Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EST Thu Mar 09 2023 Valid 12Z Mon Mar 13 2023 - 12Z Fri Mar 17 2023 ...Overview... Today's models and ensemble means agree in principle on deeper upper troughing becoming established over and south of the mainland over the course of next week, but with uncertainty over embedded shortwaves/lows and how these features will affect surface low centers over the northeastern Pacific/Gulf of Alaska. There is a general signal that the mean trough axis may retrograde a bit after midweek, supporting an increase of moisture reaching the southern coast and Panhandle at that time. ...Guidance Evaluation/Predictability Assessment... The past couple days of ensemble mean runs have been trending deeper with the evolving mean trough, supporting the operational model consensus that is deeper than the means. Within the operational cluster, there are still some noticeable timing/amplitude differences with the first strong bundle of energy/upper low dropping south over the western mainland early next week. Overall the GFS/GEFS mean have been fastest and most amplified while the ECMWF and especially ECMWF mean have been slowest and least amplified. The 12Z CMC/CMCens/UKMET favor an intermediate idea, with 12Z ECMWF/ECMWF mean trends nudging a little toward the guidance average versus the 00Z run. This leaves a guidance composite as a reasonable way to depict latest trends toward greater definition of this feature versus forecasts from 24 hours ago. By day 6 Wednesday this yields an upper low near Kodiak Island with surface low pressure consolidating just to the east, essentially replacing a leading system expected to track from a similar position southeastward across or near Haida Gwaii during the first half of the week. Not surprisingly, the spread is greater for the next upper low that could drop into the mean trough. The 12Z ECMWF/CMC track the feature southward over the eastern Bering Sea to a position just south of the eastern Aleutians/Alaska Peninsula by next Friday. In contrast, the 06Z GFS brought the upper low only as far as the northwestern mainland while the 12Z/18Z runs have a more progressive upper trough with the embedded low staying just north of the northern coast. Ensemble spread is currently great enough that the means do not have a defined upper low, but rather just a hint of shortwave energy that serves to reinforce the overall trough and pull the trough axis gradually westward. In addition to the increasing uncertainty for upper trough/low details by the latter half of the week, by Friday there is an added complication involving how the southern part of the mean trough may interact with a strengthening Pacific system. The combination of specifics in the 12Z ECMWF/CMC lead to a strong surface low reaching the southern coast by later Friday while latest GFS runs have ranged among a strong but farther southwest surface low in the 06Z/12Z versions and a weaker compact Gulf wave in the new 18Z cycle. The 12Z GEFS/CMC means and 00Z ECMWF mean were remarkably agreeable with a surface low position south-southeast of Kodiak Island as of 12Z Friday, even with some differences in their upper trough details. The new 12Z ECMWF mean is briefly a little east of that but similar to the prior run by late Friday. After midweek, prefer to capture some aspects of operational guidance but with enough ensemble mean input to reflect only the most agreeable themes versus some of the lower predictability specifics of the operational model runs. Thus the total GEFS/ECens/CMCens weight reached 60 percent by day 8 Friday. ...Weather/Hazard Highlights... The evolving upper trough and one or more northeastern Pacific/Gulf of Alaska surface low centers should support a gradual increase of precipitation along the southern coast and Panhandle over the course of next week. Expect highest totals to be in the Thursday-Friday time frame but it will take additional time to resolve specifics to a greater degree. Associated low pressure by Friday could also be on the strong side but again confidence is low, so monitor forecasts in the coming days. Regardless of exact details of surface lows next week, a fairly strong gradient will exist between the Gulf and high pressure along/north of the northern coast. This may lead to periods of strong offshore gap winds especially across the Panhandle. Elsewhere, the system dropping south over the far western mainland may produce an a period of snow early in the week. Meanwhile 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. Eastern parts of the mainland could see scattered light snow on Monday with a weakening front. The forecast pattern will likely lead to a broad area of below normal temperatures, especially for daytime highs during the early-mid week period. Anomalies for lows should be less extreme and may stay above normal over parts of the northwestern mainland. The slight retrogression of the mean trough aloft may lead to a moderation of temperatures over Southcentral and the Panhandle later in the week. Rausch Hazards: - Much below normal temperatures across portions of interior mainland Alaska, Tue-Wed, Mar 14-Mar 15. - Much below normal temperatures across the Alaska Panhandle, Sun-Tue, Mar 12-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