Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 645 PM EST Sun Jan 17 2021 Valid 12Z Thu Jan 21 2021 - 12Z Mon Jan 25 2021 ...Overview and Guidance/Uncertainty Assessment... Guidance still appears to be in a state of flux regarding various aspects of the forecast. In terms of the large scale flow the most pronounced change over the past day is a trend toward weaker/less amplified mean ridging that builds in behind a shortwave that passes through the longer-term mean ridge around Fri-Sat. This trend results in significantly more progression across the Bering Sea and Aleutians by next Sun-Mon versus what most models and means were showing yesterday. Earlier in the forecast period, models are becoming better defined and with a farther north track for a low pressure center now expected to track from the North Pacific through the southeastern Bering Sea and into the western mainland from Thu into early Sat. Due to dataflow problems preventing use of the 12Z ECMWF, today's forecast used input from the 00Z ECMWF along with the 12Z GFS/CMC/UKMET and 12Z GEFS/00Z ECMWF means. For the concentrated low pressure initially moving northward over the Bering Sea there is still some divergence for track. Latest GFS runs have pulled back toward the means while other models to varying degrees are still farther east. The UKMET was a fast extreme. Meanwhile the operational models are trending steadily better defined and clustered for the North Pacific into western mainland system. The 00Z ECMWF was the fast extreme with this feature. Based on guidance comparisons for these two features the first half of the period emphasized the 12Z GFS/CMC somewhat more than the 00Z ECMWF/12Z UKMET. The 12Z ECMWF changed only modestly for the Bering low but trended favorably slower for the second system. There is better continuity for a wave evolving near the southern coast by early Sat but consensus has slowed its southeastward departure by perhaps half a day. Agreement/continuity are fairly good in principle for low pressure emerging into the western Bering Sea during the weekend. The average of guidance seems to be taking slightly longer to develop it though (12Z ECMWF slower than model/mean consensus). The less amplified trend for flow aloft now allows the surface low and its associated front to progress eastward across the Bering and Aleutians from the weekend into the start of next week. There are signals that a frontal wave could reach or develop near the eastern Aleutians/Alaska Peninsula by next Mon but of course this has very low confidence in light of recent variability with the larger scale flow. The latter half of the forecast used increasing weight of the ensemble means, reaching 60-70 percent total by day 8 Mon. ...Weather/Hazards Highlights... After midweek the combination of flow ahead of the initial system moving northward over the Bering Sea and then the North Pacific system currently expected to track northeastward into the western mainland Thu-Sat will likely spread a broad area of precipitation from the Alaska Peninsula through the Kenai Peninsula and northward across the western mainland. Locally moderate to heavy amounts will be possible over favored terrain. A modest wave forming near the southern coast and then tracking southeast should also focus precipitation (some at least moderate) over the southeastern coast and Panhandle, most likely around Fri-Sat. Expect lighter and more scattered precipitation to progress across much of the mainland during the weekend as shortwave energy aloft passes through. The system forecast to reach the western Bering Sea by this weekend should bring an increase of precipitation and winds to the western Aleutians and then continued eastward progression across the Bering Sea/Aleutians into the start of next week. The leading edge of this moisture may reach the western mainland by next Mon but with low confidence. Late this week anticipate above to much above normal temperatures over most of the state, with the most extreme anomalies likely to be over the western half of the mainland as southerly or southeasterly low level flow prevails. Passage of upper dynamics during the weekend should promote a gradual cooling trend, with Southcentral possibly seeing some areas of below normal readings from the weekend into early next week and portions of the Panhandle trending below normal as well. The northwestern half of the mainland should remain above normal but with less extreme anomalies. Rausch 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