Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Dec 23 2023 Valid 12Z Tue Dec 26 2023 - 12Z Sat Dec 30 2023 ...Heavy rain threat reaching the Mid-Atlantic by Tuesday and possibly extending northeastward into Wednesday... ...Overview... Latest guidance shows the large scale pattern settling into a fairly amplified and blocky regime dominated by mean ridging over the western U.S. into western/west-central Canada, with troughing over the eastern half of North America and Pacific shortwaves weakening as they encounter the mean ridge. Serving to establish the eastern upper trough will be an initial Plains/Midwest upper low whose associated surface system will spread rainfall of varying intensity over the East early-mid week along with the potential for lingering snow over the north-central Plains. Precipitation over the East should trend lighter and more scattered later in the week but with uncertainty in specifics. 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... Recent multi-day D+8 means have been fairly agreeable in showing a dominant core of positive height anomalies positioned over central Canada, with corresponding teleconnections consistent for surrounding negative height anomalies associated with mean troughing over the eastern U.S. and eastern Pacific. This adds to confidence in the forecast large scale pattern, but individual solutions suggest rapidly increasing detail uncertainties from midweek onward--especially for the eastern U.S. trough. Early in the period recent guidance trends have been persistent toward a deeper Midwest upper low into midweek. Then guidance has rapidly diverged over how digging Canadian flow may interact with/replace the initial upper low, and at what point the overall trough could open up (while staying amplified). At the moment, by Thursday the 00Z GFS and farther north 12Z ECMWF (a northern extreme) are the most pronounced with the idea of upper low replacement while the UKMET/CMC maintain the initial low even with some some potential for northern stream input. Consensus trends have been a little south for the upper low by late week, and interestingly four of the five ECMWF-initialized machine learning models show an upper low track that would be south of most dynamical models aside from the new 00Z UKMET. Of course the uncertainty aloft translates to decreasing confidence in surface details by late week. There is a general consensus toward best defined/wetter low pressure tracking near the east coast Tuesday-Thursday while the Midwest system weakens, with potential for some re-development somewhere near the northern half of the East Coast around Friday but with less moisture to work with. However plenty of non-consensus day-to-day solutions are plausible within the overall mean pattern. Near the West Coast, the past couple CMC runs have finally joined other guidance for the shortwave arriving Wednesday-Thursday but the new 00Z UKMET strays to the slow side. By day 7 Saturday the 12Z ECMWF shows more closing of energy within the next arriving trough versus most other guidance, favoring emphasis of other models/means. Based on 12Z/18Z guidance comparisons, the updated forecast started with an operational model composite early but then by mid-late period placed more emphasis on the GFS/GEFS mean, ECens mean, and CMC runs due to lesser confidence in ECMWF details both over the East and late in the period along the West Coast. ...Weather/Hazards Highlights... The system affecting the central and eastern U.S. through midweek will spread locally heavy rainfall over portions of the East, with lighter rain extending back into the Great Lakes/Upper Midwest and lingering snow where the air is cold enough over the north-central Plains and then possibly wrapping around into the Midwest. The Day 4 Excessive Rainfall Outlook valid on Tuesday maintains a Marginal Risk area over the Mid-Atlantic as a wavy frontal system lifts northward. Southeasterly low level flow may help to enhance totals over favored terrain along the Appalachians while potential for some modest instability may contribute to another relative maximum near the coast. Confidence in system details at the surface and aloft decreases as moisture lifts northeastward by Wednesday. With some lingering sensitivity over the region and a loose signal for potential enhancement from the northern Mid-Atlantic into southeastern Maine, the new Day 5 ERO introduces a Marginal Risk area from the New Jersey vicinity northeastward. Precipitation should become lighter and more scattered with time, with a tendency toward lake effect/terrain enhanced snow as temperatures gradually decline. Along the West Coast, the primary precipitation events should be over the central West Coast around Wednesday and Friday with arriving upper shortwaves/surface fronts. A Marginal Risk area in the Day 5 ERO reflects fairly good guidance clustering toward a brief episode of potentially heavy rainfall centered over northern California and far southwestern Oregon. Current consensus then focuses highest totals over northern California for Friday. In both events expect snow over higher elevations. The Pacific Northwest will likely see some precipitation from these events but with lighter amounts. Continue to expect unseasonably warm temperatures ahead of the system forecast to extend from the Midwest into Southeast as of early Tuesday. Morning lows should be particularly anomalous with readings 20-30F above normal from the Upper Midwest into a decent portion of the East for one or more days, with such anomalies persisting over New England at least into Thursday. Locations from the Great Lakes/Ohio Valley into the Northeast will see the best potential of daily records for warm lows. Plus 10-20F anomalies should be more common for highs. Cooler air behind the system will initially still be on the mild side, with only slightly below normal readings over the central-southern Rockies/Plains on Tuesday. As upper troughing becomes established over the East later in the week, the Southeast and vicinity should trend cooler with temperatures dropping to 5-10F or so below normal. Much of the West should see moderately above normal temperatures through the period. 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