Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Tue Jun 06 2023 Valid 12Z Sat Jun 10 2023 - 12Z Wed Jun 14 2023 ...Slow moving eastern Aleutians into Bering Sea deep layer low likely to maintain a wet pattern across much of Alaska into next week... ...Overview... Most guidance continues to show an upper low and associated surface system initially near the eastern Aleutians wobbling northward into the Bering Sea. A very gradual weakening trend could have the upper low open up around or just after the middle of next week and the surface reflection lose definition a little earlier. The pattern on the eastern side of the upper low should support a flow of moisture and shortwave energy into the state, leading to episodes of enhanced precipitation not only along the southern coast/Panhandle (especially with a potential Sunday-Monday system) but also farther north across the mainland (aided by a combination of shortwaves and anomalously high moisture). Over the Arctic, high pressure north of the mainland as of Saturday will likely track eastward into northern Canada by the start of next week. Then solutions become increasingly divergent for details of high latitude flow and effects at the surface for the northern mainland. ...Guidance Evaluation/Preferences... For about the first half of the period, guidance still clusters fairly well for the Aleutians/Bering Sea upper low. Individual solutions exhibit typical spread for the time frame involved, with specifics influenced by smaller-scale shortwave details that have low predictability several days out in time. After Monday there is a notable divergence due to a combination shortwave issues and differences in surrounding flow, with the past couple ECMWF runs most progressive for the upper low (bringing it into the western mainland) and latest GFS runs farthest southwest (stuck over the central Aleutians). With some latitude spread, the GEFS/ECens/CMCens means all recommend keeping the feature over the east-central Bering Sea and this intermediate scenario is preferred. Since yesterday the operational models have strayed apart somewhat for the track and timing of the Pacific wave most likely reaching the southern coast around Sunday-Monday. The past couple ECMWF runs have leaned to the faster side (with relative agreement from its mean and the CMC), while the GFS is a bit of a slow extreme (implied GEFS mean track being a compromise). The UKMET has switch from being deep/west to weak/east. In general a compromise among the 12Z models appeared best, nudging the system a little faster versus continuity as of Sunday, while the influence of the stronger GFS/ECMWF pulled the track somewhat westward of continuity into late Sunday. What happens from that point into Monday appears to depend on whether the shortwave energy remains more consolidated and curls around the upper low (as in the ECMWF) or stretches eastward as was the more common theme 24 hours ago. The resulting forecast blend splits the difference for the parent low, while leaving room for an early Monday wave nearing the Kenai Peninsula--technically a redevelopment but also providing continuity in case a better consensus returns to the eastern track. Arctic flow has become more of an issue in some guidance compared to yesterday. The 12Z GFS in particular brings lower heights and more progressive flow as far south as the northern mainland by the latter half of the period, leading to the arrival of a surface front. The new 18Z run has backed off with its amplitude aloft but still eventually has a front arriving. The GEFS mean has a hint of more progressive Arctic flow but otherwise is closer to remaining guidance. Meanwhile the 12Z ECMWF has an Arctic/Siberia trough approaching the western mainland by the end of day 8 Wednesday, and while the means do not appear too enthusiastic about that either, at least it fits closer to the overall large scale pattern. The 12Z CMC/00Z ECMWF drop off an upper low well westward. With these late-period issues having some effect on the Bering Sea upper low, the means again look best for the time being. Based on the above considerations, the first half of the period started with an even blend among the 12Z GFS/ECMWF/UKMET/CMC along with a little manual editing to improve definition. Thereafter, the various questionable attributes of the 12Z GFS, along with other detail question marks, favored phasing out the 12Z GFS along with rapidly increasing total 12Z GEFS/CMCens and 00Z ECens weight to 50-75 percent. ...Sensible Weather and Potential Hazards... Portions of the interior may see periods of heavy rainfall extend from Friday (just before the start of the medium range period) into the weekend. The combination of northward-moving shortwave energy and anomalously high moisture, with precipitable water values reaching as high as 2-3 standard deviations above normal for a time, will contribute to this activity. Areas along the southern coast should see lingering rain/isolated high elevation snow into the weekend, followed by a period of heavier precipitation likely to be most pronounced between Saturday night and Sunday night in association with the arrival of a Pacific wave. It will take more time for specifics of this wave to get resolved, but overall expect this moisture to affect areas from the eastern Alaska Peninsula/Kodiak Island eastward into the Panhandle. Once this system dissipates/passes through by early next week, precipitation over the state should trend somewhat lighter albeit within a general pattern favoring continued broad coverage. Slow movement of the Aleutians/Bering Sea upper low should support a prolonged period of unsettled weather with mostly light rainfall. Expect widespread clouds and precipitation to support below normal high temperatures across much of the state during the period, likely trending gradually cooler with time. Coolest anomalies will tend to be over southern areas. Lows should start out on Saturday mostly above normal with a few pockets of below normal, followed by a trend toward a mix of moderately above/below normal anomalies and possibly with more below normal coverage next week. Rausch Hazards: - Heavy rain across portions of interior Alaska, Fri-Sat, Jun 9-Jun 10. - Heavy rain across portions of coastal Southcentral Alaska, Sun, Jun 11. 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