Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 801 PM EDT Mon Jul 18 2022 Valid 12Z Sat Jul 23 2022 - 12Z Wed Jul 27 2022 ...Multi-day, heavy rain possible across portions of coastal southern Alaska and the Southeast... ...General Overview... Through the course of extended period an anomalously deep upper level low will begin to weaken as it moves southwards over the western half of the Mainland. This will equate to widespread cool temperatures by July standards and an increase in showers and a few thunderstorms. Multiple impulses tracking around the southern periphery of the main low will increase the likelihood for heavy rain from the Kenai Peninsula to the Panhandle region with prolonged moist, onshore flow expected. ...Guidance/Predictability Assessment... Model consensus shows a highly anomalous upper low (below 540 dm) positioned over the northern Bering at the start of the extended forecast. With time, the guidance show increasing spread with the orientation of trough. As previous noted, multiple runs of the GFS hold on to a solution that takes the surface low further east as well as the cold front crossing the Gulf of Alaska by next weekend when compared to other global and ensemble guidance. Therefore, it continues to be somewhat of an outlier. The ECWMF also continues to be stronger with a low that emerges from south of the Aleutians by early next week with a more amplified trough in place. Although these two solutions lead to some differences in timing/location, they were still utilized for the blend. WPC went with a blend of the CMC/UKMET/GFS/ECWMF that incorporated the GEFS mean/EC ensemble means for the middle/latter periods. ...Sensible Weather... A large portion of the Mainland will have daily temperatures averaging 10 to 20 degrees colder than those typical of mid/late July- a majority of location in the 50s to 60s. Maximum temperatures are expected to moderate some during the extended period, however the overnight lows will trend colder with time to the 30s and 40s. In addition to the cooler temperatures, much of the southern coastal areas and the Southeast will be in a prolonged wet period as multiple rounds of showers and thunderstorms pass through. Multiple impulses aloft will assist in generating lift with steep lapse rates in place. Although there is some uncertainty in the mesoscale aspects, confidence is increasing for a multi-day heavy rainfall event across portions of southeastern Alaska, and especially the windward terrain of the Chugach Mountains where moisture and lift will likely be maximized from atmospheric rivers. The potential exists for some 4-8 inch rainfall totals during the Thursday through Sunday time period for these areas, and may lead to some flooding problems. Precipitation may be in the form of snow in northwestern and north-central Alaska for the higher elevations, and also the highest terrain of the St. Elias Mountains. Campbell/Hamrick 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 - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Sun, Jul 21-Jul 24. - Much below normal temperatures across portions of mainland Alaska, Thu-Fri, Jul 21-Jul 22. 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