Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Sat May 09 2020 Valid 12Z Tue May 12 2020 - 12Z Sat May 16 2020 ...Below average temperatures expected to persist across portions of the central/eastern U.S. through early next week... ...Stalled frontal boundary to bring potentially heavy rains to South Florida next week... ...Overview and Guidance/Predictability Assessment... Through the medium range period (Tue-Sat May 12-16), the large scale pattern is expected to remain relatively stable and persistent. With Rex blocking situated over western Canada, the flow across the CONUS is largely quasi-zonal with some suggestion of troughing over the western US and ridging over the southeast US. At the start of the period, a large closed low over the Hudson Bay will keep much colder than normal temperatures in place for the central/eastern US with several fast moving shortwaves rounding the base of the trough. By mid/late week, that upper low lifts out and allows energy off the Pacific NW to move eastward across the northern tier US. Pieces of that energy will spawn a couple of surface lows over the central Plains that eventually move toward the Great Lakes region, all along a lingering/stalling frontal boundary. The day 3 forecast guidance showed improvement and better agreement with the pattern compared to recent runs and was consistent enough to go with a multi-model blend comprised of the 00Z ECMWF, 00Z UKMET, 00Z CMC, and 06Z GFS with some higher weights given to the higher performing ECMWF. By Day 4/5, spread was noted with the speed of shortwaves moving across the northern tier and central/eastern US, where the GFS showed a fast bias. So while in the large scale sense, models have similar patterns, the 00Z ECENS offered better consistency from the previous forecast cycle, and has increasingly higher weights through the period. For added detail, the 00Z ECMWF was kept somewhat higher percentage of blend. All in all, forecast confidence was average to slightly above average through the period. ...Weather/Hazard Highlights... Well below average temperatures will continue across the eastern CONUS through early next week, with more record cold temperatures possible. Anomalies will range from 10 to as much as 20 degrees below normal on Tuesday. As high pressure shifts offshore the eastern US, some moderation in return flow is expected. As far as precipitation goes, the lingering/stalled frontal boundary over South Florida will continue to be the focus for widespread, persistent showers and thunderstorms and some of which could produce locally heavy rainfall. Areas from central/northern California and the Pacific Northwest to the northern Rockies will see fairly widespread precipitation (snows are likely at the higher elevations of the northern Rockies) Tue-Wed as the upper low/shortwave move inland. Across the central US, several waves of low pressure riding along a slow moving frontal boundary may be the focus for some locally heavy rainfall and potentially severe weather according to the Storm Prediction Center extended Day 5 outlook. With several episodes of thunderstorms possible, some areas could see excessive rainfall and flooding but forecast confidence details remain below average at this time. 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