Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EST Wed Dec 15 2021 Valid 12Z Sun Dec 19 2021 - 12Z Thu Dec 23 2021 ...Likely precipitation with significant amounts and strong southwesterly winds over the western mainland on Sunday and Monday... ...Overview... A significant pattern change is still expected over Alaska early next week. After periods of shortwave troughing moving across the state during the short range period, deep layer ridging from the surface to upper levels will set up across the Pacific, with its northern periphery spreading northeastward and pressing across the mainland Sunday-Monday. This pattern will lead to strong, moist, and unseasonably warm southwesterly flow over much of the state for the early part of next week, causing the potential for gusty winds and heavy precipitation along with much milder temperatures. A quick shortwave trough could disrupt the pattern on Tuesday before another bout of ridging sets up for midweek. ...Guidance Evaluation and Preferences... Guidance continues to have a good signal for the aforementioned Pacific ridging coming into the state and directing southwesterly flow initially to the Aleutians/western mainland and spreading eastward. However, there are some model differences especially west of the state with the details of low pressure systems. Latest 12Z guidance generally advertises two lows tracking northward across eastern Siberia and into the Chukchi Sea early next week, one behind the other taking generally the same track, and consolidating later on Monday. There remain some slight timing and position differences, but all guidance has the same general idea, so went with a multi-model deterministic blend (favoring the 06Z/12Z GFS and 12Z ECMWF) for the early part of the medium range period. NAEFS and ECMWF Ensemble Situational Awareness Tables agree that water vapor transport values will be nearing their climatological maxima for parts of the western/central/northern mainland on Sunday and Monday. By Tuesday and Wednesday, an upper low tracking to the north of Alaska is forecast to send energy southward and create a shortwave trough passing through the state cutting through the ridge axis (with additional ridging behind it moving into western Alaska early Wednesday). Model guidance remains somewhat inconsistent with the position and especially with the strength of this shortwave. The 12Z CMC did not line up as well with its shortwave axis, with the GFS runs and ECMWF showing somewhat better agreement--though GFS runs are notably stronger with closing off a mid-upper low offshore of the Panhandle by early Wednesday compared to the open ECMWF, along with some differences in timing of strengthening of a surface low and with its position. Ensemble mean guidance is of course open with its troughing but likely due to model differences averaging out rather than a true signal. The WPC model blend took a middle ground approach, transitioning the CMC out of the blend but keeping both the GFS and ECMWF as components, and ending up with percentages of 30 GFS/30 EC/20 GEFS mean/20 EC mean in the blend by day 8. Overall model guidance is reasonably well clustered for a high latitude medium range forecast. ...Weather/Hazard Highlights... Precipitation is likely for western and central parts of the mainland on Sunday and Monday given the increasing deep layer moist southwesterly flow into the state to the west of the Pacific ridge. From the Brooks Range southward into parts of western and central Alaska, snow may be heavy over terrain favored by strong southwesterly winds on Sunday and Monday. Brisk low level winds are expected, which along the coast may even lead to areas of coastal flooding. Areas with a combination of snow and strong winds could see blizzard conditions. Precipitation and winds should trend lighter on Tuesday after the cold front passes, though some could linger for the eastern part of the state including the Panhandle. Another round of precipitation may affect the Aleutians and southwestern Alaska around next Wednesday-Thursday, but those details will have to emerge in future forecast cycles. With the relatively warm inflow for Alaska, the western half of the mainland will see above normal temperatures Sunday, increasing and spreading eastward on Monday and Tuesday. Temperatures could be 20 to as much as 40 degrees above average, with some temperatures rising above the freezing mark especially in the southwest part of the state. The Panhandle will be the main area with below normal temperatures through the first half of next week, while parts of the southeastern mainland could be around normal. By Wednesday temperatures should moderate closer to normal for much of the state after a cold frontal passage. 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 snow across portions of mainland Alaska, Sun, Dec 19. - Heavy snow across portions of mainland Alaska, Sun-Mon, Dec 19-Dec 20. - Heavy snow across portions of mainland Alaska, Mon, Dec 20. - High winds across portions of mainland Alaska and the Aleutians, Sat-Sun, Dec 18-Dec 19. - High winds across portions of mainland Alaska, Sat-Mon, Dec 18-Dec 20. - High winds across portions of mainland Alaska, Tue-Wed, Dec 21-Dec 22. 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