Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Tue Oct 24 2023 Valid 12Z Sat Oct 28 2023 - 12Z Wed Nov 01 2023 ...Overview... During the Saturday-Wednesday period, the best clustering of guidance shows a persistent but gradually weakening mean ridge aloft over and southeast of the mainland while an upper low wobbles over the Bering Sea. The initial pattern should focus moisture into the Alaska Peninsula with some precipitation extending farther north over the western mainland. Then expect precipitation to extend eastward across the southern coast with lighter amounts eventually reaching into the Panhandle, as one or more surface waves lift northward around the western side of the weakening upper ridge. Meanwhile latest guidance is suggesting increasing potential for an Arctic shortwave tracking around the top of the ridge to bring a surface front at least into the far northern mainland by next Monday. The forecast pattern will likely support above to much above normal temperatures over most of the state aside from below normal readings over the southern half or more of the Panhandle. ...Guidance Evaluation and Preferences... The first forecast consideration involved the extent to which shortwave energy, surface low pressure, and moisture will push eastward into the slowly weakening mean ridge aloft. Overall the latest GFS runs and the 12Z CMC were the most aggressive with bringing features eastward into the ridge--thus leading to quicker eastward progression of heavier precipitation across the southern coast and into the Panhandle. At least during the first half of the period the multi-day trend of the GEFS/ECens means has been toward a stronger upper ridge. This trend combined with the historical tendency for some models to be excessively quick to break down strong ridges would favor leaning more toward latest ECMWF runs, the 12Z UKMET, and 12Z GEFS/00Z ECens means. The primary surface wave of interest originates from the east-central Pacific, with ECMWF runs and the 12Z UKMET bringing it to the Alaska Peninsula by early day 6 Monday. Ensemble means are slower and ultimately either lose the wave or merge it with North Pacific low pressure approaching from the west. The mean flow aloft seems to support maintaining surface definition longer/farther north than the ensemble means. The new 12Z ECens mean is not there yet but has trended a little closer to the favored models than the 00Z run. Farther north the 12Z guidance is increasing the potential for an Arctic shortwave to brush the far northern mainland around Sunday-Tuesday, with an associated front reaching the area. This trend is particularly noticeable in the 12Z ECMWF versus the old run that kept the shortwave well northward. The latest GFS runs are generally on the faster/deeper side of the envelope. Overall prefer only a modest trend for the Arctic shortwave at this time while awaiting better continuity and agreement. Elsewhere, models have been very inconsistent for the ultimate path of the Bering Sea upper low. The 12Z CMC lifts it northward into eastern Siberia (as did the 00Z ECMWF run, with the UKMET leaning north through the end of its run). The 12Z/18Z GFS runs and 12Z ECMWF plus CMCens mean keep it farther south. Prefer an intermediate solution given the ongoing spread and variability. Flow to the south will likely track one or more systems beneath the Aleutians but with a portion of the spread extending the systems' influence far enough northward to affect the Aleutians for a time. This aspect of the forecast has particularly low confidence given the spread for the Bering Sea upper low. The composite of forecast considerations led to the starting blend employing half total weight of latest ECMWF runs (12Z and 00Z) along with 30 percent 12Z UKMET and 20 percent 12Z GEFS mean (more representative of the preferred pattern than the GFS) for the first half of the period. With decreasing confidence for specifics later in the forecast, the blend transitioned to 70 percent weight of the 12Z GEFS/00Z ECens means by day 8 Wednesday. ...Weather and Hazards Highlights... The forecast pattern should initially lead to the best moisture focus and locally moderate to heavy precipitation over southward/eastward-facing terrain over and near the Alaska Peninsula during the weekend, with some precipitation of varying types extending northward across the western mainland. Expect moisture to extend farther eastward along the southern coast and eventually into the Panhandle during the early-middle of week. Highest totals then should be between the eastern Alaska Peninsula and the Kenai Peninsula. However there is some uncertainty over the distribution and magnitude of precipitation across the southern coast and Panhandle during the period, with a minority scenario suggesting quicker eastward expansion of the precipitation shield. Some of the western mainland moisture could interact with a front reaching the North Slope to produce mostly light snow around late weekend or early next week. Temperatures should be above to much above normal over most of the state, with the primary exception being below normal readings over the southern half to two-thirds of the Panhandle. The most extreme warm anomalies will likely be for min temperatures over the northern and western mainland. Rausch No significant hazards are expected over Alaska during this forecast period. 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