Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 647 PM EDT Thu Oct 29 2020 Valid 12Z Mon Nov 02 2020 - 12Z Fri Nov 06 2020 ...Guidance/Predictability Assessment... Today's model and ensemble guidance shows good agreement and continuity for the large scale pattern evolution aloft during the period. A deep upper low along the southern coast of the mainland for about the first half of the week will finally depart after upstream Siberia/Arctic energy drops southward over the far western mainland and into the overall mean trough. The resulting trough should continue southeastward as an upper ridge building across the Bering Sea through midweek continues its eastward push into the mainland by Thu-Fri. To the west of this upper ridge a strong Kamchatka/Siberia upper low will support a similarly energetic surface system whose leading circulation will spread a broad area of moisture and enhanced winds across the Bering Sea and parts of the Aleutians, and eventually into portions of the mainland. One persistent uncertainty in the surface forecast regards the system originating from the short-range system near the Aleutians. Operational models have been varying with the timing of upper support and relative emphasis of the parent low versus leading triple point wave. Current 12Z majority cluster has this system tracking over or a little north of Haida Gwaii on Tue. The UKMET diverges from the other models, splitting into two waves with the leading one most similar to other solutions but weaker and on a more suppressed track. The GEFS/ECMWF means imply a somewhat faster evolution toward the Panhandle and ultimately merging into mean low pressure over the northeastern Gulf. Typically better refinement of important details in operational guidance during the first half of the extended period continues to favor the operational runs but persistence of the ensemble mean scenario maintains some doubt in the forecast. Meanwhile there is still spread and variability with specifics of low pressure just north of the mainland through midweek, with an intermediate solution providing a reasonable starting point. The same is the case for the wave that drops southward over the far western mainland around Tue, with some improvement in definition since yesterday. Finally, during the latter half of the period clustering is better than average for the strong Kamchatka/Siberia surface low along with its associated leading frontal system and broad area of strong winds. Based on guidance comparisons the forecast started with a 12Z operational model blend for day 4 Mon and then subtracted the UKMET while otherwise maintaining operational model focus for days 5-6 Tue-Wed. Once again operational models compared favorably enough to the means to require only minority input (up to 30 percent) of the ensemble means by days 7-8 Thu-Fri. ...Weather/Threats Highlights... Heavy precipitation over the Panhandle through the weekend should trend less extreme early next week but with some activity continuing. Uncertainty regarding Pacific low pressure that may track near or south of the Panhandle around Tue lowers confidence for precip specifics at that time. Expect a drier trend during the latter half of the week as mean low pressure progresses southeastward away from the area. Meanwhile mostly light precipitation should accompany the wave dropping rapidly southward over the far western mainland around Tue. Arctic low pressure/frontal system could promote some light precip as well. By mid-late week the broad circulation ahead of strong Kamchatka/Siberia low pressure will bring an episode of precipitation and strong winds to the western half of the Aleutians, much of the Bering Sea, and then parts of the mainland--especially western areas. Favored terrain in the west/northwest could see significant totals by Thu. Over most areas the temperature forecast has been fairly consistent since yesterday. During Mon-Wed the pattern should be fairly stable with much below normal readings prevailing over the south/east and eventually extending into the Panhandle, while the North Slope sees above normal temperatures and the far west sees more modest positive anomalies. As the pattern becomes more progressive after midweek expect a warming trend to commence from west to east Thu-Fri. By Fri only about the southeast third of the state should be below normal while the North Slope and western areas see further warming relative to normal. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Sun, Nov 1. - Heavy precipitation across portions of the Alaska Panhandle, Sun-Tue, Nov 1-Nov 3. - Flooding possible across portions of the Alaska Panhandle, Sun, Nov 1. - Much below normal temperatures across portions of mainland Alaska, Sun-Thu, Nov 1-Nov 5. - Much below normal temperatures across portions of the Alaska Panhandle and mainland Alaska, Tue-Thu, Nov 3-Nov 5. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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