Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Thu Apr 27 2023 Valid 12Z Mon May 01 2023 - 12Z Fri May 05 2023 ...Overview... Most guidance shows an initial southeastern Bering Sea system tracking into the southwestern mainland early-mid week, with the leading flow and wavy front bringing enhanced precipitation to some southern areas. This system will steadily weaken with time, with low pressure emphasis shifting into western Canada by Wednesday-Thursday and leaving a more ill-defined surface pattern over the mainland by late week. The initial upper low, combined with some northern stream energy, should yield a persistent western mainland upper trough that will likely weaken gradually with time. Meanwhile expect a system to come into the picture over the western Aleutians/Bering Sea by around midweek, but there is a lot of guidance spread and run-to-run variability for the evolution and timing of this system thereafter. ...Guidance Evaluation/Predictability Assessment... The 12Z GFS/ECMWF/UKMET and the GEFS/ECMWF/CMC means offered a dominant cluster for the overall evolution of the system forecast to track into the southwestern mainland during the first half of the week. Latest CMC runs have been the one holdout, keeping the upper/surface lows on a farther south track. Over the past day or so, there has been a pronounced trend for faster progression of the overall system. Within this trend the GFS/GEFS in particular have accelerated toward the ECMWF/ECMWF mean that had been on the faster side of the spectrum. The result of this trend is to push the best precipitation focus somewhat farther eastward, with latest guidance generally showing a frontal wave reaching into the Gulf of Alaska around Tuesday helping to direct the primary axis of moisture into the far southeast corner of the mainland and portions of the Panhandle. With typical detail differences, the models and ensemble means show better than average agreement for the lingering western mainland upper trough through the rest of the week, composed of the leading system's initial upper low along with one or more pieces of northern stream energy (which could also be in the form of an upper low for a time). A blended approach provides a good representation for the upper trough evolution over the course of the week. Farther west, there is some measure of clustering for the system may reach near the western Aleutians by early Wednesday. At the same time there are emerging signals from latest guidance that a weak wave could form along the leading warm front. Thereafter guidance rapidly diverges for the timing and evolution of the overall system, including the parent low (and possible influence from an arriving western Pacific wave) and leading frontal wave. Prior to the 12Z cycle, the primary trend of note was for GFS/GEFS solutions adjusting somewhat northward toward the ECMWF/ECMWF mean. Then some of the 12Z guidance has introduced a faster trend for the overall system, including the CMCens/ECens means and the operational ECMWF. These trends have left the 12Z GFS/GEFS mean on the slow side of the envelope while the new 18Z GFS has returned closer to the 06Z GFS. Based on guidance available before arrival of the 12Z ECMWF mean, preference was for an intermediate solution that already provided somewhat faster timing than previous forecast while awaiting improved clustering for possible additional faster adjustment. Forecast considerations during the first half of the period led to a starting blend emphasizing the 12Z GFS/ECMWF with minority input of the 12Z UKMET. After early day 6 Wednesday, preferences for the Aleutians/Bering Sea system led to splitting GFS input between the 12Z/06Z runs and ECMWF input between the 12Z/00Z runs while gradually increasing 00Z ECMWF mean weight (20 percent for day 7 and 40 percent by day 8). The GEFS/ECMWF means were similar over the mainland aloft late in the week, so avoiding the GEFS for depicting the Aleutians system had minimal effect on the mainland portion of the blend. ...Weather/Hazard Highlights... The system forecast to track from the southeastern Bering Sea into the southwestern mainland during the first half of the week will likely spread enhanced rain and higher elevation snow across portions of the southern mainland into the Panhandle. The best potential for highest 24-hour totals currently exists over the far southeast corner of the mainland into the Panhandle with a frontal wave lifting into the area around Tuesday. The initial storm may produce some areas of brisk to strong winds early in the week as well. A lingering upper trough over the western mainland should maintain an unsettled pattern the rest of the week, but with gradually decreasing precipitation amounts across the south. This trough will also support areas of mostly light precipitation over other portions of the mainland. One or more waves may spread some rainfall across the Aleutians and possibly into the Alaska Peninsula from Tuesday onward. Confidence in specifics is well below average at this time. Expect the forecast pattern to bring increasing coverage of below normal high temperatures with time, with coolest anomalies initially over southern areas (including the Panhandle) and then spreading northward. Morning lows will tend to be above average early in the week and then some moderately below normal readings should begin to move into some southern/western areas by mid-late week. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue, May 2. 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