Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 728 PM EDT Mon Sep 19 2022 Valid 12Z Fri Sep 23 2022 - 12Z Tue Sep 27 2022 ...Periodic heavy precipitation is likely late week into early next week across the southeastern mainland and Panhandle... ...Overview... A persistent upper low is forecast to meander over the northern Bering Sea and the western Alaska mainland late this week into early next week, allowing for low pressure at the surface in those areas and toward the Gulf of Alaska. Deep southwesterly flow will bring Pacific moisture to southeastern Alaska as frontal systems serve to focus the moisture, leading to potential for heavy precipitation (lower elevation rain and higher elevation snow) across southeastern portions of the mainland through early next week. Lighter precipitation is possible in the western mainland, while an upper shortwave and low pressure system may enhance precipitation a bit across the Aleutians early next week. ...Guidance/Predictability Assessment... Model guidance seems to be in better agreement than typical with the overall pattern during the Alaska medium range period. Models show the upper low centered over the northern part of the Bering Sea on day 4/Friday, with energy elongating north-south somewhat over the weekend. At the surface, a strong low pressure system (but not nearly as deep as Merbok's remnants) should be stacked with the upper low Fri and should gradually weaken as it descends toward/along the western coast of the mainland over the weekend. Surface lows are forecast over the Gulf as well, anchoring frontal systems, with fairly minor model spread. For the early part of the medium range period, the WPC forecast used a multi-model deterministic blend of the 12Z guidance led by the GFS and ECMWF, incorporating ensemble means a bit by day 6/Sun. By early next week, models show a potent shortwave and surface low tracking across the Kamchatka Peninsula Sunday and into the Bering Sea while the initial upper low's troughing shifts eastward a bit. Current model guidance has pretty good consensus for these features for a day 6-8 forecast, though there are some differences in the timing of the surface low potentially crossing over the Aleutians into the Gulf around Tuesday. Thus the WPC model blend kept over half deterministic models through day 8 for added definition, but with some EC/GEFS ensemble incorporation to reduce individual model intricacies. The main change from continuity was a stronger second surface system given the better agreement in its existence and timing/track. Despite fairly good model agreement currently, the details are still subject to change given the inherent uncertainties when dealing with Arctic energy. ...Weather/Hazards Highlights... The first low pressure system could spread some light precipitation to the western mainland and Alaska Peninsula through the latter part of the week. Meanwhile this system looks to form a triple point low in the Gulf Friday and start off the pattern of frontal systems moving across the Gulf through early next week. As ample moisture streams into the southeastern part of the state, moderate to heavy precipitation amounts are likely generally from the eastern side of the Kenai Peninsula across the southeastern mainland and through the Panhandle Friday into early next week in multiple rounds with multiple fronts. Rain is expected for lower elevations while higher elevations could see heavy snow. While there are some model discrepancies for the details of exactly where precipitation will be the heaviest, the pattern is favorable for heavy amounts. Then as another low pressure system moves near and possibly across the Aleutians early next week, this could provide some focus for precipitation there, but with lower confidence in the details at this time. The forecast pattern with the upper low aloft will support below average high temperatures across the southern mainland and Panhandle through the period, while central/northern parts of the mainland should see at least a mix of above/below normal highs if not more above normal readings. Overnight/morning lows should be mostly above normal however, with the highest anomalies tending to be over the North Slope, but a few pockets of slightly below normal lows are possible over far southern Alaska and the Panhandle. Tate 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 Hazards: - Heavy precipitation across portions of southeast mainland Alaska and the Alaska Panhandle, Thu-Mon, Sep 22-Sep 26. 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