Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 740 PM EDT Tue May 17 2022 Valid 12Z Sat May 21 2022 - 12Z Wed May 25 2022 ...Overview and Guidance/Predictability Assessment... Most guidance maintains a fair degree of blockiness in the large scale pattern, with only slow evolution over the course of the period. The current majority cluster has the upper high over the mainland as of early Saturday retrograding and opening up as it reaches the Bering Sea. This opening ridge may merge with another ridge building in from the west to yield a west-east oriented upper ridge over the Bering Sea by Monday-Wednesday. As a result, an upper low and surface low pressure may meander to the south of the eastern Aleutians for a majority of the period. A leading front may sweep through the northeastern Pacific around Sunday though. As the initial mainland ridge gives way, some degree of cyclonic flow aloft may develop over the eastern mainland for a time. Then the details of flow aloft become increasingly uncertain around the top of the Bering Sea ridge and through the mainland. Arctic high pressure should provide a southward push to a cold front over the Interior through the weekend. Weakening/progression of the high should eventually allow the front to retreat northward somewhat by midweek. A 12Z operational model blend provided a good starting point for the first half of the period, Saturday into Monday. The primary forecast consideration in this time frame was that the GFS strayed to the north of other models and ensemble means with the North Pacific system by Monday--due to differences in how it handled upper ridging over the mainland and Bering Sea. Thus the GFS weight in the blend was sufficiently low for it not to have undue influence on the forecast. The operational model blend quickly transitioned to 40-55 percent total ensemble mean input (12Z GEFS/00Z ECens) by days 7-8 Tuesday-Wednesday as operational models strayed apart for low-predictability details from the mainland westward/northwestward. For example, the last two ECMWF runs became nearly 180 degrees out of phase by day 8 while the 12Z CMC and 12Z/18Z GFS offered opposing ideas over the mainland as well. At that time the favored blend provided flat ill-defined flow while awaiting better detail agreement. ...Weather and Hazard Highlights... Rain associated with the North Pacific system will likely affect the central/eastern Aleutians and possibly parts of the Alaska Peninsula during the weekend. Brisk easterly flow could produce some localized enhancement over favored terrain. Expect a drier trend over this region next week, though a minority scenario would have greater persistence/extent of rainfall. Some precipitation is also likely for at least the southern Panhandle from around late weekend onward with moisture along a front anchored by the North Pacific system. Exactly how far north this moisture may extend is uncertain at this time. Meanwhile scattered rainfall may develop over southern areas next week, depending on uncertain specifics of flow aloft. River flooding will continue to be an issue due to snow melt and ice jams, notably over the Tanana Basin. Below normal temperatures should prevail over the North Slope for most of the period. Farther south, decreasing heights aloft and the cold front pushing southward over the Interior should yield a cooler trend for highs from above to near/below normal levels. Southwestern areas may stay above normal though. The Panhandle should see a mix of above/below normal highs on Saturday followed by below normal readings. Expect somewhat greater coverage of above normal lows but some areas of negative anomalies should still develop over parts of the northern two-thirds of the mainland and Panhandle. Rausch Hazards: - Flooding likely across portions of mainland Alaska, Fri-Sat, May 20-May 21. 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