Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 731 PM EDT Sat Sep 07 2024 Valid 12Z Wed Sep 11 2024 - 12Z Sun Sep 15 2024 ...Heavy rain likely along the Southern Coast for midweek... ...Overview... As the medium range period begins midweek, an upper trough will be pushing across the Mainland, which along with a surface low should spread moderate to heavy rain especially across the Southern Coast. Upper ridging is forecast to push through behind this trough and in response to additional energy and troughing coming into the Bering Sea late week. These upper and surface lows in the Bering gradually moving east will promote increasing precipitation chances across the Aleutians late week and spreading into western and central parts of the Mainland into next weekend. ...Guidance Evaluation/Preferences... While recent model guidance has general agreement with the large scale features of note, even as the period begins Wednesday there are some model differences. Namely, the 12Z ECMWF and the past couple of runs of the EC ensemble are deeper with the surface low pushing into the Lower Kuskokwim region compared to continuity and other model suites. It seemed reasonable enough to include as part of a model blend, so did trend deeper but not to the extent of the ECMWF. This low looks to push into the Gulf by Thursday (supported by a compact upper low within the trough) with some timing/strength differences that could be handled by a model blend. Models are showing upper ridging moving through the state late week into the weekend, though possible shortwaves moving around the ridge are uncertain. Meanwhile farther upstream, broad energy and troughing is likely across the Bering Sea though with uncertainty in the details. GFS and CMC have two separate upper lows that track east from the Kamchatka Peninsula to support the trough atop the Bering, but the ECMWF shows one upper low. Ensemble means are indicating one main low, but likely just from averaging together the individual members that may have multiple lows. Regardless, at least the overall pattern of troughing there is agreeable and will continue to monitor to nail down the details. The WPC forecast was based on a blend of the 12Z deterministic models early, with increasing proportions of the GEFS and EC ensemble means as the period progressed to over half means by Day 8. ...Weather/Hazards Highlights... Notable precipitation is expected to spread across much of the Mainland into Wednesday and progress farther east into Thursday as a weak atmospheric river pushes through ahead of the first upper trough. Models have varied with rainfall amounts across the Alaska Peninsula/Kodiak Island area over the past couple of days, but with a stronger surface low forecast now, amounts have trended up there for some locally heavy amounts in the typically favored upslope/onshore areas. The Kenai Peninsula to Prince William Sound to Yakutat can also expect some heavy rain Wednesday-Thursday. Moisture spreading into the Alaska Range in colder temperatures will allow for snow in the higher elevations. Lighter precipitation is expected north of there into much of the Mainland as well. Some precipitation should spread into Southeast into Thursday, and the northern part of the Panhandle still looks to see more rainfall than southern parts. Then as the broad upper trough and surface low pressure system get established in the Bering Sea, precipitation is likely to affect the Aleutians late week and track east into western and then central parts of the Mainland next weekend. This pattern may also be favorable for high winds depending on the strength of the low and potentially some minor coastal flooding along the western coast, so will continue to keep an eye on those potential threats. High temperatures should generally be in the 50s across lower elevations of the state next week. This is generally near to slightly below normal for highs. However, the North Slope with highs in the 40s will be above normal. Prevalent clouds and precipitation will favor mostly above normal morning lows, with only portions of the Panhandle and a few other isolated pockets over the mainland seeing below normal lows. High temperatures look to gradually warm a bit into next weekend under the ridge aloft. 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