Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Dec 24 2023 Valid 12Z Wed Dec 27 2023 - 12Z Sun Dec 31 2023 ...Some areas of enhanced rainfall possibly reaching the northern Mid-Atlantic/Northeast by Wednesday... ...Two episodes of locally heavy precipitation possible for the central West Coast mid-late week... ...Overview... Models and ensembles continue to show the large scale pattern settling into an amplified and blocky regime dominated by mean ridging over the western U.S. into western/west-central Canada, which will weaken incoming Pacific shortwaves, and troughing over the eastern half of North America. An initial Plains/Midwest upper low will initially establish the eastern upper trough but then the details of the overall trough and associated surface reflection become very uncertain by late week through the weekend. A leading wave near the East Coast should spread rainfall northward into the Northeast during mid-late week while the upper low may produce lingering light snow over the north-central Plains and then a mix of precipitation types to the east. Over the West, most precipitation should focus near the West Coast with a couple frontal systems tending to produce the highest totals over/near northern California. ...Guidance/Predictability Assessment... In some respects the forecast remains in a holding pattern as multi-day means/teleconnections relative to strong positive height anomalies/ridging over Canada support the expected eastern trough/western ridge along with broad troughing over the eastern Pacific, but a lot of detail spread persists. Guidance is still having a difficult time resolving how northern stream flow will evolve and possibly interact with the initial upper low over the Midwest after Wednesday. Recent ECMWF/ECMWF mean runs have generally been on the fast side to eject the initial upper low and then leaned on the northern side of the spread with an upper low that digging northern stream flow may produce. The new 00Z ECMWF still exhibits some of these characteristics but has trended slightly toward other solutions. Meanwhile the GEFS mean has been fairly consistent with a farther south track of the upper low with the new 00Z GFS adjusting southward close to the GEFS path. A minority of solutions including a subset of the 12Z ECMWF-initialized machine learning models and 00Z UKMET/ICON would offer potential for an even farther south track. In different ways the 00Z UKMET/CMC ultimately develop a second upper low over the Great Lakes by Friday. Additional detail spread continues through the weekend. The wide array of guidance in the 12Z/18Z cycles favored an intermediate approach that kept the forecast fairly close to continuity. This would have a leading wave track northward near the Mid-Atlantic/New England coast and then trailing low pressure crossing New England. If trends for upper low track end up gravitating toward the southern side of the spread, the trailing surface system may develop more over/offshore the Mid-Atlantic. Over the West, a consensus approach looks good for resolving ongoing low-predictability detail differences for the weakening shortwave crossing the Northwest Wednesday-Thursday as well as some timing and depth differences for the next more amplified trough likely to reach the West Coast by around early Saturday (with the 18Z GFS/00Z UKMET on the slow side and the 12Z ECMWF perhaps a bit deep). Then spread rapidly increases by Sunday as the envelope ranges between recent GFS/ECMWF/ECens runs that undercut the western U.S. ridge, bringing some energy into the Southwest, and 00Z GEFS mean/CMC that hold onto a ridge. For now preference is to lean toward a modest amount of undercutting energy. The updated forecast started with an operational model composite early, with a little less 12Z ECMWF weight than average, followed by incorporation of some 18Z GEFS/12Z ECens-CMCens means later in the period. ...Weather/Hazards Highlights... The system forecast to affect the central and eastern U.S. mid-late week will spread rainfall from the Mid-Atlantic into Northeast and back into the Great Lakes/Upper Midwest, with snow trending lighter and more scattered over the north-central Plains and parts of the Upper Midwest. Rainfall may be locally heavy at times along the East Coast, with the Day 4/Wednesday Excessive Rainfall Outlook depicting a Marginal Risk area from parts of the Mid-Atlantic into New England. Differences in upper level and surface details have generally decreased confidence in this area, but there is still some agreement toward an axis of anomalous moisture crossing the region while the 12Z ECMWF indicated potential for some instability within the moisture shield. Precipitation specifics become even more uncertain after Wednesday. At the very least there should be some lake effect/terrain enhanced snow later in the week and possibly into the weekend as temperatures gradually decline and one or more upper features move overhead. Over the West, the forecast remains on track for two primary precipitation events to be focused over the central West Coast around Wednesday and Friday with the arrival of upper shortwaves and associated surface fronts. The Day 4/Wednesday Excessive Rainfall Outlook maintains the Marginal Risk area depicted in yesterday's Day 5 outlook, including parts of northern California and far southwestern Oregon. Aside from latest GFS runs trending lighter, remaining model/ensemble guidance maintains reasonable agreement toward a brief episode of locally heavy rainfall within the outlook area. Currently the Friday event looks like it may produce similar totals. In both events expect snow over higher elevations. The Pacific Northwest will likely see some precipitation from these events but with lighter amounts, while the Friday event (perhaps lingering into Saturday with lighter amounts) may spread a little moisture into the southern half of California but with lower confidence. There is also not much confidence in coverage of any mostly light precipitation that could reach farther inland during the weekend. Temperatures will be unseasonably warm on Wednesday from the Upper Midwest into Northeast and south into the Mid-Atlantic, with morning lows up to 20-25 degrees above normal and daytime highs up to 10-15 degrees above normal. Later in the week, the greatest anomalies will become more confined to New England and gradually decline. Some daily records for warm lows will be possible Wednesday into Thursday from the Great Lakes eastward. Establishment of upper troughing over the East after midweek will favor persistent chilly conditions with coolest anomalies of 5-10 degrees below normal over the Southeast and into the central Appalachians. Continue to expect moderately above normal temperatures over the West for most of the period, though perhaps trending closer to normal by next Sunday. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw