Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EDT Sat Apr 20 2019 Valid 12Z Tue Apr 23 2019 - 12Z Sat Apr 27 2019 ...Overview and Guidance Preferences... Upper-level flow during the extended period will be largely separated into a progressive northern stream across Canada and the U.S. northern tier, and a much less progressive southern stream, with the potential/tendency for systems to close off aloft. Models continue to struggle with this flow pattern, with predictability reductions resulting from timing/amplitude differences within each stream further amplified by potential interaction (or lack thereof) between the streams. As a result, forecast confidence/predictability quickly decreases to below average by the latter portion of the forecast period. Models have settled on a slower progression of a compact upper low/surface low pressure system away from the northeastern U.S. Tue-Wed, and solutions now handle this feature comparably. The evolution/progression of an upper low initially across the Four Corners region on day 3 (Tue) remains the most significant problem for this forecast period. Deterministic solutions handle this feature similarly through early Wed as the feature elongates and moves into the Southern Plains. Guidance has overall trended slightly quicker with the progression of this system than the ECMWF showed at this time last night, but remains slower than some of the much more progressive solutions shown previously by the GFS. Differences continue to grow by Thu-Sat, with the GFS continuing to favor a more dominant/amplified northern stream and a much more progressive southern stream wave, quickly crossing the Southeast by Thu night. The ECMWF continues to favor more separation between streams, keeping a closed upper low across the lower Mississippi Valley/Southeast through Fri-Sat. Given observation of previous similar systems over recent weeks, continue to favor more separation between streams, and a solution more along the lines of the ECMWF and ECENS ensemble mean. Farther west, there is general agreement among the guidance that northern stream height falls should approach the Pacific Northwest by Fri-Sat, with timing/structural differences among deterministic solutions by that time favoring an ensemble approach. The same holds true for another round of southern stream energy potentially nearing California by Sat. Given these considerations, the forecast during days 3-5 (Tue-Thu) was based on a multi-model (ECMWF/GFS/CMC) deterministic blend, with a shift to majority weighting of ECENS/NAEFS ensemble means during days 6-7 (Fri-Sat). ...Weather Highlights/Threats... The system crossing the Southern Plains Tue-Wed is expected to produce widespread showers and thunderstorms, with models indicating the potential for heavy rainfall from central Texas north into Oklahoma. Scattered to numerous showers and thunderstorms are expected to accompany the system eastward into the lower Mississippi Valley and Southeast Thu-Fri, but any signal for heavy rainfall is much less clear in the guidance given increased spread on the specific evolution of the system. Assuming the slower/more amplified solutions currently favored verify, would expect the continued potential for at least areas of locally heavy rainfall across portions of the Gulf Coast and Southeast. Above average temperatures are expected through much of the period across portions of the western U.S. as upper ridging builds in behind the southern stream trough. High temperatures are forecast to be 10 to 20 deg F above average from California to the Great Basin on Tue, with warm temperatures progressing east into the Rockies and portions of the Central Plains Wed-Thu. The arrival of a cold front by later in the week should bring a return to near or slightly below normal temperatures to the Pacific Northwest and northern Great Basin. 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