Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Thu Feb 15 2024 Valid 12Z Sun Feb 18 2024 - 12Z Thu Feb 22 2024 ***Multiple atmospheric river events possible for portions of California with heavy rain and Sierra snow*** ...General Overview... An upper level trough initially over the Great Lakes and Northeast U.S. this weekend is forecast to retreat northward and allow for more of a zonal flow pattern across the eastern U.S. to begin next week. The surface low from the Gulf will cross Florida early on Sunday and then move offshore, keeping most of the adverse weather impacts away from the southeast U.S. coast. Meanwhile, an upper level ridge is expected to be in place across the Rockies and portions of the Intermountain West early in the week, and this will tend to slow the overall progression of Pacific disturbances. Overall, the West Coast region is expected to be quite unsettled with multiple rounds of moderate to heavy rainfall with atmospheric river events, and heavy snow for the Sierra and northern California mountain ranges, and moderate snow for portions of the central and northern Rockies by mid-week as the upper level ridge breaks down. ...Guidance/Predictability Assessment... The 00Z model guidance suite is is good overall synoptic scale agreement to begin the forecast period on Sunday, and a general model blend suffices as a good starting point in the forecast process. This also holds true going into Monday, although differences in the placement of the surface low off the California Coast begin becoming more apparent. Looking ahead to the middle of the week, the best overall model agreement is across the Eastern U.S. with the surface high stretched from the Gulf Coast to New England, but more dispersion with the surface low that is likely to develop across the Midwest and then the Ohio Valley region. The ECMWF is more progressive with the Pacific/California trough moving inland across the Intermountain West by the end of the forecast period compared to the CMC/GFS. By the Wednesday-Thursday time period, the ensemble means accounted for about 50-65% of the forecast blend owing to timing and placement differences with the low over the Midwest. ...Weather/Hazards Highlights... Most of the significant rainfall associated with the Gulf low on Sunday should be confined to the southern half of the Florida Peninsula, and then moving offshore entirely by Monday. However, there will likely be some heavier convective elements across the Gulf Stream and over coastal portions of the South Florida metro areas, and this could result in a potential for some flooding if this convection trains over the same areas, so the Marginal Risk area will be maintained for the Day 4 period from St. Lucie down to the greater Miami metro area. The prospects for heavy rainfall and potential flooding are even greater across much of California going into Sunday and even more so on Monday. A Slight Risk is planned for the central California coast and coastal mountains from San Francisco to near Santa Maria on Sunday where the best moisture flux and highest PW anomalies are expected. A Slight Risk area is also planned for the coastal mountain ranges, including the Transverse Ranges, for the Day 5 period Monday-Monday night, including the greater Los Angeles metro area, as the atmospheric river and associated anomalous moisture flux is directed at the terrain and produces 2 to locally 4 inches of rainfall during this time. Heavy snow is expected for the Sierra, and moderate snow farther inland across the higher terrain of the Intermountain West and eventually the Rockies. After the brief cold spell across the north-central U.S. on Saturday, a warming trend is expected going into Sunday, and especially early next week as the upper trough lifts out and more westerly flow from the Pacific moves in and modifies the air mass. High temperatures could be 15-25 degrees above average across portions of the Central Plains and Midwest by Tuesday. Clouds and precipitation across the West Coast region will tend to hold temperatures more in check, with slightly below average readings likely. Hamrick 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