Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 531 PM EDT Fri Apr 21 2023 Valid 12Z Tue Apr 25 2023 - 12Z Sat Apr 29 2023 ...Heavy rainfall expected mid to late next week 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 excellent agreement through late next week. A blend of the deterministic guidance was used from Monday into Wednesday for the pressures, winds, and 500 hPa height pattern. Because very good agreement continued through late next week, increased the use of the 12z NAEFS and 00z ECMWF ensemble mean for the above fields to only 40%. The remainder of the grids were mostly based on the 19z NBM, with some 12z ECMWF and 12z ECMWF ensemble mean included. ...Weather/Hazard Highlights... Colder than average temperatures are likely across much of the mainland behind the Gulf upper low. 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. Southcentral Alaska to the Panhandle may see below average temperatures in terms of highs but closer to, or even above (by late week), normal for lows. 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 Wednesday which will support heavy rainfall particularly across Southcentral Alaska mid to late week onward and modify the cold air mass inland. Local liquid equivalent precipitation amounts in the 5-10" range in orographically favored spots are forecast. Rain is also likely to track east across the Aleutians and Alaska Peninsula Monday and next Friday with a pair of low pressure/frontal systems. The various low pressure systems should lead to periods of gales, with storm force gusts, across portions of the Gulf of AK and Bering Sea. Roth Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Apr 27-Apr 28. - Much below normal temperatures across portions of mainland Alaska, Mon-Wed, Apr 24-Apr 26. 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