Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EST Thu Dec 24 2020 Valid 12Z Sun Dec 27 2020 - 12Z Thu Dec 31 2020 ...Significant California to Midwest system to bring potential for snow from the central Plains to the Great Lakes midweek late next week... ...The same system could spread heavy rain across the Deep South into the East Coast during the latter half of next week... ...Overview and Model Guidance Evaluation/Preferences... Moderately progressive mean flow will carry a couple of significant upper troughs with associated impactful weather across the country during the medium range period. The first upper trough moving from the Great Lakes through New England early next week shows decent model agreement with the typically faster GFS relative to the ECMWF. Meanwhile, the next significant trough will likely move onshore across California. Models indicate good agreement that this system will develop into a significant low pressure as it moves across the central Plains toward the Great Lakes during the latter half of next week. There has been a general faster trend in the guidance to carry the system into the Great Lakes, with a sharper cold front heading faster into the eastern U.S. The WPC morning medium range package was based on a blend of the 00Z ECMWF/00Z EC mean and the 06Z GFS/06Z GEFS. The latest WPC prognostic charts now indicate a faster forward motion of this late-week system as a result. ...Weather/Threats Summary... The dominant system of interest during the period will likely bring rain and high elevation snow into California (including southern parts of the state) by late Sun-Mon, with precipitation extending into parts of the Great Basin/Four Corners states and vicinity early next week as well. Amounts will generally be moderate but with some localized enhancement over terrain favored by southwesterly flow aloft. This should be welcome rainfall given the relatively dry start to the season so far, but just a modest start in addressing the long-term drought over this part of the country. Expect precipitation to expand and intensify from the Plains eastward by Tue/midweek as the system from the West begins to draw Gulf moisture northward. Currently the best potential for significant snowfall extends from near the central Plains into the Upper Midwest (see days 4-7 Winter Weather Outlook for probabilities of 0.25 inch or greater liquid) while areas to the south of this axis may see heavy rainfall. Winds could become brisk to strong as well depending on how the system develops. Precipitation will eventually spread into the eastern U.S. and again possibly with multiple precip types for the Northeast into the Appalachians with areas of heavy rain potential farther to the south. From Sun into Tue the strengthening low tracking northeastward from the Midwest should bring mostly light precipitation to the northeastern quadrant of the lower 48, with northward multi-day trends for this system leading to a corresponding decrease in precip totals. This system may bring a period of brisk/windy conditions along with a brief episode of lake effect snow after the system passes by. Areas from the Pacific Northwest into northern California should see a return of rain/mountain snow of varying intensity by around Tue-Wed with highest totals over the favored terrain near the coast and in the Cascades. Somewhat lighter activity may reach into the northern Rockies. Temperatures will vary with system progression during the period. From Sun into Mon above normal readings will prevail in the warm sector ahead of the leading Midwest/Great Lakes system and trailing front. Best potential for plus 10F or greater anomalies for min and/or max temps will extend from the central/southern Plains into the Ohio Valley/Lower Great Lakes. Behind this system the northeastern Plains/Upper Mississippi Valley may see a day or two with temperatures at least 10-15F below normal. Moderately below normal highs will spread across a majority of the West as the California system arrives and continues eastward. Parts of the central Plains may see highs of 10-15F below average in the cold sector of this system by Tue-Wed. Meanwhile leading southerly flow will raise temperatures from the Mississippi Valley into the East by Wed-Thu with min temperatures likely to exceed 10F above normal over a broader area than daytime highs. Kong/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 Central Great Basin, California, and the Southwest, Sun-Tue, Dec 27-Dec 29. - Heavy precipitation across portions of the Central Rockies and the Central Great Basin, Sun-Mon, Dec 27-Dec 28. - Heavy precipitation across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Upper Mississippi Valley, Tue-Wed, Dec 29-Dec 30. - Heavy precipitation across portions of the Northeast and the Great Lakes, Thu, Dec 31. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Tue, Dec 29. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Wed, Dec 30. - Heavy rain across portions of the Southeast, the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Thu, Dec 31. - Heavy snow across portions of the Pacific Northwest, Wed, Dec 30. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Wed-Thu, Dec 30-Dec 31. - Heavy snow across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Tue-Wed, Dec 29-Dec 30. - Heavy snow across portions of the Northeast, Mon, Dec 28. - Flooding possible across portions of the Great Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians, and the Ohio Valley. - Flooding occurring or imminent across portions of the Pacific Northwest. - Heavy precipitation across portions of mainland Alaska, Mon-Tue, Dec 28-Dec 29. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml