Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EDT Sat Jun 27 2020 Valid 12Z Tue Jun 30 2020 - 12Z Sat Jul 04 2020 ...Heavy Rainfall Threat for the Northern Plains Tuesday-Wednesday... ...Overview and Guidance/Predictability Assessment... Expect fairly amplified and blocky flow at the start of the period Tue to transition toward a less amplified pattern more typical of summer. Ejection of multiple impulses from the deep upper trough initially over the West along with an upper ridge that builds across northern Mexico and the southern Rockies/majority of the Plains will ultimately lead to retrogression of the mean trough axis to near the Pacific Northwest coast. Meanwhile at least the first half of the period will feature an upper low over the Northeast, downstream from a ridge centered over the western Great Lakes and Ontario. Then by late week the upper low should open up and depart with some help from an amplifying eastern Canada trough while the ridge will likely weaken/merge with the ridge building into the Plains. Some of the ejecting western energy early in the period will support a slow-moving front over the Northern Plains with an associated heavy rain threat. Parts of the Northeast may see one or more days of convection with the early-mid period upper low while a broad area along/south of a weakening Midwest to Carolinas front should see one or more episodes of convection/rainfall. Within this latter area there is an uncertain potential for some enhanced rainfall associated with ill-defined energy aloft (having low predictability due to its small scale) that may settle over or near the lower half of the Mississippi Valley after midweek. The 00Z/06Z guidance suite was adequately clustered except for the 00Z Canadian to start the medium range period. Thus, a blend of the other deterministic models offered a reasonable starting point. Area of greatest contention in the models was over the Pacific Northwest, with a trend toward a deeper trough settling along or just off the coast next Thu-Sat. The ensemble trend has been fairly steady while some models have been less consistent. With a building upper ridge into the Gulf of Alaska, the consensus supports this troughiness between the downstream ridge from the southern Rockies to the Upper Midwest by late in the week. Incorporated more ensemble weighting with time to account for system timing/amplitude differences. In the Northeast, the ECMWF guidance was slower to raise heights by next Fri/Sat compared to the GFS/GEFS camp. Split the difference given uncertainty in how the southern extension of the upper trough across the Southeast may modulate the northern stream. ...Weather/Hazard Highlights... Thus far the forecast has been fairly consistent for the heavy rainfall threat over parts of the Northern Plains around Tue-Wed. The surface front at the leading edge of ejecting western U.S. upper trough energy will be very slow during this time frame and may allow for some training/repeat activity. After midweek the front should accelerate a bit and weaken, leading to lower rainfall totals to the east. Some lingering precipitation will be possible in the northern Rockies early in the period but with less intensity than expected in the short range period. Meanwhile (especially during Tue-Wed) the upper low over the Northeast should lead to diurnally-favored rainfall of varying intensity. Signals are mixed regarding where heaviest rainfall may occur within the broad area of rainfall potential near and south of a weakening front from the Midwest into the Carolinas. Locations over/east of the southern half of the Mississippi Valley will need to monitor the possibility for energy aloft to settle near the region by late week and promote areas of heavier rain. Parts of the Appalachians and Southeast could see meaningful five-day totals as well. Some moisture rotating around the upper high over western Texas will aide in producing some scattered showers for the southern Rockies/eastern Four Corners region later in the week Temperature anomalies over the lower 48 should become less extreme with time as the pattern aloft trends toward a more typical summer appearance. Portions of the Rockies and Interior West will see highs up to 10-20F below normal on Tue and to some extent into Wed, followed by a trend toward near normal readings. The warm sector ahead of the Northern Plains front will see more extreme morning lows (plus 10-20F anomalies) relative to daytime highs (generally plus 5-13F). Parts of the central/southern Plains may also see plus 5-12F anomalies for highs early in the period. Northern tier warmth should extend into the Great Lakes and eventually Northeast once the upper low opens up/departs, though a front dropping south from eastern Canada may make the warmup fairly brief over New England. Periods of clouds and rainfall will likely promote near to slightly below normal highs over the southeast quadrant of the country, except for Florida which may see slightly above normal temperatures through the period. Fracasso/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