Extended Forecast Discussion NWS Weather Prediction Center College Park MD 103 AM EDT Fri Apr 03 2020 Valid 12Z Mon Apr 06 2020 - 12Z Fri Apr 10 2020 ...Significant precipitation potential over parts of California Sunday 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 centered over the south central/southeastern states. Embedded within the broad western trough, models show strong consensus that an upper-level low will drop southward along the West Coast Mon-Wed (days 3-5), before slowly moving east across the interior West Thu-Fri (days 6-7). Farther north, an active northern stream jet is expected from the Gulf of Alaska, north of the Pacific ridge, east across western/central Canada. Amplifying northern stream shortwave energy is expected to cross the northeastern U.S. on Mon, with a second more significant shortwave amplifying across the Great Lakes and Northeast Thu-Fri. The surface low pressure system associated with this late week shortwave is forecast to cross the north central U.S. Tue-Wed, reaching the eastern U.S. on Thu, with potential development of a low off the Northeast U.S. coast Thu night-Fri. The most significant model differences during the extended forecast period surround the speed with which the western upper low begins to move eastward during the mid to late part of next week. The GFS has remained much faster than all other deterministic guidance to move the shortwave east across the West Wed-Thu and into the central U.S. Fri. Meanwhile, the ECMWF is nearly two days slower. The overwhelming ensemble consensus supports something much closer to the ECMWF (even the GEFS ensemble mean provides little to no support for the operational GFS). With the northern stream features, differences among the guidance are much smaller, and model consensus is above average (especially considering the time range) with respect to the amplifying Great Lakes/Northeast trough days 6-7. Solutions are, as would be expected, a bit more variable when it comes to the specifics of whether cyclogenesis occurs off the Northeast coast, and the track of the potential system. Given the above considerations, chose to begin the forecast period with a blend of the 12Z ECMWF/18Z GFS (more weight toward the ECMWF) during days 3-4. During days 5-7, the forecast continued to weight the ECMWF heavily, but with gradually increasing influence from the ECENS and GEFS ensemble means through time, and majority ensemble means on days 6-7. ...Weather/Hazard Highlights... The upper-level low moving south along the West Coast and eventually inland across the West will result in an extended period of onshore flow for much of California, with the potential for heavy rain (and snow in the Sierra) Mon-Wed. Farther east, showers and thunderstorms with locally heavy rain are expected to focus from the lower Mississippi Valley to the Southeast Mon-Tue along a northward-moving warm front. Relatively lighter precipitation is expected across the Upper Midwest and Great Lakes Tue-Wed in association with a surface frontal system ahead of the amplifying northern stream shortwave. 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, although forecast confidence is not particularly high at this point in time with respect to the specifics of precipitation axis location and type. This system does have the potential to produce some accumulating snows across portions of New England, especially interior/higher elevation locations. Also by late next week as the southwestern upper low begins to nudge eastward, moisture return is expected to increase across the Southern Plains, perhaps leading to an increased risk of showers and thunderstorms by Thu or Fri. A broad area of above average temperatures is expected across much of the central and eastern U.S. Mon-Wed, with forecast highs 5 to 15 deg F above average possible. By late 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. 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 10 to nearly 20 deg F below average are forecast through much of next week. 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