Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Fri Jul 16 2021 Valid 12Z Mon Jul 19 2021 - 12Z Fri Jul 23 2021 ...Hot temperatures from the northern Great Basin to the northern Rockies/northern Plains... ...Overview... A strong upper high anchored over or near southern Wyoming will dominate the pattern next week. This high will support well above normal temperatures from the northern Great Basin through the northern Plains and to some degree into the Upper Midwest, while downstream upper troughing will bring a series of fronts into the East along with near to moderately below normal temperatures. Some energy within the western side of the trough could form an upper low that reaches the southern Plains/Rockies by mid-late week before weakening--resulting in some locally enhanced rainfall and below average high temperatures. An upper low off the coast of British Columbia/Haida Gwaii should eject into western Canada during the latter half of the week, with the trough to its south possibly lowering heights over the Northwest slightly. ...Guidance/Predictability Evaluation... The relative agreement and continuity of models/ensembles for the overall pattern would suggest better than average predictability but of course confidence is lower for specifics of smaller-scale shortwaves. For the updated forecast based on 12Z/18Z guidance, an operational model composite early followed by a trend toward a model/model ensemble mean blend represented the most common ideas of guidance while representing an intermediate approach where differences existed. The 12Z CMC strayed east of consensus with the Rockies upper ridge for the latter half of the period so the forecast phased out its already minimal weight at that time. Toward the end of week solutions diverge for how much troughing remains off the West Coast depending on the details of upstream flow. The GFS/GEFS runs through the new 00Z cycle maintain a deeper trough while the 12Z ECMWF mean is noticeably weaker and the 12Z ECMWF even develops a shortwave ridge. The 12Z CMC mean is conveniently between the extremes. This difference has persisted for multiple runs so an average of these scenarios appears best for now. Within the eastern mean trough, latest GFS runs could be a bit fast with leading energy over the Northeast as of early Monday. By day 7 Friday the 00Z CMC trended toward latest GFS runs and the 12Z ECMWF for what may be the strongest energy of the period to reach southeastern Canada and the northeastern U.S. However the new 00Z ECMWF keeps the feature farther north. ...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. Rausch/Fracasso 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