Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Wed Mar 27 2024 Valid 12Z Sat Mar 30 2024 - 12Z Wed Apr 03 2024 ...Heavy rain and high elevation snow likely across California, the Great Basin/Southwest, and into the Rockies... ...Wintry precipitation possible in the north-central Plains this weekend and spreading across the northern tier early next week... ...Overview... An upper low is forecast to be just offshore of California as the medium range period begins Saturday and move eastward into the Four Corners region early next week. This feature and its surface low will direct moisture in the form of a weak atmospheric river into California and produce heavy rain and higher elevation snow for California, much of the Southwest/Great Basin, and into the Rockies. Farther downstream, increasing moisture should combine with northern stream troughing to produce low pressure that tracks east from the central Plains Sunday through the Northeast Tuesday- Wednesday. Expansive precipitation coverage is likely with this system, with snow and freezing rain possible on its north side. ...Guidance/Predictability Assessment... Recent model guidance continues to be in fairly good agreement through at least the weekend on the pattern consisting of an upper low coming into California, a broad ridge ahead of it across the Southeast, an upper low exiting the Northeast, and northern stream shortwaves through the far northern tier. A multi-model blend of the deterministic models worked well for the first half of the medium range period. By Monday-Tuesday, there are more differences which arise as the Western low moves inland and energy combines/interacts with a northern system dropping into the Great Lakes. The details of possible phasing of this system are very uncertain, with the CMC suggesting the least amount of phasing with a faster northern stream upper low through the Great Lakes/Northeast Tuesday- Wednesday. GFS and ECMWF are similar enough, but 06z GFS was displaced more south with the upper low than the ECMWF (which had better support from the ensemble means and the AI model). 12z GFS came in to better agreement with the ECMWF. Elsewhere, there are also timing differences with the next shortwave into the the Northwest. The late period blend heavily favored the ECMWF with greater contributions from the ensemble means in an attempt to mitigate detail differences. This maintained good continuity from the previous WPC forecast as well. ...Weather/Hazards Highlights... For the western U.S., an active and unsettled weekend is expected as a persistent jet on the southeast side of an upper low directs moisture into California and eastward. A modest atmospheric river should take aim at California Saturday, with anomalous moisture possibly reaching above the 95th percentile. Favorable upslope flow into the Transverse Ranges will likely bring a period of heavy to excessive rainfall, so a Slight Risk is continued in the Excessive Rainfall Outlook for the urban coastal areas into upslope portions of the Transverse Range below the snowy higher elevations. A Marginal Risk extends north and east into parts of the Desert Southwest as moisture reaches farther inland. Heavy snow is likely to continue Saturday for the Sierra Nevada while beginning in the Intermountain West as well. By Sunday, the atmospheric river will be pushing east into the Southwest, and a Marginal Risk is in place for southeast Arizona where heavy rainfall may occur. Additionally, though the atmospheric river will have passed beyond southern California by Sunday, convective rainfall could occur as steeper lapse rates come in with the upper low overhead. These showers may have some higher rain rates that could cause some isolated flooding concerns especially after a wet couple of days, so a Marginal Risk is delineated in southern California too. Ample moisture across much of the West will also produce moderate to heavy snow in higher elevations over the weekend. As the upper low shifts inland Sunday-Monday, and with developing low pressure in the Central U.S., a period of gusty winds are possible across much of New Mexico and southwest Texas. A drying trend is expected in the West Monday- Tuesday, except for the Pacific Northwest that could see increasing support for precipitation by Tuesday. Farther east, shortwaves generally riding along the northern side of a south-central U.S. ridge could combine with moisture to produce some rain over the weekend across the Midwest and Great Lakes region into the Ohio Valley and Mid-Atlantic. For the Ohio Valley in particular, there may be a swath of relatively heavy rain on Sunday that could train because of the west-east orientation, but there remains enough uncertainty still to preclude any risk in the Excessive Rainfall Outlook at this point. More widespread, possibly heavy, precipitation is likely Monday-Wednesday from the Ohio/Tennessee Valleys and eastward with a developing low pressure system. Chances for wintry precipitation are increasing across the north-central Plains by the weekend, stretching into the Upper Midwest and Great Lakes region Monday and higher elevations of the Northeast on Monday or Tuesday. However, there is still considerable uncertainty in the placement and amounts of snow and ice, so continue to monitor forecasts. Chilly temperatures linger across the north-central U.S. through Monday, but farther south from the central to southern Plains/Mississippi Valley, notable warmth is expected into the weekend as upper ridging builds. Temperatures of 10-20 degrees above average will become widespread, with highs in the 70s reaching Kansas and Missouri while highs climb into the 90s if not 100F in southern Texas. The above normal temperatures are likely to expand east into the Ohio/Tennessee Valleys, Mid-Atlantic, and Southeast by early next week. A cold front is forecast to finally drop temperatures in the south-central U.S. by Tuesday, focusing this warmth in the Southeast before the front pushes through there too by midweek. Meanwhile in the Southwest, cooler than average temperatures particularly in terms of highs are likely underneath the upper low. The Pacific Northwest could see above average temperatures though, especially into early next week. Santorelli/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 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