Extended Forecast Discussion NWS Weather Prediction Center College Park MD 121 AM EDT Sat Apr 04 2020 Valid 12Z Tue Apr 07 2020 - 12Z Sat Apr 11 2020 ...Significant precipitation potential over central and southern California through the middle of next week... ...Overview and Guidance/Predictability Assessment... An upper-level ridge is forecast to persist across the northeast Pacific, west of British Columbia and south of Alaska, during the extended forecast period. This blocking ridge will favor persistent downstream troughing along the U.S. West Coast and broad ridging/anticyclonic flow over the south central and southeastern states. Embedded within the broad western trough, models show strong consensus that an upper-level low will drop southward along the West Coast Tue-Wed (days 3-4), before slowly moving east across the interior West Thu-Fri (days 5-6), and into the central U.S. by Sat (day 7). Farther north, an active northern stream jet is expected from the Gulf of Alaska, north of the Pacific ridge, east across western/central Canada. A fairly significant shortwave is still expected to amplify across the Great Lakes and Northeast Thu-Fri. The associated surface low pressure system is forecast to cross the north central U.S. Tue-Wed, reaching the eastern U.S. on Thu. Guidance continues to suggest potential formation of a low off the coast of New England Thu night into Fri. The most significant model differences during the extended forecast period continue to surround the speed with which the western upper low begins to move eastward during the mid to late part of next week. The GFS continues to be much faster than all other deterministic guidance to move the system eastward Wed-Fri. The 18Z GFS moved the wave east quickly enough to allow phasing with northern stream energy across the central U.S., and the formation of a deep cyclone in the Great Lakes by late in the week. All other deterministic guidance remains slower than the GFS, with the CMC and the UKMET on the slow end of the spread, and the ECMWF in the middle. A look at trends among ensemble members over the past 24 hours shows somewhat of a convergence toward the center, providing continued support for the solution shown (fairly consistently) by the ECMWF and ensemble means (even the GEFS mean). Despite being faster with the western system, the GFS also leaves a portion of the energy trailing behind, and eventually amplifies it into a closed low near the Southwest U.S./Mexico border late next week, something which is also reflected in the GEFS ensemble mean to some extent. Guidance continues to show more consensus with the northern stream feature at least through Wed/early Thu (aside from the phasing shown by the GFS). Some spread emerges by later next week with respect to how deep of a trough gets carved out across the eastern U.S. by the wave. The CMC and GFS have been more aggressive with amplifying a deep trough in the East late next week, while the ECMWF solutions is somewhat more moderate in that context, with support from ensemble means as well. Given the above considerations, chose to weight the ECMWF heavily through days 3-4, with a gradual trend toward heavier weight placed on ensemble means (especially the ECENS mean) during days 5-7. ...Weather/Hazard Highlights... The western upper-level low is forecast to result in an extended period of onshore flow for much of California, with the potential for heavy rain (and snow in the Sierra) Tue-Wed. Farther east, showers and thunderstorms with locally heavy rain are expected to focus from the lower Mississippi Valley to the Southeast Tue in association with a northward-moving warm front. The northern stream system is expected to bring showers to portions of the Upper Midwest and Great Lakes Tue-Wed. As a potential surface low develops off the Northeast coast Thu into Fri, guidance shows the potential for an area of enhanced precipitation across New England, some of which may fall as snow, especially at interior/higher elevation locations. By Thu-Sat, as the southern stream upper low/trough moves east into the central U.S., increasing moisture return and a developing area of low pressure in the Southern Plains/lower Mississippi Valley will result in increasing coverage of showers and thunderstorms across much of the Gulf Coast and into the Southeast. A broad area of above average temperatures is expected across much of the central and eastern U.S. Tue-Wed, with forecast highs 5 to 15 deg F above average expected (perhaps as high as 20 deg above average for some areas). By later in the week as the northern stream trough amplifies and the surface cold front advances, the above average temperatures should become more confined to the Southeast, with high temperatures 5 to 15 deg below average overspreading areas from the Midwest to the Northeast. Meanwhile, the slow-moving upper-level low will bring persistent below average temperatures from much of California east into the southwestern states, where high temperatures are forecast to be 10 to nearly 20 deg below average through much of next week - moderating by next weekend as the system moves east. 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