Extended Forecast Discussion NWS Weather Prediction Center College Park MD 517 PM EDT Wed Jul 21 2021 Valid 12Z Sat Jul 24 2021 - 12Z Wed Jul 28 2021 ...Locally heavy rainfall threat through the weekend in an active Southwest/Four Corners Monsoon pattern... ...Heat will continue over the northern/central Plains into the Upper Midwest... 1830 UTC update... Models and ensembles continue to show very good agreement on the large scale upper level pattern, with lingering uncertainty in the details through the medium range period. Only minor changes were made to the previous WPC forecast based on the newest guidance runs. Today's update to WPCs surface and upper level charts used a general model blend days 3-5, with more inclusion of the ECENS/GEFS means days 6-7. See previous discussion attached below for details regarding the overall pattern, guidance evaluation, and the associated weather hazards. Santorelli Previous discussion below issued at 0700 UTC... ...Overview... The large scale upper pattern will remain fairly similar from the weekend into the middle of next week but there will be some adjustments along the way. Initial ridging aloft that covers the Great Basin/central Rockies and central/southern Plains should consolidate into a central Plains ridge likely to strengthen by midweek with guidance developing/expanding a 594 dm contour at that time. This pattern will keep well above normal temperatures over the central/northern Plains and Upper Midwest through the period. To the south of the initial western ridge, moisture and a compact upper low over Arizona and vicinity will extend the threat of locally heavy monsoonal rainfall through the weekend. Then the consolidation of upper ridging over the Plains should push the moisture/upper energy to the west and north thereafter, bringing a lighter trend to rainfall over the Southwest. Uncertainties over specifics of how a Northeast Pacific trough/upper low may open up after the weekend will affect the forecast across southern Canada and the northern tier U.S. after the weekend. Mean troughing should persist over the Northeast, with one shortwave departing early in the weekend followed by one or more features that should maintain cyclonic flow on average. ...Guidance/Predictability Evaluation... The updated forecast based on 12Z/18Z guidance continues to reflect a consensus model approach early in the period followed by a trend to a model/ensemble mean combination. Details are still in flux for the surface evolution near the southeastern coast. Recent trends have led to clustering in the southern half of the prior spread for initial low pressure (closer to the GFS than ECMWF from earlier runs). Latest solutions offer the potential for a defined surface low to reach into the Gulf of Mexico, whether from motion of the initial low or by redevelopment, in contrast to the wave staying near the Southeast coast. This cycle's manual forecast attempted a conservative trend by way of a more extensive trough while awaiting better continuity. Meanwhile there are only typical run-to-run adjustments for the frontal evolution from the northern Plains through the Northeast/Mid-Atlantic during the weekend into early next week. Fast/low amplitude nature of Pacific flow rounding the central U.S. ridge continues to suggest low predictability for surface details influenced by this stream. The signal persists for a wave to emerge from Montana around the start of the week and then track into Canada, but the models have not been consistent or agreeable with the supporting dynamics. The energy associated with the northeastern Pacific upper low expected to open up may also come into play by next Tuesday or Wednesday but again guidance differs on the strength/timing of the resulting shortwave(s). ...Weather Highlights/Hazards... Latest consensus shows the potential for areas of heavy rainfall within the Southwest/Four Corners Monsoon to extend into the weekend and still with central Arizona likely to see the highest totals from this event. Some areas will be very sensitive to additional rain given already wet soil from prior rainfall. Rain should trend lighter next week as upper level energy departs and the core of greatest deep moisture/rainfall lifts northwestward and becomes more diffuse. A combination of fronts should bring an organized area of rain and thunderstorms from the Great Lakes into the Northeast/Mid-Atlantic during the weekend into next week. Some rainfall could be locally moderate to heavy. Another episode of showers/storms could affect the Upper Midwest/Great Lakes next week depending on uncertain wave/front details. Portions of the Florida Peninsula may see enhanced rainfall to the south of a surface trough/possible embedded low whose details are very uncertain at this time. Expect a hot pattern to continue from the northern/central Plains into the Upper Midwest through the period with temperatures 5-15F above normal, including at least isolated Plains locations reaching 100F on one or more days. Additional humidity will make temperatures feel even hotter, with heat indices reaching 105-110F at a few locations in the southern portion of this region. Fronts reaching the northern tier will provide a little variability from day to day but without any meaningful cooling trend. Interior portions of the Northwest will also see fairly hot conditions but with anomalies tending to stay in the single digits above normal. Clouds and rainfall over the Southwest and vicinity will keep highs there up to 10-15F below normal through the weekend. Temperatures over the Southwest should trend somewhat closer to normal next week as the best moisture departs to the northwest. Most of the eastern U.S. should see near to slightly above normal readings except for some pockets of below normal highs over New England during the weekend. Rausch Hazards: - Heavy rain across portions of the Southern Rockies, the Central Great Basin, and the Southwest, Sat-Tue, Jul 24-Jul 27. - Excessive heat across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Jul 26-Jul 28. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat-Mon, Jul 24-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-Wed, Jul 25-Jul 28. 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