Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Wed Oct 23 2019 Valid 12Z Sun Oct 27 2019 - 12Z Thu Oct 31 2019 ...Heavy coastal rain and mountain snow for portions of western and southern coastal areas next week... A very amplified pattern to start the period on Sunday may only slowly weaken by late next week. The ensembles were in good agreement overall on a trough (upper low)/ridge couple along 170W/140W that will edge eastward Sun-Tue. Next Wed/Thu, another reinforcing trough southwest of the Aleutians should again favor ridge building into the Gulf. The 12Z and 00Z ECMWF/Canadian clustered with the 00Z ensemble means to start the period with the latitudinally-stretched system astride the southwest coast of Alaska. This will set up a long fetch of moisture from the sub-tropics northward into southwestern and southern coastal areas where precipitation (coastal rain, mountain/inland snow) may be heavy. Both precipitable water anomalies and M-Climate QPF were at very high levels (some at the highest for this time of year) which suggests a much higher than average chance of an extreme precipitation event. Details will change over the next few days which will affect impacts. The atmospheric river setup will drift eastward next Tuesday as the frontal system weakens toward the Panhandle. By then, the 00Z ECMWF/Canadian best clustered with the ensembles back toward/south of the Aleutians as the 12Z GFS took a surface low toward the Gulf whereas the ensembles suggest building ridging there. Thus, trended toward a majority ensemble weighting and took the next system toward the AkPen next Wed/Thu. Temperatures will rise to well above average levels west of ~150W due to strong southerly flow with the largest anomalies perhaps along the North Slope once the warmer air mixes down to the surface. This may change initial snow ahead of the warm front to rain until cooler air comes back around the main surface low behind the cold front. Temperatures over the Panhandle will be cooler than average to start with high pressure overhead. Fracasso 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 Alaska Hazards... - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Oct 26-Oct 28. - Heavy rain across portions of mainland Alaska, Sat-Mon, Oct 26-Oct 28. - Much above normal temperatures across portions of mainland Alaska, Sun-Tue, Oct 27-Oct 29. - High significant wave heights for coastal portions of mainland Alaska and the Aleutians, Sat, Oct 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