Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Tue May 02 2023 Valid 12Z Sat May 06 2023 - 12Z Wed May 10 2023 ...Overview... As of the start of the forecast early Saturday, guidance consensus shows a weakness/trough aloft over the far western mainland and an upper low over the northeastern Pacific/southern Gulf of Alaska, while ridging prevails over Canada and from the Aleutians into the Bering Sea. Going forward in time the most likely evolution would have the mainland weakness broaden for a time but weaken as the Canada ridge eventually extends into portions of the mainland, while troughing/low pressure crossing the Bering Sea ultimately connects with the upper low persisting over the northeastern Pacific to yield a broad elongated axis of relatively lower heights aloft/surface low pressure by the middle of next week. Meanwhile surface high pressure should begin to dominate north of the mainland early-mid week in the wake of a developing Arctic system to the northeast of the mainland. However guidance has been divergent and erratic for various details, so confidence in specifics is somewhat below average. Today's forecast challenge was to represent the most coherent and intermediate evolution taking into account latest guidance trends and the wide range of solutions for specifics over some areas. ...Guidance Evaluation/Predictability Assessment... Overall the latest average of model and ensemble guidance holds onto the deep layer low over the northeastern Pacific/Gulf longer than in forecasts from yesterday that suggested a farther southeast track. There are still a lot of uncertain detail uncertainties regarding possible interaction among the initial low and energy that could drop into the area from the western mainland trough. Meanwhile there had been a pronounced contrast between the GFS/GEFS and other guidance for a leading system nearing the western Aleutians as of early Saturday. The 06Z/12Z GFS/GEFS runs and the new 18Z GFS have adjusted more to the ECMWF cluster that has been showing dissipation of the parent low/front and then a suppressed track for leading waviness (due to the strong upper ridge to the north). In fact, most of the 12Z guidance has doubled down on this suppression with a track even south of the 00Z ECMWF mean/12Z CMC and the new 12Z ECMWF mean has indeed trended south with this wave as well. Then there is a gradually improving signal for a fairly strong system to track into the Bering Sea by early next week. However by Tuesday onward the 12Z GFS strayed north of other solutions for the low track, leaving the 06Z GFS as the better option for including as part of the forecast blend. The new 18Z GFS has returned its track to a latitude more similar to the ensemble means. There is still considerable timing spread, with some potential influence of upstream energy that could either reinforce the Bering Sea system in place for a time or else draw the best defined surface low pressure farther eastward. The elongation of most ensemble means by day 8 Wednesday represents this uncertainty. Detail questions exist over the mainland and Arctic as well. From late Sunday onward the 12Z CMC strays increasingly westward of other guidance for Arctic low pressure that should track northeastward away from the mainland. Then with different timing early in the week both the 12Z CMC and ECMWF bring some northern stream shortwave energy into the mainland, with the ECMWF solution quite different from its stronger ridging in the previous run. The new 18Z GFS deposits some energy over the northern tier by midweek as well. An ensemble mean/multi-model approach seems best for the time being until better detail clustering develops. Based on the various areas of focus, today's forecast started the first half of the period with 70 percent operational models (12Z-00Z ECMWF, 06Z GFS, 12Z UKMET) and 30 percent means (12Z GEFS/CMCens and 00Z ECens) to capture the most common themes in latest guidance while downplaying any stray solution for a particular feature/region. Then the blend shifted to a nearly even weight of the two ECMWF runs/06Z GFS and the three ensemble means for the rest of the period. ...Weather/Hazard Highlights... The flow around a low in the northeastern Pacific/southern Gulf of Alaska may bring some precipitation to areas along the southern coast and Panhandle through Saturday. A drier trend should take place at least along the southern coast by Sunday, with specifics for the Panhandle depending on exact track of the low. Farther north, northward-lifting shortwave energy may produce areas of precipitation over parts of the southern/eastern mainland during the weekend. The far northeastern mainland may see some moisture focus behind a developing system tracking away from the area. Around early next week a potentially strong storm may spread a broad area of precipitation and brisk to strong winds across the Bering Sea and Aleutians. Guidance is still in the process of latching onto the evolution of this system, tempering confidence in specifics at this time. The system should weaken toward midweek with the pattern evolving toward an elongated axis of low heights aloft/surface low pressure from the Bering Sea into the northeastern Pacific. Toward the middle of next week it will be worth monitoring how much easterly gradient sets up over the Alaska Peninsula/Kodiak Island/Kenai Peninsula, as the strongest side of the envelope would bring a decent moisture focus to this area. For now the most likely scenario would be a weaker gradient or positioning of embedded surface lows that would not lead to such focus. The state should see a mix of above or below normal temperature anomalies for daytime highs, perhaps leaning toward slightly greater coverage of below normal readings. On the other hand above normal morning lows should be more common, with best potential for pockets of below normal lows to be over parts of the west and interior. Rausch No significant hazards are expected over Alaska during this forecast period. 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