Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Mon Mar 13 2023 Valid 12Z Fri Mar 17 2023 - 12Z Tue Mar 21 2023 ...Overview... Most guidance has been fairly consistent with the large scale pattern over the past day. The majority cluster shows an upper low persisting south of the Alaska Peninsula/Kodiak Island late this week through the weekend and then departing to the southeast early next week. Even though models/ensembles are similar with the overall upper low, surrounding small-scale shortwaves continue to make it difficult to resolve Northeast Pacific/Gulf of Alaska surface low details that will influence precipitation emphasis from the Panhandle to potentially as far west as the Alaska Peninsula. Meanwhile an upper trough or col region over the mainland should give way to ridging by Sunday with this ridge hanging on over portions of the mainland and northwestern Canada into Tuesday. This ridge and departure of the upper low will lead to drier conditions along the southern coast early next week. More progressive flow over the Aleutians/Bering Sea will likely bring a weakening front across the region during the weekend, followed by a strong western-northern Bering Sea storm whose associated front and leading flow may bring increasing precipitation and winds across the Aleutians and eventually into the far western mainland. The forecast pattern evolution will generally support a warmer trend for temperatures over most of the state. ...Guidance Evaluation/Predictability Assessment... In a continuing story from recent days, there is decent consensus for the general upper low to the south of the Alaska Peninsula/Kodiak Island but a seemingly random assortment of surface lows that could either track northward into the southern coast or wrap back into the upper low. Thus far there have not been enough ensemble members to agree on a specific wave tracking far enough north to reach the coast for the corresponding means to show a wave, instead favoring a track back to the west toward or south of Kodiak Island. As for specifics within this theme, 06Z/12Z GFS runs and slightly weaker/north 12Z CMC are strongest with a leading low while the 12Z ECMWF/UKMET/CMC more strongly develop a trailing system that could reach close to Kodiak Island by day 6 Sunday (supported by the new 12Z ECMWF mean as well, a little south of the operational run's track). The new 18Z GFS appears to be trying to trend a little toward the other guidance, weakening the leading wave some and developing a more defined low just south of Kodiak Island by Sunday. Starting with a blend of 70 percent total 12Z GFS/ECMWF/UKMET/CMC and 30 percent total 12Z GEFS/CMCens plus 00Z ECens for the first half of the period provided a surface evolution remarkably close to continuity for days 4-5 Friday-Saturday, while adjusting the low south of Kodiak Island a little northward by early day 6 Sunday per the latest guidance signals. Also of note by Sunday and continuing the rest of the period, the model/mean average has trended stronger with the upper ridge that builds over the mainland and northwestern Canada. In the past couple cycles there has been a stray model run (12Z CMC, 00Z ECMWF) trying to open up the northern side of the low and possibly have it interact with a progressive shortwave approaching from the Bering Sea by Monday. Thus far the means have not supported that scenario, so the forecast phased out 12Z CMC input after Sunday. Guidance clustering for the western-northern Bering Sea storm and leading front has improved dramatically over the past 24-36 hours. Current consensus timing is somewhat in the faster half of the prior spread, while a significant deeper trend in the ensemble means is suggesting decreased spread among the individual members. The 00Z ECMWF had strayed somewhat on the fast side by next Tuesday but the 12Z run is now close to the 12Z/18Z GFS runs and the ensemble means. For days 7-8 Monday-Tuesday, a nearly even weight of the ensemble means and 12Z GFS/ECMWF provided a good balance of retaining some detail for the Bering Sea storm while resolving differences in strength/position of the upper ridge over the mainland and northwestern Canada. ...Weather/Hazard Highlights... There is still considerable uncertainty with the specifics of the precipitation forecast along the Panhandle and southern coast from late this week through the weekend, given model spread and variability for Pacific/Gulf surface systems. At least the latest guidance signals generally agree on heaviest emphasis over the Panhandle leading into the start of the forecast period late this week, followed by a general westward shift along the southern coast and possibly into the Alaska Peninsula and Kodiak Island. One or more surface lows could also produce brisk to strong winds along and offshore coastal areas, but again with low confidence in specifics. Southeastward departure of the upper low initially south of the Alaska Peninsula/Kodiak Island along with an upper ridge building over the mainland should support a drier trend early next week. The storm forecast to track across the western-northern Bering Sea early next week should push a leading front across the Aleutians/Bering Sea, with associated enhanced winds and moisture possibly extending into the far western mainland by next Tuesday. The forecast pattern evolution will support a warming trend over the course of the period. Temperatures should still be well below normal over interior through southwestern mainland areas on Friday. On the other hand the North Slope should already be above normal at that time and at least min temperatures may be above normal over the Panhandle and southeast corner of the mainland. Expect most areas to be above normal by early next week, aside from the southern Panhandle possibly staying below normal for highs. The North Slope will likely see the warmest temperature anomalies. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Mar 16-Mar 17. - Much below normal temperatures across portions of mainland Alaska, Thu-Fri, Mar 16-Mar 17. 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