Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Feb 15 2024 Valid 12Z Sun Feb 18 2024 - 12Z Thu Feb 22 2024 ...Atmospheric river to bring heavy rain and mountain snow to California early next week... ...General Overview... An upper-level trough initially over the the northeastern U.S. is forecast to pull away to the northeast and allow a more zonal flow pattern to set up east of the Rockies Sunday into Monday. In Florida, a surface low will track across the peninsula along a slow-moving cold front, which will trigger a round of rainfall over the weekend. Meanwhile, a deep upper low will approach the West Coast, steering a stream of Pacific moisture into California that will result in heavy rain and mountain snow on Sunday and Monday. This low is forecast to quickly weaken mid-week as the associated surface cyclone pushes onshore. By Thursday, this system will push past the Rockies and develop a low pressure system in the southern Plains and Midwest. Late next week, another upper low will approach the West Coast that could bring another round of heavy rainfall, and a cold front will drop south into the north-central and northeastern U.S. that will bring the next chance of rain to the East. ...Guidance/Predictability Assessment... Model guidance is in fairly good agreement on the evolution of the upper level pattern through the period. A general deterministic model blend comprised of the GFS/ECMWF/CMC/UKMET was used for Sunday and Monday. Differences between the models become more noticeable by Tuesday into Wednesday with slight differences in the placement of the upper low near the West Coast. The deterministic GFS pulls the low more south than the European, Canadian, and ensemble means which could affect the placement of the eventual low pressure system developing downstream over the Plains. For this reason, the WPC model blend put less weight on the GFS than the ECMWF Tuesday through Thursday and introduced ensemble means to the blend to smooth out small differences between the deterministic models. ...Weather/Hazards Highlights... Most of the significant rainfall associated with the Gulf/Florida low on Sunday should be confined to Central and South Florida, and by Monday, the low will be offshore and the front will push south of the state. A lack of instability will limit rain rates, but training convection could cause isolated flash flooding if the move over urban and/or poor drainage areas. In the Day 4 Excessive Rainfall Outlook (12Z Sunday to 12Z Monday), a Marginal Risk area is in place along the Central and South Florida Atlantic coasts from Titusville 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 of excessive rainfall is in place from San Francisco to Los Alamos on Sunday where the best moisture flux and highest PW anomalies are expected. Rainfall rates in this region have the potential to reach 0.5-1" per hour. Another Slight Risk area is in place in on Monday for part of Southern California from Conception to Anaheim, including the Transverse Ranges and the Los Angeles metro area. The atmospheric river and associated anomalous moisture flux will be directed at the terrain and could produce 2 to locally 4 inches of rainfall. Heavy snow is expected for the Sierra Sunday through Tuesday, then moderate snow is forecast 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. Dolan/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