Alaska Extended Forecast Discussion National Weather Service College Park MD 744 PM EDT Wed May 28 2025 Valid 12Z Sun 01 Jun 2025 - 12Z Thu 05 Jun 2025 ...Heavy rain will impact portions of the Alaska Peninsula and Southwest and Southcentral Alaska into early next week... ...Overview... Energies from a couple of upper lows are forecast to interact and consolidate by Sunday or Monday, forming an upper low centered over the southern Bering Sea that is relatively deep for early June. This will support a deep surface low near the Aleutians and its associated fronts, while drawing in Pacific moisture in mean layer southerly flow between the upper low and and an eastern Alaska upper ridge. Thus heavy rain is forecast to focus across portions of the Alaska Peninsula, Y-K Delta, Kodiak Island, and into Southcentral early next week. Cooler than average temperatures are likely across southern areas as well as along the North Slope as a cold surface high pokes in. ...Guidance Evaluation/Preferences... Recent model guidance has trended toward developing a strong upper low (around 1.5-2 standard deviations below normal for this time of year) centered over the southern Bering Sea early next week. The consolidated upper low will provide support for a surface low that is also deep for early June--generally in the upper 970s per model guidance. As the extended range period begins Sunday, there is some spread in exactly when energies will combine to create one upper low. Early Sunday the 12Z GFS and ECMWF both show northern and southern lobes of the upper low that consolidate by Monday. Meanwhile the 12Z CMC and UKMET indicate one upper low already on Sunday (though by Monday models/means are really quite similar with their patterns with the combined upper low). This may play a role in the CMC's ejection of the upper/surface lows east more quickly by Tuesday and beyond compared to other guidance. The 12Z GFS and ECMWF were fortunately pretty similar to each other and their ensemble means on the timing of the eastward movement of the upper low. There is still some spread in its depth as it moves east. The 12Z GFS for example got weaker than the ECMWF, possibly influenced by some stronger upper troughing in north-central Canada, so tended to prefer the ECMWF's depiction but with low confidence. Though the GFS and ECMWF happened to agree on timing, there is certainly still a fair amount of ensemble spread and will have to watch future model trends for adjustments. Thus the WPC forecast began with a blend of the GFS/ECMWF/CMC, but eliminated the CMC from the blend by Day 6 while slowly decreasing the GFS and EC proportions as well, in favor of the EC/GEFS ensemble means. The CMC mean was faster like its deterministic run. The ensemble means comprised 60 percent of the blend by Days 7-8 with some GFS and ECMWF still in for system definition. ...Weather/Hazards Highlights... The ample moisture streaming into parts of southern Alaska in the form of a weak to moderate atmospheric river will produce areas of rainfall that could be heavy across the Alaska Peninsula and north toward the Kuskokwim Delta this weekend, spreading eastward into the favored terrain of Southcentral Alaska early next week. Light to moderate precipitation is likely to expand north into the Interior. Some higher elevation precipitation would be snow in the higher elevations of the Alaska Range. Modest rain is forecast to remain over the southern half of Alaska as next week continues, while the upper low gradually shifts east. Light rain may reach Southeast Alaska by Monday, and moderate to perhaps locally heavy rain could occur in Southeast Alaska around midweek depending on the timing of lows. The deeper than normal surface low may produce strong winds in the Alaska Peninsula and Y-K Delta on Sunday. Winds at this point are expected to peak less than 50 knots, more in the 30-40s kt range. Meanwhile possible light precipitation in the North Slope early Sunday should yield to mostly dry conditions through much of next week under the southern end of the Arctic high pressure. High temperatures near and under the primary upper low are likely to be cooler than average, covering southern Alaska next week. Lows will likely be closer to average. The Seward Peninsula to the western and possibly central Interior will be the main area of above normal temperatures in the southerly flow just ahead of the upper low, especially early next week before moderating. The North Slope should also see colder than normal temperatures under its Arctic high influence. Tate 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