Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Wed Jan 27 2021 Valid 12Z Sat Jan 30 2021 - 12Z Wed Feb 03 2021 ...Significant precipitation event likely along the central West Coast from late weekend into early next week... ...Plains to western Atlantic system will spread a broad area of rain and wintry weather across the eastern half of the lower 48 with cyclogenesis possible off the Northeast coast... ...Weather Pattern Overview and Guidance/Predictability Assessment... Models and ensembles agree that two dominant features will highlight the Sat-Wed period. One will be an eastern Pacific upper trough gradually sharpening during the weekend and then moving into the western U.S./northern Plains by Tue-Wed. The upper trough and associated surface systems should initially focus precipitation over the Pacific Northwest followed by a more concentrated emphasis near the central West Coast. Then moisture will spread across much of the West with somewhat less precipitation intensity as the upper trough moves inland. Farther east, low pressure emerging over the northern Plains early Sat will continue into the East before giving way to a decent model consensus on coastal low development somewhere just off the coast of the northeastern U.S. The typically fast GFS has continued to be near the fast end of the guidance envelop while the 00Z ECMWF slowed down this system further (although not as slow as it ensemble mean position). The 00Z CMC and its ensemble were comparable to the GFS. The preference is to lean slightly toward the slower ECMWF solutions for this system. In any case, guidance continues to suggest potential for a band of significant snowfall/wintry weather where a sufficient combination of moisture and cold air exists such as along the central Appalachians. Farther south, some areas of moderate to heavy rainfall are possible. Toward the middle of next week, there is good model consensus for an upper trough to amplify and dip into the Southwest as a low pressure system gradually becomes consolidated over the central High Plains. Widespread precipitation, locally heavy, over the Pacific Northwest will advance eastward into the Rockies and the northern Plains by midweek. The EC mean, GEFS, and to a lesser degree the Canadian mean agree with this general scenario. The deterministic solutions exhibit noticeable run-to-run variability with this system at this point. The WPC medium-range forecast package was based on the consensus of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, a smaller contribution from the 00Z CMC/CMC mean, and the 13Z NBM, but leaning slightly toward the slower solutions from the ECMWF. More of the ensemble means were used on Days 6 and 7 to handle the increasing model uncertainty. ...Weather/Threats Highlights... Precipitation coming into the West during most of this weekend will primarily focus over the Pacific Northwest with some areas of moderate to heavy rain and mountain snow. Lesser amounts may reach down into northern California. Amplification/sharpening of the offshore upper trough will likely begin to support a heavier precipitation focus over or near southwestern Oregon and northern California around late Sunday or Sunday night, most likely continuing into Tuesday. This general scenario has been fairly consistent in the guidance but there is still some fine-tuning to be done for specifics of the upper trough and associated rain/snow. Heading into midweek expect progression of the upper trough to spread moisture eastward/southeastward across the West into the Rockies. Precipitation should be less intense than along the West Coast but some localized enhancement along favored terrain is possible. Regarding the system tracking eastward from the Plains and then redeveloping near the Mid-Atlantic/New England coast, probabilistic winter weather guidance shows the best potential for meaningful snow extending from the Midwest through parts of the Ohio Valley and then from the interior Mid-Atlantic/central Appalachians and across New England. There is still some uncertainty regarding the rain/snow transition zone. Meanwhile the potential complexity of the evolution aloft over the East Coast further tempers confidence in specifics, especially given how sensitive precipitation location/type/intensity will be to important details that will likely take into the short range time frame to resolve. There may be a small margin for error between a significant snowfall and a modest one. Farther south expect the highest rainfall totals to extend from near the east-central Plains/Mid-Mississippi Valley into North Carolina or vicinity. Some of this activity may be moderate to heavy but its relatively swift motion should keep rainfall totals from becoming too extreme. Expect the warmest temperature anomalies during the period to be over the central U.S., supported by flow ahead of the initial Plains system and then again ahead of the system reaching into the West/northern Plains by next Wednesday. In general the anomalies for morning lows should be greater than those for daytime highs--the former seeing broad coverage of plus 10-20F or even greater versus normal on some days in contrast to smaller areas of highs 10-20F above normal. The eastern U.S. will tend to be near to below normal during the period with coldest readings likely over the Northeast into Mid-Atlantic early in the weekend. Cool highs over the southwestern U.S. should moderate some from the weekend into Monday. Then expect a cooling trend over much of the West by midweek, with some highs next Wednesday possibly 10F or more below normal over the southern half of the West. Kong/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 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