Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Tue Apr 13 2021 Valid 12Z Sat Apr 17 2021 - 12Z Wed Apr 21 2021 ...Overview... Latest guidance continues to show a broad area of mean ridging aloft from the Bering Sea across Mainland Alaska and western Canada. This ridge will support high pressure for most of the period over the Bering, central-northern parts of the mainland, and northwestern Canada. To the north of the mean ridge expect progressive flow to carry along a series of fronts that should at least brush northern areas, with the possibility that one or more of these fronts could extend farther south. Meanwhile an inverted upper trough will rotate across the southeastern Bering early in the weekend and then a closed low (with corresponding low pressure at the surface) should lift into a position south of the Alaska Peninsula/eastern Aleutians by next Monday-Wednesday. ...Guidance/Uncertainty Assessment... The progressive nature of northern stream flow lends itself to low predictability for resolving specifics of individual shortwaves and thus associated waves/frontal systems. By day 6 Monday the 12Z UKMET becomes the strongest solution with its shortwave reaching the mainland so preference is to phase out its contribution to the forecast at that time. After Monday ECMWF/CMC runs (CMC deeper and later than the ECMWF) have been suggesting that some shortwave energy will drop into a weakness in the mean ridge, over the eastern Bering and/or the mainland. On the other hand the GFS maintains an east-west ridge. GEFS/ECMWF/CMC ensemble means all depict a modest weakness in some fashion, a reasonable intermediate solution given the full spread. Best consensus indicates that the front reaching the mainland by midweek may reach farther southward than its predecessors. Continuity/trends in the ensemble means over the past couple days have tended to favor the stronger side of the spread for the overall area of upper ridging. This is particularly the case for the western Bering Sea, where the GEFS mean has been notably lagging most other guidance in ridge strength. Farther south and from the start of the period early Saturday, there is a strong majority cluster showing that a system near the western Aleutians should drop southward more quickly than in the GFS/GEFS mean. This precludes use of the GFS during the weekend. The model is more acceptable as part of a blend/compromise for North Pacific low pressure by Monday-Wednesday though. The above considerations led to starting the blend with the 12Z ECMWF/UKMET/CMC during the weekend. Then the forecast phased out the UKMET while bringing in the GFS by early day 6 Monday. The rest of the way, ensemble mean input (more 00Z ECMWF mean than 12Z GEFS mean) increased to yield a nearly even blend of means and models (last two ECMWF runs plus 12Z GFS) by midweek. ECMWF/GFS runs essentially canceled out over the mainland to reflect the means while providing some detail elsewhere. Depth of the CMC trough over the mainland favored removing its influence by day 8. ...Weather/Hazards Highlights... The relative dominance of high pressure will keep many areas dry during the period. The primary focus for some precipitation is most likely to be over the Alaska Peninsula and eastern Aleutians early-mid week in the brisk flow around North Pacific low pressure. Lower confidence specifics of flow aloft will determine if any moisture extends farther northward-northeastward and/or becomes locally heavier, or alternatively is suppressed farther south. Scattered light precipitation may accompany northern stream fronts that reach the mainland. Expect most of the state to see above normal temperatures into Monday with anomalies for morning lows tending to be greater than for daytime highs. Also the North Slope should see higher anomalies than other areas. On the other hand a few pockets of slightly below normal highs may exist over the southeastern half of the mainland and into the Panhandle. The potential break in mean ridging aloft may promote a cooling trend by next Tuesday-Wednesday. 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