Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sun Jun 28 2020 Valid 12Z Wed Jul 01 2020 - 12Z Sun Jul 05 2020 ...Overview and Guidance/Predictability Assessment... Latest models and ensembles continue to show a fairly quick pattern transition Wed-Fri with persistence of the new regime through the weekend. The remains of a western U.S. trough will lift into Canada while the combination of Northeast Pacific/northwestern North America ridging and a broad ridge building from northern Mexico across the southern half of the Rockies and a large portion of the Plains will set up a new mean trough axis near the West Coast. An upper low over New England should open up and depart, replaced by a mean trough covering extreme eastern Canada and the Northeast. Guidance still indicates potential for diffuse energy aloft/troughing to settle somewhere between the lower half of the Mississippi Valley and southern Appalachians/Southeast. This feature could ultimately lead to some of the higher five-day rainfall totals over the lower 48 but low predictability of its details may make it challenging to resolve important specifics for a while. Early in the period the Northeast upper low and a front crossing the Northern Plains/Upper Mississippi Valley may produce meaningful rainfall over some areas. Within the upper trough setting up near the West Coast, guidance is generally maintaining recent trends toward one or more embedded upper lows that reach near the Pacific Northwest coast/Vancouver Island. It remains to be seen whether the end result is a single upper low or one being replaced by another, as either is plausible within the mean pattern. For the upper trough becoming established over the Northeast U.S./eastern Canada, latest GFS runs have finally joined consensus after a couple days of being reluctant to amplify the cyclonic flow. The 12Z CMC leaned to the amplified extreme of the spread though. Meanwhile there is a fair majority of models/means indicating the potential weakness aloft setting over the Southeast and/or Mississippi Valley but latest UKMET runs are weak/progressive. Latest array of guidance favored starting with a blend of the 18Z GFS and 12Z ECMWF/UKMET early in the period followed by a transition toward half model/half mean weight later in the period as details became more uncertain. ...Weather/Hazard Highlights... There will be a fairly broad area of potentially significant rainfall along and south of a Midwest through southern Mid-Atlantic front that will likely dissipate by the end of the week. The front may provide some focus for activity while it still exits, while energy aloft should become the primary influence by the latter half of the period. The low-predictability nature of the diffuse feature(s) aloft keeps confidence low for determining a more precise location and timing of heaviest rainfall between the lower half of the Mississippi Valley and southern Mid-Atlantic/southeastern coast. Some locally heavy rainfall may accompany the front crossing the Northern Plains/Upper Mississippi Valley early in the period followed by a gradual lighter trend as the front should trend a little faster and eventually weaken. One or more upstream fronts may approach the Northern Plains later in the period but with low confidence in eastward extent and available moisture. Expect the New England upper low to produce some showers/thunderstorms in its vicinity on Wed before the feature opens up and departs. Most rainfall should be light with the Canadian front that crosses the region late in the week. By late week/weekend expect enough moisture to exist under the upper high building over the Rockies and vicinity to allow for diurnal convection of varying intensity. The most persistent anomalous temperatures during the period should extend across the Upper Mississippi Valley through the Great Lakes with some readings up to 10-15F above normal on one or more days. Some of the above normal temperatures may extend back into the Northern Plains at times. New England will see some of this warmth mainly on Thu, followed by a cooling trend as the eastern Canada cold front drops southward and eventually stalls over the northern Mid-Atlantic. Portions of the Rockies and Interior West will see a moderating trend after a cool start on Wed when areas from the northern Rockies into Utah may see highs up to 10-15F below normal. Periods of clouds and rainfall may tend to keep the southeastern U.S. and vicinity modestly below normal for highs though Florida may see slightly above normal temperatures. 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