Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 549 PM EST Thu Nov 26 2020 Valid 12Z Mon Nov 30 2020 - 12Z Fri Dec 04 2020 ...Deep North Pacific storm to bring a broad area of strong winds along with heavy precipitation focused over the southern coast/Panhandle early-mid week... ...Overview and Guidance/Predictability Assessment... Today's guidance continues to show a transition from an amplified western mainland trough/western Canada ridge configuration with high predictability (aside from minor details, forecast with remarkable agreement and continuity since eight days out) toward gradual relaxation of the pattern and decreasing predictability. The decline in predictability extends across the Pacific mid-late week as well. For the deep storm initially south of the Aleutians early Mon, multi-day trends have favored a slower and farther south track which ultimately leads to a delayed arrival into the southern coast and today's forecast blend reflects this. The southern trend in the GFS over the past 2-3 days is particularly noticeable while the ECMWF has averaged farther south but temporarily reversed the trend in the 12Z/25 run. By day 6 Wed the 12Z GFS/GEFS mean/CMC stray to the western side of the full guidance spread, which recently has not held up well in successive cycles. Preferred forecast follows closer to the past two ECMWF runs for track/timing while while leaning closer to the deeper GFS/UKMET for depth. GFS input transitions to the more compatible 06Z run by this time while ECMWF input starts to include a little 00Z ECMWF as well, heading into the less confident late week forecast. There are multiple late week uncertainties. One involves exactly how the upper trough over/near the far western mainland weakens, whether by ejection of initial energy or filling in place. There is also a signal from sporadic operational runs for a possible frontal wave reaching near the Panhandle (strongest in the 12Z ECMWF). Upstream there is a general area of shortwave energy that reaches east-central parts of the Pacific late in the week, with decent agreement that surface development should occur in some fashion but clustering and run-to-run consistency have been quite poor thus far. CMC/CMC mean runs have tended to be on the fast extreme with low pressure generated by the leading edge of this energy. Therefore the forecast quickly increases ensemble mean weight, reaching 70 percent total by day 8 Fri. At that time the most common theme is for low pressure to consolidate close to Kodiak Island and greater consistency of the ECMWF mean in recent runs has merited a little more weight versus the GEFS mean whose 12Z run is closer to the ECMWF mean than in some prior runs. ...Weather/Threats Highlights... The deep storm to the south of the Aleutians at the start of the week will bring a broad area of strong winds from the southern Bering Sea/Aleutians/North Pacific into the Gulf of Alaska and southern coast/Panhandle. Accompanying precipitation will extend from the Aleutians eastward, with the greatest moisture feed likely to focus over the Panhandle and to some degree the southern coast around Tue. The flow of moisture may become somewhat less concentrated mid-late week but the general pattern still favors the potential for meaningful to significant precipitation over some locations along the Panhandle/southern coast. Confidence is low regarding one or more potential waves that could end up providing renewed focus of heavy precipitation for at least a brief period of time. Lesser totals will reach farther north into the mainland though with some sensitivity to exact surface low track. Much of the North Slope as well as the eastern two-thirds of the remainder of the mainland should see above to well above normal temperatures through the period with highest anomalies likely to be for lows (20F or more above normal in some cases). The Panhandle will tend to be above normal as well except for some below normal areas that are most likely early in the week. Expect the best potential for below normal temperatures over the west/southwest part of the mainland and through the Peninsula/Aleutians. Rausch 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 Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun-Wed, Nov 29-Dec 2. - High winds across portions of mainland Alaska and the Aleutians, Sun-Mon, Nov 29-Nov 30. - High winds across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Nov 30-Dec 1. 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