Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Dec 23 2024 Valid 12Z Thu Dec 26 2024 - 12Z Mon Dec 30 2024 ...Unsettled weather for the West Coast with periods of heavy rain and mountain snow... ...General Overview... The general flow pattern will deamplify/become more zonal with time as ridging in the East moves into the western Atlantic as a relentless series of Pacific systems shoves the downstream ridge along. This will allow progressive systems to move fairly quickly for the most part, with the main exception being a low pressure system moving north from TX into the Upper Midwest ahead of a broad but negatively tilted upper trough. For the most part, this should be a mild pattern for the Lower 48 with much of the country experiencing above normal temperatures. ...Guidance/Predictability Assessment... The latest guidance continues to show fairly good agreement on the large scale pattern, but plenty of uncertainty in the details of individual systems. The initial system lifting out of the Southern Plains into the Midwest later this week has some timing uncertainty, but should weaken pretty quickly as it heads into the mean ridge. The next system into the West should result in broad troughing across the Central U.S. by the weekend, but individual pieces of energy rotating through the main trough leads to still very uncertain details especially as it pertains to surface features. This system should progress eastward, kicking the Eastern ridge away, but still considerable questions on timing of this and how sharp/amplified that trough becomes. Yet another shortwave will move across the West Coast Sunday-Monday, and the guidance right now has a good handle on this feature. The WPC forecast for today used a blend of the deterministic guidance through day 5. For days 6 and 7, blended in some of the ensemble means with the GFS and ECMWF to help smooth out the details differences. Overall, this maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Multiple rounds of moderate to heavy rainfall are expected to impact the West Coast later this week as multiple frontal systems approach the coast, with the signal for heavy rainfall becoming stronger by the day between Thursday and next Sunday and eventually concentrating near the California/Oregon border. Flooding will be possible, especially where burn scars exist and possibly near the steep terrain of the Olympics and coastal mountain ranges and into the foothills of the Cascades. A Marginal Risk of Excessive Rainfall is in effect for coastal Washington, Oregon, and northern California on Thursday and Friday. Moisture spreading inland in the West will likely produce heavy snow over the higher/highest terrain of the Cascades and Rocky mountains, and portions of the Sierra Nevada. Heavy snow will focus over the Northwest on Christmas and expand through the Intermountain West/Rockies later this week. Tight pressure gradients may result in some periodically gusty winds for some locations as well. Downstream, multiple frontal systems will bring rain and thunderstorm chances to the Plains, Mississippi Valley, and Southeast. Southerly flow will bring moisture up from the Gulf of Mexico into the Ark-La-Tex region where multiple rounds of rain are expected. A Marginal Risk of Excessive Rainfall exists for that area on Thursday/Thursday night. Rainfall should eventually spread into the East as well. Widespread above normal/mild temperatures by late December standards are expected across much of the CONUS. The greatest positive anomalies are likely across portions of the Plains-Upper Midwest and extending into the Ohio Valley by Saturday. Readings closer to climatology are expected near the West Coast. Temperatures across the Southern Plains will trend warmer with time, with daytime highs 20-25 degrees above normal by early next week as upper ridging slides over the region. Santorelli/Roth 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw