Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM 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 with embedded upper low settles over the Northeast and a deeper trough over the West gradually becomes 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. There will be a tendency for less extreme rainfall totals from the Midwest through the Mid-Atlantic/Southeast and over the Northeast. The most noticeable differences/trends in recent runs have been within the western trough aloft. Over the past day the GFS has trended toward the ECMWF idea that the initially deep upper low will eject its energy into Canada instead of getting stuck over the Northern Plains. Latest GFS runs have been on the aggressive side of the spread (though with the 12Z UKMET not too far behind) with the eastward extent of height falls as the feature lifts northward. However the new 00Z ECMWF shows some GFS trends in that regard. Timing-wise the 00Z GFS strayed faster to eject the low versus the intermediate ECMWF/UKMET. The 00Z CMC is much more reluctant to eject the upper low. There are additional uncertainties with specifics of upstream impulses feeding into the overall trough. Meanwhile the consensus for the Northeast trough/upper low is decent into about Thu though with some typical spread/variability for precise upper low position. Then spread develops for the amplitude of an eastern Canada trough and its possible effect on the Northeast feature. Most solutions suggest greater amplitude for the late-week eastern Canada trough versus the 18Z GFS. The updated forecast started with an operational blend early in the period followed by an increase of 18Z GEFS/12Z ECMWF mean input, reaching 50 percent by day 7 Fri, to reflect the most agreeable aspects of the large scale evolution as confidence decreased for embedded details. ...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. 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