Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM 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 may weaken some later in the week but still linger over the southern Rockies/High Plains. This feature could support some areas of locally enhanced rainfall in its vicinity, with clouds/rain from this system and monsoon activity farther west keeping 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 uncertainties. Within the eastern trough aloft, at the start of the period early Tuesday the 12Z/18Z GFS runs were somewhat faster than most other guidance with a leading shortwave ejecting from the East Coast. The new 00Z GFS has adjusted somewhat closer to consensus. Solutions cluster well for a trailing midweek shortwave while some spread and variability continue for northern Canada energy that reinforces the upper trough later in the week. The southern Plains weakness/upper low clusters fairly well in latest guidance and there is also decent agreement into at least Thursday for the upper low that tracks from offshore British Columbia into Canada. How much eastward progression occurs after Thursday is somewhat of a question mark. Faster trend of the 18Z GFS versus the 12Z run (and maintained in the new 00Z run) has led to similar solutions in the GFS/ECMWF and the ensemble means though the 00Z CMC is a bit slower by Saturday. Toward the end of the period there is now a more pronounced consensus advertising development of a new upper trough south of Alaska, which should help to erode/eject the initial trough off the West Coast to a greater degree than seen in the GEFS mean. An operational model composite from the 12Z/18Z cycles trending to a model/mean blend later in the period (a little more ECens relative to GEFS) reflected the most common elements of guidance while downplaying uncertain smaller-scale details and less confident aspects of one solution or another depending on the feature. This approach maintained reasonable continuity with only typical run-to-run adjustments. ...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 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. 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. Rausch/Kong 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