Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 647 PM EST Fri Mar 10 2023 Valid 12Z Tue Mar 14 2023 - 12Z Sat Mar 18 2023 ...Overview... Guidance continues to show a mean trough aloft over and south of the mainland during the days 4-8 (Tuesday-Saturday) period, with the majority highlighting an upper low that is likely to settle into a position near the Alaska Peninsula/Kodiak Island from Wednesday onward. The mainland part of the trough axis may retrograde a little mid-late week and then stabilize. There is still a good signal for a wetter/snowier period along the southern coast and Panhandle due to the expected pattern but detail uncertainties for northeastern Pacific surface lows temper confidence in specifics. ...Guidance Evaluation/Predictability Assessment... Early in the period (Tuesday into Wednesday), an operational model average provides good continuity for the upper low expected to drop from the southwestern mainland to just south of the Alaska Peninsula/Kodiak Island. Continued deeper trends in the ensemble means over the past day have finally brought their forecasts into closer agreement with the operational runs. Among the latter, the GFS/GEFS remain a bit on the faster/southern side of the guidance spread but overall the current solutions are well within typical guidance error ranges at the days 4-5 time frame. Perhaps due in part to the GFS handling of this feature, the model has also been showing more of a tendency to curl a system initially southwest of Haida Gwaii northwestward to a greater degree than most other guidance. Regardless of the details, there is decent agreement for low pressure to be just east of Kodiak Island by early Wednesday. After midweek there is still a lot of spread and run-to-run variability for how upstream energy may reinforce the mean trough/upper low, as well as for surface low details over the northeastern Pacific. In varying ways, latest GFS/CMC runs are suggesting that some of the upstream energy could just be part of a separate trough that may be anchored by an Arctic upper low with run-to-run variability over how much energy on the western fringe may amplify farther south. The ECMWF is consistent in showing greater digging of Arctic energy that arrives by Thursday, though it has been trending somewhat slower and eastward with this energy. Individual ensemble member spread continues to yield a very weak signal in the means, just a hint at some possible reinforcement of the trough/upper low late in the week--but persistence of the upper low near the Alaska Peninsula/Kodiak Island. Meanwhile multiple ECMWF/CMC runs have been showing a potentially quite strong low-mid latitude system lifting northward into the northeastern Pacific/Gulf of Alaska. Individual model runs have been waffling on whether this low reaches the southern coast or curls back toward Kodiak Island. ECens/CMCens means hint at this evolution in weaker form with a track that stays south of the coast and east of Kodiak Island. On the other hand, GFS runs have been either much more disjointed with this development (pushing one or more weaker waves farther east) or more recently showing a somewhat better defined system but still with a farther east track. The new 18Z GFS is still well east into early Friday but then shows redevelopment northwest over the Gulf. A multi-model/multi-run average in combination with the ensemble means (with less emphasis on the weaker GEFS mean due to being weakest with surface low pressure by Friday-Saturday) helps to provide good continuity while awaiting any guidance shifts/consolidation that could provide confidence adjustments. Regardless of the details, it seems reasonable to expect low pressure to gravitate toward the consensus upper low in some fashion. The above considerations led to starting with the 12Z ECMWF/GFS/CMC/UKMET (in order of more to less weight) from early day 4 Tuesday into early day 5 Wednesday. After that time, the forecast phased out what little of the UKMET was initially incorporated due to poor comparisons for Arctic flow. In addition the mid-late period portion of the blend split the GFS among 06Z/12Z runs and ECMWF among 00Z/12Z runs (while keeping a little of the 12Z CMC) to cancel out lower confidence details on the operational model side, while increasing the total weight of 12Z GEFS/CMCens and 00Z ECens means to 40-50 percent by days 7-8. ...Weather/Hazard Highlights... The evolving upper trough/low and one or more northeastern Pacific/Gulf of Alaska surface low centers should support increasing precipitation along the southern coast and Panhandle over the course of next week. Expect some rain over southern Panhandle/coastal areas and snow over inland/higher elevation areas. The best guidance signal continues to point toward highest totals during the Thursday-Friday time frame and from the eastern Kenai Peninsula through the Panhandle, but with enough embedded uncertainty to keep confidence in specifics moderate at best. In addition there is a persistent potential for low pressure reaching the northeastern Pacific/Gulf of Alaska by Friday to be on the strong side but again without great confidence yet. Regardless of exact details of surface lows next week, a sufficiently strong gradient may lead to periods of strong offshore gap winds especially across the Panhandle. Elsewhere, the overall pattern will likely lead to multiple days of brisk to strong northwesterly winds over the Bering Sea and Aleutians, with some variability depending on individual systems diving down the west side of the upper trough, along with periods of precipitation. A little moisture may extend over parts of the mainland late week into the weekend depending on exactly how the upper trough evolves. The expected pattern will support below normal temperatures over many areas through midweek, with coldest anomalies likely over parts of the interior and the Panhandle. Anomalies for lows should be less extreme and may be above normal over parts of the northwestern mainland. Temperatures will likely moderate after midweek, with North Slope and Southcentral/Panhandle areas potentially seeing above normal lows and by the weekend possibly even a few pockets of slightly above normal highs. Rausch Hazards: - Much below normal temperatures across portions of interior mainland Alaska, Tue-Wed, Mar 14-Mar 15. - Much below normal temperatures across portions of the Alaska Panhandle, Mon-Tue, Mar 13-Mar 14. 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