textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Message:

- Strong, southeast gap winds return to Southcentral as a gale force front lifts across the northern Gulf and through Southcentral.

- One to three inches of rainfall is expected for most communities along the Northern Gulf coast, Kodiak Island, and Prince William Sound by Sunday morning. Western Prince William Sound will see locally higher amounts.

- Much less rainfall is expected farther inland. Western Kenai Peninsula could see 0.2 to 0.7 inches and Anchorage through the MatSu, a quarter of an inch or less. Amounts for these communities will depend on how much rainshadow effect/downslope drying they experience.

Discussion:

A gale force front extending from a very strong Bering low with a corridor of moderate to heavy rainfall is lifting north across the Gulf this morning. Cloud cover ahead of the front has steadily spread across Southcentral.

Coastal ridging is on track to build ahead of the front across the northern Gulf coast, which will lead to increasing gap winds for the Copper River Basin, Knik River Valley, and Turnagain Arm as pressure gradients tighten. The Knik River Valley wind is the area of the biggest uncertainty Friday afternoon as there is a chance that a few elevated wind gusts move into Palmer, but some models show that the strong down-inlet component to the wind prevailing and essentially bending the wind direction southward before making it into the Matanuska Valley. Due to the parent low location out west signaling down-inlet flow, the Anchorage Bowl seems to be less of a concern than the Hillside, where at elevation wind gusts will be ticking up to 50 mph. Overall consensus remains short of Wind Advisory criteria for the Hillside. Current forecast confidence is still in the medium range (about 55 to 65 percent) for Hillside wind gusts. Higher profile vehicles may struggle with those gusty winds on the Seward Highway along the Turnagain Arm, especially later today into Saturday morning as the highway could become clipped with strong wind gusts though the Arm. The low location out west will be a key factor in determining the extent of impacts from the Turnagain Arm winds, and there is no real indication in the models of the Bering low driving north of 60N, where in doing so would bend winds more into west Anchorage, but that chance right now is low (20 percent).

Through the short term, periods of locally heavy rain are expected with strong orographic lift along the eastern Kenai Peninsula. By Sunday morning, Seward, Cordova, Valdez, and the eastern portions of Turnagain Arm are on track to see 1 to 3 inches of rainfall. In contrast to coastal locations, southeasterly winds ahead of the front will create a "rain shadow" effect in lee of the mountains for the western Kenai Peninsula, Anchorage Bowl, the MatSu, and Copper River Basin, keeping these areas mostly dry through Saturday morning. As the front continues north, however, some of the energy could push moisture northward up the Cook Inlet, bringing periods of brief, light rainfall to Kenai, Anchorage, and the MatSu Saturday afternoon through Sunday. The pressure gradient will show signs of gradual relaxing Saturday night through Sunday morning, but strong winds will continue to push east to southeast winds into Prince William Sound.

-AM/Rux

LONG TERM FORECAST (Days 4 through 7)

The long term begins late Monday with the longwave trough axis moving east and extending from the Al-Can border south to the eastern Gulf of Alaska. The result is low pressure over the Gulf with showers and winds wrapping around the low and across the immediate Southcentral coast. This feature will continue to move east toward the Alaska Panhandle for Tuesday, with northwesterly flow aloft and at the surface in the wake of the low and the upper-level trough. The northwesterly flow will advect weaker waves from northwest Alaska across interior Southwest and Southcentral, allowing for periods of rain showers and higher elevations snow showers along the mountains and across the Copper River Basin Tuesday into Wednesday. Northwesterly gap winds will also pick up along the coast across the Barren Islands through this same time period. A stronger upper-level shortwave deepens into an upper-level low as it drives across the Southern Mainland for Thursday keeping showers, gap winds, and colder air in place.

Farther west, strong northwesterly flow over the eastern Bering and AKPen weakens as the aforementioned waves move east. A ridge of high pressure settles over the western Bering and Aleutians Monday and moves east through Tuesday as a Kamchatka Low moves toward the forecast area with a gale-force front swinging out ahead of it. The front reaches the western Aleutians by late Tuesday moving to the eastern Aleutians by late Wednesday. Model agreement then wanes with respect to how quickly this front and its parent low move east across the Bering and Aleutians. Whatever the solution, conditions for these areas look to remain unsettled with steady rain and wind through much of the latter half of next week.

-TM

AVIATION

PANC...There is some uncertainty to the duration of LLWS throughout the TAF period as gusty, southeast winds through Turnagain Arm could make it over the terminal. Otherwise, light and variable winds at the surface will shift northwesterly at times beneath continued southeasterly flow aloft. VFR conditions will persist through the TAF period.

-BL