Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Sun Jul 17 2022 Valid 12Z Thu Jul 21 2022 - 12Z Mon Jul 25 2022 ***Multi-day heavy rainfall event becoming more likely across portions of coastal southern Alaska*** ...General Overview... A persistent and anomalously deep upper level low will be in place over the western half of the Alaska mainland through next weekend, and then tending to weaken some by early next week. This will equate to widespread cool temperatures by July standards and an increase in showers and a few thunderstorms. Multiple impulses are expected to track around the southern periphery of the main low, and this will increase the prospects for heavy rain from the Kenai Peninsula to the southeast Panhandle region with prolonged moist, onshore flow expected. ...Guidance/Predictability Assessment... The 12Z model guidance suite strongly supports a highly anomalous upper low (below 540 dm) over the northern Bering by Thursday, albeit with some trough orientation differences that increase going through the end of the week. The 18Z GFS is a bit farther to the east with the surface low and cold front crossing the Gulf region by next weekend compared to the other guidance. Meanwhile, the ECMWF becomes stronger with a low south of the Alaska Peninsula by next Monday with a more amplified trough in place. Taking these factors into account, the WPC forecast is initially derived from mainly a CMC/GFS/ECMWF blend, and then transitioning to more of the ensemble means whilst still incorporating some of the CMC/ECMWF/GFS, which maintains reasonable continuity with yesterday's WPC forecasts. ...Sensible Weather... Multiple rounds of scattered showers are likely across mainly central and southern portions of the state through the forecast period, with multiple impulses aloft helping to generate 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. In terms of temperatures, the main headline will be the persistence of high temperatures running 10-20 degrees below July averages owing to the upper low and also increased cloud cover, with readings mainly in the mid 50s to mid 60s for the valley locations. Low temperatures are also forecast to cool to below average levels, although the anomalies will be less compared to the afternoon highs. Some moderation in temperatures is likely by next Sunday and Monday. 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 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