Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 709 PM EDT Thu Oct 14 2021 Valid 12Z Mon Oct 18 2021 - 12Z Fri Oct 22 2021 ...Stormy Pattern Continues from the Aleutians through Southern and Southeast Alaska... ... Guidance and Predictability Assessment... The extended range starts Monday with a low pressure system in the Gulf of Alaska and another one moving into the Bering Sea. Both systems should weaken by midweek as the next shortwave drops in across the Aleutians and into the Gulf. This in part may help to reinvigorate whatever might be left of T.S. Namtheun (currently in the West Pac) by that time, eventually resulting in a deep surface low (12z ECMWF shows 950mb central pressure, 12z GFS/CMC are in the low 970s) well south of Alaska by Thursday/day 7. The latest 12z models actually show fairly good agreement on this pattern, so a general model compromise worked as a good starting point for the WPC progs days 4-6. After that, the models begin to really diverge on bringing the low more north towards the Panhandle (ECMWF/CMC) or keeping it south (GFS). There is also a lot of spread across the Bering Sea at the same time as another surface low enters the region. The ECMWF and CMC are much faster with this, and has it moving across the AK Pen by day 8/Friday, while the GFS keeps it well north in the Bering. The ensemble means show good agreement somewhere in the middle. To help mitigate these late period differences, WPC leaned heavily on the ensemble means days 7-8. This approach allowed for good continuity from yesterdays forecast, as well as with downstream across the CONUS. ...Pattern Overview and Weather/Hazard Highlights... The mean low pressure pattern over the Aleutians and Gulf will keep cloudy/unsettled conditions through the extended period. Gusty winds with scattered to widespread precipitation (locally heavy) will continue into the early part of next week across parts of the southern Coast and the Panhandle region ahead of the weakening initial low pressure system in the Gulf. Snow is also possible in the higher elevations of southern and southeast Alaska. Northerly flow around the backside of the low is also likely to lead to gusty winds across parts of the Aleutians as well, especially into Monday. At the same time, upstream flow drive a deep storm into the Bering Sea by the start of the period with some light precipitation possible across far western mainland Alaska. This system should weaken as a stronger surface low moves south of the Aleutians. This shouldn't have much impact on sensible weather across the Aleutians/southern coast except for a few fairly light and scattered precipitation. Yet another deep cyclone makes its way into the Bering late in the period, which likely brings another round of gusty winds and moderate precipitation to parts of the Aleutians. There's potential for impacts to spread farther downstream towards southern Alaska, but this remains highly dependent on exact placement/track of the low, which is very uncertain at this time. Temperatures from the Aleutians to the Panhandle should remain below normal through the period as a parade of storms moves through these regions. Into interior and northern Alaska, a warming trend is expected by the early to middle part of next week as upper level ridging tries to work over the state. Santorelli 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 Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Wed, Oct 18-Oct 20. - Heavy precipitation across portions of mainland Alaska, Sun, Oct 17. - High winds across portions of mainland Alaska and the Aleutians, Sun, Oct 17. 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