Extended Forecast Discussion NWS Weather Prediction Center College Park MD 131 PM EST Tue Dec 27 2022 Valid 12Z Fri Dec 30 2022 - 12Z Tue Jan 03 2023 ...Multiple rounds of precipitation are likely in the West with lower elevation heavy rain/flooding and higher elevation heavy snow... ...Heavy rainfall and flooding are possible across the Gulf Coast states late this week with a heavy rain threat reemerging over the Lower Mississippi Valley in early 2023... ...Overview... It remains the case that the medium range period begins Friday with upper troughing across the central U.S. that should lift over the weekend. Ahead of this trough and frontal systems at the surface, Gulf moisture will stream into the eastern third of the U.S. through late week causing potentially heavy rainfall and possible flooding with a focus across the Gulf Coast states. Notably warmer than average temperatures will ensure that most of this precipitation falls as rain even in much of the northern tier. Meanwhile in the West, considerable moisture influx from a long fetch of Pacific moisture will lead to additional precipitation late week, with flooding concerns in lower elevations of California in particular and heavy snow across much of the higher elevations. By the start of 2023 early next week, a surface low pressure system could consolidate around as the central High Plains in response to another upper trough, and lift northeast across the central U.S. to Midwest for increasing chances of snow/wintry weather on the backside and another round of possibly heavy rain centered over the Lower Mississippi Valley region around Monday. ...Guidance/Predictability Assessment... Recent model cycles have kept with the pattern of relatively good agreement with the large scale pattern of central U.S. troughing with upper ridging ahead of it in the eastern U.S. late week lifting by early next week, while another trough digs into the western U.S. Sunday and traverses eastward. But there are still some differences in the details like exact shortwave/low pressure/frontal placements. Overall and considering lingering smaller scale system interactions/developments, the WPC medium range product suite was mainly derived from a composite blend of best clustered guidance from the 06 UTC GFS/GEFS mean and 00 UTC ECMWF/ECMWF ensemble mean along with the compatible 13 UTC National Blend of Models through medium range time scales. This solution maintains excellent WPC product continuity and tends to mitigate detail variance consistent with a pattern with seemingly average to above average overall predictability despite complex and active/stormy flow. Latest 12 UTC models and ensembles seem in line with this forecast plan. ...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 coastal California and along the foothills of the Sierra Nevada due to bouts of heavy rain moving in during those days. Once the rain gets into southern California Saturday, burn scars across that region will be particularly vulnerable to flooding. Additionally, significant snow amounts are likely in higher elevations of the West through the latter part 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 the Southwest. After that brief break, another round of precipitation is forecast for the West Coast states by Monday. East of the Rockies, 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 Thursday across the central Gulf Coast region, where a Slight Risk of excessive rainfall is in place for these areas as instability could produce high rainfall rates that may cause flooding/flash flooding. North of there, 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 lower. Most precipitation should stay rain but with some chance of minor snow accumulations in the Upper Midwest and the highest peaks of the interior Northeast. Better but still uncertain chances for snow may be across the central High Plains by Sunday and spreading into the north-central U.S. Monday as another upper trough provides colder air there, and there are increasing probabilities for a low pressure system to consolidate and lift toward the Midwest. Farther south the cold front associated with this low could provide a focus for another bout of heavy rain across parts of the Mississippi Valley by Monday-Tuesday. After the recent cold spell, the temperature pattern in the medium range period will significantly change across much of the nation. Warmer than average temperatures will be widespread across the eastern half of the country late this week into early next week, with lows around 15-25F above average and highs around 10-20F above average. 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 around 5-10F below normal. Lows should generally be a bit above average until early next week as the second large-scale upper trough moves through, bringing lows within a few degrees of normal. Schichtel/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