Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 630 PM EDT Sun Sep 12 2021 Valid 12Z Thu Sep 16 2021 - 12Z Mon Sep 20 2021 ...Pattern Overview and Model Guidance... An active weather pattern is expected to be in place across the Alaska domain during the medium range period. A strong low pressure system is forecast to track from the Bering to the northern Gulf through the middle to end of the week, followed by another low that will likely take a track farther south by next Sunday/Monday. Meanwhile, a rather significant pattern change is expected to materialize across much of mainland Alaska as a deep Arctic trough builds in and heralds the arrival of much colder temperatures for the second half of the forecast period. Given the more amplified upper level pattern in place by mid-late week, the 12Z guidance suite is in better agreement on the synoptic scale compared to previous days. Some of the timing differences in the guidance early on can probably be attributed to the potential double low structure of the storm system on Thursday, which evolves into one main parent low over the northern Gulf as it consolidates. For the second storm system next weekend, the GFS is suggesting a farther south track compared to the more northern placement of the ECMWF/CMC, and the ECENS even farther to the north. For the polar low settling in across the North Slope region, the GFS is the most impressive with the southward extent of sub-522 dm heights. The WPC fronts/pressures forecast was primarily derived from a CMC/GFS/ECMWF blend with some of the UKMET to begin, followed by slightly more of the ECMWF compared to the CMC/GFS and increased use of the ensemble means. ...Sensible Weather... Periods of moderate to heavy rainfall are likely from the Kenai Peninsula to the southeast Panhandle owing to onshore flow from the Gulf system for the Thursday-Saturday time period, with perhaps 1 to 3 inches of rainfall across the coastal ranges. The heaviest rainfall with the following storm system should stay mainly offshore for the time being. Some strong winds are also possible for portions of the Aleutians and coastal southern Alaska. Another thing that will likely make weather headlines will be the colder temperatures in the wake of a strong cold front dropping south from the Arctic. Readings will likely still be near to above normal on Thursday for most of the state, followed by well below average temperatures by next weekend and into next Monday. Accumulating snowfall is also looking increasingly likely across portions of the Interior, Brooks Range, and North Slope, along with widespread subfreezing overnight lows. Hamrick 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