Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EDT Fri Sep 8 2023 Valid 12Z Tue Sep 12 2023 - 12Z Sat Sep 16 2023 ***Long duration heavy rainfall likely over portions of South-central to Southeastern Alaska into next week*** ...Synoptic Overview... An upper trough is expected to extend from northwest to southeast from Siberia into the western Mainland and the Gulf of Alaska through much of next week, with embedded upper lows. Surface lows focused in the Gulf directing moisture inland into coastal areas could lead to multiple rounds of heavy rainfall through the period. The highest precipitation totals are likely from parts of South-central into Southeast Alaska. ...Guidance Evaluation/Preferences... Recent model guidance has persisted in indicating a mean troughing pattern across much of the state, with the negatively tilted orientation of the trough possibly allowing some ridging from western Canada to push into the northeastern part of the Mainland. Differences become more substantial from Wednesday on across the Bering Sea, Aleutians, and into the Gulf of Alaska regarding the timing of upstream shortwaves translating southeastward from Siberia with increasing spread in the guidance. Noted outlier solutions in the deterministic guidance include the CMC mid-next week, which has remained the most out of phase by indicating a trend towards ridging over the Aleutians and the southern Bering Sea as opposed to the mean negatively oriented troughing through the region. In addition, by next Friday, the GFS becomes much stronger with a low developing over the Gulf of Alaska compared to the model consensus. Thus, the updated WPC forecast begins with a consensus deterministic guidance blend with the GEFS/ECENS ensemble means gradually increased as the period progresses and uncertainty in timing/phasing of embedded systems within the mean troughing increases, ending up to just over half by the end of the period. ...Weather/Hazards Highlights... The southeast portion of the mainland and the southeast Panhandle region will remain rather unsettled for much of next week. Onshore flow associated with low pressure systems traversing the northern Gulf of Alaska will advect multiple rounds of enhanced moisture inland with the potential for an atmospheric river event or two. While totals have trended downward a bit, some locations could still see upwards of 3 to 5 inches of rain over the entire forecast period, with the greatest totals expected across the windward terrain of the coastal mountain ranges. Otherwise, much of the remainder of the state looks to remain mostly dry through the period. An upstream system moving in from the Bering Sea late next week may increase precipitation chances for the Aleutians/southwest Mainland, though uncertainty with the evolution of this system is notably higher. In terms of temperatures, expect readings to generally be slightly below average across western portions of the state and slightly above for the eastern Interior to start the week. Going into the middle to end of the week, warmer conditions are likely across the northern two-thirds of the state due to noted ridging over the north from western Canada, and slightly below average for the southern coastal areas owing to increased clouds and rainfall coverage. Putnam/Hamrick Hazards: No significant hazards are expected over Alaska during this forecast period. 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