Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Sat Sep 14 2019 Valid 12Z Wed Sep 18 2019 - 12Z Sun Sep 22 2019 Guidance continues to show varying ideas for an Arctic upper low expected to start the period near the northwest corner of the mainland at the start of the period early Wed, along with individual systems carried along by progressive North Pacific/Bering stream flow. After showing considerable spread/variability through yesterday's 12Z run, guidance has come into alignment for the system likely to be approaching the southwest mainland as of early day 4 Wed. The ECMWF/ECMWF mean were the last solutions to join the current solution cluster. This system should make rapid progress through the mainland early in the forecast period. The next system, tracking near the Aleutians mid-late week, originates from an area of disturbed weather (98W) to the south-southeast of Japan with some interaction with mid-latitude dynamics occurring by Mon-Tue. To some degree as with the preceding system, there have been some flow interaction details that could take added time to work out given their scale. Up to arrival of the 12Z GFS, the GFS runs were the slowest and most southern/separated solution while the more phased 00Z ECMWF was on the faster side. The 00Z CMC had elements of both as it resembled the GFS into Thu but then accelerated. At the time of forecast preparation the guidance spread suggested that a compromise approach would be best for a single deterministic forecast--and provide only modest adjustment from continuity. Remaining 12Z runs including the ECMWF have bolstered support for an evolution closer to the GFS at least into early Thu with some spread (and timing a little faster than the GFS) developing thereafter. For the final system of the period there is still a good signal for its existence. Details have varied though, as over the past day through the 06Z cycle the GFS/GEFS mean have trended noticeably weaker while remaining solutions are still rather well defined. The 12Z GFS has returned to a stronger feature--offering further support for keeping the system fairly well defined. As the system reaches the vicinity of the Alaska Peninsula by day 8 Sun a model/mean blend provides a reasonable starting point, with the components of operational runs providing a little enhancement to the means. A blend/ensemble mean approach still looks best for the upper low initially near the northwest corner of the mainland. As a whole the average of guidance through arrival of the 12Z GFS had adjusted somewhat westward from yesterday, though GFS runs have been erratic with the 00Z and 12Z runs well westward of the 06Z run. Other 12Z models show potential for even greater retrogression. Based on forecast preferences the forecast blend started with a blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET early in the period and then transitioned to a model-mean blend with the means represented by the 06Z GEFS/00Z ECMWF mean. Guidance consensus still shows that the pattern will provide the best potential for heaviest precipitation over the southeastern coast and Panhandle with significant totals also extending farther north into the southern and eastern mainland. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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