Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 559 PM EST Thu Dec 17 2020 Valid 12Z Mon Dec 21 2020 - 12Z Fri Dec 25 2020 ...Guidance and Uncertainty Assessment... Upper troughing across the North Slope into the Bering Sea will consolidate near the western Aleutians and ease southeastward by late next week. To the east, upper ridging will build from the Gulf Monday into eastern areas/Panhandle and into northwestern Canada in response to the troughing upstream. The models and ensembles generally show this progression but vary on the details for much of the period. Lead system will meander its parent low near the western Aleutians as the occlusion pushes into the AKPen. 12Z guidance was close enough (minus the Canadian) such that a blended solution sufficed as they each differed on timing/track of the parent low and other areas of low pressure around it, perhaps along the front itself. Without any clear trends, could not weight one solution much more than the other early in the period. That system will move into the Gulf and perhaps dissipate as it encounters upper ridging, opening the door for another system from the central North Pacific. That system should lift northeastward toward the southern Gulf per the ensemble consensus. No use in trying to favor one model over the other (GFS was quicker, ECMWF slower and closer to the ensemble means) given the pattern and lead time. ...Weather/Hazards Highlights... Areas along the southern coast and to the Panhandle could have moderate to perhaps locally heavy precipitation early in the week as the frontal system (and developing triple point low) move across the Gulf. Several inches of rain/melted snow are likely over the multi-day period, focused over the southeastern Kenai peninsula. Lighter showers are likely over the Aleutians and southwestern areas of the mainland under the upper low. Temperatures are forecast to be much below normal over the central/eastern Interior will trend much milder (about 10-20 degrees above normal) for the middle and end of the week as southerly flow overspreads the area. Fracasso Hazards: - Heavy precipitation across the Alaska Peninsula into southern Alaska, Mon-Wed, Dec 21-Dec 23. - Heavy precipitation across portions of the Alaska Panhandle into southern Alaska, Wed, Dec 23. - High winds across the Alaska Peninsula into southern Alaska, Mon-Tue, Dec 21-Dec 22. - Much below normal temperatures across much of the lower elevations of eastern Alaska, Sun-Tue, Dec 20-Dec 22. 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 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