Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Fri Aug 18 2023 Valid 12Z Tue Aug 22 2023 - 12Z Sat Aug 26 2023 ...Overview... An Arctic low looks to track eastward north of the state around Tuesday-Wednesday, supporting a frontal system pushing into northern Alaska. Farther south, a weak low or two should track through the northern Pacific/Gulf of Alaska. But the main impactful part of the pattern will be upper troughing developing and getting reinforced over the Bering Sea through much of the week, with the possibility of surface low/frontal systems tracking northeastward with the mean southwesterly flow providing moisture and increased rain chances across much of the state generally spreading west to east. ...Guidance Evaluation/Preferences... Model guidance is in fairly good agreement with the pattern described above, considering the lead time. The track of the Arctic upper low has shown somewhat better consensus in the recent models, moving from north of Siberia day 4/Tuesday eastward across the Chukchi and Beaufort Seas before shifting back north, though the 12Z GFS was slightly south of other models with that feature. Models indicate upper troughing developing across the Bering Sea, possibly helped by energies from the north and west and south combining, and this trough gets reinforced with a slow shift eastward during the latter part of the week. Recent models have been weaker with an associated surface low moving northeast across the Aleutians into the Bering Sea and southwestern Mainland, with actually increased spread compared to yesterday's guidance. This will impact the details of areas of enhanced rainfall within this moist southwesterly flow generally favorable for rain. Then by late next week, uncertainty remains with how far inland the trough axis makes it and the timing of its progression east. Guidance like the GFS starts to split the energy in the trough, taking the northern stream troughing east perhaps influenced by its nearer Arctic low, while some spins near and south of the Aleutians. Meanwhile the ECMWF keeps the trough more phased with a steady slow progression of the trough. This may stay a model diagnostics issue in the next couple of days. Thus was able to start with a blend of the deterministic guidance favoring the ECMWF and GFS early in the period, but with reduction especially of the GFS component as the period progressed, with gradually increasing weight of the GEFS and EC ensemble means, ending up at just over half by the end of the period. ...Weather/Hazards Highlights... Rain totals may be relatively light around Tuesday-Wednesday between major storm systems, but with the northwest part of the Mainland favored for a bit of enhancement as a front draped across the northern tier may provide some added focus, and favored windward terrain in the Brooks Range may see relatively higher totals. Then rain chances should increase and become more widespread for the latter half of the week as the Bering Sea upper trough provides moist southwesterly flow. Generally the western half of the state will be most affected at first, and rain should spread gradually eastward with time as the trough edges east. Today's forecast favors the western half of the Brooks Range a little less so, but with increases through the Alaska Range and Southcentral Alaska, but this will continue to be monitored. Southeast Alaska is likely to be a drier area with just a few showers there through much of the period. The northern mainland should see the greatest negative anomalies for high temperatures during the period, though the coastal communities may stay warmer than average. The eastern interior and southern Panhandle will see the best potential for above normal highs. Low temperatures look to be more above normal, with just scattered pockets of modestly below normal anomalies. Tate 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