textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Messages:

- Despite drier conditions today, rivers in Susitna Valley may still be high this weekend as water level trends lag behind rainfall.

- This weekend's gusty northerly to westerly offshore winds will be strongest along the coast and in marine waters. Areas to watch include Resurrection Bay, Prince William Sound (including Passage Canal), Kachemak Bay, Kamishak Bay, and bays and passes along Shelikof Strait.

- Colder conditions Saturday night through early next week, including the potential for snow in mountain passes early Monday morning and near freezing temperatures for lower valleys Tuesday morning.

Discussion:

Last night's system is quickly pushing east this morning with skies trying to clear behind the trough as drier northwesterly flow takes over. As the trough continues to move through the Copper Basin this morning a compact surface low has developed near the Copper River Delta with a moderate rain band wrapping around the low. The low is currently moving inland and will weaken some as it moves into the southeast corner of the Copper Basin and lower Copper River Valley. While chances are low given lack of instability, there is still a chance that this low is dynamic enough to squeeze out a lightning strike of two as it moves inland. This system is moving out faster than models are forecasting so have cut down on rain chances much quicker today over the Copper Basin.

For the rest of Southcentral, today will some drier weather and clearing skies as drier westerly to northwesterly flow takes hold leading to gusty offshore winds, mostly along the coast and extending into the Gulf. Inland areas will see light winds and with minimal cloud cover later tonight, good radiational cooling will allow temperatures to drop into the 40s for most inland locations with the exception of the Copper River Valley where lows are expected to drop into the 30s. Sunday will see another day starting out dry but increasing cloud cover as the next system drops south from the Seward Peninsula. This system will bring another round of more widespread rainfall for Southcentral and a cooler airmass. This will likely lead to higher elevation snow/termination dust, and perhaps even snow at higher elevation mountain passes, like Broad, Tahneta, and Thompson Passes. Whether the snow will stick will depend on road surface temperatures - this early in the season, probably not. For lower elevations, expect widespread rain and a resurgence of gusty offshore winds. This system will move through rather quickly and depending on how quickly we can clear out cloud cover we could see temperatures drop into the 30s and frost developing Tuesday morning for many lower elevations across inland Southcentral.

- PP

LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)

The overall upper-level pattern through Wednesday will be dominated by a ridge of high pressure extending from the North Pacific across the Aleutian Islands and southern Bering Sea with a negatively tilted trough with axis extending from the Chukchi Sea southeast to the eastern Gulf of Alaska.

Multiple shortwaves and vorticity maxima with move from northwest Alaska into the Gulf, embedded in the northwesterly flow aloft. For Tuesday, the first of these features will help maintain gusty gap winds from Alaska Peninsula east across the immediate Southcentral coast to Prince William Sound. A transient ridge in the wake of this wave will provide some warm air advection, allowing snow level to rise from an expected minimum around 3,000 ft on Monday. A second, weaker wave moves through on Wednesday, allowing for a return of scattered showers across the Alaska Range and the higher terrain surrounding the Copper River Basin.

By late Wednesday, the upper-level pattern looks to become more amplified, with the North Pacific Ridge moving east and lifting across the Southern Mainland Thursday morning. The eastward progression of this ridge will allow a trough over the western Bering to deepen, resulting in a stout southerly wind flow between the two features. A strong shortwave will move north along a stalled frontal boundary, leading to the development of a surface low somewhere between the Pribilofs and Kuskokwim Delta coast. This feature, along with its surface front, will drive into Southwest Alaska with another round of heavy rain and gusty southwesterly winds along the coast for Thursday.

By late Thursday, model agreement begins to break down more significantly, particularly with regard to the eastward progression of the upper-level ridge. The GFS and Canadian models are favored for the long term forecast, both of which slide the ridge axis over the Alaska Panhandle by Friday. The resultant flow aloft becomes more southwesterly across the Alaska Peninsula and Southern Mainland as the trough also slides east. Multiple shortwave then looks to lift from the North Pacific to the Alaska Peninsula by the end of the week, with more rain and gusty winds for much of Southwest and western half of Southcentral Alaska as yet another storm system drives north across western Alaska.

- TM

AVIATION

PANC...VFR conditions and light winds anticipated through the period.