Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 753 PM EDT Wed Sep 04 2024 Valid 12Z Sun Sep 08 2024 - 12Z Thu Sep 12 2024 ...Possible heavy rain and mountain snow threat along the southern coastline by latter portion of next week... ...Overview... As the medium-range period begins later this weekend, a low in the Gulf of Alaska will be dissipating. The next system of concern will be swinging across eastern Siberia with an occluded front moving onshore across southwestern Alaska on Sunday. Meanwhile, a frontal wave from a former tropical depression that has passed east of Japan will be tracking eastward across the Aleutians. An occluded cyclone should then consolidate and slide east-southeast across the northern portion of Bering Sea early next week before sliding across southwestern Alaska by midweek. Moisture ahead of a developing triple-point low is forecast to spread heavy precipitation from west to east along the southern coastline toward the Panhandle by the latter part of next week. Temperatures will generally be above normal for morning lows and below normal to slightly above for daytime highs across the entire state. ...Guidance Evaluation/Preferences... Deterministic model solutions start out with a fair amount of spread regarding the next system of concern over eastern Siberia during the weekend. This system is forecast to generally slide east-southeast into the Bering Sea early next week with increasing solution spread. The 12Z Canadian model offered the southern- and western-most solution, tracking the cyclone center near/along the Aleutians. The GFS offered the eastern-most solutions with the core of the cyclone moving into southwestern Alaska on Wednesday. The 12Z ECMWF had the center still off the West Coast Wednesday morning before pushing the cyclone onshore. The ensemble means were much more agreeable with one another even out to Day 8. They show good consistency with a blend of the ECMWF and GFS solutions, yielding a double-barrel occluded cyclone to slide into and across southwestern Alaska by midweek next week. The next cyclone on the horizon appears to move into western Bering Sea by Day 8 next Thursday. With these considerations in mind, the WPC medium-range forecasts were derived from 40% of the latest ECMWF/EC mean, 40% from the latest GFS/GEFS, and 20% from the 00Z CMC mean. The Canadian deterministic solution (CMC) was considered an outlier as noted above. Nevertheless, its ensemble mean solution was usable and comparable with the 00Z EC mean, and the 12Z GEFS mean. This blend yielded a solution consistent with yesterday's WPC forecasts. ...Weather/Hazards Highlights... Residual rainfall associated with the dissipating low centered over Gulf of Alaska will affect mainly the Alaska Panhandle Sunday into Monday. Widespread light to moderate rainfall can be expected across western and southwestern Alaska to start out the medium-range period on Sunday while moderate rainfall will be possible along the Aleutians with the passage of the former western Pacific tropical depression. Additional moderate precipitation can be expected along the island chain early next week as the Bering Sea low is forecast to slide toward southwestern Alaska. The next wave of moderate rainfall could reach the western Aleutians by later next week as the next cyclone is forecast to move into the western Bering Sea. A heavy rain area has been introduced today on the WPC Hazards map near the eastern shore of the Alaska Peninsula as well as Kodiak Island for Day 6 later on Tuesday before spreading east along the southern coastline and nearby mountains for Day 7 Wednesday. This is in response to moisture interacting with the terrain ahead of the developing triple-point low over the Gulf of Alaska. The precipitation could be in the form of heavy snow over the higher terrain. The heavy precipitation could reach into the northern Panhandle by Day 8 as the triple-point low moves east and weakens. Meanwhile, the synoptic pattern will favor greater coverage of below normal daytime high temperatures during the period. Some areas over the eastern Interior and along the northern coast should see moderately above normal highs. Prevalent clouds and precipitation will favor mostly above normal morning lows, with only portions of the Panhandle and perhaps a few other isolated pockets over the mainland seeing below normal lows. Kong 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