Extended Forecast Discussion NWS Weather Prediction Center College Park MD 133 AM EDT Fri Apr 19 2019 Valid 12Z Mon Apr 22 2019 - 12Z Fri Apr 26 2019 ...Overview and Guidance Preferences... Upper-level flow across North America 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. Overall, models have struggled with a similar flow pattern for the past week or two, and this certainly seems to be the case going forward as well. Predictability reductions resulting from timing/amplitude differences within each stream are further amplified by potential interaction (or lack thereof) between the streams. As a result, confidence/predictability at the beginning of the extended forecast period is average at best, and quickly decreases to well below average by later in the period. A multi-model deterministic blend (including the 12Z ECMWF/UKMET/CMC and 18Z GFS) was favored during days 3-4 (Mon-Tue). Deterministic solutions and ensemble members vary with respect to how quickly a compact upper low initially across the Mid-Atlantic region on Mon will lift out, but a consensus solutions close to the ensemble means was preferred. A trend toward a somewhat slower progression of this feature Mon-Tue was noted among much of the guidance and ensemble members over the past couple runs. This process is related to potential interaction with progressive northern stream energy crossing north of the Great Lakes on Tue, which also shows timing/amplitude differences among the guidance. Farther west, models are in relatively good agreement that another slow-moving southern stream system should cross the Four Corners region Mon-Tue, with differences (at least early on) confined to how amplified the feature becomes. From day 5 (Wed) onward, model spread quickly grows with respect to the southern stream feature approaching the southern/central plains by that time. The precise timing/amplitude of the feature (which remains uncertain) will play a significant role in whether or not any interaction occurs with progressive northern stream flow and ultimately how quickly the feature moves eastward. A significant trend was noted in the ensemble guidance toward a slower/deeper southern stream system with less northern stream interaction. This has been a trend among the guidance with a couple recent similar systems, so given the trend was inclined to lean more heavily toward that idea, as shown by the 12Z ECMWF and ECENS mean. Additionally, models generally agree on another round of Pacific shortwave energy approaching the West Coast by Thu or Fri, although given the time range, solutions vary widely on the nature and timing of the feature. What does seem at least a bit more clear is that a strong upper-level jet should be accompanied by height fall and a surface cold front pushing into the northwestern U.S. by Thu-Fri. Given these considerations, the forecast during days 5-7 was trended heavily toward ensemble means (the ECENS mean in particular), with some continued weight from the deterministic ECMWF as well. ...Weather Highlights/Threats... The southern stream system slowly moving from the Southwest into the central U.S. is expected to produce widespread showers and thunderstorms with potentially heavy rainfall. The most widespread/heaviest rains should be across the southern/central plains by Mon night/Tue, and spreading east toward the Gulf Coast and lower Mississippi Valley by Wed-Thu. Meanwhile, well above average temperatures are expected across portions of the western U.S. from Tue onward, with high temperatures expected to be 10-20 deg F above average for some areas. The warm temperatures should gradually spread east toward the central U.S. by later next week, with a cold front bringing a return to near normal temperatures for the Pacific Northwest. 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