Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EST Tue Dec 03 2024 Valid 12Z Sat Dec 07 2024 - 12Z Wed Dec 11 2024 ...Heavy precipitation and gales expected along the southern coast late this weekend/early next week... ...Overview... The ensemble means remain consistent and agreeable in showing a persistent upper ridge from the Southeast and western Canada northwest through much of Mainland Alaska (though with some variation in shape). Multiple shortwaves/upper-lows to the west rotate around persistent, mean low pressure over the Bering Sea in a gyre-like pattern, with increasing uncertainty on the timing/evolution of the shortwaves/upper-lows into next week. The southern coast and Panhandle look to see a brief break in the short-term wet/snowy pattern for at least a part of the weekend but the pattern evolution appears likely to support a surge of moisture from Sunday into Monday or Tuesday with strong southerly flow anchored along the coast between the upper-ridge to the east and persistent low pressure to the west. ...Guidance Evaluation/Preferences... The deterministic guidance showed much better pattern agreement with the updated 00Z/12Z runs with respect to the forecast this weekend. An initial upper low/surface system lingering over the eastern Gulf and Southeast will weaken/move inland as ridging builds northward in its wake, setting up broad-scale mean ridging over the Panhandle/eastern mainland/western Canada with persistent waves of low pressure approaching from the west. The 00Z/12Z ECMWF/GFS/CMC runs depict the first such wave deepening over the western Aleutians with strong surface cyclogenesis/low consolidation from energies in the northeastern Pacific by early Saturday morning. This system moves eastward south of the Aleutians before lifting northward towards the Alaska Peninsula and into the Bering Sea by early Monday. This will help to focus strong, southerly moist flow along the east side of the system directed at the southern coast later this weekend and into early next week. The guidance then begins to diverge with the phasing of the next system, as the GFS is faster in bringing a second wave/developing surface low pressure towards the western Aleutians by early Monday while the timing is delayed by about 12-24 hours in the ECMWF/CMC. The ECMWF/CMC remain in good agreement through late Monday but then begin to diverge into Tuesday as well, with the wave/surface system in the ECMWF following a more southerly track along/south of the Aleutians and towards the Gulf of Alaska while most of the upper-level energy and surface system in the CMC lift northward and consolidate with mean low pressure over the Bering Sea by Wednesday. The CMC does at least hint at some shortwave energy continuing east towards the Gulf of Alaska. The GEFS, ECens, and CMC means depict a similar pattern to the deterministic guidance through the weekend but the lack of detail with any subsequent systems is indicative in a significant increase in uncertainty in the pattern into early next week. The timing of secondary energy does look to support the phasing of the ECMWF/CMC through Tuesday, and does suggest some energy may continue eastward towards the Gulf of Alaska by mid-next week. The updated WPC forecast for Alaska began with a composite blend of the 12Z ECMWF/GFS/CMC through the weekend given generally good agreement. Contributions from the ECMWF/CMC are retained into early next week while the GFS is removed as the forecast starts to significantly diverge in timing/evolution compared to the other models. The GFS is replaced with the ECens to begin to help tamper down the more specific details of the ECMWF/CMC as uncertainty increases, though with due weight still given to the ECMWF/CMC given their retained good agreement. A heavy emphasis on the ECens and GEFS means is used by mid-next week as all three deterministic models diverge, with only the ECMWF retained as it most follows the means in suggesting a more southerly wave/surface system track into the Gulf of Alaska late in the period. ...Weather/Hazards Highlights... Latest trends for features over and possibly south of the Gulf of Alaska have generally added to recent signals suggesting a short break in the overall wet/snowy pattern along the southern coast and Southeast the first half of this weekend. Precipitation chances from a system weakening/moving inland along the eastern Gulf look to linger longest in vicinity of Prince William Sound and the far Southeast. Then, confidence continues to increase in a mid-latitude Pacific system lifting northward near the western Alaska Peninsula on Sunday 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. Continue to monitor forecasts as details become better resolved over the coming days. For the period as a whole, expect mostly rain along coastal/low elevation areas with snow elsewhere aside from the Southeast where milder conditions should promote broader rain coverage with higher snow levels. Depending on the strength of the Sunday system, there may be some areas of gale force winds from the Alaska Peninsula east long the southern coast as well. Away from the southern mainland moisture focus the details become more uncertain, but expect at least light precipitation chances for the western mainland, with the chance of some more moderate to heavy totals here as well in the Sunday-Monday time frame. Some areas of light snow will also be possible for north-central parts of the mainland. Uncertainties in the pattern over the northeastern Pacific and into the Gulf of Alaska/Bering sea lead to lower confidence in forecast specifics into mid-next week, though expect at least moderate precipitation chances will linger along the southern coast and potentially into the Southeast as the overall pattern featuring moist, southerly flow into the Gulf remains consistent. Expect above to well above normal temperatures to persist across the state through the period, with anomalies for min temperatures tending to be somewhat greater than those for highs. Putnam/Rausch 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