Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Jul 17 2021 Valid 12Z Tue Jul 20 2021 - 12Z Sat Jul 24 2021 ...Heat continues from the northern Great Basin to the northern Rockies/northern Plains... ...Overview... The strong upper high forecast to be centered near the central Rockies will maintain well above normal temperatures over the northern Rockies/Plains, with some of this heat extending into the Upper Midwest and less extreme anomalies reaching back into the Great Basin. An upper low forecast to track from offshore British Columbia toward central Canada should gradually flatten the ridge to the north of the upper high, with a cold front beginning to erode the western side of the heat by next Saturday. Downstream mean troughing will prevail over the East, with the northern portion likely to trend a little deeper with time. The trough will support a series of fronts and accompanying episodes of showers/thunderstorms along with near to moderately below normal temperatures. Meanwhile energy pulling away from the trough by the start of the period early Tuesday should form a southern Plains upper low that lingers and slowly weakens with time. This feature could support some areas of locally enhanced rainfall over eastern to southern Texas as clouds/rain from this system together with monsoon activity farther west keep high temperatures below normal from the southern Plains into the Southwest. ...Guidance/Predictability Evaluation... Most guidance continues to offer similar ideas for the large scale pattern along with embedded small scale uncertainties. A blend of the 00Z ECMWF, 06Z GEFS, and 00Z CMC mean together with the deterministic components yielded a solution that maintains good continuity with previous forecast cycles. The 00Z GFS appeared to be an outlier late in the forecast period with too much frontal wave development across the northern Plains toward the Great Lakes, and thus was not included in the blend. The 00Z ECMWF and the CMC tended to keep an axis of heavy rainfall across eastern to southern Texas from mid to late next week on the eastern flank of the upper vortex. This scenario was not supported by both the EC mean and GEFS mean however. Thus, the heavy rain axis over northeastern Texas was downplayed for the current QPF blend. For the rest of the CONUS, models exhibited reasonable variability/uncertainty for the medium range forecast period so that a general model compromise trending toward their ensemble means should offer a good starting point. ...Weather Highlights/Hazards... The greatest northern tier temperature anomalies (plus 10-20F) associated with the central Rockies upper high should extend from the eastern half or more of Montana into the Dakotas and parts of the Upper Midwest. Some locations in Montana and the Dakotas may see one or more days with highs reaching at least 100F. Best potential for daily records will be in the short-range period but a few isolated records could extend into at least Tuesday. A cold front expected to reach the northern Plains by Saturday should bring some moderation to the heat for Montana at that time. Southern Plains upper low/wavy front may produce some areas of heavy rainfall Tuesday-Wednesday followed by a lighter trend as these features weaken. The remaining thunderstorms should tend to drift east into the lower Mississippi Valley toward the Southeast for the latter part of next week south of the dissipating front. Meanwhile, rain and thunderstorms should continue within an area of monsoon moisture over parts of the Four Corners states, possibly extending into parts of California/Nevada after midweek. Coverage and intensity of rainfall will likely increase somewhat with time, and with a potential contribution from the southern Plains moisture/upper level energy. The unsettled pattern over the southern Plains/Southwest will lead to highs up to 5-10F or so below normal for one or more days. Expect lingering rain/thunderstorms with a leading front that settles over the Mid-Atlantic and South by Tuesday. Two trailing fronts dropping southeastward from Canada will bring rainfall of varying intensity, mainly to the Great Lakes/Northeast but possibly farther southward at times. Temperatures over the East will likely be near to moderately below normal through the period. Kong/Rausch 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