Extended Forecast Discussion NWS Weather Prediction Center College Park MD 135 AM 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 expand during the extended forecast period, promoting a broad region of downstream cyclonic flow across much of the CONUS. 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 speed with which the upper low moves east and interacts with the northern stream remains the point of greatest contention among the guidance. 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 (the CMC has waffled between the two extremes, but has spent more time on the slow side of the spread). Trends among ensemble members suggest a slow convergence toward the center, which has been consistently occupied by the ECMWF. In the northern stream, model consensus remains relatively good on a shortwave expected to dig across the Upper Midwest/Great Lakes and Northeast Wed-Thu night, with a deepening low pressure system expected to cross eastern Canada, and a surface low expected to develop off the coast of Maine Thu night. Guidance spread is higher with the next northern stream shortwave, forecast to dive southeast across western Canada Fri/Fri night, and into the U.S. northern tier on Sat. There is a quite a bit of spread on this feature will amplify and dig a bit more southwesterly into the eastern side of the Pacific ridge (GFS) or will be less amplified and more quickly move east into the north central U.S. (ECMWF). The ECENS and GEFS ensemble means generally reflect similar ideas to their respective deterministic runs. Guidance does generally show consensus, however, that a fairly broad region of cyclonic upper-level flow will develop across the central/eastern U.S. by next weekend, which along with the northeast Pacific ridge, will favor southward transport of polar air into much of the CONUS. The WPC forecast was based heavily on the 12Z ECMWF during days 3-4 (Wed-Thu). During days 5-7 (Fri-Sun), a gradual trend toward more ECENS/GEFS ensemble mean weighting was shown, to compensate for increasing spread in the northern stream late in the week. 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 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 the Florida Peninsula by next weekend, keeping a chance for showers and thunderstorms in place along most of the Gulf Coast. Farther north an area of enhanced precipitation appears increasingly likely across New England Thu/Thu night as the surface low develops off the coast of Maine. Some of the precipitation across New England is expected to fall as snow, especially at interior and higher elevation locations, where some areas may see at least a couple inches of snow. 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 attempting to nose southward in the lee of the Rockies. This system will increase chances for mountain snow/valley rain across the northern Rockies by 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 some Fri through the weekend. Ryan 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