Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Mon Jan 20 2025 Valid 12Z Thu Jan 23 2025 - 12Z Mon Jan 27 2025 ...Bitterly cold temperatures to continue through Thursday for parts of the East and the South... ...Overview... Mean troughing over the Central U.S. on Thursday will progress eastward and cross the East Coast by the weekend. Behind this, some weak upper ridging or at least flatter flow should move in, finally helping to moderate the bitterly cold late week temperatures across the East and South. A shortwave reaching western Canada by Thursday will split, sending northern stream energy through the northern U.S. tier/Great Lakes/Northeast as trailing energy dives south and likely forms a closed low over or near California into the Southwest this weekend. The upper low should persist into next Monday, and may bring some much needed precipitation to parts of southern California which has been stricken with devastating wildfires lately. Meanwhile a front reaching the Mississippi/Tennessee Valley region may serve to enhance rainfall over parts of the southern tier around the start of next week. ...Guidance/Predictability Assessment... With guidance gradually clustering better for the shortwave crossing the central/eastern portion of the country from Thursday into early Saturday, the primary forecast discrepancies involve the potential upper low closing off along or inland from the West Coast during the weekend and the character of northern stream troughing over the central/eastern U.S. by next Monday. 00Z/06Z guidance used for the updated forecast, as well as new 12Z runs, show typical spread for the western upper low. ECMWF runs and the ICON are near the California coast while GFS/GEFS runs are more inland. Latest machine learning (ML) models show a similar spread though overall with a little more tilt toward the western half of the envelope. Latest CMC runs have been somewhat erratic, holding the upper low north of consensus by next Monday in the 00Z run and then delaying closed low formation by a day or so in the new 12Z run. An upper low path most similar to the 00Z ECens mean provides a reasonable intermediate solution at this time and is close to continuity. Farther east, by next Sunday-Monday the models and ensembles begin to diverge for the amplitude and timing of northern stream troughing over parts of the central and/or eastern U.S. GFS/GEFS mean runs are flattest, ECMWF runs lean somewhat on the amplified side, and the CMC/CMCens mean are most progressive. ML models offer good support for an intermediate solution between these various extremes, perhaps closest to the 00Z ECens mean for amplitude and timing. Specifics of this trough will affect the surface front and embedded waviness reaching the southern/eastern states by the start of next week, along with precipitation details. Dynamical/ML consensus suggests that by early next week the 13Z NBM and earlier GFS/GEFS runs may be overdone with amounts in the northern half of the precipitation shield and light farther south. Partial adjustments were made in light of consensus ideas, with leeway for additional trends pending new guidance. Forecast considerations led to starting with the 00Z ECMWF/06Z GFS/00Z CMC, in order from more to less weight, from Thursday into early Saturday. Then the forecast quickly moved toward increasing ensemble weight (more 00Z ECens relative to 06Z GEFS) and eventually some WPC continuity. Means/continuity comprised 80 percent of the blend by next Monday. ...Weather/Hazards Highlights... Much colder than normal temperatures will linger into Thursday over parts of the East and South. Readings of 15-25 degrees below normal should not be as extreme as prior days but a few daily record low max and min temperatures are still possible. Expect temperatures to continue moderating thereafter, finally reaching near normal this weekend. The cold anomalies should be most stubborn along the Gulf Coast and over the far Southeast. Meanwhile, California to the Great Basin can expect warmer than normal temperatures through the rest of the workweek before the West cools underneath the upper trough next weekend. The eastern Rockies/High Plains will also see temperatures drop below normal (by up to 15-20 degrees) this weekend. Precipitation across the lower 48 during the medium range period should be relatively quiet. Some rain is possible across Florida on Thursday associated with a lingering frontal boundary, with rain or light snow up the Southeast to Mid-Atlantic coast with weak frontal waviness offshore. A couple cold fronts/clipper systems may produce some light/scattered precipitation from the Northern Plains to Midwest/Great Lakes and Ohio Valley, but specifics for locations seeing measurable totals will likely wait until the short range to be resolved. More widespread rain and mountain snows will move into the West by Thursday-Friday and shift south with time, likely providing some much needed rain to Southern California next weekend. The tight gradient behind a cold front dropping south through the West and Plains may produce an episode of strong winds along parts of the Rockies late this week and over California and vicinity by Saturday. Farther east, some Gulf moisture will likely interact with a front approaching/reaching the Lower Mississippi Valley to Tennessee and Ohio Valleys by this coming weekend/early next week. Latest guidance shows potential for some locally enhanced rainfall over parts of the southern tier. Snow will be possible possible farther north across the Great Lakes, with a wintry mix possible in-between depending on currently uncertain upper level and surface details at that time. Rausch/Santorelli 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