Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 535 PM EST Sat Dec 09 2023 Valid 12Z Wed Dec 13 2023 - 12Z Sun Dec 17 2023 ...Multi-day Heavy Precipitation and High Wind Threat from South-Central/Alaska Range to Southeast Alaska... ...Guidance Evaluation/Preferences... Forecast spread remains lower than normal Wednesday/Thursday with the depiction of a wet/active and blocky pattern for Alaska. A composite blend of best clustered model guidance from the 12 UTC GFS/ECMWF/UKMET offers a good forecast basis with reasonable detail that also maintains good WPC product continuity in flow with above normal predictability. Growing guidance differences and run to run continuity issues with system interactions and block transition Friday and next weekend portends transition to 12 UTC GEFS/ECMWF ensembles. Applied double weighting to the ECMWF ensembles in this blend to allow a slightly slower breakdown of the block. This type of pattern change often occurs on the slower side of the forecast envelope, but is shown to occur in most guidance. ...Weather/Hazards Highlights... An unsettling upper trough position should weaken/linger well into next week over the mainland. This may lead to periods of light to locally enhanced snows scattered over the Interior, especially midweek along with windy conditions upstream from the Bering Strait into the Bering Sea. Meanwhile, a main storm genesis pattern will emerge just to the south of the state next week with the development and poleward track across the Gulf of Alaska to the mainland of a series of complex and deepened low pressure/frontal systems as stream flows uncertainly interact. The systems will offer a maritime to coastal high wind/wave threat. Deep layered and long fetch moisture will also feed inland by enhanced wind flow around these storms, with main focus over South-Central/Alaska Range through Southeast Alaska past midweek. Expect a multi-day period with periodic surges of heavy precipitation in the form of coastal rains with potential runoff issues due to the protracted accumulation that will also be a factor for heavy inland/terrain/mountain snows. Additionally, gusty terrain enhanced winds are expected for this region into this period to include gap and downslope Chinook winds to the lee of the Alaska Range into the southern Interior. Later, ample system trough energies are expected to eventually work eastward through the Bering Sea and along/south of the Aleutians into next weekend as the leading/blocking upper ridge also shifts eastward and deamplifies while also splitting off a closed upper high over the Arctic Ocean to the northwest of the state. The advent of this transition would allow for uncertain northern Pacific/Aleutians and Bering Sea travelling systems to bring new organized precipitation more eastward from these areas to the mainland into next weekend. Schichtel Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html