Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 PM 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... The flow pattern will settle into a western ridge/eastern trough pattern through the medium range period, and potentially beyond, per the latest Climate Predication Center 8-14 day forecast. 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. ...Guidance/Predictability Assessment... At the start of the medium range period /day 3 Saturday/, low pressure is expected to be across western KY where the latest guidance has improved and clustered together on a solution. As that system progresses to the Mid-Atlantic, there remains some speed and latitudinal spread, particularly seen in the 00Z/06Z GFS which maintains a fast bias with some northward spread as well, which has largely been discounted given the relatively above average agreement among the ECMWF/CMC/UKMET. The CMC/UKMET however become out of favor as the system moves up the East Coast and the consensus/compromise approach mirrors the deterministic ECMWF. Across the western U.S., guidance shows pretty good agreement through about day 6 at which the ECMWF builds the western ridge considerably more than the GFS, which leans toward more zonal flow across the Pacific NW and northern Rockies. Looking at the ensembles and teleconnections, the stronger ridge axis makes more sense and was favored in the blend (higher inclusion of the 00Z ECMWF and less GFS/GEFS). As shortwave energy moves into the central Plains, low pressure is forecast develop across the central US, Ohio Valley, Great Lakes area. There is considerable uncertainty with a large spread in the model guidance. With the blend favoring ECMWF, low pressure would track from the Mid-MS River Valley toward the Great Lakes, but overall forecast confidence at day 7 is quite low. In summary, the WPC blend for the medium range period initialized with the 00Z ECMWF, 00Z GFS with some components of the CMC/UKMET but quickly transitioned to a majority blend of the ECMWF/ECENS by day 5 onward. Higher percentages of the ECENS mean was incorporated for day 6/7. ...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 across the interior Northeast and possibly the Appalachians could see meaningful 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/Taylor 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