Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EDT Sat Jul 24 2021 Valid 12Z Tue Jul 27 2021 - 12Z Sat Jul 31 2021 ...North-central U.S. triple-digit heat may exceed records into early-mid next week... 1800 UTC Update... No major changes were needed for this update of the WPC medium range progs. Models continue to advertise strong upper ridging holding over the central U.S. while mean troughs become established across the East Pacific and the Northeast U.S.. A blend of the deterministic models served as a good starting point for days 3-5, with some inclusion of the ECENS/GEFS ensemble means later in the period to help smooth out the harder to resolve detail differences. See previous discussion attached below for additional details regarding the pattern, uncertainty, and notable weather hazards. Santorelli Previous Discussion below issued at 0646 UTC... ...Overview with Weather Highlights/Hazards... Latest guidance continues to advertise a hot ridge dominating much of the west-central U.S. into next week while multiple shortwaves push downstream to sustain general troughing across the Midwest/Northeast, where models continue to struggle with resolving the various shortwave interactions and arrive at a common and stable solution. Rain associated with wavy and slow moving fronts can be expected to spread mainly from the Midwest/Great Lakes to the Northeast/Mid-Atlantic with considerable uncertainty in its timing and intensity. Meanwhile, lead scattered thunderstorms can also be expected to push slowly south into the Southeast by midweek. In addition, the National Hurricane Center continues to monitor the potential for tropical cyclone formation east of northern Florida. The disturbance may intensify and drift toward the southeast U.S. coast early to middle of next week along with deepened moisture availability to fuel local downpours. Meanwhile, a heatwave is forecast to peak Tuesday for the northern Plains/Rockies and into the northern Great Basin. High temperatures for the northern Plains are forecast to exceed 100F into Tuesday and Wednesday, likely producing some record values. Gradual moderation is forecast for the north-central U.S. later next week, but it appears that anomalous heat will once again intensify across the interior Pacific Northwest. An upper low drifting offshore into the Pacific and the strengthening of the west-central U.S. upper ridge into next week should act to spread moisture to the north and west out from the Southwest. Expect modest monsoonal activity will extend into California, the Great Basin, then northward and onward into the Rockies then Plains. ...Guidance/Predictability Evaluation... The medium range pattern evolution at mid-larger scales overall seems to offer slightly above average predictability Days 3-7. However, the least certain mass field part of the forecast remains across the Midwest/Northeast where models continue to struggle with resolving the various shortwave interactions converging along a mean trough. A wide variety of solutions regarding the timing and intensity of the upper trough and surface based low pressure system developments. Prefer an upper trough amplitude at least on the more amplified side of the full guidance envelope given the staying power and amplitude of the building upstream upper ridge. Of the models, a blend of the 18 UTC GFS and 12 UTC ECMWF seems to best provide this. In view of forecast spread, also recommend inclusion of the 18 UTC GEFS and 12 UTC ECMWF ensemble means to the composite along with the 01 UTC National Blend of Models. Respective 00 UTC models stayed in line. Schichtel 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