Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1254 PM EDT Mon Apr 13 2020 Valid 12Z Thu Apr 16 2020 - 12Z Mon Apr 20 2020 ...Overview and Guidance/Predictability Assessment... Models and ensembles show good agreement in broad troughing across most of the CONUS Thursday-Friday downstream of an amplified northeast Pacific mean ridge. A mid-upper level trough progresses through the Rockies on Thursday, crossing the central Plains Fri, Oh Valley to the Mid Atlantic Sat., and then off the east coast. The 06z GFS/00z ECMWF/00Z Canadian global all have different timing and amplitude of the 500 mb wave. The ECMWF ensemble members split into different camps, with one set looking like the GFS and another like the operational Canadian. A blend of the models and 06z GEFS/00z ECMWF ensemble means was used, with confidence slightly below average due to model differences plus the big run to run difference from yesterday's 12z ECMWF to last night's 00z ECMWF run, which was an outlier on the strength of the closed low off the southeast New England coast 12z Sun 19 Apr. Upstream, models are coming into better agreement compared to previous days on the handling of shortwave energy undercutting the Pacific Ridge late this week, likely closing off into an upper low. This low should slide southward along or just offshore California Thu-Fri before progressing inland across the Southwest and southern tier states next weekend. Once the system gets into the southern Plains to lower MS Valley, there is potential for a frontal wave to develop and move across the southeast Sun 19 Apr and off the east coast Mon 20 Apr. There are model and run to run variances on the strength of the low, so low weighting was given to any one model and more weighting to the 06z GEFS and 00z ECMWF ensemble mean forecasts. The models show a broad upper ridge over the northeast Pacific into the northwest US. Shortwave energy may build down the backside of the ridge into WA/OR Days 6/7. The models/ensembles show a closed low drifting east from the Pacific towards northern CA. Since the low is moving east under the upper ridge, movement is expected to be slow. Manual progs used a blend of the 00z ECMWF/06z GFS and their respective ensemble means to handle typical timing/phasing differences. ...Weather/Hazard Highlights... Shortwave energy digging southward through the Intermountain West will spread snow potential into the Central Rockies on Thursday. As this system move east across the central Plains to the Mid MS Valley, Oh Valley, and mid Atlantic, rain showers are expected along the path of the system into the warm sector, with light snow possible in the cold sector north of the low track. As an unsettled eastern Pacific closed low/trough enters the Southwest/Four Corners region towards the end of the week, it should bring showers and below normal temperatures. Over the weekend, it should also tap a return of downstream Gulf of Mexico moisture to fuel showers and thunderstorms over the South Sunday 19 Apr. The threat winds down the system moves off the east coast Mon 20 Apr. The closed low off the west coast may get close enough to bring a threat of showers in northern CA next Sun 19 Apr into Mon 20 Apr. For Temperatures across the CONUS, locations from the Rockies to the Midwest will be well below normal Thursday into Friday with daytime highs averaging 15 to 25 degrees below normal. Values should moderate back towards normal by this weekend and into early next week. The Great Lakes to northeast should stay cooler early next week as the next upper trough builds across these areas. Meanwhile, temperatures should trend warmer in the north west as upper ridging develops, with warmer than normal conditions day 6/Sun in WA/OR, and expanding further east onto the northern high Plains Day 7/Mon 20 Apr. Petersen/Santorelli 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