Extended Forecast Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Thu Jul 15 2021 Valid 12Z Sun Jul 18 2021 - 12Z Thu Jul 22 2021 ...Hot temperatures from the northern Great Basin to the northern Rockies/northern Plains... ...Overview... An upper high over the Four Corners region Sunday will drift into Wyoming, maintaining well above normal temperatures for much of the region into the northern Plains. A deep upper low off the coast of British Columbia/Haida Gwaii will very slowly wobble inland over the period as the upper ridge over the Rockies flattens a bit. In the East, upper troughing will be reinforced from Canada and likely detach its southern extent westward across the Lower Mississippi Valley and southern Plains toward the upper Rio Grande, favoring a wetter and cooler than average pattern. ...Guidance/Predictability Evaluation... The ensembles remained in very good agreement overall with the longwave evolution and saw no reason to deviate from the consensus. The 00Z ECMWF and somewhat the Canadian offered the best clustering with the means as the GFS runs were more aggressive with troughing in the east as a result of a more amplified pattern upstream (slower with the upper low off BC). Multi-day ensemble trend was slightly more amplified overall but it is unclear if this will continue or if it was just within the typical noise of a day 6-7 forecast. ...Weather Highlights/Hazards... The potent upper ridge building from the Great Basin to the northern Rockies/northern Plains will sustain very warm to hot temperatures across these regions--90s and lower 100s at lower elevations this weekend into next week, which are 10-20 degrees above normal. This may near or exceed daily records especially Sun/Mon. The above normal area will expand eastward into MN and WI around midweek. Temperatures in the Southern Tier/Southwest will be slightly below normal due to increased cloud cover and higher rainfall chances as a cold front settles (and dissipates) into the Southeast Mon-Wed. Rainfall could be locally heavy in parts of the central to southern Plains and MS Valley Sun-Tue along and ahead of the front. Farther west, the monsoon season has been more active than last summer in AZ and nearby areas with conditions favorable to persist, enhanced by the retrograding weakness aloft. Rainfall could even extend into Southeastern California later in the week. A wavy and slow-moving lead front will through the Northeast with its tail-end over the southern Mid-Atlantic as it settles over the South/Southeast, with scattered to numerous showers and thunderstorms and locally heavier amounts where thunderstorms locally train. High temperatures are forecast to fall below normal into early next week due the clouds and showers but near normal in its wake. Another cold front out of Canada could provide additional cooling of a few degrees around the eastern Great Lakes/Northeast later next week in what is typically they hottest time of the year. Fracasso/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 Hazards: - Heavy rain across portions of the central Plains to the lower Mississippi Valley, as well as portions of the Mid-Atlantic, Sun-Mon, Jul 18-Jul 19. - Heavy rain across portions of the southwestern Texas, Tue-Thu, Jul 20-Jul 22. - Heavy rain across portions of the Mogollon Rim to the Gila Mountains in Arizona, Sun-Thu, Jul 18-Jul 22. - Flooding possible across portions of the mid-Mississippi Valley, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of Florida and the mid-Mississippi Valley. - Excessive heat across portions of the Snake River valley and northern Rockies, Sun-Mon, Jul 18-Jul 19. - Excessive heat across portions of the northern High Plains, Sun-Tue, Jul 18-Jul 20. - Much above normal temperatures across portions of the central and northern Great Basin, the northern to central Rockies, across the northern Plains, and into the upper Midwest, Sun-Thu, Jul 18-Jul 22. - Much above normal temperatures across portions of the central Rockies, northern Plains, and into the upper Midwest, Wed-Thu, Jul 21-Jul 22. 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