Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 forecast to be 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 weaken and progress around the western periphery of the building Rockies/Plains ridge, 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 lifts northwestward. ...Guidance/Predictability Evaluation... Once again the full array of guidance provides above average confidence in the large scale pattern but below average confidence in the details, especially with shortwave specifics around the northern periphery of the Rockies/Plains upper ridge. 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. Most models from the 12Z/18Z cycles showed some splitting of the energy while the majority was slower than the GFS runs for the overall evolution. This consideration, along with the fact that the GFS strayed to the weak side of guidance with the Plains/Rockies ridge late in the period, led to a forecast approach that started with mostly operational model focus early and then trended toward the 18Z GEFS/12Z ECMWF means and lingering operational input from the 12Z ECMWF/CMC. Even with the southern Canada details looking more ambiguous as a whole after Monday, the blend still yielded some elements of continuity. Among new 00Z runs the GFS is still a bit on the fast side with the Northeast Pacific upper low but the CMC has trended faster to make the GFS appear less extreme. The ECMWF is still fairly slow with the low and also eventually builds the downstream ridge more into Canada than consensus/continuity. Meanwhile the 00Z GFS has adjusted stronger toward consensus for the Rockies/Plains upper ridge by the end of the period. Farther south the small scale of the feature initially expected off the southeastern coast/Florida will likely lead to low predictability until the short range time frame. The National Hurricane Center's 06Z Tropical Weather Outlook shows a 30 percent chance of development over the next five days. ...Weather Highlights/Hazards... The potential for some locally heavy monsoonal rainfall should continue over Arizona through Sunday as a compact upper low tracks 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. 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. 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