Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 441 PM EDT Sat Apr 22 2023 Valid 12Z Wed Apr 26 2023 - 12Z Sun Apr 30 2023 ...Heavy rainfall expected late next week into next weekend across south-central Alaska... ...Overview... An upper low will combine with a shortwave rounding its periphery and a blocking high to the north to lead to retrogression of the feature slowly through the Bering Sea, which will allow ridging aloft to build across eastern and interior AK. Cold temperatures are likely across much of the mainland through around midweek, while a persistent wet pattern is expected across Southcentral Alaska to the southeastern mainland which should pick up in intensity mid next week, with rounds of rain also possible for the Aleutians/Alaska Peninsula and the Panhandle. ...Guidance Evaluation/Predictability Assessment... The model guidance was in reasonable agreement into next weekend. A blend of the deterministic guidance was used from Tuesday into Thursday for the pressures, winds, and 500 hPa height pattern. To deal with increasing uncertainty in details of the forecast, increased the use of the 12z NAEFS and 00z ECMWF ensemble mean for the above fields to 50%. This generally maintained good continuity. The remainder of the grids were mostly based on the 19z NBM, with some 12z ECMWF included. ...Weather/Hazard Highlights... Colder than average temperatures are likely across much of the mainland. Expect well below average temperatures by 20-40F in between the Brooks and Alaska ranges, especially for eastern and central parts of the mainland, likely lasting into mid next week before modifying. Colder than average temperatures should make it all the way west to southwestern Alaska early next week as troughing overtakes the late week ridging, but could be shorter-lived than farther east. Cold air (though less anomalous) will affect the North Slope as well, with highs in the single digits and perhaps even staying below 0F in some areas. The Alaska Range is likely to block the core of coldest air from coming into southern Alaska. Starting mid-week, as the mid to upper cyclone retrogrades into/slowly through the Bering Sea, the mean flow will become more southerly into south-central AK and bring onshore low as well as a low pressure system on later on Wednesday which will support heavy rainfall particularly across Southcentral Alaska late next week into next weekend and modify the cold air mass inland. Local liquid equivalent precipitation amounts in the 5-10" range in orographically favored spots remain forecast. The remainder of southern AK should remain wet as assorted cyclones impact the AK Panhandle and Peninsula. The various low pressure systems should lead to periods of gales, with the possibility of storm force gusts, across portions of the Gulf of AK and Bering Sea. Roth 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