Extended Forecast Discussion NWS Weather Prediction Center College Park MD 106 AM EDT Mon Jul 12 2021 Valid 12Z Thu Jul 15 2021 - 12Z Mon Jul 19 2021 ...Western Heat Persists This Week in the Great Basin and northern Rockies and Spreads into the Northern Plains... ...Weather Highlights/Hazards... The modest northwest US trough is forecast to gradually retrograde, with the forecast 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 into early next week. As the upper ridge amplifies next Monday, the anomalies gradually increase, peaking 15-20 degrees above normal in eastern Mt next Mon 19 Jul. Lingering upper ridging over the western Atlantic and into the Southeast may also support above normal temperatures this week from the Mid-Atlantic to the Northeast. Over next weekend, the upper trough drifts east from the Lakes toward 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/New England. The forecast does indicate potential for temperature moderating showers/thunderstorms over Arizona/Southern Rockies this week with pockets of enhanced 700 mb relative humidity in place to support diurnal activity with instability peaking each afternoon/evening. This may lead to local downpour and terrain enhancing runoff issues, as recent activity has moistened soils. With a wavy 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 rainfall maxima gradually shifting south across the the Mid-MS/OH Valleys late week and next weekend with slow frontal progression. The rain threat area drifts south next weekend and Monday 19 July down the Plains, MS Valley and from the OH Valley into the TN Valley, central-southern Appalachians, and mid Atlantic. ...Guidance/Predictability Evaluation... Guidance forecast spread and uncertainty remains below normal this week with a main exception of shortwaves progressing through a mean Great Lakes 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 12 UTC ECMWF/12z ECMWF Ensemble Mean/18z GEFS Mean. 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