Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 728 PM EDT Mon Sep 13 2021 Valid 12Z Fri Sep 17 2021 - 12Z Tue Sep 21 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 across the northern Gulf through the end of the week, followed by another low that should track just south of the Aleutians and reach the Gulf region by next Sunday/Monday. By the end of the forecast period next Tuesday, a third system could be affecting the central and western Aleutians. 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 next weekend and into early next week. The 12Z model guidance suite is in decent overall synoptic scale agreement for much of the forecast period, although some timing and latitudinal differences remain. The CMC is slower with bringing the next storm system to the Gulf region on Sunday and the GFS on the faster side of things. Most of the guidance keeps the core of the polar low just north of the Arctic Coast, whereas the GFS is more amplified and brings the low closer to the Brooks range and thus a colder overall solution. 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 on Friday, with perhaps 1 to 2 inches of rainfall across portions of the coastal ranges. The heaviest rainfall with the following storm system should mainly stay offshore until Monday when the plume of moisture brings potentially heavy rainfall to the southeast Panhandle region for early next week. 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 and perhaps Friday 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 Hazards: -Heavy precipitation across portions of southern mainland Alaska, Thursday, Sep. 16 -Heavy rain across portions of the Alaska Panhandle, Monday, Sep. 20 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