Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Fri Jun 26 2020 Valid 12Z Mon Jun 29 2020 - 12Z Fri Jul 03 2020 ...Heavy Rainfall Threat for the Northern Rockies and Northern Plains... ...Overview and Guidance/Predictability Assessment... Latest guidance agrees upon a fairly amplified/blocky regime for most of next week, with the pattern possibly opening up some by next Fri. An upper ridge will persist over Ontario and the western Great Lakes for multiple days while a trough may develop into a cut-off low over the Northeast. Meanwhile, a deeper trough over the West should gradually become more shallow as embedded features eject and a Gulf of Mexico ridge builds into northern Mexico/southern Rockies and southern/central Plains. The slowly evolving pattern should be particularly favorable for areas of heavy rainfall over northern parts of the Rockies and Plains--near a surface front at the leading edge of the western trough aloft. Farther east, there will be a swath of less extreme rainfall extending from the Midwest toward the Mid-Atlantic/Southeast near a stationary front, as well as initially over the Northeast associated with a slowly departing low. The morning model guidance has shown a better agreement with the synoptic pattern mentioned above compared with previous forecast cycles. Toward the latter part of next week, the overall amplitude of the upper-air pattern appears to decrease and lift closer toward the Canadian border. The GFS has joined the ECMWF in keeping a more closed off solution for the upper trough/low over the Northeast compared with earlier runs. Given the high amplitude omega block pattern through the middle of next week, it appears a more closed off solution is more viable for the Northeast. st noticeable differences/trends in recent runs have been within the western trough aloft. The updated WPC forecasts is a multi-model consensus of the 00Z ECMWF, 06Z GFS, 00Z CMC, and 00Z UKMET together with the ensemble means from the EC, GEFS and the CMC. More of the ensemble means were incorporated for Day 5-7. ...Weather/Hazard Highlights... Expect the northern Rockies/Plains to see the most focused heavy rainfall potential during the period. An ejecting upper low will promote one area of enhanced rain over the northern Rockies/Montana early in the week while a slow wavy front will likely bring one or more episodes of heavy rain to parts of the Northern Plains through at least midweek. Meanwhile showers and thunderstorms of varying intensity will be possible near and south of the eastern U.S. surface front that will define the southwest periphery of the Northeast upper trough. Some rainfall/convection should also be possible at times over the Northeast under the trough itself. Areas from the northern Rockies to southern California will see unseasonably cool highs early next week. Locations over/near the northern Great Basin may see highs as much as 20-25F below normal on Mon and there should still be decent coverage of minus 10-20F anomalies for highs over the northern Rockies/Interior West into Tue. Some daily records for cool highs will be possible. On the other hand, areas from the Northern Plains into Upper Midwest will see above normal temperatures, though with the Northern Plains gradually moderating with the eastward drift of the front across the region. Readings will be as warm as 10-20F above normal on one or more days with low temperatures tending to be more anomalous than the highs. Some of the warmth will extend into the Great Lakes, and eventually the Northeast as the upper trough weakens late in the week. In contrast, much of the Mid-Atlantic, Southeast, and into the Deep South should experience cooler than normal afternoon temperatures due to persistent chance of precipitation. Kong/Rausch 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