textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Message:

- Gap winds return to Southcentral ahead of a gale force front that will lift northeastward across the northern Gulf through the short term period.

Discussion:

A broad surface trough draped northwestward from a weakening low pressure system located in the eastern Gulf is sending waves of rainfall towards the Prince William Sound. Aside from that, a gale force front extending from a very strong Bering low with a corridor of moderate to heavy rainfall is entering the western Gulf this afternoon and will move across Kodiak Island through Friday morning before moving northeastward towards coastal 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 (50 to 60 percent) for Hillside wind gusts. Higher profile vehicles may struggle with those gusty winds on the Seward Highway along the Turnagain Arm, especially Friday afternoon into Saturday morning as the highway could become clipped with strong wind gusts - though the strongest core of winds will remain located over 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%). Stay tuned.

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 very light rainfall to Kenai, Anchorage, and the MatSu Saturday afternoon. The pressure gradient will show signs of gradual relaxing Saturday night into Sunday, 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...VFR conditions will persist through the TAF period, although increasing mid and high level clouds are expected. Light and variable winds early may shift and become southeasterly for a time later this evening. Any period of Turnagain Arm winds clipping the terminal later this evening should end as winds bend back down Cook Inlet for Friday morning. This shift in wind at the surface will be met with a continued southeasterly winds aloft resulting in an extended period of LLWS at the terminal for Friday.

-TM