Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1255 PM EDT Fri Apr 03 2020 Valid 12Z Mon Apr 06 2020 - 12Z Fri Apr 10 2020 ...Significant precipitation potential over central and southern 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 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 southwest and possibly high Plains of CO Thu-Fri (days 6-7). 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 later 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. The 00z ECMWF/Canadian/UKMET are much slower, and the 06z GEFS Mean was even slower than the ECMWF, clustering well with the UKMET. The overwhelming ensemble consensus supports something much closer to the ECMWF, with confidence below average given the continuing spread of solutions. On Mon, a 500 mb shortwave builds south from western Canada and moves across the northern Rockies, continuing across the northern Plains Tue and upper MS Valley/Upper Lakes Wed. As the developing closed low moves across New York and into New England Thu-Fri, a triple point low is expected to develop off the Northeast U.S. coast Thu night-Fri. With this system, the models and ensembles cluster fairly well Mon-Wed., then separate as the GFS and Canadian model differ in the timing and phasing of the wave. The GFS has a faster low, and the 06z GEFS Mean better matches the slower timing of the ECMWF/UKMET. Given the historic fast bias in the GFS and GEFS Mean slower like other solutions, more weighting was given to the UKMET/ECMWF/06z GEFS Mean. This group also provides good forecast continuity. ...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 central to southern California, with the potential for heavy rain (and snow in the Sierra) Mon-Wed, with lighter precip spreading into NV. As the low moves inland across the southwest Thu-Fri 10 Apr., valley rain showers and mountain snow showers are expected across NV/UT/AZ. Farther east, showers and thunderstorms with locally heavy rain are expected to focus from the lower Mississippi Valley to the Southeast Mon 06 Apr - Tue 07 Apr along a band of confluent flow and return moisture fluxes that support showers/storms. Precipitation is expected across the Upper Midwest and Great Lakes Tue-Wed in association with a frontal system ahead of the amplifying northern stream shortwave. Snow is possible in northern WI to the UP of MI. As the wave crosses the northeast the surface low develops off the Northeast coast Thu into Fri. Guidance shows the potential for an area of enhanced precipitation across New England. This system does have the potential to produce some accumulating snows across interior New York and New England, especially in higher elevations. An area of well above average temperatures is expected across much of the central to southern high Plains Mon-Tue, with forecast highs 10 to 20 deg F above average. The cluster of above average temperatures then drifts east across the lower MS Valley Wed., the southeast Thu, and then south into FL Fri 10 Apr. The slow-moving upper-level low will bring persistent below average temperatures from much of central and southern California into the southwestern States, where high temperatures 10 to nearly 20 deg F below average are forecast through mid week. As the low moves inland, the core of colder air continues east out of CA and NV into UT/AZ/CO. 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