Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Dec 26 2022 Valid 12Z Thu Dec 29 2022 - 12Z Mon Jan 02 2023 ...Multiple rounds of moderate to heavy precipitation likely along the West Coast, central Rockies, and the Southwest for the rest of this week before possibly spreading into the northern Plains to begin 2023... ...Heavy rainfall and flooding possible west of the lower Mississippi Valley and across the eastern Gulf Coast region to end 2022 before the next heavy rain threat re-emerges to the west of the lower Mississippi Valley to ring in the new year... ...Overview... A transition from a ridge-trough to a trough-ridge upper-level pattern is forecast to occur across the mainland U.S. by the time medium-range period begins midweek. Multiple Pacific systems pushing onshore into the western U.S. will act to build and maintain a synoptic scale trough farther south into the southwestern U.S. with time, bringing multiple episodes of heavy precipitation mainly across the Southwest, the Great Basin, then toward the central Rockies, and possibly reaching into the northern Plains by the time 2023 begins. On the other side of the continent, a building ridge will keep mild air in place for much of the eastern and southern U.S. while three episodes of heavy rain appear to transpire across the South. By early next week, the next Pacific system appears to bring the next surge of moisture farther up the West Coast from northern California to the Pacific Northwest. ...Guidance/Predictability Assessment... Model guidance this morning is in relatively good agreement with one another with the general pattern evolution as described above. The largest discrepancy is noticed across the central to eastern U.S. regarding the speed of frontal waves and upper-level shortwaves moving toward the Great Lakes, with the fastest being the 00Z CMC. In general, the models are trending toward a more sheared and positive-tilted upper-level pattern leading to a weaker cyclone to track across the Great Lakes this weekend. As of the West, models are in surprisingly good agreement on the timing and location of features associated with the low pressure system to track across the Southwest this weekend and reaching into the southern Plains by early next week. The WPC forecast was composed based on a blend of the deterministic and ensemble means from the 00Z ECMWF, 06Z GFS, with a smaller contribution from the 00Z CMC, leaning slightly more toward the ensemble means by Days 6 and 7. ...Weather/Hazard Highlights... A series of Pacific fronts/systems should continue to sustain a multi-day heavy precipitation event across the western U.S. While the strongest atmospheric river expected is now fully within 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. Slight Risk areas are shown in both the Days 4 and 5 experimental Excessive Rainfall Outlooks across southwestern Oregon/coastal California due to bouts of heavy rain moving in during those days. Additionally, significant snow amounts are likely in higher elevations of the West through the latter half of the week and into the weekend. Exact timing and placement details of the heaviest precipitation still show some differences, so continue to monitor forecasts for specifics as the details come into better focus over coming days. However, there is a general trend of precipitation lightening in northern areas by the weekend while the better focus for precipitation becomes across California through the central/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 into Thursday. More notably around Thursday-Saturday, 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 heavy 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/Southern Appalachians. Slight Risks of excessive rainfall are also in place for these areas as instability could produce high rainfall rates that may cause flooding/flash flooding. Progression of a potential Plains into Great Lakes system next weekend should spread moisture through the rest of the East as moisture streams northward, but with maximum rainfall totals likely to trend lower. Expect most wintry weather from this system to be confined to the northern Plains/Upper Midwest through late this week. Better but still uncertain chances for snow may be across the central High Plains by Sunday into next Monday as another upper trough provides colder air there. After the recent cold spell, the temperature pattern in the medium range period will significantly change across much of the nation. A widespread area of above normal temperatures in the central U.S. will be expanding eastward through the latter part of the week and continuing into early next week. Low temperatures look to be 15-25F above average over the Plains/Mississippi Valley Thursday 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. Daily records for warm lows may be widespread 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. Lows should generally be a bit above average until early next week as the second large-scale upper trough moves through, bringing highs to around 10F below normal as well. Kong/Tate/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, 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