Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM 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 updated forecast based on 12Z/18Z guidance started with a model consensus on day 3 Tue followed by elimination of models whose solutions strayed from consensus for either the western trough or eastern trough/upper low, along with increasing ensemble mean input. The 12Z CMC strayed to the west/southwest for the Northeast upper low after Tue (corrected in the 00Z run) while the 12Z UKMET become a flat extreme with flow coming into the Northwest trough after Wed. Meanwhile the blend favored the 12Z GFS versus the 18Z run, as the former was closer to consensus for the Northeast upper low and better reflected the majority of guidance that was more progressive with individual features embedded within the Northwest trough during the latter half of the period. Also the 12Z GFS was somewhat closer (but still somewhat flat) versus other solutions for the eastern Canada upper trough that brings a front into the Northeast late in the week. The 12Z GEFS mean (given somewhat less weight than the ECMWF mean) held onto the eastern trough a little longer than the 18Z version. Total ensemble weight reached 60 percent by day 7 Sat. Relative stability of the ensemble means provide at least a moderate degree of confidence in the large scale evolution. However operational model detail differences/variability for embedded features, as well as some more significant changes between the 12Z and new 00Z ECMWF runs over the Northeast Pacific and northwestern North America, temper confidence for the details. ...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. 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 5-12F). 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. 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