Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 PM EDT Sun Apr 05 2020 Valid 12Z Wed Apr 08 2020 - 12Z Sun Apr 12 2020 ...Significant precipitation potential over central and southern California through Wed... ...Overview and Guidance/Predictability Assessment... A broad upper-level ridge across the northeast Pacific Ocean is forecast to persist through next week. An upper-level low initially off the coast of southern California on Wed (day 3) is forecast to slowly move east across the Southwest through late in the week, eventually getting absorbed into a broad northern stream trough forecast to develop across the central/eastern U.S. The models have a little less spread on the timing of the upper low moving east across the southwest, but significant timing/phasing differences as the system comes into the eastern US. Models have generally followed into the same camps as recent days, with the GFS consistently quite a bit faster than other guidance, and the UKMET on the slow side. Trends among ensemble members suggest a slow convergence toward the center, which has been consistently occupied by the ECMWF. For this forecast, sufficient agreement existed among the operational ECMWF and its ensemble mean to blend the two solutions days 3-5. Spaghetti plots show large spread days 6-7 in the southeast/Appalachians with little overlap among the ensemble means/operational run, so confidence remains low on the evolution of the system from the southern Plans east to the southeast. Consequently, the blend used greater weighting towards the ECMWF Ensemble Mean solutions days 6/7, with less weight to the operational ECMWF. In the northern stream, model consensus remains relatively good on a lead shortwave developing Wed morning off the northern Mid Atlantic coast/south of southern New England/Long Island, with the wave moving east offshore by Wed night. As a closed low digs across the Upper Midwest/Great Lakes Wed and Northeast Thu, a triple point low develops likely as the wave crosses southern New York/New England, with wave quickly strengthening as it moves off the coast of Maine Thu night. Guidance agreement is better than average so a consensus of the models/means sufficed for minor timing/intensity differences. Guidance spread is higher with the next northern stream shortwave, forecast to dive southeast across western Canada Fri/Fri night, into the U.S. northern Plains on Sat, and upper MS Valley/Upper Lakes Sun 12 Apr. The 00z ECMWF flipped continuity to become more amplified, with a shorter wavelength. The ECENS and GEFS ensemble means generally reflect similar ideas, with a broad region of cyclonic upper-level flow will develop across the central U.S. next weekend, which along with the northeast Pacific ridge, will favor southward transport of polar air into the northern to central Plains. The WPC forecast was a blend of the 00Z ECMWF/ECMWF Ensemble Mean during days 3-4 (Wed-Thu). During days 5-7 (Fri-Sun), a gradual trend toward more ECENS/GEFS ensemble mean weighting was used. The GFS was excluded based on its persistent fast solution with the southern stream upper-level low/shortwave. ...Weather/Hazard Highlights... The potential for heavy rain (and mountain snow) will persist across southern California and portions of the Southwest through Wed night/early Thu as the upper low passes overhead. Farther east, the trailing cold front south of the deepening eastern Canada surface low is expected to focus showers and thunderstorms across portions of the Gulf Coast, lower Mississippi Valley, and Southeast Thu-Fri. The front is forecast to stall across the Gulf of Mexico and Florida Peninsula by next weekend, keeping a chance for showers and thunderstorms in place. Wave along the front have the potential to allow rain to occur further north into the lower MS Valley/southeast/southern Appalachians next weekend . Farther north an area of enhanced precipitation appears increasingly likely across New York/New England Thu/Thu night as the low develops and then moves off the coast of Maine. Some of the precipitation across northern New York /New England is expected to fall as snow, especially in higher elevations. A few solutions are showing potential for heavy snow in Maine. The shortwave expected to dive into the northwestern/north central U.S. late in the week will be accompanied by a cold front and a polar air mass moving southward in the lee of the Rockies. Temperatures are forecast to drop to 10-15 degrees below normal next weekend in MT/ND, with locally greater colder conditions. This system will increase chances for mountain snow/valley rain across the northern to central Rockies/adjacent foothills/high Plains next weekend. A few solutions show potential for snow in the cold sector of the frontal wave in the northern Plains next weekend. Above average temperatures are expected across much of the central and eastern U.S. on Wed, with forecast highs 5 to 15 deg F above average. As the cold front sweeps southward and eastward, the above average temperatures will be confined to the Southeast and southern Mid-Atlantic region by Thu. A cooler air mass in the wake of the front (highs 5 to 15 deg below average) is forecast to overspread much of the central U.S. on Thu, reaching the eastern U.S. on Fri and persisting into Sat. Farther west, highs are forecast to be 10 to 20 deg below average across southern California and much of the Southwest Wed-Thu as the upper low passes. Temperatures across these areas should moderate Fri through the weekend. Above average temperatures are expected Fri and Sat in OR/northern NV/southern ID, moderating as the height falls occur Sun. Petersen Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml