Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Wed Feb 14 2024 Valid 12Z Sat Feb 17 2024 - 12Z Wed Feb 21 2024 ***Multiple atmospheric river events possible for portions of California with heavy rain and Sierra snow*** ...General Overview... Broad cyclonic flow aloft is expected across the north-central U.S. for the weekend, and an amplified upper ridge axis across the Rockies and Intermountain West. The trough over the Great Lakes gradually lifts north into Canada going into early next week and a subsequent abatement in the colder temperatures, and a southern stream upper trough is progged to cross the Gulf on Saturday and induce a relatively weak surface low that should track across Florida and then offshore. Meanwhile, a big surface high from Canada will initially be situated across the Southern Plains and the Deep South region this weekend, with chilly conditions on Saturday followed by a warming trend. The West Coast will likely become quite unsettled going into the weekend and especially into early next week with increasing chances of multiple atmospheric river events for portions of central and southern California. ...Guidance/Predictability Assessment... Overall, the 00Z model guidance suite is in decent synoptic scale agreement to begin the period on Saturday. At the time of fronts/pressures compilation, the 12Z CMC was out of phase across portions of the central and eastern U.S. and did not fit the better clustered GFS/ECMWF/UKMET/ICON solutions and the ensemble means, so a non-CMC blend was used as a starting point in the forecast process. However, the 00Z CMC is in better agreement overall compared to its previous run. Going into early next week, the fronts/pressures were primarily derived from a GFS/ECMWF/previous WPC blend with steadily increasing contributions from the ensemble means, which accounted for about 40-50% of the forecast blend going into early-mid next week due to the greater mesoscale differences. ...Weather/Hazards Highlights... The low pressure system developing over the Gulf Saturday morning has maintained some continuity now over the past few model runs, with consequential rainfall mainly confined to northern Florida on Saturday and then South Florida on Sunday. Although rainfall totals are generally expected to be on the order of 1 to 2.5 inches, there may be potential episodes of training convection across northern Florida on Saturday to justify a Marginal Risk there. Some slower moving cells may develop across the metro areas of Palm Beach, Broward, and Miami-Dade Counties, and this may locally produce enough heavy rainfall on Sunday to warrant a Marginal Risk area for Day 5. A more substantial axis of heavier rainfall is becoming more likely across the northern half of California going into Saturday and extending inland across upslope areas of the northern Sierra, with some rainfall totals approaching two inches, and therefore enough confidence in having a Marginal Risk for the Day 4 period. The axis of highest PW anomalies will likely be oriented towards the higher terrain of the coastal mountain ranges between San Francisco and Eureka, where a Slight Risk of excessive rainfall is planned for the Day 4 period. Looking ahead to Sunday, another atmospheric river event is becoming more evident in the latest model guidance as a slow moving Pacific low approaches California, with anomalous moisture flux directed towards the coastal terrain of central California. It appears the highest rainfall totals will be on Tuesday for the Transverse Ranges where several inches of rainfall is a good possibility, so this will continue to be monitored closely. Heavy snow is also likely for the Sierra and northern California mountains during this time period. The surface high settling south across the southern tier states this weekend will likely be accompanied with temperatures running about 5-15 degrees below mid-February averages from Texas to the Ohio Valley on Saturday behind the cold front. A warmer trend commences across the Plains and Midwest as the airmass modifies by Sunday and into early next week. In fact, most of the nation should be be warmer than average by the middle of next week east of the Rockies, perhaps up to 20 degrees above average across portions of the central and southern Plains, and slightly below average for the West Coast where clouds and precipitation will keep temperatures in check. 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