Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EST Tue Dec 28 2021 Valid 12Z Sat Jan 01 2022 - 12Z Wed Jan 05 2022 ...Overview... Guidance continues to show an evolution toward a sharply amplified and blocky upper pattern, with a ridge extending from the Pacific through the Bering Sea into the Arctic. The ridge will promote the south-southeastward progression of initial Mainland Alaska upper energy over the Gulf of Alaska and Northeast Pacific. Gulf of Alaska low pressure should bring precipitation (the majority being snow) to Southcentral and especially the Panhandle during the weekend. Once this system departs, expect fairly dry conditions while high pressure tracking across the northern half of the state should push well below normal temperatures into southern/eastern areas. By next Tuesday-Wednesday questions arise over the shape of the upper ridge and surrounding flow. ...Guidance Evaluation and Preferences... Today's forecast started with a 40/30/30 blend of the 12Z ECMWF, 06Z GFS, and 12Z CMC through the entire period. This approach provided a good representation of the most common ideas of guidance or the most reasonable compromise as the case may be, and maintained definition of features and surface gradients. Note that the 12Z UKMET was not available due to a local data issue and the 12Z CMC mean was delayed. With consensus good for the strong storm tracking south-southeast from the Gulf of Alaska, there were two primary question marks both regarding the building upper ridge--its longitude and whether the northern part would close off a high (with the high's location being an embedded question). Many recent runs of models/means have indicated the upper ridge should not reach much farther eastward than 170W longitude while the ECMWF/CMC means have been showing a high closing off late in the period. ECMWF runs have gone along with the longitude of the ridge but not so much for the closed high until the new 12Z run. At least a couple of CMC runs have closed a high. Meanwhile GFS runs have been very erratic with both longitude and shape. Some runs including the 12Z version pushed the ridge axis east of consensus and did not close a high, while the 06Z run was much closer to consensus in both regards (hence its inclusion in the blend). GEFS runs have tended to be less extreme with eastward extent of the ridge but not surprisingly have been later to close off a high if at all. 00Z ECMWF/CMC mean runs were farther north than operational runs with the eventual upper high, making the preferred model blend a good compromise for position (over the Bering Strait by early day 8 Wednesday) given the uncertainty over whether a high would close off. Means tend to be weak with ridges that are agreeably strong in operational models as well. New 12Z ECMWF/CMC means did trend somewhat south and a bit stronger for the upper high next Wednesday, adding support for the manual forecast. It is worth noting that even with the better than average agreement from the models used in the blend and some support from ensemble mean trends, there is usually a fair amount of uncertainty with the exact details of ridge shape and surrounding flow in this type of evolution. Thus some meaningful changes for aspects of the forecast such as the upper weakness that may develop over or near the Gulf of Alaska and how suppressed surface features and moisture will be over the Bering Sea and Aleutians may still be possible in future runs. ...Weather/Hazard Highlights... The fairly strong storm tracking south-southeastward from the Gulf of Alaska will likely produce some snow over Southcentral and heavier precipitation (snow, with some rain possible at low elevations) over the Panhandle during the weekend. Otherwise a majority of the state should see fairly dry conditions during the period. The one exception will be the western half of the Aleutians where flow around the western side of the upper ridge may promote organized precipitation and brisk winds. High pressure crossing the northern half of the state will bring well below normal temperatures especially into southern and eastern areas (including the Panhandle) from west to east. Northern areas will be cold in absolute terms for a time but with less pronounced anomalies, and then should trend warmer than normal after passage of the surface high. The strong gradient between the high pressure (possibly exceeding 1050mb for a time) with Northeast Pacific low pressure may produce a period of strong winds across the southern mainland and Panhandle. Rausch Hazards: - Heavy snow across portions of the Alaska Panhandle and mainland Alaska, Fri-Sat, Dec 31-Jan 1. - High winds across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, Jan 1-Jan 2. - Much below normal temperatures across portions of the Alaska Panhandle and mainland Alaska, Sun-Tue, Jan 2-Jan 4. 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