Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM 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 heaviest rain/snow to areas from the Plains eastward Tuesday into late next week... ...Overview and Model Guidance Evaluation/Preferences... Moderately progressive mean flow will carry a series of systems, with an amplifying upper trough/embedded low and associated surface reflection expected to reach California by early day 4 Mon and continue generally eastward thereafter likely generating the broadest area of significant weather during the period. Ahead of this system, low pressure emerging from the Plains/Mississippi Valley on Sun should strengthen with a track through the Great Lakes and just west/north of New England as sharpening southern Canada-northern tier U.S. energy increases its influence. After the California system exits the West, an upper shortwave and associated frontal system should bring some moisture into mainly the northern-central West Coast. Previously lower confidence for the leading system tracking northeastward from the Midwest is increasing now that guidance is more strongly clustered. The current consensus track is well northward versus what many of the quite varied solutions had been advertising over recent days. The interaction of southern Canada/northern U.S. energy with the originally supporting shortwave ejecting from the West in the short range period still provides some uncertainty though. Recent trends yield decreasing coverage/amounts for precipitation with this system. A consensus blend or multi-model/multi-run average has held fairly steady over recent days for the evolution of the system forecast to amplify into California early in the week and reach the Plains by midweek. However over the past day or so there appears to be increasing sensitivity to specifics of shortwave energy that may try to round or push into the northern part of the upstream ridge. Recent GFS runs had been on the faster side of the spread along with the 00Z/23 ECMWF. However the new 00Z CMC and ECMWF have both trended faster than their 12Z runs, and it will be interesting to see if this trend persists or reverses. Based on guidance available through the 18Z cycle, the 12Z GFS was favorably a little slower than the 18Z run by late in the period. Guidance as a whole is showing a decent signal for the shortwave energy expected to reach the Northwest late in the period but with various ideas for timing and amplitude/shape. Latest ensemble means generally recommend a compromise between the slower 12Z ECMWF and faster GFS runs of late. Exactly how this shortwave energy evolves may also have at least some indirect influence on timing of the system reaching the central U.S. late in the period. Model/ensemble comparisons led to the updated forecast starting with a blend of the 18Z GFS and 12Z ECMWF/UKMET/CMC early-mid period followed by a transition to the 12Z GFS/ECMWF/CMC and some 18Z GEFS/12Z ECMWF mean input. ...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. 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. 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 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