Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Jul 24 2024 Valid 12Z Sat Jul 27 2024 - 12Z Wed Jul 31 2024 ...Hazardous heat to shift into the northern-central Plains... ...Showers/storms with locally heavy rainfall possible from the western Gulf Coast into the Southeast and northward into the Upper Midwest... ...Overview... Guidance shows a somewhat complex evolution over the eastern half of the country and western Atlantic during the weekend into the start of the week, while progressive northern tier flow will carry energy ejecting out of a persistent West Coast mean trough. By next Wednesday the merger of a retrograding and strengthening Southeast/Gulf coast upper ridge with a separate ridge lingering over the southern Rockies should ultimately yield a strong southern Rockies/Plains ridge with a hint of troughing over the Great Lakes/Northeast. This evolution will support an expanding area of hot weather across the northern-central Plains and into the Great Lakes/Northeast. Locations from the South through the east-central U.S. into the Upper Midwest may see one or more days of locally heavy showers/storms, while monsoonal convection over the West will likely be more confined to southern locations after Saturday. ...Guidance/Predictability Assessment... Dynamical and machine learning (ML) guidance is still showing a fair degree of spread for specifics of the upper low/shortwave energy ejecting northeastward from the Plains along with separate East Coast energy that may form a weak upper low along or offshore the coast. For the ejecting Plains energy, the UKMET has tended to be on the strong side and the CMC on the weaker side. Farther east, the average of 12Z ECMWF-initialized ML models would suggest an upper low would most likely form/track over a position south/southeast of New England by early next week and then lift northward as ridging to the east becomes more pronounced. This feature would then eject northeastward with the approach of upstream flow (faster than the 12Z ECMWF, and the new 00Z run has trended faster). Specifics of southern tier ridging and the progressive flow on its northern periphery have low predictability at extended time frames. Also, the details of Pacific Northwest mean trough become increasingly ambiguous by the early-middle part of next week, with various ideas for individual shortwaves that may feed into/eject from the feature. 12Z/18Z guidance comparisons led to starting the updated forecast with an operational model composite early in the period (with more GFS/ECMWF weight relative to the CMC/UKMET) followed by a trend toward 60 percent total weight of the ensemble means by next Wednesday. ...Weather/Hazards Highlights... For the Days 4-5 Excessive Rainfall Outlooks valid this weekend, one area of focus will be over the northern Plains/Upper Midwest where shortwave energy and a wavy surface front will interact with enhanced moisture, with ground conditions still on the wet side. Although guidance signals for QPF have become a bit more diffuse during Saturday versus continuity, moisture anomalies/instability still seem to support the ongoing Marginal Risk area (Level 1 of 4). The GFS/ECMWF and their ensembles suggest greater heavy rainfall potential by Sunday but remaining guidance differs in some respects, so for now the plan is to include the region as part of a larger scale Marginal Risk area with the understanding of upgrade potential to a Slight Risk pending improved clustering. Farther south, the weekend will start with a broad area of locally heavy rainfall potential with anomalous moisture extending from the western Gulf Coast to the southeastern coast (the latter with a front sinking southward) and extending north into the Middle Mississippi Valley/Lower Ohio Valley (along/ahead of an upper low/trough ejecting northeast from the central Plains). The Day 4 ERO depicts a broad Marginal Risk area across these areas with one or more embedded Slight Risk areas possibly arising if short term guidance refines areas of best focus. By Day 5 the corridor of best moisture/instability should extend from the Southeast into the Midwest, possibly getting close to connecting with the northern tier moisture. Lingering moisture over southern Texas will support a Marginal Risk area there as well. Shortwaves and moderately above-climatology moisture will support a Marginal Risk area in the Day 4/Saturday ERO over and near the southern two-thirds of the Rockies. Much of the West will see a push of drier air by Day 5 Sunday, confining the Marginal Risk area to southeastern Arizona/southwestern New Mexico. Expect the evolving pattern to support an expanding area of hot weather over portions of the Plains and extending into the Great Lakes and Northeast. The most persistent and extreme high temperature anomalies should be over the central Plains where some locations may see multiple days with highs 10-15F above normal. The Northeast could see some highs reach 10F or so above normal after Sunday. The experimental HeatRisk likewise reflects an expanding area of Moderate to Major (Level 2-3 of 4) risks of heat-related impacts during the weekend through next Wednesday, and even some pockets in the extreme category over the central U.S. by next week. Forecast temperature anomalies would yield highs in the upper 90s to low 100s over the central Plains and upper 80s to around 90 farther northeastward. Climate Prediction Center forecasts indicate some variation of this pattern may persist well beyond next Wednesday. Much of the West will likely see near to slightly below normal highs through the period. The Southwest could be a little warmer on Saturday, and then again by midweek as southern Rockies upper ridging rebuilds. Lingering rainfall over the southern tier should support near to below normal highs, especially over southern/eastern Texas especially during the weekend. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw