Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EST Sun Dec 25 2022 Valid 12Z Wed Dec 28 2022 - 12Z Sun Jan 01 2023 ...Multiple rounds of moderate to heavy precipitation are likely across the West this week... ...Rainfall could be heavy across the southeastern quadrant of the country late this week... ...Overview... Many parts of the lower 48 should see an active weather pattern from midweek through next weekend. A rapid sequence of Pacific systems will bring multiple episodes of precipitation to much of the West, with some areas of potentially heavy rain/snow depending on elevation. There will likely be a tendency for greater moisture focus over southern parts of the region by the start of the weekend. The energy from these systems will feed into a long-term upper level mean trough becoming established over the Rockies/Plains while an upper ridge builds over the East Coast/western Atlantic. The flow between the trough and downstream ridge aloft, and between one or more central U.S. systems and high pressure to the east at the surface, will spread precipitation across an increasing portion of the eastern half of the country while promoting unseasonably warm temperatures that should lead to a majority of this precipitation falling as rain. The best potential for highest rainfall totals should extend from the Gulf Coast northward through the southern half of the Mississippi Valley and eastward into the far southern Appalachians. ...Guidance/Predictability Assessment... In a continuation from recent model cycles, there is a consistent and agreeable depiction of the overall mean pattern but with some spread for individual features. Guidance from the 00Z/06Z runs used for the updated forecast, combined with new 12Z runs, suggest some details may be starting to come into better focus but of course with recent guidance behavior still tempering confidence somewhat. Regarding the sequence of Pacific systems, consensus looks good for the initial upper trough moving into the West. The majority of guidance shows the next feature as a frontal system that could have some embedded waviness but with modest definition and a track near or north of Vancouver Island if there is one. There is still moderate spread for the next system forecast to arrive Friday, with the new 12Z UKMET joining latest GFS/ECMWF runs in showing a fairly vigorous system reaching the Pacific Northwest while CMC runs and the ensemble means show or imply any surface low would track well to the north. Finally, there has actually a little better clustering for the trailing system that is most likely to track farther south into California and beyond during the weekend. However the new 12Z ECMWF has shifted somewhat north of the majority, but supporting dynamics are still well within typical detail variability for that time frame. Meanwhile ahead of the developing mean trough, guidance has been waffling over the last few days with respect to whether northern tier energy could support low pressure that would lift northeastward into southern Canada late this week. In the ongoing multi-day oscillation, latest trends have shifted toward better potential of such a feature in contrast to more persistent eastern Canada high pressure. The new 12Z ECMWF has strayed on the fast side of guidance. There has been a little more consistency with the idea for a well-defined low to emerge from the Plains and then track over the Mississippi Valley into the Great Lakes by the weekend. There is spread/variability for timing and track but the ensemble means depict this system well in principle. The updated forecast started with a 00Z/06Z model composite early and then transitioned to a blend of the 06Z GFS/00Z ECMWF and their respective ensemble means. The 00Z CMC strayed from the primary cluster over the eastern half of North America by the latter half of the period so it was phased out of the blend then. ...Weather/Hazard Highlights... A series of Pacific fronts/systems should bring a multi-day period of notable precipitation to much of the West. While the strongest atmospheric river expected has now shifted into the short range period, additional rounds of heavy rainfall along coastal areas may cause flooding problems in areas that will be hydrologically sensitive by that time. A Slight Risk area was maintained in the experimental Day 5 Excessive Rainfall Outlook for southwestern Oregon/coastal California as most model guidance shows another bout of heavy rain moving in by then, though there remains some uncertainty with the timing. Additionally, significant snow amounts are likely in higher elevations of the West through much of the week. Exact timing and placement details of the heaviest precipitation through late week and the weekend still show some differences, so continue to monitor forecasts for specifics as the details come into better focus over coming days. However for next weekend the latest guidance does show a general theme of precipitation trending lighter over northern areas, with greater focus from California through the southern Great Basin and Southwest U.S. East of the Rockies, a weak leading wave lifting northeastward may produce some precipitation (likely at least some of it snow) across the northern Plains around Wednesday-Thursday. By the latter half of the week, developing moisture inflow from the Gulf of Mexico into the Mississippi Valley and vicinity ahead of one or more Plains low pressure/frontal systems and supporting upper trough will lead to increasing coverage and intensity of precipitation, most of it rain. Within this expanding moisture shield, rainfall could become locally moderate to heavy by Friday-Saturday with highest rainfall totals currently expected to be approximately from the Gulf Coast through the Lower Mississippi Valley and into the Tennessee Valley and Southeast. Latest guidance is starting to cluster around the potential for a band of enhanced rainfall over and near Arkansas Thursday-Thursday night, leading to the introduction of a Slight Risk area in the experimental Day 5 Excessive Rainfall Outlook. A broader area of heavy rainfall will be possible across the South shortly thereafter. Progression of a potential Plains into Great Lakes system next weekend should spread moisture through the rest of the East but with maximum rainfall totals likely to trend lower as moisture streams northward. Expect most wintry weather from this system to be confined to the northern Plains/Upper Midwest. After the recent cold spell, the temperature pattern in the medium range period will significantly change across much of the nation. Though the Southeast may still have some below normal readings for midweek, a widespread area of above normal temperatures in the central U.S. will be expanding eastward through the latter part of the week. Low temperatures look to be 15-30F above average over the High Plains Wednesday with highs around 10-20F above average, and temperatures of this magnitude should shift east underneath the upper ridge into the east-central U.S. late week and the East by next Sunday. Some daily records for warm lows may be possible as the warmest air reaches the East, while a few isolated daily record highs cannot be ruled out. Though meanwhile in the West, the wet/snowy pattern will tend to keep daytime temperatures within a few degrees of normal, though lows should generally be a bit above average. Rausch/Tate 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, experimental excessive rainfall outlook, 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/#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