Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 26 2022 Valid 12Z Thu Dec 29 2022 - 12Z Mon Jan 02 2023 ...Multiple rounds of moderate to heavy precipitation are likely across the West this week... ...Heavy rainfall and flooding are possible across the southeastern quadrant of the country late this week... ...Overview... An active pattern is likely across much of the lower 48 late this week into early next week. 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 Thursday-Saturday 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. low pressure 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. By early next week this trough is expected to lift into the Midwest while another longwave trough deepens over the West. ...Guidance/Predictability Assessment... Model guidance has been persistent in showing reasonably good agreement on the large scale but with some spread for individual features. Differences in shortwaves within the main trough as early as day 3/Thursday are not anomalously large for the medium range, but do translate into some spread at the surface for surface low tracks and frontal positions in a reasonably complex pattern within mean low pressure in the central CONUS. The main outlier initially Thursday-Friday at the surface seemed to be the 12Z ECWMF with a fast low track across the Midwest into eastern Canada where other models were maintaining a high. But other than that, there did not seem to be major outliers in the guidance, just general variations. There has been some signal for a surface low to lift across the Midwest/Great Lakes over the weekend, though the newer 00Z model cycle shows this to be generally weaker. For the West, similarly there are some differences in the details, but the pattern favoring multiple rounds of precipitation and a shift toward favoring the southern half of the West by early next week is persistent and agreeable. Models have waffled with indicating stronger surface low features at various times in the eastern Pacific; for example the 12/18Z GFS runs showed a strong low offshore of the Pacific Northwest 12Z Friday but this is not favored in the newer model runs. A model compromise seemed to do a fine job to mitigate these differences. There is reasonably good agreement given that it is the day 6-7 timeframe for another synoptic scale round of troughing to dig across the West and track east with its axis around the Rockies by next Monday. The WPC forecast was able to use a multi-model deterministic blend of guidance early in the period given the large-scale model agreement, with the blending process serving to create a compromise solution. GEFS and EC ensemble means were introduced and increased in the blend by days 5-7 to mitigate model variations but was able to keep deterministic models as the majority through the period. ...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 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. 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