Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM 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... Expect little change in the long-term mean pattern through the period, with a persistent upper high anchored over or near the central Rockies and mean troughs over the northeastern Pacific and eastern U.S./Canada. An upper low just off the British Columbia coast as of early Wednesday will track into central Canada, pushing a front into the northern Plains. This front should bring only slight moderation of the hot weather over the northern Plains though. On the other hand the combination of an upper weakness/low over 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 may provide a focus for showers and thunderstorms. ...Guidance/Predictability Evaluation... The best consensus of 12Z/18Z operational models early in the period followed by a trend to a model/ensemble mean blend late (60 percent total ensemble mean weight by day 7 Sunday) maintained reasonable continuity in principle with only typical run-to-run adjustments. As has been the case lately, agreement is good for the mean pattern while some spread exists for the embedded details. One area of uncertainty involves initial Hudson Bay energy that drops into the mean trough, with various ideas between sheared energy and a deeper compact upper low. New 00Z guidance tilts more toward the compact upper low scenario, though it remains to be seen if it tracks as far south as latest ECMWF runs (clipping Maine). Later in the period, there was not great support for the surface low offshore New England on Saturday in the 18Z GFS while over the past 12 hours the GFS has been backing off from low pressure the 12Z run had brought into the Northeast by Sunday. The upper low tracking into central Canada still has some timing variability by late in the period. On the positive side the 00Z CMC has trended more progressive to compare better with other models and the GEFS/ECMWF means. The upper feature evolving over the southern Plains as of midweek has generally trended a little southward early in the period and appears slightly weaker after Wednesday. Finally, ahead of the reloaded Northeast Pacific trough the 00Z CMC brings more height falls versus consensus into the Northwest next weekend. ...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 westward as well. 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. 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