Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Thu Sep 05 2024 Valid 12Z Mon Sep 09 2024 - 12Z Fri Sep 13 2024 ...Heavy rain likely along the Southern Coast Tuesday-Wednesday along with mountain snow... ...Overview... The medium range period begins on Monday with quasi-zonal upper flow, with just some low amplitude troughing over the Mainland and ridging behind it over the Aleutians. Upstream, a more amplified trough will swing east across the Bering Sea anchored by an upper low atop eastern Siberia/Bering Strait. This trough and associated surface lows/fronts will direct a weak atmospheric river promoting heavy precipitation Tuesday-Wednesday especially across the Southern Coast. The trough should be reasonably progressive and cross the Mainland as it weakens into later week, as possible additional but uncertain troughing enters the Bering Sea once again. ...Guidance Evaluation/Preferences... Recent model guidance shows relatively good agreement with the large scale features of note at the beginning of the period. Models agree in principle that an upper low will slowly drift from eastern Siberia toward the Bering Strait while swinging a trough more progressively across the Bering Sea early week into the Mainland midweek and eastward while weakening. The first model to diverge from consensus was the 12Z UKMET, which brought energy within the trough much farther south and closed off an upper low near Kodiak Island by early Wednesday. But was able to maintain a GFS/ECMWF/CMC blend with small portions of the EC and GEFS ensemble means early in the forecast period. This worked to allow for a double barrel surface low around midweek, as the northern low entering the northwestern Mainland weakens in favor of the triple point low in the Gulf of Alaska. By around Thursday/Day 7, the 12Z ECMWF took the weakening troughing east across the Gulf into Canada faster than the other deterministic models, the AIFS, and the ensemble means, so it was not favored. Then the 12Z CMC split some upstream energy weirdly to lead to a different pattern across the Bering Sea compared to consensus. Thus gradually increased the proportion of ensemble means to well over half by Day 8 given the increasing spread among the deterministic models. ...Weather/Hazards Highlights... Into Monday, a frontal wave from a former tropical depression that has passed east of Japan will be tracking eastward across the Aleutians, spreading moderate rainfall there. Meanwhile light to modest precipitation is also possible across much of the Mainland with some focus across areas of upslope flow, and Prince William Sound with weak low pressure in the vicinity. More significant precipitation is expected to spread into the western Mainland with a forming occluded low pressure system ahead of the upper trough late Monday into Tuesday and progress across the state through midweek. The heaviest rain totals will focus along the Southern Coast where a weak atmospheric river will focus moisture and a triple point low is forecast to form. 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 the central and eastern Mainland. Some precipitation should spread into Southeast by around Wednesday-Thursday, with some uncertainty in amounts depending on the atmospheric river weakening. Generally northern parts of the Panhandle like the Yakutat area are forecast to see the most rainfall compared to southern parts. Another round of precipitation is possible across the Aleutians to western Mainland and Alaska Peninsula by late week, but the details remain uncertain. 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 upper 40s will be above normal. 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. 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