Extended Forecast Discussion NWS Weather Prediction Center College Park MD 124 AM EDT Sat Jul 10 2021 Valid 12Z Tue Jul 13 2021 - 12Z Sat Jul 17 2021 ...Southwestern Heat Persists into Next Week... ...Midwest Heavy Rainfall Threat Persists Into the Middle of Next Week... ...Pattern Overview with Weather Highlights/Hazards... The heat wave now underway under upper ridging will continue for the Great Basin to CA and the Southwest into early-mid next week with temperatures 10-15F above normal. Several daily high temperature records are likely through Tuesday in these areas. The model consensus is leading to gradually lowering heights with a developing modest northwest US trough, with the highest heights shifting south into southern CA/AZ. There is potential for rebuilding heights across the northwest US as a closed low develops off the west coast of Canada next Fri-Sat 17 July. Above normal temperatures are expected across much of the northern Great Basin, northern Rockies, and northern Plains next Fri 16 July- Sat 17 July. Lingering upper ridging over the western Atlantic and into the Southeast may support above normal temperatures next week from the Mid-Atlantic to the lower Great Lakes and into the Northeast. This includes the Washington, DC to Baltimore, Philadelphia, and New York City corridor. The forecast does indicate potential for temperature moderating showers/thunderstorms over Arizona/Southern Rockies next week with pockets of enhanced 700 mb relative humidity in place to support diurnal activity with instability peaking each afternoon/evening. However, this could also lead to additional local downpour and terrain enhancing runoff issues. There is also a persistent signal a multi-day stretch of episodes of heavy rainfall in the Midwest that begins in the short range, with more episodes occurring in the medium range Wednesday-Friday as moisture and instability pool ahead of a wavy front. The models still have rainfall maxima in Iowa/Missouri/Illinois Wed-Thu., with a shift into adjacent portions of the Mid-MS/OH Valleys Fri 16 July-Sat 17 July with slow frontal progression. This offers excessive rainfall and resultant flooding threats where heavy rainfall falls over short range time frames. ...Guidance/Predictability Evaluation... Model and ensemble forecast spread into next week remains low with the exception of waves progressing through the mean Midwest trough. The persistent differences between the GFS, ECMWF and their ensemble means on the wave timing suggests blending the operational and ensemble mean solutions until better clustering develops. The WPC medium range product suite was primarily derived from a blend of guidance from the 18 UTC GFS/GEFS, 12 UTC ECMWF/12z Canadian and ECMWF/GEFS ensembles along with the 01 UTC National Blend of Models. This broad composite maintains good WPC continuity and acts to mitigate smaller scale guidance differences. Petersen Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and Tennessee Valley, Mon, Jul 12. - Heavy rain across portions of the Middle Missouri Valley and the Middle and Upper Mississippi Valley, Wed, Jul 14. - Heavy rain across portions of the Central Plains, Middle Mississippi Valley, Ohio Valley, and Great Lakes, Thu-Fri, Jul 15-Jul 16. - Flooding possible across portions of the Northeast, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the Southeast, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Southeast, the Middle Mississippi Valley, and the Southern Plains. - Flooding likely across portions of the Southeast and the Southern Plains. - Excessive heat across portions of California, the Southwest, the Central and Southern Great Basin, and the Central Rockies, Mon-Tue, Jul 12-Jul 13. - Excessive heat across portions of the Interior Pacific Northwest, Mon-Wed, Jul 12-Jul 14. - Excessive heat across portions of the Central and Northern Great Basin, Mon-Fri, Jul 12-Jul 16. - Much above normal temperatures across portions of the Northern High Plains, Thu-Fri, Jul 15-Jul 16.