Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Tue Sep 05 2023 Valid 12Z Sat Sep 09 2023 - 12Z Wed Sep 13 2023 ...Heavy rainfall likely over portions of the Southcentral to Southeastern Alaska this weekend into early next week... ...Overview... A upper low is forecast to spin atop much of the state this weekend into the first half of next week, with a mean position atop Bristol Bay or so. Multiple shortwaves rotating around this feature along with surface low pressure/frontal systems may reorient the axis of the main upper low at times and also provide a focus for precipitation that could be heavy. The highest precipitation totals are likely from parts of Southcentral into Southeast Alaska with Gulf of Alaska lows, with some rain likely to affect the eastern Aleutians/Alaska Peninsula as well. ...Guidance Evaluation/Preferences... Model guidance is in reasonably good agreement with this mean upper low pattern, with more differences regarding shortwave troughs and energy rounding the feature and the accompanying surface lows. For this weekend, the first trough of concern will be pivoting eastward through the northeast Pacific along with a surface low. A multi-model blend of deterministic guidance seemed to handle that well, despite some minor differences. Upstream, another shortwave looks to come in on the western side of the low across the Bering Sea already as the period begins Saturday. This feature has converged much better in the model guidance compared to a day ago, with models agreeable that the associated surface low should track southeast across the Aleutians or Alaska Peninsula and arrive in the northern Pacific by Sunday, staying somewhat strong in the Gulf of Alaska through around Tuesday along with upper-level energy. This could reorient the main trough somewhat southeast to northwest, and allow for more upper ridging from western Canada to come into the northeastern part of the Mainland. Another change to the forecast is that yet another shortwave moving through the Bering Sea early next week looks to be moving faster than it was in yesterday's guidance. Model guidance pretty much all agreed on this faster trend, but particularly the CMC and ECMWF were faster than the GFS runs with this energy and frontal system. The WPC forecast used the multi-model deterministic blend mainly favoring the ECMWF/GFS/CMC for the early part of the period, with a gradual introduction and rampup of the EC and GEFS ensemble means to just over half by the end of the period. Given the uncertainty of shortwaves arising from Siberia and the vicinity, confidence is not terribly high, but at least there was some good model clustering to key on. ...Weather/Hazards Highlights... As the period begins on Saturday, heavy rain is likely to be ongoing from an area around Prince William Sound east into the Yakutat area and northwestern parts of the Panhandle as a low pressure system directs moisture (likely as an atmospheric river) into the region. Gusty winds may also accompany this low pressure system. Meanwhile farther west, a reasonably strong low tracking southeastward across the Aleutians this weekend will lead to some moderate to heavy rainfall there and into the Alaska Peninsula and the southwestern Mainland, and then once again increases rain chances for Southcentral into Southeast Alaska as the low tracks into the Gulf early to mid next week. Altogether, rain amounts could add up to multiple inches. For this cycle, forecast rain amounts increased for Southcentral. During these moisture surges, some moisture could make its way farther north into the Alaska Range where precipitation could be snow in higher elevations. Some scattered light to moderate showers are also possible across the Interior. Below normal high temperatures should prevail over most areas during the weekend into early next week, with the coolest readings likely to be into the weekend when the deepest upper troughing should be over the state. The eastern Interior could warm to slightly above average for the first half of next week though, if the orientation of the upper trough allows for ridging to poke into the region from western Canada. Low temperatures will likely be closer to normal though still a bit cool for especially the western/central Interior on Saturday, gradually moderating early next week. Tate Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri-Mon, Sep 8-Sep 11. 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