Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Jul 18 2021 Valid 12Z Wed Jul 21 2021 - 12Z Sun Jul 25 2021 ...Heat will continue over the northern Plains and extend into the Upper Midwest... ...Overview... Models and ensembles continue to advertise a recurring synoptic scale pattern across the the U.S. through the medium range period where a persistent upper high anchored over or near the central Rockies will be contrasted with multiple shortwaves traveling downstream toward a mean trough across the Northeast into eastern Canada. Meanwhile, an upper low just off the British Columbia coast in the short-range period is forecast to push a front against the upper high and should bring only slight moderation of the hot weather into the northern Plains. On the other hand the combination of an upper vortex sliding across the southern Plains and monsoonal moisture over and just west of the Four Corners states will promote episodes of rain and thunderstorms along with below normal high temperatures to the south of the upper high. The eastern U.S. should see near to below normal temperatures under mean troughing aloft while multiple fronts will likely provide episodes of showers and thunderstorms. ...Guidance/Predictability Evaluation... As has been the case lately, models have generally locked into the aforementioned synoptic pattern in which agreement is good for the mean flow while some spread exists for the embedded details. The faster northwesterly flow with multiple shortwaves downstream from the upper high continues to introduce some uncertainty into the forecast. The 00Z EC mean was faster in carrying a batch of rain across the Great Lakes late next week than the 06Z GEFS mean. A compromise of the two solutions was adopted. Over the southern Plains, models are now more eager to push the pre-frontal rainfall farther south into southern Texas midweek with a flatter frontal wave. Meanwhile, there has been a general increase in the monsoonal rainfall amounts across central Arizona where heavy rain threat appears to have increased through the medium range period. By next weekend, some deterministic models are indicating low pressure development across the Great Lakes but this scenario is not supported by the ensemble means at this time. Finally, models have trended slightly faster with the low pressure wave over the northern High Plains next weekend. 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 smaller weight was given to the 06Z GFS beginning on Day 5 when the predicted flow pattern appears too fast across the Northeast. 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... Portions of the northern Plains into Upper Midwest will likely see highs up to 10-15F above normal on multiple days. Highest anomalies and best potential for parts of eastern Montana and the Dakotas to reach at least 100F should be during Wednesday-Friday. A front extending from Canadian low pressure will likely bring temperatures down a little from west to east into the weekend but perhaps by only a few degrees or so. Plus 10F anomalies for highs may also reach southward into the central Plains around late week. Another period of enhanced monsoonal rainfall will be possible over the Four Corners states after midweek, with some moisture possibly reaching a little farther northward toward the northern Rockies as well. There is an increasing threat of heavy rain across central Arizona where monsoonal moisture has increased. The unsettled regime will keep highs up to 5-10F or so below normal over the southwestern U.S. Over Texas expect rainfall to continue trending lighter and more scattered with time while below normal highs trend somewhat closer to normal. Lingering moisture along and south of a dissipating southern tier front may provide localized enhancement to diurnal showers/thunderstorms into the end of the week. The northeastern quadrant of the lower 48 should see areas of rain/thunderstorms accompanying the fronts pushed along by flow within the mean trough aloft. Some rain may be locally moderate to heavy but confidence in specifics is fairly low at this time. Eastern U.S. temperatures should be near to moderately below normal through the period where another episode of rainfall is possible by next weekend. 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