Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Tue Jan 21 2025 Valid 12Z Fri Jan 24 2025 - 12Z Tue Jan 28 2025 ...Overview... Mean troughing over the east-Central U.S. on Friday 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 mid- week temperatures across the East and South. A shortwave into the northern Rockies 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 week, likely bringing 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 by Sunday. ...Guidance/Predictability Assessment... With guidance clustering better for the shortwave crossing the eastern portion of the country from Friday into early Saturday, the primary forecast discrepancies involve the potential upper low closing off along or inland from the West Coast during the weekend. Models continue to show good agreement on the presence of an upper low, but uncertainty on how long it lingers over California or shifts gradually eastward. The 12z/Jan 20 CMC was slowest with this feature, but the 00z CMC came in weaker and more progressive. 00z GFS trended a bit faster too. So still a lot of uncertainty late period for this. To the north, a shortwave should drop into the northern Plains late week and progress eastward with time. But by Monday, there is a lot of discrepancy in the timing and amplitude of the shortwave over the East/Mid-Atlantic and also if/how additional energy dropping into the mean trough will affect its evolution/characteristic. The WPC progs for tonight featured a general model blend through Day 5, increasing to 60 percent of the ensemble means with 40 percent of the GFS/ECMWF. This worked well as a good middle ground/starting point for the forecast and maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Colder than normal temperatures will linger into Friday over parts of the East and South, especially near the Gulf Coast. Readings of 5-15 degrees below normal should not be as extreme as earlier in the week and temperatures should moderate, reaching near normal this weekend. 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 wise, 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 the end of the week 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 continues to trend heavier with enhanced rainfall over parts of the southern tier. Snow will be 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. Some moisture spreads back into the Eastern U.S. next week. 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