Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 733 PM EDT Mon Aug 21 2023 Valid 12Z Fri Aug 25 2023 - 12Z Tue Aug 29 2023 ...Wet pattern for much of the state with Bering Sea troughing and surface low pressure systems pulling in moisture... ...Overview... Troughing over the Bering Sea will send a relatively strong surface low into western Alaska this weekend with the threat for heavy rainfall across especially northwest Alaska and parts of the Brooks Range. To the south, additional shortwave energy towards the Alaska Peninsula and southern Coast will bring rain, heavy at times, to portions of this region Friday-Sunday. By early next week, an upper low will move into the Bering Sea sweeping a cold front through the Aleutians. Southeast Alaska should have comparatively drier and warmer conditions with ridging overhead. ...Guidance Evaluation/Preferences... On the large scale, the guidance shows generally good agreement on the overall pattern, though with plenty of differences in the details and timing of systems, especially the latter half of the period. The WPC forecast used a blend of the 12z ECMWF with the 12z GFS and CMC days 4-6. The UKMET was excluded from the blend today due to it being slower with initial shortwave energy into the Gulf early in the period, and some notable differences with the surface low into western Alaska. By around day 5/Saturday, the GFS becomes notably faster with the upper low into the western Bering Sea, eventually bringing it across the central Aleutians by Day 8. The ensemble means suggest a slower solution, much closer in line with the ECMWF. Farther east into the Gulf, the CMC becomes fast with troughing late period, and is also more aggressive to break down the ridge over Southeast Alaska. Eliminated the CMC completely from the blend after day 6, with increasing contributions from the more agreeable ensemble means. Favored the ECMWF over the GFS with minor inclusion in the blend late period to help better define systems washed out by the ensemble means. ...Weather/Hazards Highlights... Initial surface low into northwest Alaska on Friday will bring increased rain chances across much of the state, with a threat for heavy rainfall especially across portions of northwest Alaska and western parts of the Brooks Range. To the South, another surface low and potential atmospheric river will direct at the southern Coast region with several inches of rain possible from the Kenai Peninsula to the Southern Coast, with enhancement over terrain regions. Rain could sneak as far east as Yakutat and the surrounding areas, but the rest of southeast Alaska is likely to be much drier. General troughing sliding across northern Alaska will keep showers in the forecast into the weekend/early next week with the next system into the Bering spreading rain chances across the Aleutians. In terms of temperatures, the eastern interior and southern Panhandle will see the best potential for above normal highs, while other areas should be near to below normal. The coolest anomalies are likely to be over the Brooks Range to North Slope to the north of a front, though the coastal communities may stay closer to average. Low temperatures look to be more above normal until early next week, though with the possible exceptions of the Seward Peninsula and northwestern Mainland. 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 rain across portions of mainland Alaska, Thu-Fri, Aug 24-Aug 25. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat, Aug 26. 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