Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EDT Sun Apr 21 2019 Valid 12Z Wed Apr 24 2019 - 12Z Sun Apr 28 2019 ...Overview and Guidance Preferences... Models continue to struggle with interaction between a more progressive northern stream and less progressive southern stream through much of the extended period. Timing and amplitude differences of features within each stream are compounded by differences in the potential interaction (or lack thereof) between streams. As a result, forecast confidence/predictability continues to quickly decrease by the latter portion of the forecast period. Southern stream shortwave energy is forecast by all guidance to cross the Southern Plains Wed/Wed night and move into the lower Mississippi Valley on Thu, with perhaps a closed center developing at 500 hPa (although this continues to vary by model and from run to run). In general, timing of the system has continued to drift toward a middle ground, and a consensus blend of deterministic guidance should now represent the system reasonably well at least through day 4 (Thu). Timing differences continue to grow from Fri onward, generally related to interaction with northern stream energy crossing the Great Lakes. Preferred to gradually lean more toward ensemble timing of this system starting on Fri and through next weekend, as most solutions show a larger degree of phasing occurring as the system passes New England. Farther west, models show general agreement that a rather amplified northern stream shortwave should approach the Pacific Northwest on Fri, moving inland Fri night-Sat, lowering heights and pushing a cold front inland. Solutions vary widely as to what will occur with the southern stream, however. Models have oscillated between a cutoff upper low approaching California late Sat into Sun, or a more elongated and progressive feature that quickly moves inland. At this time, prefer to lean heavily toward ensemble means given the spread and inconsistency of solutions with the southern stream feature. Given these considerations, the forecast during days 3-4 (Wed-Thu) was based heavily on a deterministic multi-model blend, including the 12Z ECMWF/UKMET and 18Z GFS. By days 5-7 (Fri-Sun), weighting of ECENS and NAEFS ensemble means was significantly increased to account for increasing spread/uncertainty. ...Weather Highlights/Threats... The system crossing the Southern Plains and lower Mississippi Valley Wed into Thu is expected to produce widespread showers and thunderstorms, with models showing a strong signal for heavy rainfall from portions of central Texas east toward the Mississippi River. Severe thunderstorms are also possible. Please see convective outlooks issued by the Storm Prediction Center for further details on the severe weather threat. Scattered to numerous showers and thunderstorms are expected to accompany the system eastward into the Southeast as well as the Tennessee/Ohio Valleys by late Thu into Fri, but any signal for heavy rainfall is much less clear in the guidance given increasing forward speed of the system and inability of models to resolve convective details at that time scale. Nonetheless, would expect continued potential for at least locally heavy rainfall. Farther west, the energetic upper-level trough entering the northwestern U.S. late in the week is expected to produce rain and mountain snow from the Northwest to the northern Rockies. As the energy moves into the central U.S. by next weekend, rain and possibly even snow may spread into areas from the Northern Plains to the Upper Midwest/Great Lakes. Above average temperatures are expected through much of the period across portions of the western U.S. High temperatures are forecast to be 10 to 20 deg F above average from California to the Great Basin Wed-Thu, with these temperature anomalies spread into the Rockies and portions of the High Plains by Fri-Sat as cooler temperatures overspread the Northwest/northern Rockies in association with the arriving upper trough/cold front. 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