Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 653 PM EST Mon Dec 02 2024 Valid 12Z Fri Dec 06 2024 - 12Z Tue Dec 10 2024 ...Potential for heavy precipitation along the southern coast and Panhandle early next week... ...Overview... During the Friday-Tuesday period, the ensemble means have been fairly consistent and agreeable in showing a persistent upper ridge from the Panhandle and western Canada northwest through much of Mainland Alaska (though with some variation in shape), while multiple shortwaves/upper lows to the west eventually become overwhelmed by flow around a large scale upper low settling in the vicinity of Kamchatka and the western Aleutians. The southern coast and Panhandle may 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. Operational models have been showing a lot of spread and run-to-run variability for embedded features though, tempering confidence in some specifics of the forecast. ...Guidance Evaluation/Preferences... Operational models have shown some significant trends/divergences in the past 12-24 hours for what may be two separate northeastern Pacific/Gulf waves, along with a mid-latitude Pacific system that should reach close to the Alaska Peninsula by early Sunday and then trailing features over the North Pacific/Aleutians/Bering Sea. In order to maintain the best continuity possible while guidance is in the process of sorting out the specifics, today's preference was to stay as close to the means as possible in concept and supplement that template with the most agreeable operational model runs. During Friday-Saturday, recent guidance has been changing/diverging with respect to evolution of what may be a leading wave near the Alaska Peninsula/Kodiak Island and a separate wave to the south as of early Friday. Latest trends have generally been weaker with the upper dynamics/surface low with the northern feature while models offer a host of possibilities for what may become of the southern one. Ensemble means and 00Z machine learning (ML) models recommended a more consolidated evolution (whether merging the two or weakening the northern one in favor of the second) reaching the Gulf by early Saturday and quickly weakening/departing thereafter. ECMWF runs best fit this idea. Not surprisingly, the new 12Z ML models available after forecast preparation has diverged some like the dynamical runs. For the system most likely to reach just south of the Alaska Peninsula by early Sunday, the ensemble means and ML models have offered the most consistent theme over the past couple days with a mostly northward track through the rest of Sunday into Monday. ML models have tended to maintain somewhat more definition by Monday than some other guidance, by way of slightly later arrival of overwhelming large scale cyclonic flow. As of early Sunday ML average depth was in the upper half of the 970s mb, so the 12Z ECMWF is in the neighborhood. The 12Z GFS tracked the system well east into the Gulf, with minimal support from other guidance while the 12Z CMC seems to stray too far north. The 06Z GFS and new 18Z GFS compare better to the means and ML models in principle but may be too deep. Individual model solutions become even more chaotic behind this system by Monday-Tuesday, with potential for one or more surface lows anywhere between the northern Pacific and the Bering Sea. Consecutive GFS/ECMWF/CMC runs have all been very erratic, while latest ML runs hedge toward the North Pacific and the AIFS mean has tended to have its main low center over the Aleutians by early next Tuesday. The more consistent and agreeable ensemble means provide the best option to depict the large scale pattern for now until specifics become better resolved. The early part of the forecast emphasized the 00Z/12Z ECMWF runs early in the period to reflect preferences over the Northeast Pacific, with a 30 percent ensemble mean weight (12Z GEFS/CMCens and 00Z ECens) introduced by day 4 Saturday. By Sunday the blend transitioned to nearly half means and the rest 12Z ECMWF and 06Z GFS for the system forecast to be near the Alaska Peninsula. The increasingly poor model agreement after Sunday led to increasing the ensemble means to 75 percent with the rest comprised of the 12Z ECMWF on Monday and a split of the last two ECMWF runs on Tuesday. ...Weather/Hazards Highlights... Latest trends for features over and possibly south of the Gulf of Alaska from Friday into the weekend have generally added to recent signals suggesting a short break in the overall wet/snowy pattern along the southern coast and Panhandle. However a leading wave that may be near Kodiak Island as of early Friday could still produce some localized enhancement of precipitation around Prince William Sound. The most likely scenario of a mid-latitude Pacific system lifting northward near the western Alaska Peninsula on Sunday and persistent southerly mean flow farther east thereafter should bring enhanced moisture and heavy precipitation into the southern coast and Panhandle from Sunday into the first part of the next work week. Guidance signals are currently favorable enough for the Days 3-7 Hazards Outlook to depict a heavy precipitation area from the eastern Alaska Peninsula/Kodiak Island to the northern Panhandle valid Sunday-Monday. 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 southern Panhandle 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 brisk to strong winds. Away from the southern mainland moisture focus, some areas of mostly light snow are possible during the period over the western and north-central parts of the mainland. North Pacific into Bering Sea system uncertainties lead to low confidence for sensible weather specifics over the Aleutians where there should be at least some periods of unsettled conditions. 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. 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