Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EST Fri Dec 10 2021 Valid 12Z Tue Dec 14 2021 - 12Z Sat Dec 18 2021 ...Overview... Today's guidance generally maintains the previously advertised pattern evolution during the period. Initial moderately amplified and progressive flow across the North Pacific/Bering Sea should extend into the mainland around midweek or so, with an upper ridge displacing an Arctic/Mainland Alaska trough in place as of early Tuesday. Expect a brief flattening of mean flow late in the week and then renewed amplification as a deep storm near Kamchatka builds an upper ridge into the Aleutians/Bering Sea by next Saturday. The ridge should in turn begin to deepen a trough from the mainland southward. This forecast pattern will carry along a series of lows/frontal systems that will tend to bring varying amounts of moisture to the Aleutians, western/southern parts of the mainland, and the Panhandle. In addition temperatures should moderate after the well below normal readings forecast early in the week. ...Guidance Evaluation and Preferences... A composite of the 12Z operational models provided better than average continuity from yesterday for significant features between Tuesday and early Thursday. Meaningful model discrepancies become evident by early Thursday, with additional divergence for systems over the Bering Sea and Pacific thereafter in the brief period of flatter mean flow as well as where amplifying features set up by Saturday. Thus the early-period model blend transitioned steadily toward somewhat greater emphasis on the means by day 8 Saturday in light of very good agreement among the means for large-scale features. One area of notable uncertainty involves what may become of a mid-latitude Pacific wave that could get picked up by the frontal system extending from the midweek Bering Sea low. The wave would then contribute to the surface low that consensus has been indicating over the Gulf of Alaska by early Friday. The majority clustering over the past couple days has been showing at least the warm front even if not a very well defined anchoring wave, while latest GFS runs (which keep the wave suppressed) stray flatter than most other guidance (including the GEFS mean) for the overall upper trough. Thus some indication of such a wave seems reasonable. Interestingly the new 18Z GFS has become even more pronounced with its flatter/faster flow aloft. On the other hand the 12Z ECMWF mean maintains a decent hint of the ECMWF scenario. Meanwhile there is now enough support to include a wave which the majority brings to the western Aleutians by early day 6 Thursday and into the Bering Sea thereafter--though still with a lot of spread for specifics by that point. Again the GFS is on the fast side of the envelope. Ensemble members are diverse enough for the means not to have a defined wave but do suggest a front about where the non-GFS cluster shows it. This feature should quickly lead to another Gulf wave by early next weekend. Keeping modest influence of the operational guidance late in the forecast provides a little enhancement to the upper ridge expected to build over the Aleutians/Bering Sea by Saturday. Latest GFS runs have been on the deeper/slower side of the full guidance spread for the amplifying trough to the east. ...Weather/Hazard Highlights... Shortwave energy and a weak surface wave may support light to locally moderate precipitation along the southern coast/Panhandle on Tuesday. A Bering Sea storm and its leading front will bring light precipitation across the Aleutians Tuesday-Wednesday and light-moderate activity into western and southern parts of the mainland during mid-late week. Still expect a wave to reach the Gulf of Alaska or southern coast by early Friday and help to focus precipitation along the southeastern coast and Panhandle around that time. However precip totals are uncertain due to lack of confidence in whether this low will reflect just northern stream dynamics or have a contribution from a mid-latitude Pacific wave (which would lead to somewhat higher amounts). Another wave/frontal system may bring light to locally moderate precipitation to the Aleutians and western/southern mainland as well as the Panhandle late in the week. After below normal temperatures cover much of the state on Tuesday, with some well below normal readings possibly lingering in the southern half of the mainland, there should be a moderating trend from southwest to northeast during the latter half of the week. This could allow for a few pockets of slightly above normal highs to develop by Friday while coverage of above normal lows could be greater at that time. A slight cooling trend from the northwest is possible by next Saturday. Rausch Hazards: - Much below normal temperatures across portions of mainland Alaska and the Aleutians, Mon, Dec 13. 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