Extended Forecast Discussion NWS Weather Prediction Center College Park MD 531 PM EDT Thu Jul 22 2021 Valid 12Z Sun Jul 25 2021 - 12Z Thu Jul 29 2021 ...Locally heavy rainfall threat through Sunday in the Southwest/Four Corners Monsoon followed by a lighter trend... ...Heat will continue over the northern/central Plains into the Upper Midwest... ...Overview... Expect upper ridging to consolidate and strengthen over the central Rockies/Plains by midweek after initially extending from California and the Great Basin into the central/southern Plains on Sunday. An upper low drifting westward near the western U.S.-Mexico border as of early Sunday will continue to enhance Southwest Monsoon rainfall through the end of the weekend. This feature should then move off the southwest U.S. into the Pacific, leading to a lighter trend for rainfall over Arizona and vicinity. Flow around the northern side of the ridge will feed into a broad Northeast mean trough aloft, while an initial northeastern Pacific low should eventually open up but with considerable uncertainty over how/when its energy will become incorporated within the progressive flow along/north of the U.S.-Canada border. This overall pattern should lead to a meandering surface low/front over the northern Plains--generally providing only modest variability within a lengthy period of hot weather--while fronts periodically reach the Northeast/Mid-Atlantic. Finally, guidance continues to suggest that a surface trough with one or more embedded lows may extend from off the Southeast/eastern Florida coast into the Gulf of Mexico to start the week and then weaken as it edges closer to the southeast U.S. coast. ...Guidance/Predictability Evaluation... Guidance this morning continues to indicate noticeable spread regarding the details of the upper-level confluent flow across the northern tier states through much of the medium-range period. From the larger-scale perspective the primary issue for this stream is when/how the upper low initially over the northeastern Pacific opens up and its energy becomes embedded within the progressive flow. The deterministic 00Z ECMWF solution began to deviate further and further from its ensemble mean on Day 5 to the extent that a cyclone develops over the Northeast midweek only in the deterministic solution. On the other hand, the GFS did not exhibit such deviation for the low over the Northeast but for the next cyclone moving into the northern High Plains midweek. In view of these model spread, more weights from the ensemble means were applied starting on Day 5. This resulted in a blend that yielded a lingering low pressure wave near the Great Lakes, with a warm front lifting faster across the upper Midwest. Otherwise, changes to the remaining parts of the country were relatively minor through the medium range period. The WPC medium range forecast package was derived from a general model compromise among the 06Z GFS/GEFS, 00Z ECMWF/EC mean, and smaller contribution from the 00Z CMC/CMC mean. A greater weight toward the ensemble means was used from Day 5 onward to handle the uncertainty. ...Weather Highlights/Hazards... The potential for some locally heavy monsoonal rainfall should continue over Arizona through Sunday as a compact upper low tracks generally westward along the southern border of the state. This area will be sensitive to additional rain given the significant totals already expected in the short range time frame. The strengthening/consolidation of the Rockies-Plains upper ridge next week should force the weakening upper feature to the west and spread moisture to the north and west. Thus the Southwest will see rainfall trend lighter after Sunday. Activity that extends into California, the Great Basin, and northward over the Rockies should be less intense than with the Arizona event. Showers/thunderstorms will accompany a front pushing from the Great Lakes/Plains into the East from Sunday onward. By Tuesday-Thursday another front may bring rainfall of varying intensity to areas from the Upper Midwest through New England. With considerable uncertainty in the specifics, the weak surface feature off the southeast Coast and possibly into the eastern Gulf may enhance rainfall at some locations. Please monitor National Hurricane Center outlooks for latest information on development potential. Within the persistent hot pattern from the northern/central Plains into the Upper Midwest, expect Monday through Wednesday to feature the greatest coverage of plus 10-15F or so high temperature anomalies--with some locations in the Plains reaching at least 100F. A meandering surface low/front across the the northern tier will likely provide some day-to-day variability for temperatures but not a meaningful cooling trend. The one exception could be over the northern High Plains on Thursday when highs may decline to only a few degrees above normal. There is still uncertainty in the surface front/temperature details though. From Sunday into Monday the northern Great Basin and interior portions of the Northwest will also see hot conditions with highs possibly reaching 10F or so above normal. A few locations could approach or reach daily record values. The moisture over the Southwest will initially support highs up to 10-15F below normal followed by a moderating trend. Northwestward progression of this moisture will bring temperatures down somewhat over the Great Basin and interior Northwest after Monday. Most of the eastern U.S. should see near to somewhat above normal readings. Kong/Rausch Hazards: - Heavy rain across portions of the Southern Rockies, the Central Great Basin, and the Southwest, Sun-Mon, Jul 25-Jul 26. - Heavy rain across portions of the Southeast, Sun, Jul 25. - Heavy rain across portions of the Mid-Atlantic, Mon-Tue, Jul 26-Jul 27. - Excessive heat across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Sun-Wed, Jul 25-Jul 28. - Excessive heat across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Jul 26-Jul 28. - Excessive heat across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Tue-Thu, Jul 27-Jul 29. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun-Mon, Jul 25-Jul 26. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, the Southern Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Great Lakes, Sun-Thu, Jul 25-Jul 29. 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