Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EST Sat Jan 15 2022 Valid 12Z Wed Jan 19 2022 - 12Z Sun Jan 23 2022 ...Heavy precipitation threat along the southern coast and Panhandle late week into the weekend... ...Overview... Latest guidance still shows a mean ridge aloft over the far eastern Pacific extending its influence farther north/northwest into Mainland Alaska and northwestern Canada by late week into the weekend, leading to period of warmer and wetter weather. Consensus shows the best potential for heavy precipitation will be along parts of the southern coast and Panhandle from Friday into the weekend. Meanwhile the North Pacific will be active during the period. One wave is likely to track into the Gulf of Alaska by Thursday followed by another system that may affect portions of the Aleutians and Alaska Peninsula/far southwest mainland and then yet another storm that comes into the picture south of the Aleutians next weekend. There is still some uncertainty over what becomes of an elongated upper trough over the northwestern mainland as of early Wednesday but there are some hints at gradual convergence among latest solutions. ...Guidance/Uncertainty Assessment... For the evolution over the mainland early in the period, it is still a story of the operational models versus the ensemble means. Models continue to form a compact closed low from the energy within the early Wednesday elongated upper trough with slightly delayed influence from the upper ridge to the southeast while the means still depict a mere weak trough with stronger ridging to the southeast. Today's 12Z cycle provided a notable westward adjustment in the ECMWF that had been on the southeast side of the spread with its upper low (keeping it over the mainland for a longer time). By Thursday-Friday the latest consensus has yielded a moderate southwestward adjustment from yesterday's preference for the upper low position but maintains the theme of having the upper low retrograde into the Bering Sea by late in the week/weekend. During the latter half of the period some of the model/mean differences persist with the 12Z GFS/ECMWF keeping the upper low east of the means. The new 18Z GFS has trended westward though. Forecast preferences still reflect the better detail of the models early in the period followed by a transition to a model/mean compromise. Various detail differences persist for North Pacific systems. The model blend for the leading wave expected to reach the vicinity of the Gulf of Alaska by Thursday provides good continuity in principle (with typically improving definition getting closer to the valid time) and fits within the pattern of the means. There are small-scale details that lead to relatively lower predictability so some changes in specifics may be possible for a while. Differences with the stronger system farther west derive in part from the issues noted between the models and means farther north. In general the models suggest a triple point wave may track toward the Alaska Peninsula by Friday-Saturday while the parent low remains farther southwest, in contrast to the means that show the parent low reaching farther north near the eastern Aleutians. The time frame of interest is during the transition between model emphasis and model-mean compromise, with the end result somewhat closer to the model cluster. The next system comes into the picture next weekend. Thus far some GFS runs have been on the deep and northwestern side of the spread while it remains to be seen if it merges into the overall mean low pressure the way CMCens/ECens means depict. A model/mean blend provides a reasonable starting point given the various uncertainties. Guidance preferences led to starting the forecast with the 12Z GFS/ECWMF/UKMET/CMC from Wednesday into early Thursday, followed by gradually increasing 12Z GEFS/00Z ECens inclusion so that the means had 50-60 percent weight by days 7-8 Saturday-Sunday. ...Weather/Hazard Highlights... The leading Pacific system tracking into the Gulf of Alaska by Thursday should produce an initial episode of precipitation, mainly from the eastern Kenai Peninsula through the Panhandle. Some activity could be locally moderate to heavy. Then guidance shows a persistent and perhaps increasing signal for heavier precipitation from the southern coast into Panhandle from Friday into the weekend as strengthening southerly winds over the northeastern Pacific ahead of trailing North Pacific low pressure bring in more moisture. Most solutions suggest the highest precipitable water values will be around Friday with latest GFS/ECMWF runs reaching at least three standard deviations above normal over some locations and the GEFS/ECens somewhat lower as is typically the case. To a lesser extreme the moisture anomalies generally remain elevated through the weekend. Mean flow between the upper low expected to be over the Bering Sea and the ridge to the east/southeast may also bring some moisture farther north into the mainland. Early in the period the combination of the upper trough/low over the northwestern mainland and a front to the southeast may produce some light snow over the Interior. The Aleutians may see periods of unsettled weather though with uncertainty in effects from individual systems. During mid-late week the North Slope and parts of the far western mainland will tend to see below normal temperatures while above normal readings should be more prevalent elsewhere. Temperatures may rise slightly during this time frame. As the upper ridge becomes more established over the mainland and western Canada, expect a more pronounced warming trend next weekend with increasing coverage and magnitude of positive anomalies. Warmest readings versus normal should be over Southcentral. In most cases the anomalies for min temperatures will be warmer than those for highs. Rausch 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 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