Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Thu Jul 02 2020 Valid 12Z Sun Jul 05 2020 - 12Z Thu Jul 09 2020 ...Overview and Guidance Assessment... The operational models and ensemble means maintain good agreement and consistency for the large scale pattern evolution. Expect some amplification of a persistent West Coast/eastern Pacific upper trough during the period while the center of a ridge initially over the Southwest/southern Rockies and northern Mexico shifts back to the southern Rockies and southern half of the Plains. Downstream a central Canada ridge and eastern Canada/Northeast U.S. mean trough should give way to increasingly progressive nearly zonal flow. A diffuse upper weakness/trough over the southeastern quadrant of the lower 48 this weekend will trend steadily weaker and/or shear out with time. Within this background pattern the forecast challenges for embedded details are similar to past days and fairly low in predictability due to their relatively small scale--impulses within and ejecting from the West Coast trough as well as specifics within the initial Southeast upper trough--or the evolution toward progressive and low amplitude mean flow as is expected across the northern tier U.S./southern Canada. Thus it is not surprising that continuity has been less than optimal for frontal position/timing from the Northern Plains through the Northeast/Great Lakes. Over the past day a consensus blend has held up fairly well for low pressure from the Southeast to East Coast and possibly western Atlantic but there is still a fair amount of spread for where a low center may be at a particular time. The agreeable large scale pattern and low confidence in specifics favor maintaining a forecast approach that incorporates the 18Z GFS and other models from the 12Z cycle early in the period followed by a gradual transition toward a blend of models and 18Z GEFS/12Z ECMWF means. Later in the period ECMWF input included both runs up to the 12Z cycle due to timing differences with an impulse expected to eject from the West Coast trough and produce a northern tier wave by day 7 Thu. Regarding this feature the 12Z ECMWF/ECMWF mean were on the leading side of the spread while 12Z/18Z GFS runs seemed a bit slow to eject it. Minor edits yielded a compromise timing. The new 00Z GFS compares better to other solutions, showing better definition of energy dropping into the core of the upper trough off Vancouver Island by next Thu and in turn faster ejection of the shortwave of interest. ...Weather/Hazard Highlights... The Northern Plains and possibly into the Upper Mississippi Valley, along with an area from the central/eastern Gulf Coast to the Southeast Coast will be the two regions in focus for potentially heavy rainfall. Frontal specifics over the northern tier have varied from day to day but there has been more consistency from a multi-day rainfall perspective due to initially slow progression of a leading Northern Plains front and then the trailing part stalling as it awaits ejection of shortwave energy from the West Coast upper trough. Confidence remains low for specifics of Southeast rainfall due to the small scale of features embedded within the overall weakness aloft and possible dependence on mesoscale influences on rain/thunderstorms. Less organized rainfall of varying intensity will be possible over parts of the Rockies and Central Plains as well as the Northeast/Mid-Atlantic. Continue to expect the highest temperature anomalies to be centered over the Great Lakes during the period with some daytime highs up to 10-15F above normal. A broader area covering much of the northeast quadrant of the lower 48 will see slightly less extreme anomalies. New England will start closer to normal but trend warmer by midweek as flow aloft flattens. Most of southern Florida and the Southwest will be moderately above normal for high temperatures but heat index values may be sufficiently high to represent excessive heat potential. Somewhat below normal highs will be possible near the West Coast mean trough aloft, over the northern High Plains during the first half of the period (behind the leading Northern Plains front), and with clouds/rainfall over parts of the Southeast. 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