of rain (or rain/snow mix at times) could fall across the Cook Inlet region as well.

The precipitation forecast across the western Kenai, Anchorage, and the MatSu remains fairly uncertain as model guidance continues to struggle to come to a consensus on a storm track in the Gulf. A farther west storm track in the Gulf could lead to a much more substantial second round of precipitation for these areas for Tuesday and Tuesday night, with temperatures likely cooling back down enough by this time to see wet snow or a rain/snow mix near sea level. A more eastward track scenario would largely clear precipitation out of these areas by Tuesday, with the Gulf coast, Prince William Sound, and Copper Basin continuing to see a bulk of the precipitation.

Winds will also be quite strong for a few locations as well, with strong easterly winds ahead of the first front today through Prince William Sound and across Turnagain Arm and the Anchorage Hillside. Strong winds and gusts over the Gulf will then be reinforced by the rapidly deepening secondary surface low with a likely core of storm-force winds over the open waters. Gusty westerly winds will develop by Tuesday morning for Kodiak with a period of strong northerly winds for Cook Inlet and Seward Tuesday morning as well. This low will keep strong winds across Prince William Sound into Wednesday morning.

LONG TERM FORECAST (Days 4 through 7: Thursday through Sunday)

Key Messages:

* A strong storm moves across the Bering Sea and into Alaska from Thursday through the weekend, bringing winds as high as 40 to 60 mph.

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

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, including as it moves across the Alaska Peninsula and Southwest Alaska coastline. 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 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 advection along the front, which will bring periods of moderate to heavy precipitation to the Aleutians, Alaska Peninsula, and Southwest Alaska. Precipitation amounts are a bit 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.

-KC

AVIATION

PANC...A frontal system continues to move northeast into Southcentral today bringing rain into the terminal. Low level temperatures have remained warm overnight and will continue through the day today keeping rain the main precipitation type. Low level wind shear is possible through this afternoon. Mostly MVFR conditions will prevail through Tuesday afternoon. Cooler air moves over the terminal this evening leading to a change to snow/rain mix into Tuesday morning as a low moves through the Northern Gulf.