Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 AM EDT Sun Jul 11 2021 Valid 12Z Wed Jul 14 2021 - 12Z Sun Jul 18 2021 ...Western Heat Persists Thru Next Week and Spreads into the Northern Plains... ...Midwest Heavy Rainfall Threat to Redevelop Midweek... ...Weather Highlights/Hazards... Another significant heat wave now underway over the West under upper ridging will continue from the Great Basin to CA and the Southwest into early-mid next week with temperatures 10-15F above normal. Several daily temperature records are likely into Tuesday/Wednesday. Guidance 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 over the Great Basin to the central Rockies and northern Plains as a closed low persists off the west coast of Canada next weekend. Above normal temperatures are expected across much of the northern Great Basin and northern Rockies/High Plains next Fri 16 July- Sun 18 July, with cluster of high temperatures reaching 10-15 degrees above normal next weekend. Lingering upper ridging over the western Atlantic and into the Southeast may also 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. Over next weekend, the upper trough drifts east from the Lakes into the northeast, driving a cold front across the region. This increases shower/storm chances with the front, followed by post-frontal cooling in the lower Lakes and then next Sunday 18 July into the adjacent northern mid Atlantic. 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. With a wave front in the Midwest to the central Plains, additional rounds of heavy showers and storms are forecast near the meandering front, instability and upwards of 1.75" precipitable water values. The models have renewed rainfall maxima gradually shifting from the Upper Midwest by midweek to adjacent portions of the Mid-MS/OH Valleys late week with slow frontal progression. The rain threat area drifts south next weekend down the plains, MS Valley and from the OH Valley into the TN Valley, central-southern Appalachians, and mid Atlantic. ...Guidance/Predictability Evaluation... Model and ensemble forecast spread and uncertainty remains below normal next week with exception of shortwaves progressing through a mean Midwest trough and whether a closed low develops within the 500 mb trough next weekend near the Lakes/adjacent Canada. The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the 18 UTC GFS/GEFS, 12 UTC ECMWF/UKMET and ECMWF ensembles along with the 01 UTC National Blend of Models. The composite maintains good WPC continuity and acts to mitigate smaller scale guidance differences consistent with predictability. Petersen/Schichtel 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