Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Wed Apr 22 2020 Valid 12Z Sat Apr 25 2020 - 12Z Wed Apr 29 2020 ...Extended Period of Much Above Normal Temperatures Expanding Across the West... ...Cool and Unsettled Period Across the Eastern Half of the Lower 48... ...Overview... Models and ensemble means continue to show the upper flow settling into a western ridge/eastern trough pattern for a lengthy period of time. This will support an increasing area of much above normal temperatures over the West, with some of this warmth possibly extending into the Northwest by the middle of next week after shortwave energy rounding the ridge brings a period of precipitation and moderate temperatures to that region during the weekend and early next week. Farther east expect below normal temperatures and a couple episodes of meaningful precipitation as one system tracks northeastward from the south-central Mississippi Valley during the weekend into the start of next week and another developing system likely reaches the eastern states by next Wed. Some variation of this mean pattern may extend well beyond day 7 per the Climate Prediction Center 8-14 day forecast. ...Guidance/Predictability Assessment... The latest array of guidance is generally maintaining the recent improvement in clustering for the significant features during the period, though some detail differences/uncertainties remain. The 12Z ECMWF/CMC and 18Z GEFS/12Z ECMWF means offered the most consistent and intermediate solution for the leading system affecting the East, with GFS runs over the past day straying a bit to the fast side and the UKMET at times exhibiting its recent slow tendency with eastern U.S. systems. Farther west the GFS/GEFS were still a bit more amplified with shortwave energy coming into the Northwest early in the period but they have backed away fractionally over the past day. Then they have recently trended toward less amplitude for the trailing shortwave. Expect another system to develop over the central/eastern U.S. next Tue-Wed as the shortwave energy amplifies downstream from the rebuilding ridge. By next Wed latest GFS runs and 12Z ECMWF are on opposite sides of the envelope for upper trough amplitude, the GFS on the shallow side and the 12Z ECMWF on the amplified side. Compared to the full spread of guidance the 12Z ECMWF appeared more extreme. The new 00Z ECMWF has adjusted a little flatter. Meanwhile the 18Z GFS was on the fast side/GEFS on the slow side for the system tracking into the East. This left the 12Z ECMWF mean as the middle ground for flow amplitude and eastern system evolution by the end of the period. An operational model blend with some of the 18Z GFS component replaced by the GEFS mean represented preferences well early-mid period. Then the forecast increased weight of the ensemble means, reaching 70 percent by day 7 Wed as detail uncertainties increased for the system moving into the East. The updated forecast yielded good continuity. ...Weather/Hazard Highlights... The system forecast to track from the south-central Mississippi Valley to near the New England coast during the weekend into Mon should bring highest precipitation totals to areas from the Midwest/Ohio Valley through the northern Mid-Atlantic and Northeast. Some of the higher elevations in the Northeast and possibly the Appalachians could see some snow from this system. There are lingering detail uncertainties that offer potential for at least modest adjustment of this axis in future model runs. The Storm Prediction Center is monitoring severe potential in the warm sector to the south but predictability of important ingredients is fairly low so far. Check future outlooks for latest details on any severe threats as specifics become more clear. Farther west the guidance is making progress in converging (between the two extremes seen yesterday) for the precipitation forecast over the Pacific Northwest and northern Rockies as shortwave energy crosses the region Sat-Mon. Highest totals are still most likely to be along favored terrain in western Washington. As the shortwave energy continues eastward and amplifies Tue-Wed another central-eastern U.S. low pressure system and associated fronts should focus areas of rain and thunderstorms with some activity possibly moderate to heavy depending on specifics of system evolution. Details are too uncertain at this time to specify most favored areas for highest rainfall totals at this time. The expansion/intensification of well above normal temperatures over the West should occur in two phases. During Sat-Mon expect coverage of plus 10-15F anomalies to extend farther eastward across the southern half to two-thirds of the region, eventually extending into the High Plains. The Northwest should be closer to normal due to clouds and precipitation. Then strengthening of the upper ridge during Tue-Wed should support increasing anomalies and northward expansion of warmth. By next Wed there should be considerable coverage of highs 15-25F above normal with locations in and near the Great Basin seeing the highest potential for plus 20F or greater anomalies. Some daily records for highs/warm lows are possible through the period and likely a greater number heading into Tue-Wed. Meanwhile within the generally cool pattern over the East expect an area of minus 10-15F anomalies for highs to progress from the Midwest through the Mid-Atlantic/Northeast during Sat-Mon, followed by single-digit anomalies Tue-Wed. Rausch 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