Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 AM 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... An amplifying trough tracking across parts of the western and central lower 48 will be the main upper-level feature of concern for the latter half of this week into the weekend. The trough and upstream rounds of Pacific frontal systems and energy is forecast to lead to multiple rounds of precipitation with potentially heavy rain along the West Coast and heavy snow in higher elevations. Just ahead of the trough, a couple of frontal systems should develop in the central U.S. while the pattern promotes multiple days of deep-layer southerly flow from the Gulf, spreading precipitation over an increasing portion of the east-central U.S. late in the week with best potential for heavy rainfall over the Gulf Coast/Lower Mississippi Valley region. Meanwhile pronounced ridging developing ahead of the trough will bring a notable warming trend to the central-eastern U.S. for widespread above normal temperatures. ...Guidance/Predictability Assessment... Model guidance continues to show reasonably good agreement with the large scale pattern of the western to central U.S. upper trough and downstream ridging, but with typical differences in the details of individual features, including shortwaves as well as surface low and frontal positions. This is true both within the broad area of mean low pressure that develops over the central U.S. and for the specifics of surface lows in the Pacific. While there is some increasing signal for a consolidated surface low to lift northeast into the Midwest this weekend, differences in the track and depth of the low are seen along with differences in the trough aloft (GFS runs have been consistent with keeping the depth of the trough deeper through next Sunday while the CMC is much weaker; the 12Z ECMWF weakened the trough more quickly than the newer 00Z EC). The details of surface lows and fronts before that this week are actually even more nebulous given the even smaller scale shortwave differences and uncertainties with features' interactions. But at least there is agreement for lower pressures in general in the central U.S. ahead of the trough leading to increased moist flow from the Gulf. Meanwhile, recent guidance has shown better agreement for a surface low off the Pacific Northwest around day 5/Friday, but then what appeared to be fairly good model agreement for another reasonably strong Pacific surface low coming into California next weekend ended up showing a weakening trend in the newer 00Z guidance. Additional changes will likely be needed with all these features with time. The WPC forecast stuck with a blend of the 12/18Z GFS and 12Z ECWMF and CMC early in the period, trusting the blending process to smooth out smaller differences and come to a consensus. The 12Z UKMET was not used given its differences from consensus early in the period for a surface low position along the U.S.-Canada border. The ensemble means from the EC and GEFS were introduced and increased in the model blend for the latter part of the period as spread increased. ...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 was added to the experimental Day 5 ERO 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. East of the Rockies, most areas should be generally dry Wednesday other than some light precipitation chances across the northern tier. By the latter half of the week, moisture inflow from the Gulf of Mexico is forecast to return to the Mississippi Valley ahead of one or more Plains low pressure/frontal systems and supporting upper trough. 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. 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. 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. 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