Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 650 PM EST Fri Jan 15 2021 Valid 12Z Tue Jan 19 2021 - 12Z Sat Jan 23 2021 ...Overview and Guidance/Uncertainty Assessment... Today's guidance continues to show a pattern change toward lowest heights aloft/deepest low pressure tending to be over the northwestern Pacific/western Bering Sea while mean ridging aloft becomes more prominent over the northeastern Pacific and Mainland Alaska. As this new pattern becomes established there are still likely to be progressive shortwaves that will bring waves/frontal systems into portions of the mainland and Gulf of Alaska. However these features should be weaker than some of the northeastern Pacific systems in the recent past and into the short-range part of the current forecast. Most models/means share similar ideas for the general pattern but there are numerous detail uncertainties to be resolved. Already early in the period there has been considerable spread and run-to-run variability for a compact but possibly vigorous wave (with a narrow band of fairly strong winds ahead of it) expected to be southwest of the Alaska Peninsula as of early Tue and track northeastward thereafter. Solutions for track by early Wed have varied from the southern coast to the Bering Strait, with some possibility of merging with a wave along an initial Bering Sea front. Most possible solutions are plausible relative to the overall ensemble mean pattern, and the small scale of this feature lends itself to low predictability until it reaches closer to the valid time. Thus prefer a conservative blended approach that allows for a wave at either extreme. Strong low pressure expected to track into the western Aleutians/Bering Sea mid-late week is a consistent feature but guidance spread for track and evolution (consolidated versus elongated) has not improved over the past day. If anything, the modest weakening trend in some ensemble means over the past day or so could suggest a slight widening of the envelope for the storm's character. At the very least would favor leaning away from the 12Z GFS for this feature as it is on the fast/eastern side of the full spread. The 06Z GFS was somewhat closer to remaining guidance. Shortwave energy continuing eastward from this system should push a leading front into the southwestern mainland by day 7 Fri. There remains potential for a wave along this front but with only a modest signal in the guidance overall--in contrast to the stronger 12Z GFS. Then the majority cluster (recent GFS/ECMWF runs and GEFS/ECMWF/CMC means) has the shortwave energy cutting through the mean ridge with a corresponding surface low in the Gulf by early Sat. The full guidance average lies between the 12Z ECMWF and faster 00Z ECMWF run, with the new 12Z ECMWF mean providing confirmation of this intermediate timing. Meanwhile there is decent agreement on another system coming into the picture over the western Aleutians/Bering by the start of next weekend. In contrast to the first Bering system, the 12Z GFS compared better to consensus versus the fast 06Z GFS. Based on considerations for each system, today's blend incorporated the 12Z operational models early and then trended toward somewhat more ensemble mean focus (plus splitting ECMWF input between both recent runs) late in the period. Although not a large weight in the blend to begin with, varying GFS ideas in different runs led to 12Z run inclusion early, transitioning to the 06Z run during the middle of the period, followed by a return to the 12Z run late. ...Weather/Hazards Highlights... From Tue into Wed expect a wave/frontal system to bring moderate precipitation into parts of the Alaska Peninsula as well as the southwestern mainland/along the southern coast. There may be a band of brisk to strong winds just ahead of the surface wave. Low pressure tracking into the western Bering Sea mid-late week and its leading front will bring enhanced wind/precipitation into the western and central Aleutians by Wed. The moisture, and possibly still brisk winds over some areas, should extend into the western/southern mainland by late in the week with some localized enhancement of precip possible along and near the Alaska Peninsula. Some of the moisture may reach as far eastward as the Panhandle by the start of next weekend. Next system may bring another episode of wind/precipitation to the western Aleutians by next Fri-Sat. Above to much above normal temperatures should prevail over most areas through the period. Highest anomalies will likely be over northern/eastern areas to start the period on Tue followed by a shift into the eastern half of the mainland later in the week. Meanwhile the Panhandle may see enough of a cooling trend to bring readings into the moderately below normal range over southern locations by late week. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon, Jan 18. - High winds across portions of the Alaska Panhandle and mainland Alaska, Mon, Jan 18. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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