Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Fri Feb 16 2024 Valid 12Z Mon Feb 19 2024 - 12Z Fri Feb 23 2024 ...Atmospheric river to bring heavy rain and mountain snow to California early next week... ...Overview... Upper-level and surface lows over the eastern Pacific early next week will steer a stream of Pacific moisture into California and produce heavy rainfall with flooding potential and heavy mountain snow. These features should weaken by midweek while shortwave energy pushes eastward. This shortwave looks to combine with northern stream energy and help form a low pressure/frontal system in the central U.S. that pushes into the eastern U.S. by the latter part of next week, spreading rain and northern tier snow to some areas. Ahead of the upper trough and cold front, temperatures are forecast to be above average in the central U.S. early-mid next week and shifting into the eastern U.S. mid-late next week. ...Guidance/Predictability Assessment... As the medium range period begins Monday, model agreement is initially quite good in showing a closed upper low across the eastern Pacific, ridging atop the Intermountain West, and low amplitude troughing with embedded shortwaves across the central and eastern CONUS. But 12/18Z models start to diverge by Tuesday with the positioning of the upper low and thus the surface low over the eastern Pacific. GFS runs took a southward dive, especially in the 18Z run, while the non-NCEP guidance shifted it north with a little more phasing in the mid-upper levels. The 12Z CMC in particular migrated the low northwestward, which was not favored. The ECMWF seemed best aligned with the ensemble means. Fortunately the incoming 00Z model suite seems to show somewhat better agreement with the positioning of these upper and surface low features. By Wednesday-Thursday there are some model differences in the Pacific Northwest/British Columbia region with the potential for shortwave energy to disrupt a round of ridging ahead of another upper low tracking through the Pacific. However, farther east models show reasonable agreement on the large scale with the phasing of the northern and southern streams to form troughing, though the smaller scale details are yet to be determined, and these differences do affect the positioning of the developing central to eastern U.S. surface low and fronts. The WPC forecast was based on a blend of mainly deterministic guidance of the 12/18Z GFS and 12Z ECMWF/CMC/UKMET early on, but eliminated the CMC and UKMET by Day 5 while reducing other deterministic models in favor of the EC and GEFS ensemble means. The means reached half of the blend Day 5 and more by Days 6-7 as spread increased. ...Weather/Hazards Highlights... An atmospheric river with ample Pacific moisture will cause rounds of precipitation that will be heavy across California into Monday and Tuesday. For coastal/lower elevation areas, the Day 4 and 5 Excessive Rainfall Outlooks indicate Slight Risks across central to southern parts of California for the flooding potential. Rainfall rates in this region have the potential to reach 0.5-1" per hour, with over 1"/hour rates possible, and some areas are likely still sensitive from AR events early in the month/cool season. The Slight Risk Monday is likely more on the higher end of the probability range for a Slight across the Los Angeles metro, while Tuesday's Slight is lower-end as the expected rainfall decreases, with some shift south as well. Broader Marginal Risks cover much of the state farther north and east, until the higher elevations of the Sierra Nevada are reached where the precipitation will be in the form of heavy snow, possibly reaching multiple feet. Farther east, moderate snow is likely in the higher elevations of the Intermountain West and Rockies. Precipitation chances are likely to increase in the Ohio Valley to Great Lakes Wednesday-Thursday under the influence of a couple of low pressure/frontal systems. Precipitation is most likely to be rain except in the far northern tier. The precipitation should reach the East Thursday-Friday with some snow possible in parts of the Northeast. Continue to monitor forecasts as the details are yet to be determined. Upper ridging traversing the interior West to the central and then eastern U.S. through next week will promote above normal temperatures in those areas. Temperatures around 10-20F above normal will rise to 15-25F above normal across the Plains to Midwest on Tuesday and Wednesday. Highs are forecast to be into the 80s for parts of Texas with 70s stretching into the Southeast. Temperatures should moderate closer to normal after the cold front comes through but may still remain above normal in the central U.S., especially for lows. By Thursday the above normal temperatures are forecast to push into the East with warm anomalies of 10-20F, also moderating there by Friday. Meanwhile the West Coast should be near average for highs but somewhat above normal for lows. 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw