Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Sun Jun 14 2020 Valid 12Z Wed Jun 17 2020 - 12Z Sun Jun 21 2020 ...Heavy rain potential over the southern Mid-Atlantic and vicinity to decrease after Wed-Thu... ...Overview and Weather/Hazard Highlights... The upper low expected to be over the Carolinas at the start of the period early Wed should drift northward a bit over the following day or so before opening up and lifting away. Therefore expect the best chances for locally heavy showers and thunderstorms over the southern Mid-Atlantic and into North Carolina to be during and just after midweek, followed by lighter and more scattered rainfall. In the wake of this upper low some degree of upper ridging should begin to extend into East and South. Farther west there is still a complex evolution that involves an upper ridge over northwest Canada and two or more pieces of energy initially over the northwestern U.S. and southwestern Canada, with trailing flow that eventually brings an upper trough into the northern half of the Plains. Over recent days the differences in handling of the initial energy and trailing flow have been affecting the forecast progression of the leading surface front and exactly where it may stall over portions of the Midwest/Central Plains region. Spread and variability with specifics aloft keep confidence in the details fairly low but the general setup continues to favor the potential of one or more episodes of heavy convection with some repeat activity/training near and possibly some distance from the surface boundary. Meaningful precipitation should extend back into the northern Rockies, with duration dictated by specifics of flow aloft. A modest upper ridge will move into the Northwest in the late week/weekend time frame. Then toward the end of the period the leading edge of a Northeast Pacific trough should extend far enough southeast to bring a cold front into the Pacific Northwest around next Sun. The greatest temperature contrasts should exist around midweek. Warm flow ahead of the Northern Plains front will support max/min readings 10-20F above normal over the northern/central Plains into parts of the Upper Mississippi Valley. On the other hand upper troughing over the West will produce highs of 10-20F below normal centered over the northern Rockies while the eastern upper low will keep highs as much as 10-15F below normal in its vicinity. By late week/weekend the warmest anomalies should tend to focus over the Great Lakes and Northeast, generally up to plus 10-15F. There is some uncertainty in the eastern Canada/Northeast U.S. pattern by this time though. The rest of the East will trend warmer as well with most areas seeing single-digit positive anomalies by next weekend. Similar anomalies will extend back into the southern half of the Plains. With time the cool air over the northern Rockies will modify as it continues into the Northern Plains. The late week/weekend upper ridge over the Northwest will bring above normal temperatures to the Northwest/northern-central California and eventually into the Great Basin/northern Rockies. Some locations may see highs up to 10-15F above normal next weekend. The cold front expected to reach the Pacific Northwest next Sun should bring a cooling trend to that region. ...Guidance/Predictability Assessment... The forecast of southern Canada/northern tier U.S. upper troughing that emerges from the West into the Plains, with some influence from the ridge over northwestern Canada, is showing some gravitation toward ideas that some of yesterday's 12Z guidance had introduced. These trends increasingly favor formation of a fairly slow upper low reaching north or northeast of eastern Montana by early day 4 Thu while upstream energy becomes incorporated into the flow to the south of the upper low. In today's guidance the GFS/GEFS mean offer the notable difference in being more progressive with the southern Canada upper low, though with a slower trend versus the 12Z run from 24 hours ago. Aside from timing differences there is at least agreement in principle on a trough reaching the Northern Plains/Upper Mississippi Valley by the latter half of the period. Even with relative non-GFS agreement for the upper low north of eastern Montana, significant spread eventually develops for specifics of flow aloft over eastern Canada into the northeastern U.S. so confidence is rather low over this area. Meanwhile the GFS is still a bit on the slow side with the eastern upper low but not to the extent seen yesterday. Slightly slower timing than the remaining consensus is certainly possible but the comparisons/trends upstream lend some doubt to the GFS scenario. Forecast considerations led to primary emphasis on the 00Z ECMWF/CMC/UKMET with only modest 06Z GFS/GEFS input days 3-5 Wed-Fri. The rest of the forecast transitioned to a model/mean blend as detail confidence decreased. This yielded good continuity for the eastern U.S. upper low and its eventual departure while it refined the emerging trends upstream over southern Canada and northern tier U.S. The 12Z ECMWF/UKMET/CMC reinforce the established majority cluster for the slow upper low north of eastern Montana and slightly faster departure of the eastern upper low versus the GFS. The 12Z GEFS mean agrees with the non-GFS models for the eastern low. 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