Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM 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 situated over the central Rockies/High Plains while a combination of energy from northern/eastern Canada and flowing around the northern side of the upper high will feed into an eastern U.S. upper trough. Northeastern Pacific troughing will be in the process of reloading at the start of the period as an ejecting upper low/trough reaches western Canada as of early Thursday. This upper low should continue into central Canada and possibly merge with northern Canada flow. The upstream trough should approach the Pacific Northwest late in the period. This pattern will maintain well above normal temperatures over the northern Plains and vicinity, with a front anchored by the Canadian system likely bringing only a modest cooling trend. Fronts reaching the East will bring episodes of showers/storms and keep temperatures near to slightly below normal. Farther south, a weak upper low over the southern Plains and an active Southwest/Four Corners monsoon will promote periods of rain/thunderstorms and below normal high temperatures over those areas. ...Guidance/Predictability Evaluation... Within the agreeable mean pattern there is still a struggle to resolve embedded details whose smaller scale leads to low predictability multiple days out in time. One persistent uncertainty is the details of short-range energy from Hudson Bay and northward reaching the Northeast U.S./eastern Canada by Friday-Saturday. Models have been quite divergent and inconsistent in the path of a compact upper low which could either remain at high latitudes or drop into or just north of Maine. At the moment the GFS is the only model to drop it southward--the opposite of 24 hours ago when the majority brought down the low versus a mere shortwave in the GFS. Upstream, a model/mean approach tones down run-to-run variability for timing of the Canadian system and trailing front that extends into the northern U.S. There has been a signal for leading low pressure to track approximately from the northern Plains through the Upper Great Lakes into the Northeast U.S./southeastern Canada but with a lot of spread and variability for specifics. The 12Z/18Z GFS runs looked particularly questionable with how strongly this low developed by early next week, while the new 00Z run appears more reasonable. Meanwhile guidance displays various ideas for the path of the weak upper low over the southern Plains. The 12Z ECMWF was on its own with a more eastward drift. GFS/CMC runs offer potential for it to retrograde toward the Southwest U.S. late in the period. By next Monday the 18Z GFS was on the strong side with various impulses reaching the Southwest/Great Basin with the ridge adjusting farther east into the Plains than consensus. The various forecast uncertainties led to a blended/mean approach to represent the most agreeable aspects of the pattern and an intermediate/majority scenario for the contentious details. Guidance weight began with mostly operational model emphasis on Thursday and trended somewhat more to the 18Z GEFS/12Z ECMWF means late. ...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 another upper trough approaching the Pacific Northwest could give a northward push to the front stalled over the northern Rockies/High Plains. 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 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. 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