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.

Discussion:

Skies have struggled to scatter out throughout the day today as a persistent stratus deck continues to linger across most of Southcentral. Similar to the previous night and this morning, the lingering cloud cover will likely again prevent temperatures from dropping below the 30s across much of Southcentral overnight tonight. Looking at satellite and radar imagery this afternoon, high-level clouds and radar returns are beginning to make their way into the western Gulf, with rainfall beginning to overspread Kodiak Island as a front lifts across the AKPen, Kodiak Island, and the western Gulf this afternoon through this evening.

Widespread precipitation will continue to spread across Kodiak Island this afternoon before spreading north across the Kenai Peninsula, Susitna Valley, and Prince William Sound tonight through Monday morning. Initial downsloping may limit precipitation for a bit across the western Kenai Peninsula, Anchorage, and Mat Valley but is expected to fill in by late Monday morning. The precipitation type continues to be a forecast challenge as it will be highly dependent on the overnight and early morning temperatures. Wet snow or a rain/snow mix will be possible at the initial onset of precipitation 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 toward freezing Monday night, precipitation at higher elevations will transition back to snow. Snowfall is expected to be slower to change over to rain in the Susitna Valley Monday morning through Monday afternoon while the Northern Copper Basin remains cold enough to see all snow.

By Tuesday morning, an additional, stronger low pressure system lifts north in the Gulf and tracks toward Prince William Sound by Tuesday night, leading to an additional round of precipitation Tuesday night into Wednesday. Snowfall in the Susitna Valley and Northern Copper Basin can expect to see about 2-4" for lower elevations to 5-10" for the areas along the Denali Highway and Richardson Highway corridor near Paxson through Tuesday night. The Glenn Highway from about Sutton over to Tolsona could also see snowfall totals of between 2-4". 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.

The precipitation forecast across the western Kenai, Anchorage, and the Mat-Su remains fairly uncertain as model guidance continues to struggle to come to a consensus on a storm track in the Gulf. A further 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.

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...VFR conditions and light north winds will persist through this evening. Later tonight, a frontal system moving northeast into Southcentral will bring light precipitation to the terminal. Low level temperatures could be cold enough for precipitation to start as a slushy snow or rain-snow mix for a few hours, becoming all rain by Monday morning as temperatures slowly trend warmer. MVFR conditions are possible at times during and after the onset of precipitation starting around 8 to 10z, especially during any periods of snow reaching down to sea level. Low level wind shear will also be possible early Monday morning as southeast winds aloft pick up out ahead of the front. Southeast winds could clip the terminal for a short time close to midday Monday, but potential for winds at the surface to turn southeasterly remains highly questionable at this time.