Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Sun Jul 10 2022 Valid 12Z Thu Jul 14 2022 - 12Z Mon Jul 18 2022 ...Heavy Precipitation Threat at times for parts of the Alaskan Southern Tier... ...Overview... Within initially broad cyclonic flow covering Mainland Alaska back through Siberia, latest trends suggest that a greater percentage of emerging Siberia energy could drop southward through the Bering Sea and help to strengthen a system tracking into the Aleutians late this week. Through the weekend and the start of next week an upper low initially near the North Pole should descend to a position north of Siberia, with troughing to its south. The evolution of the upper pattern will most likely draw the Aleutians system northeastward through the Bering Sea and at least to the western coast of the mainland before merging with a front at the leading edge of the Arctic mean trough. A triple point low may form near the southern coast by early next week. Lingering moisture across the southern mainland and initial weak energy aloft, followed by the approach/arrival of the Aleutians/Bering system, may produce locally significant precipitation across parts of the southern mainland. The Aleutians system could also spread moisture farther northward over the mainland. Most of the state should see below normal highs during the period. Meanwhile, a northeastern Pacific system expected to track close to Haida Gwaii may spread some moisture into the southern Panhandle. ...Guidance/Predictability Assessment... Within what has been a moderate degree of spread for shortwave details within the broad mean trough over and west of the mainland early in the period, latest guidance agrees in concept that a greater percentage of energy emerging from Siberia should drop southward rather than progress into/through the mainland. The models still differ on the specifics, and predictability at extended time frames for resolving such cases of flow separation tend to be on the lower side, so it would not be surprising to see additional changes in future model runs. That said, the 12Z models as a whole and especially the GFS/ECMWF/CMC agree that this infusion of northern stream energy should lead to a deeper system than in earlier consensus and the overall timing/track of their average fits well within the framework of the ensemble means. This provides good support to follow the model composite as the most likely evolution at this time. By late in the period the 12Z GFS strayed to the east of other model/mean guidance for the Arctic upper low, requiring phasing out its input. The new 18Z GFS run may bring the upper low too far southward but at least brought it back to a more reasonable longitude so the late-period evolution of the Aleutians system adjusts more toward the other models. The 12Z ECMWF mean maintains good continuity. Guidance continues to oscillate from day to day for the strength of the northeastern Pacific system forecast to track near Haida Gwaii, with today's consensus adjusting a little stronger. Over the past day the previously farther north CMC has joined the majority cluster. Comparisons led to starting the forecast with a 12Z GFS/ECMWF/CMC blend for the first half of the period. Greatest weight was placed on the GFS near the start of the forecast, per Alaska WFO preference toward its solution as of late in the short range time frame. By days 7-8 Sunday-Monday the ECMWF/CMC maintained their weight in the blend given ensemble mean support, while GEFS mean input increased to 15-35 percent as it replaced the GFS due to the latter's questionable Arctic upper low track. ...Weather/Hazard Highlights... There is continued potential for some areas of locally moderate to heavy precipitation over the southern mainland during the latter half of the week. Anomalously high moisture should already be in place and guidance expects an additional stream of moisture to arrive from the west as broadly cyclonic flow aloft carries one or more shortwave impulses. Some precipitation may extend somewhat farther north as well, with a wavy front as a focus. Flow ahead of the Aleutians system may bring another surge of moisture into the southern mainland during the weekend as the storm tracks northeastward, with some coastal/terrain enhancement possible. Meaningful moisture and terrain enhancement could also extend farther northward across the mainland by Sunday-Monday. The system tracking near Haida Gwaii may spread light to perhaps locally moderate precipitation into the southern Panhandle Thursday-Saturday. Moisture from the west may arrive over parts of the Panhandle by late weekend or early next week. The Aleutians should trend gradually drier during the latter half of the period after seeing focused precipitation with the system affecting the region late this week. Expect most of the state to see below normal highs during the period. Coolest anomalies are likely to be Thursday-Friday over the North Slope and in localized areas over Southcentral and the Panhandle. It still appears likely that temperatures will moderate somewhat during the weekend/early next week as the front over the central mainland lifts northward in response to flow ahead of a front setting up near the northwest coast of the mainland. Warmer anomalies should prevail for lows, though still with below normal readings over northern-central areas into Saturday. Later in the period a majority of lows may be within a few degrees on either side of normal. Rausch 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