Alaska Extended Forecast Discussion National Weather Service College Park MD 731 PM EDT Thu May 29 2025 Valid 12Z Mon 02 Jun 2025 - 12Z Fri 06 Jun 2025 ...Heavy rain and strong winds will impact portions of the Alaska Peninsula and Southwest and Southcentral Alaska into early next week... ...Overview... A consolidated upper low centered over the southern Bering Sea/eastern Aleutians to start the period Monday will be relatively deep for early June. This will support surface lows in that vicinity, weakening by Monday but providing some strong winds to southwestern Alaska Sunday. Mean southerly flow between the upper low and an eastern Alaska upper ridge should draw in Pacific moisture and produce heavy rain focusing across portions of the Alaska Peninsula, Y-K Delta, Kodiak Island, and into Southcentral early next week. The upper/surface low is forecast to track eastward and weaken, spreading moderate precipitation toward Southeast Alaska by Tuesday as precipitation lingers farther west before tapering off later 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... Model guidance continues to show a deep upper low (likely over 2 standard deviations below normal for 500mb heights for this time of year) centered over the southern Bering Sea into Monday. The resultant surface low is expected to be at its deepest on Sunday, just before the medium range period begins, but still possibly in the upper 980s in the Southern Bering early Monday while another slightly weaker surface low may wrap in over the eastern Aleutians. Models are in rather good agreement with the upper and surface lows position and depth early on, but model spread increases with the details of the lows tracking east and weakening atop/near southern Alaska as next week progresses. The 12Z ECMWF seemed to be the fastest/farthest east outlier starting Tuesday and especially by Wednesday. This looks to be a result of the EC phasing the upper low with troughing emerging from just north of northwest Canada (Beaufort Sea), while other models keep those features separate. The 00Z ECMWF had not shown this signal and had been more agreeable with the timing, so favored it rather than the newer 12Z run. A model/mean compromise other than the 12Z ECMWF for the lows moving east tended to work. Then behind this system, models are trending toward another (weaker) trough diving across the Aleutians during the latter half of the week, but the details are yet to be determined. Thus the WPC forecast began with a blend of the deterministic models, primarily the 00Z ECMWF, 06Z/12Z GFS, and 12Z CMC. Gradually reduced the proportion of deterministic runs while increasing the GEFS/EC ensemble means as the period progressed to account for increasing spread. Means were about half the blend Day 7 and more Day 8. ...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, spreading eastward into the favored terrain of Southcentral Alaska early next week. Light to moderate precipitation is likely to expand north into the Interior and perhaps reach the Brooks Range at times. Some higher elevation precipitation would be snow in the higher elevations. 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. As the low gradually pulls away later week, precipitation chances and amounts will be reduced for most areas. However, some moderate precipitation could come into the Aleutians later week under the next trough. The deeper than normal surface low may produce strong winds in the Alaska Peninsula and Y-K Delta on Sunday. Winds are forecast to be much stronger than normal for this time of year per the ECMWF-based Extreme Forecast Index, in the 30-40s kt range, so a borderline High Winds area was added to the Hazards graphic today. Meanwhile mostly dry conditions are expected in the North Slope 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 are expected to 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