Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 736 PM EDT Sat Sep 24 2022 Valid 12Z Wed Sep 28 2022 - 12Z Sun Oct 02 2022 ...Heavy rainfall expected across southern coastal Alaska and the Panhandle through the upcoming workweek with another big storm possible in the Bering late next week... ...Synoptic Overview... An active and unsettled pattern is forecast across especially southern portions of Alaska through the extended range (Wednesday-Sunday). The period will begin with an elongated upper level trough stationed over the Bering Sea and western Alaska through later next week, as several shortwaves round the base through the Aleutians and into the Gulf. At the surface, this features a weakening surface low into mainland Alaska with triple point redevelopment across the Gulf. After this, the focus shifts to the Aleutians as a deepening surface low, with likely tropical connections to a disturbance currently in the northwest Pacific, moves through the island chain and into the Bering Sea next weekend. This is concerning just given the immense impacts from the remnants of Merbok brought to parts of Alaska last week, with this system potentially taking a similar track. Guidance currently does not suggest this system gets as deep, but its impacts (winds and rain) could certainly exacerbate still ongoing issues after Merbok. ...Guidance/Predictability Assessment... The latest suite of guidance shows pretty good agreement on the overall longwave pattern over the Alaska region through at least the first half of the period, with mostly typical timing and strength differences in the details. The WPC forecast through day 6 leaned mostly on the 12z ECMWF and CMC as the 12z GFS was a little bit stronger with a shortwave through the eastern Aleutians early on, and also maintains the initial surface low in the Bering Sea much longer than the rest of the guidance. There are some still notable differences regarding evolution of an eventual deep cyclone in the Bering Sea next weekend. Initially, this regards interactions of north Pacific tropical/extra-tropical remnants with another system dropping in from the north. The GFS is again the outlier here into day 6, as it is farther west/slower than the ECMWF, CMC, and ensemble means. By 7 and 8 however, the GFS seems to catch up and is must better aligned with the ECMWF and the ensemble means (the CMC is too quick to pull this low into western Alaska day 8). Despite some placement differences amongst the latest deterministic models, which are not atypical at these longer time scales, the models continue to show excellent agreement on some type of strong storm impacting the Aleutians, Bering Sea, and western Alaska by next weekend. For the days 7 and 8 period, the WPC blend stuck close to the ensemble means which offered a reasonable middle ground placement for this low, although manual deepening of the system was done as the means are likely much too weak. ...Weather/Hazards Highlights... Heavy rainfall (with snow in the highest elevations) can be expected next Wednesday and Thursday from southwest Alaska eastward into the Panhandle, with several inches of rainfall expected in some locations, particularly coastal Kenai and the Chugach Mountains of southern Alaska. Rainfall should taper off by Friday across southern areas, but may linger across the Panhandle for another day as weak troughing hangs on in the Gulf. Locally stronger winds may accompany this system as well. By Friday, what looks to be a very strong area of low pressure should move into the Aleutians and eventually the Bering Sea likely bringing with it widespread high winds, heavy rainfall, and coastal flooding, potentially in many of the same areas impacted by Merbok last week. 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 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