Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Mon Jul 19 2021 Valid 12Z Thu Jul 22 2021 - 12Z Mon Jul 26 2021 ...Heat will continue over the northern Plains and extend into the Upper Midwest... ...Overview... Latest guidance continues to show a persistent mean pattern featuring an upper high dominating the central Rockies/High Plains while successive shortwaves from northern/eastern Canada will tend to maintain an upper trough across the northeastern quadrant of the country. A couple of upper troughs from the northeastern Pacific are forecast to move onshore across British Columbia against the strong upper high over the Rockies during the medium range period, but bringing only a modest cooling trend to the excessive heat over the northern Plains. Multiple fronts with attached low pressure waves reaching the East will bring episodes of showers/storms while keeping temperatures near to slightly below normal. Farther south, a lingering upper disturbance moving westward across the southern Plains into the southern Rockies will promote periods of rain/thunderstorms and below normal high temperatures over portions of the Desert Southwest and the Four Corners where the monsoonal regime will remain active. ...Guidance/Predictability Evaluation... Models and ensembles this morning generally agree quite well for the synoptic pattern across U.S. into the medium range period with reasonable increase in model spread toward Day 7. Guidance has generally abandoned the idea of bringing an anomalous upper low from east of Hudson Bay down toward the Northeast U.S./eastern Canada by Friday-Saturday. This has hastened the eastward progression of the low over the Canadian Maritimes as well as the frontal passages across the Northeast behind the low. Over the northern Plains, a frontal wave is forecast to track across the region to reach the Great Lakes by the weekend where the ECMWF and GFS indicate the possibility of heavy rain. The Canadian model has sped up this frontal wave. Models then take the moisture downstream toward the eastern U.S. where a round of rainfall appears to move into the Appalachians and the East Coast early next week. Meanwhile, the ECMWF now agrees with the rest of the guidance to retrograde the weak upper low over the southern Plains toward the southern Rockies this weekend. This could bring additional instability into the southern Rockies/Southwest later this weekend and may have enhancing effects on the monsoonal rainfall already in place across the area into early next week. 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 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 Thursday-Friday, when plus 10F anomalies for highs may also extend into the central Plains. 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. Northern High Plains temperatures could rebound some next Monday as southwesterly flow increases ahead of the next upper trough approaching the Pacific Northwest. Another period of enhanced monsoonal rainfall appears likely over the Four Corners states from late this week onward. Some moisture could reach a little farther northward and westward as well. Coverage and intensity of rainfall will depend in part on specifics of low-predictability impulses tracking around the central Rockies upper high, including possibly the small upper low initially over the southern Plains. Rain/thunderstorms could be heavy at times with highest totals during the period currently expected over central Arizona. 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. Locations from the Upper Midwest through the Great Lakes/Ohio Valley into the Northeast/Mid-Atlantic should see at least a couple episodes of rain and thunderstorms with fronts and low pressure waves that reach the eastern U.S. Models are providing a general signal that some of this activity could be locally moderate to heavy but continue to vary considerably for the details which may take into the short-range time frame to come into better focus. The upper trough over the Northeast will keep highs over that region moderately below normal, especially late this week into the weekend. The remainder of the East should see readings within a few degrees on either side of normal. 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