Extended Forecast Discussion NWS Weather Prediction Center College Park MD 509 PM EDT Fri Jul 16 2021 Valid 12Z Mon Jul 19 2021 - 12Z Fri Jul 23 2021 ...Heat continues from the northern Great Basin to the northern Rockies/northern Plains... ...Overview... A strong upper high anchored over northern/central Rockies will support much above normal temperatures from the northern Great Basin through the northern Plains well into next week before spreading farther east into the Upper Midwest late next week. Meanwhile, downstream upper troughing will bring a series of fronts into the East along with near to moderately below normal temperatures. Vorticity breaking off on the western side of the trough would increase the chance of locally enhanced rainfall across portions of the southern Plains together with below average high temperatures. An upper low off the coast of British Columbia should lift into western Canada during the latter half of the week as an offshore trough edges closer toward the Northwest against the strong upper high well inland. ...Guidance/Predictability Evaluation... The relative agreement and continuity of models/ensembles for the overall pattern indicate better than average predictability with uncertainty expected regarding the specifics of smaller-scale shortwaves toward the end of the forecast period. Models have generally agreed better with the placement of an upper vorticity centered over the central Plains that is forecast to break off from the positively-tilted trough across the Northeast. This has led to better definition for a heavy rain area to develop in the models over the southern Plains between the upper vorticity and the cold front. A general model compromise among the 06Z GFS/GEFS, 00Z ECMWF/EC mean, and smaller contribution from the 00Z CMC/CMC mean was used to compose the morning WPC medium range package. A greater weight toward the ensemble means was used for Days 6-7 to handle the uncertainty. The results only deviated slightly in details compared with the previous forecast package. ...Weather Highlights/Hazards... The potent upper ridge likely to be centered near southern Wyoming for much of next week will sustain very warm to hot temperatures from the northern Great Basin through the northern Rockies/Plains with highs in the 90s up to at least lower 100s at lower elevations, which are 10-20 degrees above normal. Such readings may approach or exceed daily records especially early in the week. In less extreme fashion some of this heat will extend eastward into Minnesota and Wisconsin. On the other hand daytime highs in the Southern Tier/Southwest will be somewhat below normal (by up to 5-10F on one or more days) due to increased cloud cover and higher rainfall chances. The combination of a wavy front and possible upper low reaching the southern Plains and vicinity could produce some areas of locally heavy rainfall. Monsoon activity over Arizona and some neighboring areas should continue, with an increase in rainfall amounts possible late in the week depending on how far southern Plains moisture and upper level energy retrograde. A leading wavy and slow-moving front will extend from near the New England coast through the Mid-Atlantic into the South early in the week, accompanied by scattered to numerous showers and thunderstorms with locally heavier amounts where thunderstorms locally train. Additional fronts should reach the Great Lakes/Northeast around Tuesday-Wednesday and then Thursday-Friday. In both cases lower-predictability shortwave specifics and frontal waves will play a role in the existence/placement of any locally heavier rainfall. The mean trough aloft and series of fronts will keep temperatures over the East near to somewhat below normal next week. Kong/Rausch/Fracasso Hazards: - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Mon, Jul 19. - Heavy rain across portions of the Southern Plains, Tue-Wed, Jul 20-Jul 21. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Tue, Jul 20. - Heavy rain across portions of the Southwest, Mon-Thu, Jul 19-Jul 22. - Excessive heat across portions of the Northern Plains, Mon-Thu, Jul 19-Jul 22. - Excessive heat across portions of the Upper Mississippi Valley and the Northern Plains, Wed-Fri, Jul 21-Jul 23. - Much above normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, the Northern Great Basin, and the Upper Mississippi Valley, Mon-Fri, Jul 19-Jul 23. - Much above normal temperatures across portions of the Central Rockies, the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Wed-Fri, Jul 21-Jul 23. 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