Extended Forecast Discussion NWS Weather Prediction Center College Park MD 132 PM 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... A blocking ridge over the eastern Pacific Ocean will favor downstream trough developing along the U.S. West Coast. 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 southwest Thu-Fri (days 5-6), and possibly into the southern Plains by Sat (day 7). The most significant model differences during the extended forecast period continue to surround the speed with which the upper low moves eastward during the mid to late part of next week. The 06z GFS continued to be much faster than all other deterministic guidance to move the system eastward Wed-Fri, and is starting to slow down on the 12z run. All other deterministic guidance remains slower than the GFS, with intermediate timing shown by the 00z ECMWF and 00-06z GEFS Mean. The CMC and the UKMET remained on the slow end of the spread, so the ECMWF/GEFS Mean 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/GEFS mean). Consequently, the forecast was a blend of the 00z ECMWF/yesterday's 12z ECMWF/06z GEFS Mean, with lower weight on the Canadian and 00z ECMWF Ensemble Mean through Day 6. The Day 7 forecast used a blend of the 00z ECMWF and 06z GEFS mean, which have clustered better than usual the past few days. Confidence is still below normal on day 7, given the large timing differences with the closed low and resultant differences phasing with the northern stream trough in the eastern US. The 00z ECMWF Ensemble mean continued with faster timing than the operational 00z ECMWF, highlighting the large spread and potential for adjustments to later forecasts. Farther north, a mid-upper level trough with a surface frontal wave crosses the Northern Plains into the upper MS Valley Tue and into the Great Lakes Wed. A lead wave crosses the lower lakes with low pressure developing off the mid Atlantic coast and moving steadily offshore Thu. The 00z ECMWF was north of the ECMWF Ensemble Mean, UKMET, Canadian, and GFS, so a multi-model consensus was used due to variance in the latitude of the operational ECMWF runs from yesterday to today. As the upper low moves across the Great Lakes and Northeast Thu-Fri, another triple point low develops either in eastern New York, NJ, or southern New England, deepening as it moves slowly across New England and the adjacent coastal waters Fri, departing Sat. Significant spread emerges by later next week with respect to how long the eastern US trough persists, as the operational CMC/GFS/ECMWF/06z GEFS Mean allow more persistence into Sat, but the 00z ECMWF Ensemble Mean quickly moves the trough offshore, allowing ridging to building in Sat Apr 11 downstream from dual northern and southern stream troughs in the Plains. ...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 valley rain and mountain snow in the ranges of southern CA. Lighter snow spreads into the ranges of Nevada, and then into the mountains of northern AZ early to mid week. As the closed low move east, snow develops downstream into the ranges of southern UT, CO, and northern NM. Farther east, showers and thunderstorms with locally heavy rain are expected to focus from the lower Mississippi Valley to the Southeast Tue. The northern stream system is expected to bring showers to portions of the Upper Midwest and Great Lakes Tue-Wed. As a 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 in northern New York/northern New England, with a greater chance of snow at higher elevation locations. Once the southern stream upper low/trough moves east into the Plains, increasing moisture return might result in increasing coverage of showers and thunderstorms across much of the Gulf Coast and into the Southeast. Timing of this remains uncertain. 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 in Iowa on Day 3/Tue). 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. By Fri, only FL remains above normal. 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. Above normal temperatures are forecast under the building upper ridge in northern CA to OR/northern NV/southern ID next Fri-Sat. 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