Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Wed Apr 14 2021 Valid 12Z Sun Apr 18 2021 - 12Z Thu Apr 22 2021 ...Overview... Latest guidance continues to depict a blocky regime during the period. This pattern will be highlighted by upper ridging that covers the mainland and western Canada (possibly drifting into the northeastern Pacific for a brief time) and eventually joined by another ridge that builds over the western Bering Sea/eastern Siberia, while a consolidating Pacific mean low aloft should settle into a position well south of the eastern Aleutians and Alaska Peninsula. Shortwave energy within progressive flow around the north side of the upper ridge may push a cold front into the northern/eastern mainland by Tuesday-Thursday. Many locations should be fairly dry for most of next week with the best potential for precipitation existing over the Aleutians and Peninsula in the gradient between high pressure to the north and Pacific low pressure to the south. ...Guidance/Uncertainty Assessment... The most pronounced trend seen in the guidance over the past day has been toward a stronger ridge aloft from western Canada through the mainland from the start of the period early Sunday onward. To some degree this trend is a nod to yesterday's 12Z GFS which held the strongest ridging over the mainland into late period. By the latter half of the forecast, guidance diverges over details of the ridge that builds into the western Bering/eastern Siberia, with the more progressive GEFS mean merging it into the mainland ridge while the ECMWF/CMC means hold it farther west (near continuity) while the ECMWF and to a lesser extent CMC have it give way, allowing lower heights to extend into the Aleutians/Bering. The shape of ridging over the northeastern Pacific/Alaska Panhandle by late in the period will also have some influence on this evolution. The primary forecast consideration was to lean away from the ECMWF scenario after Monday as its farther north extent of lower heights aloft and surface low pressure from the North Pacific into the Aleutians/southern Bering during the mid-late part of the period conflicted with other operational models, the latest GEFS/ECMWF/CMC means, and recent trends for stronger upper ridging to the north. As the preferred operational model blend transitioned to a model/mean mix later in the forecast, the ensemble component tilted a bit more in the GEFS direction since trends supported its stronger mainland ridge (regardless of the specifics of how it gets there). The GFS/CMC provided some lingering model input by day 8 Thursday. Also on day 8, trends for the upper ridge seemed to favor maintaining a drier forecast over the southeast mainland and Panhandle (per operational models) versus the ensemble-based PoP guidance. Finally the stronger upper ridge early in the period deflects Arctic fronts a little northward versus continuity but there is a continued signal for the front reaching the mainland by Tuesday-Thursday--albeit with typical variability in timing. ...Weather/Hazards Highlights... Most precipitation should be confined to the Aleutians/Alaska Peninsula and vicinity, with a fairly persistent flow of moisture carried in the gradient between one or more Pacific lows and high pressure well to the north. Light to moderate amounts are most likely but some locally higher totals could be possible depending on exact strength and track of Pacific systems. Also expect a broad field of brisk winds around these lows. Most of the state should see above normal temperatures at the start of the period Sunday, with the warmest anomalies over the North Slope and for morning lows. A gradual cooling trend with time will tone down the warm anomalies somewhat while possibly bringing at least high temperatures moderately below normal over parts of the southeastern quadrant of the mainland and the Panhandle. Rausch No significant hazards are expected over Alaska during this forecast period. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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