Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM 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... ...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 coastal low development whose center is most likely to track just off the Mid-Atlantic/New England coast. Guidance continues to suggest potential for a band of significant snowfall/wintry weather where a sufficient combination of moisture and cold air exists, while farther south some areas of moderate to heavy rainfall are possible. For the eastern Pacific into the western U.S. upper trough, the primary guidance difference involves the amplitude and timing around Tue-Wed. GFS/UKMET runs have been sharper/more amplified and slower in contrast to the CMC/ECMWF. Over the past day the ECMWF mean has nudged a bit slower toward the GEFS mean (which predictably hints at the GFS scenario), but the new 00Z GFS has adjusted to a more intermediate solution versus some earlier GFS runs. Interestingly these various solutions converge in principle by day 7 Wed as the trough moves into the West. Persistence of the two camps along with the occasional adjustment of one solution or another toward the middle of the spread would favor a general blend approach at this time. The evolution affecting the eastern half of the U.S. will have some complexity in terms of contributions from individual features aloft but guidance is gradually coming closer together for the overall evolution. The GFS/GEFS mean are a bit on the fast/strong side with the core of the leading shortwave that crosses the Plains on Sat. This leading impulse may settle near the Lower Great Lakes while upstream energy feeds into an eastern U.S. trough that takes shape as an upstream ridge amplifies. Then the amplifying ridge may help another bundle of energy to drop into the eastern trough. By around early Tue there is decent clustering toward an upper low centered over the northern Mid-Atlantic coast but it remains to be seen whether the latter energy merely flows around the established upper low (as in the 00Z GFS) or leads to a new low on its own (00Z CMC). The upper low should track northeastward thereafter. For surface low pressure, issues with the GFS/GEFS aloft lead to the initial Plains to Ohio Valley system straying somewhat to the northeast side of the spread. With the coastal development by early Mon the ECMWF/ECMWF mean are a bit south of other solutions though they and the 18Z/00Z GFS cluster fairly well by early Tue--farther west/southwest than latest GEFS means. Multi-day trends have generally favored a farther southward track, though this trend appears to have stabilized over the past 24 hours. Both eastern events this week fit into the theme of colder air winning out to support a farther south low track/precipitation distribution. These considerations favored tilting the forecast somewhat (but not completely, due to the various detail uncertainties) toward the ECMWF/ECMWF mean in principle. Ultimately a 12Z ECMWF/18Z GFS/12Z UKMET blend in order from more to less weight Sat into early Mon, followed by a blend of ECMWF/GFS runs and their means with a little more ECMWF/ECMWF mean versus GFS/GEFS, led to a coastal low position closest to the ECMWF early Mon followed by a track toward an average of the 12Z ECMWF/12Z ECMWF mean/18Z GFS by early Tue. ...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. With 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 through the Mid-Atlantic and Northeast. There is still some uncertainty in specifics even west of the Appalachians. 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 progression should be sufficient to 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. 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