Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 652 PM EST Thu Dec 05 2024 Valid 12Z Mon Dec 09 2024 - 12Z Fri Dec 13 2024 ...Heavy precipitation and gales expected along the southern coast through at least early next week, additional moderate to heavy precipitation expected for the western mainland... ...Overview... Model guidance remains consistent and agreeable in showing a strong upper-ridge building northward over the northeastern Pacific/mainland Alaska and slowly translating east through the middle part of the period. Multiple shortwaves/upper-lows to the west rotate around mean low pressure in vicinity of the Bering Sea in a gyre-like pattern, with increasing uncertainty on the timing/evolution of the shortwaves/upper-lows through next week. However, the overall pattern featuring ridging along the Panhandle/eastern mainland and approach of systems to the west will support very heavy precipitation along the southern coast from Sunday through at least Tuesday with a northward surge of Pacific moisture in strong southerly flow. Confidence has also increased in the potential for moderate to heavy precipitation, including freezing rain, for western portions of the Mainland Sunday and Monday. The strong ridge will also result in well above average temperatures for the state through at least Wednesday, particularly for central and northern portions of the mainland. ...Guidance Evaluation/Preferences... Guidance has remained in good agreement on the overall pattern that will guide expected precipitation, hazardous weather, and temperatures through at least the middle of next week. A strong, broad-scale ridge will build northward through the Gulf of Alaska and mainland this weekend that will make slow eastward progression through mid-next week. At the same time, at least a couple upper-level waves/accompanying surface systems are expected to approach the state from the west. Uncertainty grows with time on the evolution of these waves, though the deterministic guidance continues to show good agreement on the track and increasing strength of an initial system lifting northward from the northeastern Pacific towards the Aleutians/Alaska Peninsula and passing into the Bering Sea Monday. This is quickly followed by another upper-level shortwave/surface system in its wake lifting towards the Aleutians, with increasing uncertainty on timing/track as it passes by the Aleutians and approaches the Alaska Peninsula. The 12Z GFS/ECMWF bring this system eastward faster compared to the CMC and better align with the timing in the ensemble means. Then, all deterministic guidance diverges on track Tuesday into Wednesday, as the CMC shows the system lifting northward into the Bering similar to the prior system, the GFS takes it into the southwest mainland, and the ECMWF shows a more southerly track eastward into the Gulf of Alaska by Wednesday morning. The 12Z ECens/GEFS means show stronger support for upper-level troughing/surface low pressure into the Gulf of Alaska, most similar to the ECMWF, and maintain a pattern that was similar to guidance from the prior couple days. The GFS, and to some degree the ECMWF, show a secondary wave that helps to reinforce upper-level troughing over the Gulf of Alaska and maintain an area of low pressure. At the same time, both the deterministic and ensemble guidance agree that the strong upper-level ridge will eventually weaken and shift eastward by later next week, allowing for this potential of a more southerly wave track from the northeastern Pacific into the Gulf of Alaska. To the west, the guidance is also in good overall agreement that ridging will build northward over the Aleutians ahead of a deepening northern Pacific low as the pattern amplifies a bit more, with the GFS/ECMWF in best agreement with the ensemble means compared to the stronger low pressure lingering in the Bering Sea for the CMC. All deterministic/mean guidance show this deepening low approaching the western Aleutians towards the end of the period Friday, though the GFS does show additional leading shortwave energy. The updated WPC forecast for Alaska begins with a blend of the deterministic guidance given the good agreement on the strength/track for the initial system into the Bering Sea Monday and subsequent system approaching the Aleutians from the southwest Monday into Tuesday. Larger contributions from the 12Z ECMWF/GFS are favored over the CMC given they both better fall within the envelope of the ensemble means. A contribution from the ensemble means is added by the middle of the period on Wednesday as uncertainty in track/evolution of the second system increases, with a more southerly/easterly emphasis into the Gulf of Alaska favored from the ECMWF/GFS and means. The contribution from the means is increased to over 50% by Friday with added detail from the ECMWF which best fits the means at this point, given uncertainty over additional energies in the GFS over the northeastern Pacific/Gulf of Alaska and also upstream ahead of the late week system over the northern Pacific/western Aleutians. ...Weather/Hazards Highlights... High confidence remains in a mid-latitude Pacific system lifting northward over the eastern Aleutians/Alaska Peninsula and into the Bering Sea as ridging builds northward to the east over the Panhandle/eastern mainland, helping to enhance/focus moist southerly Pacific flow along the southern coast and bring very heavy precipitation to the region late this weekend and into early next week. Reinforcement from a secondary system following in its immediate wake looks to continue this threat through at least Tuesday, with a gradual shift in focus from west to east along the southern coast and into the northern Panhandle with time. Expect the upper-ridge to move far enough eastward to allow for the precipitation focus to shift into the Panhandle and perhaps even as far south as British Columbia by Wednesday, with greater uncertainty with respect to coverage and amounts during this time frame. For the period as a whole, expect mostly rain along coastal/low elevation areas given warm temperatures with snow for higher mountain elevations. Strong, gale force winds can also be expected with the approach of both systems from the eastern Aleutians/Alaska Peninsula east along the southern coast Sunday through Tuesday. Away from the southern mainland hazards, the details have come into better focus regarding additional moderate to heavy precipitation, including the potential for freezing rain, as well as gusty winds, for the western coast/mainland Sunday-Monday as the first system tracks northward into the Bering Sea. Some areas of lighter precipitation will likely linger beyond Monday, and also expand into portions of the interior/North Slope. Stronger post-wave ridging over the central/eastern Aleutians/Alaska Peninsula Thursday into Friday may bring a brief break in precipitation chances for these regions. Another system over the northern Pacific may begin to approach the western Aleutians by the end of the period on Friday, bringing the potential for both heavy precipitation and strong, gusty winds. Expect above to well above average temperatures to persist across the state through most of the period, with those for morning lows even higher than those for afternoon highs. Anomalies of 10-25 degrees can be expected for afternoon highs with anomalies upwards of 20-35 degrees for morning lows, with the highest anomalies focusing across the interior mainland/North Slope. Forecast highs early to mid-next week generally range from the teens for the North Slope, the teens and 20s for the interior mainland, 20s and 30s along the western coast, 30s and 40s for southwestern portions of the state, and mostly 40s along the Aleutians, southern coast, and Panhandle. A pattern change may bring cooler, more seasonable temperatures to the northern half of the state by later next week. Putnam 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